52 Radioactive Elements

Transcription

1 Radioactive Elements Most of the elements found in nature have several isotopes. Elements with atomic number 83 (bismuth) and lower have at least one stable isotope, although some are also radioactive and unstable. From element number 84 (polonium) and upwards, all the elements lack stable isotopes. The nine elements are called naturally occurring radioactive elements. They are polonium, astatine, radon, francium, radium, actinium, thorium, protactinium and uranium. They are all treated in this chapter. There are also an additional two radioactive elements, number 43 technetium and 61 promethium. However, they have been described in Chapter 28 Technetium and Chapter 17 Rare earths, respectively. Two of the nine radioactive elements described in this chapter, uranium and thorium, were known before the discovery of radioactivity itself. In addition, polonium and radium were discovered in 1898, actinium in 1899, and the other four became known in the 20 th century. The synthetic transuranium elements are treated in section of this chapter Po Facts about Polonium Po Po The Element Symbol: Po Atomic number: 84 Atomic weight: 209 Ground state electron configuration: [Xe]4f 14 5d 10 6s 2 6p 4 Crystal structure: Cubic with a = 3.36 Å. At 36 o C it changes to rhombohedral with a = 3.37 Å and = 98.2 o Po Encyclopedia of the Elements. Per Enghag Copyright 2004 WILEY-VCH Verlag GmbH & Co KGaA ISBN

2 Radioactive Elements Po Po Discovery and Occurrence Discovery: Polonium was discovered in connection with the investigation of the ore pitchblende by the French scientists Marie and Pierre Curie in They discovered that the ore was more radioactive than its principal component, uranium, and they separated the ore into many chemical fractions in order to isolate the unknown sources of radioactivity. One fraction, isolated by use of bismuth sulfide, contained a strongly radioactive substance that the Curies showed was a new element. They named it polonium after Poland. Most important mineral: Polonium is present in pitchblende but in small quantities. The content may be 0.1 mg of polonium in a tonne of ore. It is more usual to obtain polonium through the neutron bombardment of 209 Bi. Ranking in order of abundance in earth crust: 87 Mean content in earth crust: ppm (g/tonne) Mean content in oceans: ppm (g/tonne) Residence time in oceans: Mean content in an adult human body: Content in a man s body (weight 70 kg): Po Chemical Characterization Polonium is a rare, radioactive silver-gray metal that dissolves readily in dilute acids, but is only slightly soluble in alkalis. Because most polonium isotopes disintegrate by emitting alpha particles, the element is a source of pure alpha radiation. Oxidation states: Ionization energy (kj mol 1 ): Electron affinity (kj mol 1 ): Po II as in PoO Po(g) Po + (g) + e 812 Po(g) + e Po (g) Po IV as in PoO Standard reduction potential: PoO 2 (s) + 4H + (aq) + 2e Po 2+ (aq) + 2H 2 O(l) E 0 = +1.1 V Po 2+ (aq) + 2e Po(s) E 0 = V Electronegativity (Pauling): 2.0 Radii of atoms and ions: Atomic: 190 pm (WebElements ) Po 4+ (6-coordinate, octahedral): 108 pm Po 4+ (8-coordinate): 122 pm Po 6+ (6-coordinate, octahedral): 81 pm Po

3 52.1 Facts about Polonium Po Physical Properties Po Density Molar volume Melting point Boiling point Specific heat c p at 298 K 9320 kg m cm K 1235 K (125) J K 1 kg g cm C 962 C Thermal conductivity at 298 K 20 W m 1 K 1 Coefficient of linear expansion at 298 K K 1 Resistivity at 298 K 420 n m Po Thermodynamic Properties Enthalpy of fusion H fus at m.p. 13 kjmol 1 Enthalpy of vaporization H vap at b.p. 100 kjmol 1 Enthalpy of atomization H at at 298 K 142 kjmol 1 Entropy S 0 at 298 K Molar heat capacity Cp at 298 K Po Nuclear Properties and X-ray Isotope range, all nuclides radioactive Most long-lived Nuclide Half-life Natural Nuclear Magnetic Decay Decay Decay (N/ ) spin moment µ mode reaction energy Q 209 Po 102 y 1/ Po 205 Pb MeV EC or Po 209 Bi MeV 208 Po y Po 204 Pb MeV EC or Po 208 Bi MeV 210 Po d N Po 206 Pb MeV 206 Po 8.8 d 0+ EC or Po 206 Bi MeV 206 Po 202 Pb MeV 207 Po 5.80 d 5/ EC or Po 207 Bi MeV 207 Po 203 Pb MeV Po

4 Radioactive Elements Po Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 83 Bi kev (CuKα 2 ) Po kev (MoKα 2 ) At Po Neutron absorption Thermal neutron capture cross section At 52.1 At Facts about Astatine At The Element Symbol: At Atomic number: 85 Atomic weight: 210 Ground state electron configuration: [Xe]4f 14 5d 10 6s 2 6p 5 Crystal structure: At Discovery and Occurrence Discovery: When, in 1940, D. Corson, K. MacKenzie and E. Segrè used the Berkeley 60-inch cyclotron and bombarded bismuth 209 Bi with energetic helium ions, the element number 85 was formed Most important mineral: Some isotopes of astatine are present in uranium and thorium minerals as part of their radioactive decay series. Astatine belongs to the most rare elements of all. Its total amount in the earth s crust is estimated to be less than 30 grams. Ranking in order of abundance in earth crust: Mean content in earth crust: Mean content in oceans: Residence time in oceans: Mean content in an adult human body: Content in a man s body (weight 70 kg): At

5 52.1 Facts about Astatine At Chemical Characterization At Astatine is the heaviest of the halogens. There are about 20 isotopes known, all of which are radioactive. The element name, from Greek astatos, unstable, also indicates this fact. The longest-lived isotope, 210 At, has a half-life of 8.1 hours. Astatine behaves chemically very much like iodine. People in contact with this very rare element would probably accumulate it in their thyroid gland. Oxidation states: Ionization energy (kj mol 1 ): Electron affinity (kj mol 1 ): At I as in HAt At(g) At + (g) + e 920 At(g) + e At (g) At V as in [AtO 3 ] 270 There is also an inconclusive indication of oxoastatine(vii). Standard reduction potential: At 2 (s) + 2e 2At (aq) E 0 = +0.2 V Electronegativity (Pauling): 2.2 Radii of atoms and ions: Atomic (WebElements ) Covalent: Van der Waals: At 7+ (6-coordinate, octahedral): 76 pm At Physical Properties Density Molar volume Melting point Boiling point Specific heat c p at 298 K 575 K 610 K (140) J K 1 kg C 337 C Thermal conductivity at 298 K 1.7 W m 1 K 1 Coefficient of linear expansion at 298 K Resistivity at 298 K At Thermodynamic Properties Enthalpy of fusion H fus at m.p. (15) kjmol 1 Enthalpy of vaporization H vap at b.p. 30 kjmol 1 Enthalpy of atomization H at at 298 K Entropy S 0 at 298 K Molar heat capacity Cp at 298 K At

6 Radioactive Elements At At Nuclear Properties and X-ray Isotope range, all nuclides radioactive Most long-lived Nuclide Half-life Natural Nuclear Magnetic Decay Decay Decay (N/ ) spin moment µ mode reaction energy Q 210 At 8.1 h 5+ EC or At 210 Po MeV 210 At 206 Bi MeV 211 At 7.21 h 9/2-211 At 207 Bi MeV EC or At 211 Po MeV 209 At 5.41 h 9/2- EC or At 209 Po MeV 209 At 205 Bi MeV 207 At 1.80 h 9/2- EC or At 207 Po MeV 207 At 203 Bi MeV 208 At 1.63 h 6+ EC or At 208 Po MeV 208 At 204 Bi MeV Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 84 Po kev (CuKα 2 ) At kev (MoKα 2 ) Rn Neutron absorption Thermal neutron capture cross section At

7 52.1 Facts about Radon Rn Facts about Radon Rn Rn The Element Symbol: Rn Atomic number: 86 Atomic weight: 222 Ground state electron configuration: [Xe]4f 14 5d 10 6s 2 6p 6 Crystal structure: Rn Discovery and Occurrence Discovery: F. E. Dorn in Halle, Germany, is frequently credited with the discovery in 1900, but actually Rutherford and Soddy were the first to isolate radon and also the first to really understand the nature of radon. It occurred in the first decade of 1900 at McGill University in Montreal, Canada. Most important mineral: Small quantities of radon, formed by decay of uranium minerals, are found in rock and soil, and radon makes up most normal background radioactivity. Radon is present as a dissolved gas in some spring waters. Ranking in order of abundance in earth crust: 88 Mean content in earth crust: Mean content in oceans: Residence time in oceans: Mean content in an adult human body: Content in a man s body (weight 70 kg): Rn

8 Radioactive Elements Rn Rn Chemical Characterization Radon is a colorless, odorless and radioactive gas, the heaviest of all gases. 222 Rn, the most abundant isotope of radon, has a half-life of 3.8 days and decays into an isotope of the element polonium. After inhalation of radon, this radionuclide stays locked in the tissues, e.g. in the lungs. Because of that, radon from the surrounding soil and rocks has become a safety issue around the world. Oxidation states: Ionization energy (kj mol 1 ): Electron affinity (kj mol 1 ): As a noble gas, radon ought Rn(g) Rn + (g) + e 1037 Rn(g) + e Rn (g) not to react with other elements. Fluorine is however an exception. The compound RnF 2 has been formed. Standard reduction potential: Electronegativity (Pauling): Radii of atoms and ions: Atomic: (WebElements ) Covalent: 145 No ionic radii are known for Rn Rn Physical Properties Density Molar volume Melting point Boiling point Specific heat c p (at 273 K) at 298 K 9.73 kg m cm K K 94 J K 1 kg g/l (at 211 K) 71.2 C 61.8 C Thermal conductivity at 298 K W m 1 K Rn Thermodynamic Properties Enthalpy of fusion H fus at m.p. 2.9 kjmol 1 Enthalpy of vaporization H vap at b.p. 17 kjmol 1 Enthalpy of atomization H at at 298 K 0 kjmol 1 Entropy S 0 at 298 K 176,1 JK 1 mol 1 Molar heat capacity Cp at 298 K JK 1 mol 1 Rn

9 52.1 Facts about Radon Rn Nuclear Properties and X-ray Rn Isotope range, all nuclides radioactive Most long-lived Nuclide Half-life Natural Nuclear Magnetic Decay Decay Decay (N/ ) spin moment µ mode reaction energy Q 222 Rn 3.82 d N Rn 218 Po MeV 211 Rn 14.6 h 1/ EC or Rn 211 At MeV 211 Rn 207 Po MeV 210 Rn 2.4 h Rn 206 Po MeV EC or Rn 210 At MeV 221 Rn 25 m 7/ Rn 221 Fr MeV 221 Rn 217 Po MeV Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 85 At kev (CuKα 2 ) Rn kev (MoKα 2 ) Fr Neutron absorption Thermal neutron capture cross section 0.7 barns ( 222 Rn) Rn

10 Radioactive Elements Fr 52.1 Fr Facts about Francium Fr The Element Symbol: Fr Atomic number: 87 Atomic weight: 223 Ground state electron configuration: [Rn]7s 1 Crystal structure: Fr Discovery and Occurrence Discovery: Marguerite Perey discovered the element in She worked at the Curie Laboratory of the Radium Institute in Paris. Most important mineral: Francium is formed when the radioactive element actinium disintegrates. Because of that, very small amounts of francium are found in uranium ores. The element can also be made artificially by bombarding thorium with protons. No weighable quantity of the element has been prepared or isolated. Ranking in order of abundance in earth crust: Mean content in earth crust: Mean content in oceans: Residence time in oceans: Mean content in an adult human body: Content in a man s body (weight 70 kg): Fr

11 52.1 Facts about Francium Fr Chemical Characterization Fr Francium is the heaviest of the alkali metals and the most electropositive of all elements. Chemically, it closely resembles cesium. Francium is naturally radioactive. It is the most unstable of the first 101 elements. Only two isotopes, 221 Fr and 223 Fr, are naturally occurring; all others are artificial. 223 Fr is the longest-lived isotope with a half-life of 22 minutes. Oxidation states: Ionization energy (kj mol 1 ): Electron affinity (kj mol 1 ): Fr I as in Fr + Fr(g) Fr + (g) + e 380 Fr(g) + e Fr (g) Standard reduction potential: Fr + (aq) + e Fr(s) E = 2.92 V Electronegativity (Pauling): 0.7 Radii of atoms and ions: Atomic: (Web Elements ) Fr + (6-coordinate, octahedral): 194 pm Fr Physical Properties Density Molar volume Melting point Boiling point Specific heat c p at 298 K (2410) kg m 3 (300) K (930) K (2.41) g cm 3 (27) C (657) C Thermal conductivity at 298 K (15) W m 1 K 1 Coefficient of linear expansion at 298 K Resistivity at 298 K Fr Thermodynamic Properties Enthalpy of fusion H fus at m.p. (2) kjmol 1 Enthalpy of vaporization H vap at b.p. (65) kjmol 1 Enthalpy of atomization H at at 298 K (64) kjmol 1 Entropy S 0 at 298 K 95.4 JK 1 mol 1 Molar heat capacity Cp at 298 K Fr

12 Radioactive Elements Fr Fr Nuclear Properties and X-ray Isotope range, all nuclides radioactive Most long-lived Nuclide Half-life Natural Nuclear Magnetic Decay Decay Decay (N/ ) spin moment µ mode reaction energy Q 223 Fr 21.8 m N 3/ Fr 223 Ra MeV 223 Fr 219 At MeV 212 Fr 20.0 m EC or Fr 212 Rn MeV 212 Fr 208 At MeV 222 Fr 14.2 m Fr 222 Ra MeV 221 Fr 4.9 m N 5/ Fr 217 At MeV 225 Fr 4.0 m 3/ Fr 225 Ra MeV Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 86 Rn kev (CuKα 2 ) Fr kev (MoKα 2 ) Ra Neutron absorption Thermal neutron capture cross section Fr

13 52.1 Facts about Radium Ra Facts about Radium Ra Ra The Element Symbol: Ra Atomic number: 88 Atomic weight: Ground state electron configuration: [Rn]7s 2 Crystal structure: Cubic bcc with a = 5.15 Å Ra Discovery and Occurrence Discovery: Radium was discovered in connection with the investigation of the ore pitchblende by the French scientists Marie and Pierre Curie in They discovered that the ore was more radioactive than its principal component, uranium, and they separated the ore into many chemical fractions in order to isolate the unknown sources of radioactivity. In a highly radioactive barium chloride fraction they found a new element, which they named radium, the radiator. Most important mineral: Radium is formed by the radioactive disintegration of uranium and is consequently found in all uranium ores. One tonne of pitchblende might contain 0.3 g of radium. Ores containing radium are found in Zaire, Australia, Canada and the USA (New Mexico, Utah and Colorado). Ranking in order of abundance in earth crust: 85 Mean content in earth crust: ppm (g/tonne) Mean content in oceans: ppm (g/tonne) Residence time in oceans: years Mean content in an adult human body: Content in a man s body (weight 70 kg): Ra

14 Radioactive Elements Ra Ra Chemical Characterization Radium is a white metal, but blackens on exposure to air. The chemical properties of the element are similar to those of barium. Like other heavy elements of group 2 radium salts impart a characteristic color (a carmine red) to a f lame. Radium is radioactive, over a million times more radioactive than the same quantity of uranium. It emits, and rays as well as radioactive radon gas. When a radium salt is mixed with a substance such as zinc sulfide, the substance glows in the dark. Small amounts of this mixture was used in the mid-1900s to paint the hands and numbers of watches. Exposure to radium can cause cancer. On the contrary ordinary cancerous cells may be killed by directed radium radiation. This treatment is nowadays utilized for only a few kinds of cancer. Oxidation states: Ionization energy (kj mol 1 ): Electron affinity (kj mol 1 ): Ra II as in RaO, Ra(OH) 2 Ra(g) Ra + (g) + e 509 Ra(g) + e Ra (g) and RaCl 2 Ra + (g) Ra 2+ (g) + e 979 Standard reduction potential: Ra 2+ (aq) + 2e Ra(s) E 0 = V Electronegativity (Pauling): 0.9 Radii of atoms and ions: Atomic: 215 pm Ra 2+ (8-coordinate): 162 pm Ra Physical Properties Density Molar volume Melting point Boiling point Specific heat c p at 298 K 5000 kg m cm K 1413 K 119 J K 1 kg g cm C 1140 C Thermal conductivity at 298 K 18.6 W m 1 K 1 Coefficient of linear expansion at 298 K K 1 Resistivity at 298 K 1000 n m Ra

15 52.1 Facts about Radium Ra Thermodynamic Properties Ra Enthalpy of fusion H fus at m.p kjmol 1 Enthalpy of vaporization H vap at b.p kjmol 1 Enthalpy of atomization H at at 298 K 159 kjmol 1 Entropy S 0 at 298 K 71 JK 1 mol 1 Molar heat capacity Cp at 298 K 27.0 JK 1 mol Ra Nuclear Properties and X-ray Isotope range, all nuclides radioactive Most long-lived Nuclide Half-life Natural Nuclear Magnetic Decay Decay Decay (N/ ) spin moment µ mode reaction energy Q 226 Ra 1600 y N Ra 222 Rn MeV 228 Ra 5.75 y N Ra 228 Ac MeV 225 Ra 14.9 d N 1/ Ra 225 Ac MeV 223 Ra d N 3/ Ra 219 Rn MeV 224 Ra 3.66 d N Ra 220 Rn MeV 227 Ra 42.2 m 3/ Ra 227 Ac MeV Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 87 Fr kev (CuKα 2 ) Ra kev (MoKα 2 ) Ac Neutron absorption Thermal neutron capture cross section Ra

16 Radioactive Elements Ac 52.1 Ac Facts about Actinium Ac The Element Symbol: Ac Atomic number: 89 Atomic weight: Ground state electron configuration: [Rn]6d 1 7s 2 Crystal structure: Cubic fcc with a = 5.67 Å Ac Discovery and Occurrence Discovery: Actinium was discovered in connection with the investigation of the ore pitchblende by the French scientists Marie and Pierre Curie in They discovered that the ore was more radioactive than its principal component, uranium, and they separated the ore into many chemical fractions in order to isolate the unknown sources of radioactivity. In one fraction André Debierne, a colleague of the Curies, and a specialist in chemistry, discovered a new, strongly radioactive element that was given the name actinium. The discovery year was Most important mineral: Actinium is a decay product of 235 U and is found naturally in uranium ores. Ranking in order of abundance in earth crust: 86 Mean content in earth crust: ppm (g/tonne). In uranium ores 0.2 ppm Mean content in oceans: Residence time in oceans: Mean content in an adult human body: Content in a man s body (weight 70 kg): Ac

17 52.1 Facts about Actinium Ac Chemical Characterization Ac Chemically actinium is similar to the rare earth metals, especially lanthanum. Actinium is 150 times more radioactive than radium and for that reason it is a very dangerous element. Oxidation states: Ionization energy (kj mol 1 ): Electron affinity (kj mol 1 ): Ac III as in Ac 2 O 3 and AcCl 3 Ac(g) Ac + (g) + e 499 Ac(g) + e Ac (g) Ac + (g) Ac 2+ (g) + e 1170 Standard reduction potential: Ac 3+ (aq)+e Ac 2+ ((aq) E = 4.9 V Ac 2+ (aq) + 2 e Ac(s) E = 0.7 V Ac 3+ (aq) + 3e Ac(s) E = 2.13 V Electronegativity (Pauling): 1.1 Radii of atoms and ions: Atomic: 195 pm (WebElements ) Ac 3+ (6-coordinate, octahedral): 126 pm Ac Physical Properties Density Molar volume Melting point Boiling point Specific heat c p at 298 K kg m cm K 3473 K 120 J K 1 kg g cm C 3200 C Thermal conductivity at 298 K 12 W m 1 K 1 Coefficient of linear expansion at 298 K K 1 Resistivity at 298 K Ac Thermodynamic Properties Enthalpy of fusion H fus at m.p. 14 kjmol 1 Enthalpy of vaporization H vap at b.p. 400 kjmol 1 Enthalpy of atomization H at at 298 K 406 kjmol 1 Entropy S 0 at 298 K 56.5 JK 1 mol 1 Molar heat capacity Cp at 298 K 27.2 JK 1 mol 1 Ac

18 Radioactive Elements Ac Ac Nuclear Properties and X-ray Isotope range, all nuclides radioactive Most long-lived Nuclide Half-life Natural Nuclear Magnetic Decay Decay Decay (N/ ) spin moment µ mode reaction energy Q 227 Ac y N 3/ Ac 227 Th MeV 227 Ac 223 Fr MeV 225 Ac 10.0 d N 3/2 225 Ac 221 Fr MeV 226 Ac h (1) 226 Ac 226 Th MeV EC or Ac 226 Ra MeV 226 Ac 222 Fr MeV 228 Ac 6.15 h N Ac 228 Th MeV 224 Ac 2.78 h 0- EC or Ac 224 Ra MeV 224 Ac 220 Fr MeV 224 Ac 224 Th MeV Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 88 Ra kev (CuKα 2 ) Ac kev (MoKα 2 ) Th Neutron absorption Thermal neutron capture cross section 880 barns ( 227 Ac) Ac

19 52.1 Facts about Thorium Th Facts about Thorium Th Th The Element Symbol: Th Atomic number: 90 Atomic weight: Ground state electron configuration: [Rn]6d 2 7s 2 Crystal structure: Cubic fcc with a = 5.08 Å Th Discovery and Occurrence Discovery: J. J. Berzelius discovered thorium in Stockholm in The element name was selected from Norse mythology: Thor was the god of thunder. Most important mineral: Thorite ThSiO 4 (Figure M24) and thorianite ThO 2. The primary source of thorium is monazite (Figure M25). Ranking in order of abundance in earth crust: 38 Mean content in earth crust: 9.6 ppm (g/tonne). Mean content in oceans: Residence time in oceans: 100 years Mean content in an adult human body: Content in a man s body (weight 70 kg): Th

20 Radioactive Elements Th Th Chemical Characterization When pure, thorium is a silvery white metal. In air, it tarnishes slowly, becoming gray and finally black. Thorium has isotopes ranging in mass number from 210 to 237, all isotopes being radioactive. Much of the internal heat in the earth s crust has been attributed to thorium (and uranium).thorium is a potential atomic fuel source, because bombardment of 232 Th with slow neutrons yields the fissile isotope 233 U. There is probably more energy available for use from thorium in the minerals of the earth s crust than from combined uranium and fossil fuel sources. Oxidation states: Ionization energy (kj mol 1 ):Electron affinity (kj mol 1 ): Th IV as in ThO 2 and ThI 4 Th(g) Th + (g) + e 587 Th(g) + e Th (g) Th + (g) Th 2+ (g) + e 1110 Th 2+ (g) Th 3+ (g) + e 1930 Th 3+ (g) Th 4+ (g) + e 2780 Standard reduction potential: Th 4+ (aq) + 4e Th(s) E 0 = 1.83 V Electronegativity (Pauling): 1.3 Radii of atoms and ions: Atomic: 180 pm (WebElements ) Th 4+ (6-coordinate, octahedral): 108 pm Th 4+ (8-coordinate): 119 pm Th

21 52.1 Facts about Thorium Th Physical Properties Th Density Molar volume Melting point Boiling point Specific heat c p at 298 K kg m cm K 5061 K 118 J K 1 kg g cm C 4788 C Thermal conductivity Wm 1 K K 273 K 373 K 573 K 973 K Coefficient of linear expansion K K 293 K 500 K 800 K Resistivity n m 78 K 298 K 373 K 573 K 973 K 1473 K Mass magnetic susceptibility χ mass at 293 K m 3 kg 1 Magnetic characterization Paramagnetic (as susceptibility is positive) Elastic properties Youngs Shear Bulk Poissons modulus E modulus G modulus K ratio ν 79 GPa 31 GPa 58 GPa 0.27 Th

22 Radioactive Elements Th Th Thermodynamic Properties Enthalpy of fusion H fus at m.p. 16 kjmol 1 Enthalpy of vaporization H vap at b.p kjmol 1 Enthalpy of atomization H at at 298 K kjmol 1 Entropy S 0 at 298 K JK 1 mol 1 Molar heat capacity Cp at temperature K. JK 1 mol K 298 K 600 K 1000 K 2000 K 2500 K Standard free energy G 0 of oxide formation kj/mol O 2 Reaction 298 K 500 K 1000 K 1500 K 2000 K Th+O 2 ThO Th Nuclear Properties and X-ray Isotope range, natural and artificial Naturally occurring isotopes Nuclide Half-life Abundance Nuclear Magnetic Decay Decay % 1) spin moment µ mode energy Q 232 Th y MeV 230 Th y N MeV 229 Th 7340 y N 5/ MeV 228 Th y N MeV 234 Th d N MeV 1) N = naturally occurring trace Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 89 Ac kev (CuKα 2 ) Th kev (MoKα 2 ) Pa Neutron absorption Thermal neutron capture cross section 7.4 barns Th

23 52.1 Facts about Protactinium Pa Facts about Protactinium Pa Pa The Element Symbol: Pa Atomic number: 91 Atomic weight: Ground state electron configuration: [Rn]5f 2 6d 1 7s 2 Crystal structure: Tetragonal with a = b = Å, c = 3.24 Å Pa Discovery and Occurrence Discovery: Protactinium was identified by Fajans and Göhring in Karlsruhe in 1913, who named the new element brevium. They had discovered the isotope 234 Pa with a half-life of 6.70 h. Lise Meitner at the Kaiser-Wilhelm Institute for Chemistry in Berlin separated the oxide of a more long-lived isotope from pitchblende (mixed with tantalum oxide). She published the news in 1918 together with Otto Hahn. The new element was discovered independently in the same year by F. Soddy, J. A. Cranston and A. Fleck in Glasgow. The name protactinium was selected because it was recognized as the prototype for actinium. The element was first isolated by Aristid V. Grosse in He prepared 2 mg of the metal. Most important mineral: Protactinium is one of the rarest and most expensive naturally occurring elements, found in pitchblende. Ranking in order of abundance in earth crust: 84 Mean content in earth crust: ppm (g/tonne) Mean content in oceans: ppm (g/tonne) Residence time in oceans: Mean content in an adult human body: Content in a man s body (weight 70 kg): Pa

24 Radioactive Elements Pa Pa Chemical Characterization Isotopes of protactinium ranging in mass number from 212 to 238 are known. 231 Pa is the most long-lived isotope with a half-life of years. Oxidation states: Ionization energy (kj mol 1 ): Electron affinity (kj mol 1 ): Pa IV as in PaO 2 and PaF 4 Pa(g) Pa + (g) + e 568 Pa(g) + e Pa (g) Pa V as in Pa 2 O 5 and PaCl 5 Pa III exists but is of secondary importance Standard reduction potential: Pa 4+ (aq) + 4e Pa(s) E 0 = 1.47 V Electronegativity (Pauling): 1.5 Radii of atoms and ions: Atomic: 180 pm (WebElements ) Pa 3+ (6-coordinate, octahedral): 118 pm Pa 4+ (6-coordinate, octahedral): 104 pm Pa 4+ (8-coordinate): 115 pm Pa 5+ (6-coordinate, octahedral): 92 pm Pa 5+ (8-coordinate): 105 pm Pa Physical Properties Density Molar volume Melting point Boiling point Specific heat c p at 298 K kg m cm 3 (1840) K (4500) K 120 J K 1 kg g cm 3 (1567) C (4227) C Thermal conductivity at 298 K 47 W m 1 K 1 Coefficient of linear expansion at 298 K Resistivity at 298 K 180 n m Pa Thermodynamic Properties Enthalpy of fusion H fus at m.p. 15 kjmol 1 Enthalpy of vaporization H vap at b.p. 480 kjmol 1 Enthalpy of atomization H at at 298 K 607 kjmol 1 Entropy S 0 at 298 K 51.9 JK 1 mol 1 Molar heat capacity Cp at 298 K 28 JK 1 mol 1 Pa

25 52.1 Facts about Protactinium Pa Nuclear Properties and X-ray Pa Isotope range, all nuclides radioactive Most long-lived Nuclide Half-life Natural Nuclear Magnetic Decay Decay Decay (N/ ) spin moment µ mode reaction energy Q 231 Pa y N 3/ Pa 227 Ac MeV 233 Pa 27.0 d 3/ Pa 233 U MeV 230 Pa 17.4 d (2-) 2.0 EC or Pa 230 Th MeV 230 Pa 230 U MeV 230 Pa 226 Ac MeV 229 Pa 1.50 d (5/2+) EC or Pa 229 Th MeV 229 Pa 225 Ac MeV 232 Pa 1.31 d (2-) 232 Pa 232 U MeV EC or Pa 232 Th MeV Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 90 Th kev (CuKα 2 ) Pa kev (MoKα 2 ) U Neutron absorption Thermal neutron capture cross section 200 barns ( 231 Pa) Pa

26 Radioactive Elements U 52.1 U Facts about Uranium U The Element Symbol: U Atomic number: 92 Atomic weight: Ground state electron configuration: [Rn]5f 3 6d 1 7s 2 Crystal structure: Orthorombic with a = 2.85 Å, b = 5.87 Å, c = 4.95 Å U Discovery and Occurrence Discovery: M. H. Klaproth in Berlin discovered a new element in pitchblende in He named the new element uranium after Uranus, the father of the gods in Greek mythology. Most important mineral: Pitchblende and uraninite, essentially UO 2, but usually partly oxidized to U 3 O 8 (both in Figure M76). The uranium minerals carnotite and coffinite are also important. Ranking in order of abundance in earth crust: 49 Mean content in earth crust: 2.7 ppm (g/tonne). Mean content in oceans: ppm (g/tonne) Residence time in oceans: years Mean content in an adult human body: ppm Content in a man s body (weight 70 kg): 0.07 mg U

27 52.1 Facts about Uranium U Chemical Characterization U Uranium is a silvery metal. As it is chemically reactive, it tarnishes in air. It is attacked by steam and acids but not by alkalis. Due to its radioactivity and accessibility, the element is the main fuel in nuclear reactors. Natural uranium consists of 99.3% of the isotope 238 U and 0.7% of the fissile isotope 235 U. Oxidation states: Ionization energy (kj mol 1 ): Electron affinity (kj mol 1 ): U III as in UCl 3 U(g) U + (g) + e 584 U(g) + e U (g) U IV as in UO 2 and [UCl 6 ] 2 U + (g) U 2+ (g) + e 1420 U V as in [UF 6 ] U VI as in UO 3, UF 6, UCl 6 and UO 2 (NO 3 ) 2 Standard reduction potential: [UO 2 ] 2+ (aq) + 4H + (aq) + 2e U 4+ (aq) + 2H 2 O(l) E 0 = V U 4+ (aq) + 4e U(s) E 0 = 1.38 V Electronegativity (Pauling): 1.38 Radii of atoms and ions: Atomic: 175 pm (WebElements ) Van der Waals: 186 pm U 3+ (6-coordinate, octahedral): pm U 4+ (6-coordinate, octahedral): 103 pm U 4+ (8-coordinate): 114 pm U 5+ (6-coordinate, octahedral): 90 pm U 6+ (6-coordinate, octahedral): 87 pm U 6+ (4-coordinate, tetrahedral): 66 pm U 6+ (8-coordinate): 100 pm U

28 Radioactive Elements U U Physical Properties Density Molar volume Melting point Boiling point Specific heat c p at 298 K kg m cm K (4100) K 116 J K 1 kg g cm C (3827) C Thermal conductivity Wm 1 K K 273 K 373 K 573 K 973 K Coefficient of linear expansion K K 293 K 500 K 800 K Resistivity n m 78 K 273 K 373 K 573 K 973 K 1473 K Mass magnetic susceptibility χ mass at 293 K m 3 kg 1 Magnetic characterization Paramagnetic (as susceptibility is positive) Elastic properties Youngs Shear Bulk Poissons modulus E modulus G modulus K ratio ν 180 GPa 72 GPa 120 GPa 0.25 U

29 52.1 Facts about Uranium U Thermodynamic Properties U Enthalpy of fusion H fus at m.p kjmol 1 Enthalpy of vaporization H vap at b.p. 420 kjmol 1 Enthalpy of atomization H at at 298 K kjmol 1 Entropy S 0 at 298 K JK 1 mol 1 Molar heat capacity Cp at temperature K. JK 1 mol K 298 K 600 K 1000 K 2000 K 2500 K Standard free energy G 0 of oxide formation kj/mol O 2 Reaction 298 K 500 K 1000 K 1500 K 2000 K U+O 2 UO /3U+O 2 2/3 UO U Nuclear Properties and X-ray Isotope range, all nuclides radioactive Most long-lived Nuclide Half-life Abundance Nuclear Magnetic Decay Decay % 1) spin moment µ mode energy Q 238 U y MeV 235 U y / MeV 236 U y MeV 234 U y MeV 233 U y 5/ MeV 1) = not naturally occuring Characteristic X-radiation X-ray radiation absorbed by the element Z Element Kα 2 kev Incoming radiation energy Absorption coefficient µ (cm 2 /g) 91 Pa kev (CuKα 2 ) U kev (MoKα 2 ) Np Neutron absorption Thermal neutron capture cross section 7.6 barns U

30 Radioactive Elements 52.2 Elements Known Before Radioactivity Was Discovered Uranus The Father of the Gods in Greek Mythology Martin Heinrich Klaproth ( ) grew up in a family of tailors in Harz, Germany. He became an apprentice in a pharmacy and acquired deep knowledge of chemistry mostly by studying alone. In spite of his apprenticeship and later work with his own pharmacy, he was the foremost chemist of his time regarding the analysis of minerals and ores. In 1810, at the age of 67, he became the first holder of the newly established professorship in chemistry at the University of Berlin. He worked there until his death in We met him in Chapter 18, regarding the discovery of titanium. In 1789 Klaproth obtained a sample of pitchblende for investigation. In the mineral he found the oxide of a metal that had not been described earlier. He named the new element uranium. There are various reasons mentioned for why he used that name. The astronomer, telescope-builder and organist William Herschel ( ) in London had discovered a new planet in 1781 and given it the name Uranus after the father of the gods in Greek mythology. Did Klaproth select the element name after the new planet or after the Greek god? The latter seems to have been the case, because, as he himself says, six years later, in connection with the discovery of titanium: As the case was at the naming of uranium I take the designation titanium from mythology. Klaproth reduced the new oxide with carbon at high temperature. His reaction product consisted of gray, lustrous grains, which he regarded as uranium metal. Probably they consisted of uranium carbide to a large extent. The pure metal was prepared in 1842 by E. M. Péligot in France. He reduced uranium tetrachloride with potassium metal Thor The God of Thunder in Norse Mythology A Day of the Week Why Not Also an Element? In 1815 Berzelius examined a mineral from Falun, in which he thought he had found a new element. He intended to use the element-naming philosophy utilized by Klaproth for uranium and titanium, and of his own former student Ekeberg for tantalum (Chapter 23). But why not use a god from Norse mythology? The name of the Norse god Thor had been used in the naming of numerous places and had also given its name to a day of the week, Thor s day. Why not also an element name? However, Berzelius had to change his mind when he found that the new element was not new. The mineral was an yttrium phosphate. But in 1829 it was time for an element discovery, beyond dispute.

31 52.3 Radioactivity The Reverend Finds a Mineral and Berzelius Has His New Element At this time Professor Jens Esmark was active at the University of Christiania (Oslo) in Norway. He was a leading Norwegian geologist and is remembered for his initial discoveries about ice ages. In 1824 he concluded that glaciers had once covered much of Norway and the adjacent sea f loor. A son of his became a clergyman. He had inherited his father s interest in stones and became known as a clever mineral collector with an extensive correspondence with mineralogists in other countries. He discovered a black mineral near Brevig in Norway. He thought it was gadolinite, but his father did not agree. As he could not identify it, he sent a specimen to Berzelius for examination. The latter examined it and established that it contained 60% of a new earth. He named the new oxide thoria, the mineral in which it was obtained thorite (Figure M24) and the new element thorium [52.1]. He also isolated the metal by reduction of potassium thorium f luoride with metallic potassium. The discovery of thorium was announced by Berzelius in Annalen der Physik und Chemie (Poggendorf) 16 (1829) Radioactivity Measuring Radioactivity The Becquerel Since the Chernobyl disaster in 1986, we have become accustomed to the concept of the becquerel (Bq) as a means to describe radioactivity in the environment and in food (Figure 52.1). But what is a becquerel? The answer to that question is as follows: the becquerel (Bq) is the unit used to measure the amount of radiation that radioactive substances emit. The number of Figure 52.1 As a result of the Chernobyl nuclear reactor accident, grass became radioactive, cattle ate it and the meat became unfit for human consumption. (Reprinted from ref. [52.2] with permission.)

32 Radioactive Elements becquerels is the number of disintegrations per second. An older unit is the curie (Ci), which corresponds to the rate of decay of 1 g radium, i.e. 37 billion disintegrations per second. Thus 1 Ci = Bq. Several attendant questions arise. What is radioactivity? What is it that disintegrates? Why is the decay injurious to the human and other organisms? What Is Radioactivity? The atomic nucleus consists of two types of nucleons, protons (positively charged) and neutrons (uncharged). Neutrons provide a special nuclear force that binds protons and neutrons together into stable units. Otherwise, the nucleus would disintegrate due to the repulsive electrostatic forces between the positively charged protons. A condition for a stable nucleus is that the number Z of protons (atomic number) has a certain relation to the number N of neutrons. In light atoms, N/Z=1. The nuclide 12 C, for instance, has six protons and six neutrons in its nucleus. The combination of a pair of protons with a pair of neutrons has particularly high stability. That group corresponds to the stable helium nucleus, which also remains after some radioactive decays and is emitted as alpha ( ) particles. When radium disintegrates, particles are emitted. Electrons from the environment are added to the helium nuclei and neutral helium atoms are formed. That is the reason why helium was discovered in uraninite. In heavier atoms, the repulsive forces between the protons are very high, and N/Z>1 is a condition that must be fulfilled if the nucleus is to be stable. The most usual lead isotope 208 Pb has 82 protons and 126 neutrons in its nucleus. The N/Z ratio is thus 1.54 and the isotope is stable. Above atomic number 83, all nuclei are unstable, no matter how many neutrons are present Radioactive Decay The naturally occurring radioactive substances disintegrate in different ways, as outlined below Alpha Particle Emission (Alpha or Radiation) A stable helium nucleus (an particle), thus a bundle of two protons and two neutrons, may be emitted in a decay process. For example: U Th He Both Z and N decrease by two units, and the mass number A (number of protons plus number of neutrons, A=Z+N) is four units lower than the mass number of the mother nuclide. Thus a new element located two steps to the left in the periodic table has been formed.

33 52.3 Radioactivity Beta Particle Emission (Beta-minus or Radiation) An electron may be emitted in a nuclear decay process. For example: Th Pa The process can be regarded as the emission of an electron from a neutron, which is then changed to a proton. The atomic number increases by one unit. A new element has been formed but the mass number is unchanged Positron Emission (Beta-plus or + Radiation) Emission of a positron can also occur in a nuclear decay process. For example: P Si The process can be regarded as a transition of a proton to a neutron. Also in this case a new element has been formed, one with a lower atomic number. The emitted positron is rapidly annihilated on collision with an electron. Gamma ( ) radiation is formed (see below) Electron Capture (EC) Electron capture is a process that achieves the same effect as positron emission. An electron from a shell is absorbed by the nucleus. A proton in the nucleus is changed to a neutron, and the new element formed has decreased the atomic number by one unit Gamma Rays (Gamma or Radiation) Gamma rays are physically of the same character as X-rays with a great ability to penetrate matter. The injurious effects of radioactivity are connected with this type of radiation. Immediately after a decay of or type, the daughter nuclei are excited. They have energy in excess, which leads to a regrouping of the nucleons and the emission of electromagnetic radiation with short wavelength. This can be regarded as an echo of the material processes in the atomic nucleus, which are associated with radioactive decay Activity and Dose The unit becquerel (Bq) measures the activity of the radiation. It gives no direct information about its injurious effect. A radioactive substance with high Bq value may be harmless if its distance is 100 m but highly dangerous if people eat it. To assess the health hazard, a different unit is needed, expressing the radiation energy that is absorbed by the tissues of the body. To evaluate the risk, the absorbed radiation in J/kg is multiplied by a quality factor Q (relative biological effectiveness): Q is 1 for, X and radiation, while it is 20 for radiation. The multiplication gives the equivalent dose, which is expressed in sievert (Sv) or millisievert (msv).

34 Radioactive Elements The Swede Rolf Sievert was a pioneer within international radiation protection research. He took an active part in the foundation in 1928 of the International Commission of Radiological Protection (ICRP) and was its chairman from 1956 to Radioactive Decay Series If radioactive elements disintegrate, how is it possible that they still remain in the earth s crust after 4.5 billion years, which is estimated to be the age of the earth? Yet, some are not left the exciting story about the search for element 43, technetium, tells us that (Chapter 28 Technetium). It is the same story for the rare earth element 61, promethium (Chapter 17 Rare earths). These elements may be found after reactions in a nuclear reactor, but they are short-lived. The most long-lived technetium isotope has a half-life of 4 million years; the most long-lived promethium isotope only 17.7 years. If they had been present at the creation of the earth, they would have disappeared long ago. Other short-lived isotopes can, however, still be detected in nature. The element number 86, radon (Rn), has several isotopes, the most long-lived of which has a halflife of only 3.8 days. How is it then possible that we have radon problems in our mines and our houses? The answer is that radon certainly disintegrates rapidly but is also being formed continuously. Radon is part of the radioactive uranium decay series 1) : 238 U 234 Th 234 Pa 234 U 230 Th 226 Ra 222 Rn 218 Po 214 Pb 214 Bi 214 Po 210 Pb 210 Bi 210 Po 206 Pb (stable) Uranium-238 ( 238 U) has a half-life of 4.5 billion years. This value is of the same magnitude as the age of the earth. Thus for 238 U only one half-life has passed. Referring to our radon example, 226 Ra emits particles and forms 222 Rn, which also disintegrates with the emission of particles and forms 218 Po. Other natural decay series originate with thorium and actinium. Radon is also an intermediate link in the former, the radioactive thorium decay series 1) : 232 Th 228 Ra 228 Ac 228 Th 224 Ra 220 Rn 216 Po 212 Pb 212 Bi 208 Tl 208 Pb (stable) The starting element 232 Th is very long-lived. Its half-life is 14 billion years. 1) In three positions of the uranium series and two of the thorium series, alternative decay pathways are possible.

35 52.4 Henri Becquerel Discovers Radioactivity Henri Becquerel Discovers Radioactivity Just before the start of the 20 th century, several important discoveries were made within physics and chemistry. Two dealt with radiation X-rays Wilhelm Roentgen ( ) worked in the Physical Institution at the Julius-Maximilian University in Würzburg, Germany. As many other scientists at this time, he investigated the radiation emitted from a cathode ray tube. In November, 1895, he made a remarkable discovery. Outside the tube he had placed a paper coated with barium platinocyanide. It f luoresced while the tube was in operation. He tried to stop the radiation with black paper round the tube. The platinum compound glowed in spite of that. Nor could lead stop the radiation. Wilhelm Roentgen had seen what no man had observed before: the effect of X-ray radiation. The discoverer himself coined the term X-ray, and this designation is still used in many countries. On December 28 he published his discovery in the university s own journal, and before the New Year he made an X-ray picture of his wife s hand. The bones in it were clearly visible. The picture was sent to a colleague in Vienna and the innovation was soon known even outside the circle of physicists. On January 5, 1896, the newspaper Die Presse made it known that it was now possible to look through a man. The novelty was spread over the world. In 1901 Wilhelm Roentgen became the first winner of the Nobel Prize for physics. The principle of the X-ray tube is shown in Figure It consists of a glass vacuum tube in which X-rays are generated. Electrons are accelerated from the hot cath- Figure 52.2 X-ray tube. (Reprinted from ref. [52.3].)

36 Radioactive Elements ode filament F towards the anode surface T, where X-rays are produced. A window allows the produced X-rays to escape from the tube housing Uranium Radiation Henri Becquerel ( ) grew up in Paris. His father Alexandre-Edmond Becquerel was a professor of applied physics and had done research on solar radiation and on luminescence 2). Henri studied at the Polytechnic from In 1892 he was appointed Professor of Applied Physics in the Department of Natural History at the Paris Museum. He became a professor at the Polytechnic in In 1889, still very young, Henri Becquerel was elected a member of the Académie des Sciences de France. His membership there came to be important in connection with the discovery of uranium radiation. The Becquerel family, Henri s father Alexandre-Edmond and also his grandfather Antoine-Edmond, had been passionately interested in luminescence. Henri himself had, for his father s investigations, prepared beautiful crystals of potassium uranyl sulfate, a substance with f luorescing properties. In his earliest scientific work he also was concerned with the phenomenon of phosphorescence and with the absorption of light by crystals (the subject of his doctoral thesis). When, in early 1896, Henri heard about Roentgen s discovery, he was excited. Could the X-rays be a special form of f luorescence? As Becquerel himself tells it [52.4]: The idea of examining whether bodies could emit an invisible and penetrating radiation was suggested to me by the announcement of the first experiments of Roentgen. Poincaré 3) showed the first radiographs of Roentgen at the Academy of Sciences in Paris on January 20 th, 1896, and in an answer to a question from me [Becquerel] stated that the source of the rays was the luminous spot on the wall of the glass tube, which received the cathode stream. I immediately thought of examining whether this new emission was caused by the vibratory movement, which gave rise to the phosphorescence, and whether all phosphorescent bodies could emit similar rays. At this epoch no one imagined that it was natural to suppose that a transportation of energy must be going on. 2) Phosphorescence is one type of luminescence (from the Latin lumen meaning light ). Another is f luorescence. When certain materials absorb energy (from sunshine or from a UV lamp), the electrons become excited and jump from the inner orbits of the atoms to the outer ones. When an electron falls back to its original state, a photon of light is emitted. The interval between the two steps may be short or long. If the interval is short, the process is called f luorescence. It means that the emission of light ceases as soon as the primary source, such as the UV lamp, is switched off. If the interval is long and the emission continues after the source has been switched off, the process is called phosphorescence. When yellow phosphorus oxidizes in air, emitting green radiation, this is neither phosphorescence nor f luorescence but another type of luminescence called chemiluminescence. 3) Jules Henri Poincaré became Professor of Mathematical Physics and Mechanics at the University of Paris in In 1887, two years before Becquerel, he was elected a member of the Académie des Sciences.

37 52.4 Henri Becquerel Discovers Radioactivity 1177 Just 10 days later Poincaré wrote an article on Roentgen rays in Revue Générale des Sciences, in which he said: Thus it is the glass which emits the rays, and it emits them by becoming f luorescent. May we ask, therefore, whether all bodies whose f luorescence is sufficiently intense may not emit, beside luminous rays, the X-rays of Roentgen, whatever may be the cause of their f luorescence? These phenomena would then be no more connected by an electric stimulus. It may not be very probable, but it is possible, and doubtless easy enough to verify. In those days of 1896 science stood at the crossroads. Roentgen s rays were a new hard fact. The radiation due to luminescence was well known and had been studied for a long time. We must remember that radioactivity was quite unknown. It was discovered in those days during experiments intended for quite other purposes. What Becquerel wanted to verify was the connection between the two phenomena, X-rays and f luorescence. Uranium compounds had good f luorescing properties and could be suitable to use. He had prepared, 15 years earlier, crystals of potassium uranyl sulfate for his father s investigations and he now utilized them for his own experiments. He knew very well that they phosphoresce on exposure to light. When the salts were placed near to a photographic plate, the effect was indeed confusing. Even if the plate was covered with opaque paper, the plate was fogged (Figure 52.3). Figure 52.3 Henri Becquerel ( ) discovered the radiation from uranium. In 1896 he placed small pieces of a uranium salt on a photographic plate. The plate was exposed, as shown in the figure on the right. This was the first indication of the decay of atoms. (Reprinted from ref. [52.5].)

38 Radioactive Elements Becquerel had carried out his experiments during the last days of February, 1896, and on March 2, 1896, he reported the results to the French Academy of Sciences. His very important report had the title On the invisible rays emitted by phosphorescent bodies. The translation by C. Giunta [52.6] is quoted here: In the previous session, I summarized the experiments which I had been led to make in order to detect the invisible rays emitted by certain phosphorescent bodies, rays which pass through various bodies that are opaque to light. I was able to extend these observations, and although I intend to continue and to elaborate upon the study of these phenomena, their outcome leads me to announce as early as today the first results I obtained. The experiments which I shall report were done with the rays emitted by crystalline crusts of the double sulfate of uranyl and potassium [SO 4 (UO)K+H 2 O], a substance whose phosphorescence is very vivid and persists for less than 1/100 th of a second. The characteristics of the luminous rays emitted by this material have been studied previously by my father, and in the meantime I have had occasion to point out some interesting peculiarities which these luminous rays manifest. One can confirm very simply that the rays emitted by this substance, when it is exposed to sunlight or to diffuse daylight, pass through not only sheets of black paper but also various metals, for example a plate of aluminium and a thin sheet of copper. In particular, I performed the following experiment: A Lumière plate with a silver bromide emulsion was enclosed in an opaque case of black cloth, bounded on one side by a plate of aluminium; if one exposed the case to full sunlight, even for a whole day, the photographic plate would not become clouded; but, if one came to attach a crust of the uranium salt to the exterior of the aluminium plate, which one could do, for example, by fastening it with strips of paper, one would recognize, after developing the photographic plate in the usual way, that the silhouette of the crystalline crust appears in black on the sensitive plate and that the silver salt facing the phosphorescent crust had been reduced. If the layer of aluminium is a bit thick, then the intensity of the effect is less than that through two sheets of black paper. If one places between the crust of the uranium salt and the layer of aluminium or black paper a screen formed of a sheet of copper about 0.10 mm thick, in the form of a cross for example, then one sees in the image the silhouette of that cross, a bit fainter yet with a darkness indicative nonetheless that the rays passed through the sheet of copper. In another experiment, a thinner sheet of copper (0.04 mm) attenuated the active rays much less. Phosphorescence induced no longer by the direct rays of the sun, but by solar radiation ref lected in a metallic mirror of a heliostat, then refracted by a prism and a quartz lens, gave rise to the same phenomena. I will insist particularly upon the following fact, which seems to me quite important and beyond the phenomena which one could expect to observe: The same crystalline crusts, arranged the same way with respect to the photographic plates, in the same conditions and through the same screens, but sheltered from the excitation of incident rays and kept in darkness, still produce the same photographic images. Here is how I was led to make this observation: among the preceding experiments, some had been prepared on

39 52.4 Henri Becquerel Discovers Radioactivity 1179 Wednesday the 26 th and Thursday the 27 th of February, and since the sun was out only intermittently on these days, I kept the apparatuses prepared and returned the cases to the darkness of a bureau drawer, leaving in place the crusts of the uranium salt. Since the sun did not come out in the following days, I developed the photographic plates on the 1 st of March, expecting to find the images very weak. Instead the silhouettes appeared with great intensity. I immediately thought that the action had to continue in darkness, and I arranged the following experiment: At the bottom of a box of opaque cardboard I placed a photographic plate; then, on the sensitive side I put a crust of the uranium salt, a convex crust which only touched the bromide emulsion at a few points; then, alongside, I placed on the same plate another crust of the same salt but separated from the bromide emulsion by a thin pane of glass; this operation was carried out in the darkroom, then the box was shut, then enclosed in another cardboard box, and finally put in a drawer. I did the same with the case closed by a plate of aluminium in which I put a photographic plate and then on the outside a crust of the uranium salt. The whole was enclosed in an opaque box, and then in a drawer. After five hours, I developed the plates, and the silhouettes of the crystalline crusts appeared in black as in the previous experiments and as if they had been rendered phosphorescent by light. For the crust placed directly on the emulsion, there was scarcely a difference in effect between the points of contact and the parts of the crust which remained about a millimetre away from the emulsion; the difference can be attributed to the different distance from the source of the active rays. The effect from the crust placed on a pane of glass was very slightly attenuated, but the shape of the crust was very well reproduced. Finally, through the sheet of aluminium, the effect was considerably weaker, but nonetheless very clear. It is important to observe that it appears this phenomenon must not be attributed to the luminous radiation emitted by phosphorescence, since at the end of 1/100 th of a second this radiation becomes so weak that it is hardly perceptible any more. One hypothesis which presents itself to the mind naturally enough would be to suppose that these rays, whose effects have a great similarity to the effects produced by the rays studied by M. Lenard and W. Röntgen, are invisible rays emitted by phosphorescence and persisting infinitely longer than the duration of the luminous rays emitted by these bodies. However, the present experiments, without being contrary to this hypothesis, do not warrant this conclusion. I hope that the experiments which I am pursuing at the moment will be able to bring some clarification to this new class of phenomena. His report awakened very little interest from the Academy members. The blackened plates were put into the shade by the X-rays. Six reports on the X-ray theme were read at the March 2 meeting and a further six were announced to the next. The consequence of Becquerel s results was however that the observed uranium phenomena were not caused by phosphorescence. Instead it was a question of a fundamentally new fact, the emission of radiation not earlier observed. The phenomenon was found to be common to all uranium salts, and was concluded to be a prop-

40 Radioactive Elements erty of the uranium atom. Gradually it obtained the designation uranium radiation. Becquerel had discovered radioactivity, but Marie Curie was the first to use that designation. Becquerel made an additional important discovery about his uranium radiation. A gold leaf electroscope was charged and the leaves separated. Different substances, placed in its vicinity, had no effect on the position of the gold leaves. Under the inf luence of a uranium salt, the leaves however fell down. The gold leaf electroscope is not an instrument of precision, nor is it very sensitive. The experiment however demonstrated that uranium radiation cooperates with electricity. This property made it possible for Pierre Curie to develop equipment for quantitative radiation measurement. The radiation that Becquerel had discovered was a powerful tool in the coming search for atomic structure. It gradually also became utilized in medicine and for generating energy to an extent beyond previous comprehension. But people were unaware of these things when Becquerel made his first experiments and reported the results to the Academy, listening with absent-mindedness, in Paris on March 2, Nor was the listener aware of the coming fission of uranium atoms. There were in fact good reasons for the lack of interest. Uranium salts had been known for more than one hundred years. M. H. Klaproth had discovered the element in Berlin in He named it after Uranus, the very first of the Greek gods, the father of them all. That had been over-ambitious. The insignificant metal had, during that hundred years, existed only in a remote corner of the element family. Its oxide U 3 O 8 had a certain use for coloring glass yellow and green. That was all. Becquerel was systematic, as can be expected of a professor of physics. He questioned whether the radiation he had discovered was related just to the uranium salts he had investigated, or whether it might also emanate from uranium in an ore. He obtained such ore from Joachimsthal, situated in the Bohemian part of Erzgebirge [52.7]. There an ore was won that contained pitchblende (Figure M76), a uranium oxide. It could be expected that uranium in an ore acted in the same way as uranium in a salt. The photographic effect ought however to be reduced, due to the lower content of uranium. The investigation became a great surprise. The ore from Bohemia had a much greater effect on the photographic plate than could be expected from its content of uranium. Why? Is there any substance in the ore that inf luences a photographic plate more than uranium itself? It must be investigated! In France, Becquerel was almost alone in the research on uranium radiation. At the University of Sorbonne, a young Polish woman, Marie Sklodowska, studied and worked. She had been married for one year to a Frenchman, the physicist Pierre Curie. Professor Becquerel had observed her. She seemed to be clever and skilled in experimental work. He told her about his findings and put a straight question to her. Would she work with uranium radiation? The answer was yes.

41 52.5 Marie Sklodowska-Curie s Early Years Marie Sklodowska-Curie ( ) A Young Girl Fights for Herself and Her People The teacher and principal Wladyslaw Sklodowski was about the same age as Mendeleev. He had studied science in St Petersburg in the middle of the 19 th century, when also Dimitri Mendeleev was active there. He returned to Poland and in 1868 was a teacher of physics at a high school in Warsaw. The year before, his youngest child Marie was born. His wife Bronsitwa Boguska had been working as the headmistress of a girls school but got tuberculosis and died when Marie was 11. The father now had to take responsibility, not only for teaching and duties as a principal, but also for the care of Marie and her three sisters and one brother. Three fields in Marie s education were important to her: Polish literature, poetry and prose; the political liberation of Poland; and due to her father s inf luence science. At this time Poland s independence was lost. Russia had captured Warsaw in 1831, Poland s constitution was abolished, and Polish independence was ended for more than a century. All teaching was fulfilled in the Russian language, and Polish was not allowed. About 1885, when Marie and her young Polish comrades were years of age, the situation for education and liberation was bad, especially for young women. In the last decades of the 19 th century, Europe experienced a great enthusiasm for technology and science. Considerable progress was made within physics and chemistry. Industry and communications experienced quite new conditions as a result of the development of electricity. The inf luence of all this also reached Poland. Education, especially in science, was the key to this fascinating world, but women in Poland did not have access to higher education. Marie and her sister Bronya joined other friends in attending the Floating University. It had this name because its classes met in changing locations, in order to avoid the watchful eyes of the Czarist authorities. The fact that education was given in Polish, and also that new political theories were treated, was dangerous. Paris was at this time a center of attraction regarding both science and striving for political liberty. Many Polish insurgents had emigrated, and several young people had gone to Paris for their studies. Marie s sister Bronya had done so to study medicine. Marie wished to follow her. She still lacked real laboratory experience, however, and she hoped to gain some before her departure. The Museum of Industry and Agriculture in Warsaw presented the opportunity. It was a laboratory aimed at training Polish scientists. Marie s cousin Joseph Boguski helped her. He was responsible for the laboratory and arranged for Marie to have an intensive chemistry course on Sundays and evenings. Joseph had been an assistant to Dimitri Mendeleev and could certainly convey lots of direct information that could fill his young student with enthusiasm. Finally, in the fall of 1891, at the age of 24, Maria Sklodowska set out for Paris. Some very hard and laborious years followed.

42 Radioactive Elements It is told by Jaffe [52.7] that Marie should have met Mendeleev during this preparatory time in Warsaw. The fact that his former assistant Boguski ran the Museum of Industry makes it probable that Mendeleev paid a visit there in connection with some journey. And he would have found Marie Sklodowska, daughter of the old Petersburger Wladyslaw Sklodowski, working in the laboratory. Obviously the girl had impressed him favorably. Mendeleev had encouraged her and predicted a bright future if she stayed with chemistry. Even if she took his words just as an expression of kindness from the nice old man, the speech was certainly significant for the young girl. Considering the elements that Mendeleev had anticipated before their discovery, he has been called a prophet of science. His talk to Marie Sklodowska at the end of the 1880s if it really took place also contained a prophecy that was fulfilled. Mendeleev lived to see Marie Sklodowska discover the elements polonium and radium in 1898 and be awarded the Nobel Prize in First Degrees and a Happy Time In the fall of 1891 Marie enrolled at the Sorbonne. She had to live somewhere at first in the home of her sister Bronya; after some time in a small garret, cold in winter, hot in summertime, but nearer the Sorbonne. Paris offered the possibility for studies and perhaps perhaps a career within science. That was an unrealistic dream. Science was something for men. The young Polish woman was however obsessed by her will to succeed. Had not her father, whom she admired so much, believed in her? And who, among the self-confident French students just on the point of going into the Sorbonne, had a personal exhortation of the great Mendeleev to study chemistry. None! But she had. And her diligence paid off. In 1893 and 1894 she undertook her first degrees in physics and mathematics. Her economic situation however was a permanent concern. Then she received information from Warsaw that she had been awarded the Alexandrowitch scholarship intended for Polish students abroad. A miracle! The amount, 600 roubles, was enough for her to live for 15 months. In Paris at this time existed the École de Physique et de Chimie Industrielle. There Pierre Curie ( ) worked on research on the magnetic properties of materials. He had identified a temperature above which a material s magnetic properties disappear. It is still known as the Curie point. By 1880 he and his brother Jacques had discovered piezoelectricity, showing that a pressure applied to some special crystals resulted in the creation of an electric potential. In 1894, Marie met the eight years older Pierre. They became friends and colleagues in materials research. And still more. The year after they met, they married. Marie s father arrived from Warsaw for the simple wedding celebration. After the wedding they continued their assignments. Marie completed her studies and did some work at the École Industrielle. One day Professor Henri Becquerel paid a visit to Madame Curie and asked if she would examine pitchblende. Marie and her husband Pierre realized that research on uranium radiation could be an excellent field for

43 52.6 Marie Curie s Work for Her Doctor s Degree 1183 Marie s further studies towards gaining a doctor s degree. So Marie answered yes. That decision was the start of something very important in the history of science Marie Curie s Work for Her Doctor s Degree A Simple Problem Analysis In 1897 Marie was preparing to continue her studies. The first degrees were completed, she was certificated as a teacher, and she had her first child, daughter Irène. It could be time to go further for her doctor s degree. Professor Becquerel s rays fascinated. Likewise the uranium mineral pitchblende that gave stronger radiation than pure uranium salts. It was a scientific mystery and a challenge. She also felt she had all the support she needed from her husband. In Becquerel s first investigations, important observations had been made. From a scientific point of view, however, his method of measurement was unsatisfactory. The observations were only qualitative or possibly semi-quantitative. Results of the type substance X blackens a photographic plate more than a substance Y could not be a good basis for working through the subject field thoroughly. The doctoral candidate Marie, however, had a much better foundation from which to tackle this difficult problem. Her husband Pierre and his brother Jaques had developed new equipment, an ionization chamber (Figure 52.4), which made it possible to make quantitative measurements of the effect of so-called ionizing radiation. For research on Becquerel s radiation, this was of critical importance. I E E' ionizing radiation + A V Figure 52.4 Principle of an ionization chamber.

44 Radioactive Elements An ionization chamber consists of a container, filled with gas at atmospheric pressure. Two electrodes, E and E, in the container are given a high dc potential. Ionizing radiation entering the chamber removes electrons from the outer shells of the atoms in the chamber gas. The gas becomes conductive and a current pulse is recorded. (The Geiger counter for detecting radioactivity is a modern application of this principle.) With this equipment available, and with the assistance of her husband, Marie Curie could set to work with a systematic search. First she changed the designation Becquerel radiation to radioactive radiation and she discovered that not only uranium minerals but also thorium minerals give a radioactive radiation. Marie Curie was the first to establish that radioactivity is an atomic property and independent of the origin and history of the radioactive element. Nor has chemical treatment or chemical binding any importance. For uranium radiation, Marie and Pierre made an important analysis of the problem: If the radioactivity of the ore in Joachimsthal is caused by uranium, the waste products after the uranium concentrate is produced should have no or low radioactivity. The reality was the contrary. The waste products were more radioactive than the ore. Thus one or more elements were hidden in the ore and did not follow uranium in the chemical treatment. Which elements were the carriers of this high radioactivity? Of the elements known at the turn of the century, not one had such high radioactivity that its presence in the ore could explain the strong radioactivity observed. The necessary consequence gave the scientists shudders of excitement. The radioactivity was the sign of the presence of one or more unknown elements with strong radioactivity Concentrated and Enriching Work In the uranium mine at Joachimsthal in Bohemia, an ore with up to 15% U 3 O 8 was won. With a dressing by hand, the gangue was removed and a concentrate with 65 70% U 3 O 8 was obtained. It was roasted with soda and saltpeter at 800 C. The sodium uranate formed was leached out with sulfuric acid. From the solution, uranium oxide for sale was prepared. The residue after leaching, about one-third of the original ore weight, was waste. Some tonnes of this material, containing silicates, lead and calcium sulfate, aluminum and iron oxides, were placed at the Curies disposal. The main components of this waste were of little interest. Nevertheless, the material was of utmost importance. Which trace elements did it hide? The two scientists realized that there were small prospects of success by a random search for new elements in the huge amount of waste rock. Instead, they decided to dissolve a certain quantity in acid and examine the solution with the separation methods known from classical chemical analysis. The unknown elements would then be enriched in those fractions with which they have a chemical similarity. With the ionization chamber, they could measure the radioactivity of the different fractions. It was hard work in a damp room of a shed near Pierre s School of Physics, more like a barn or a potato-cellar than a laboratory. The raw material was treated, of course manually, in batches of up to 20 kg. The fractions were saved and combined with the corresponding fractions from other

45 52.6 Marie Curie s Work for Her Doctor s Degree 1185 batches, treated in the same way. The shed was filled with large receptacles, containing precipitates and solutions. It was a problem to keep the survey and to plan for the next step. The situation was also complicated by the fact that Marie and Pierre were physicists more than chemists. Because of that, they contacted a clever chemist, André Debierne, active in Professor Friedel s laboratory at the Sorbonne. From him they got a lot of good advice and he took an active part in the research The Discovery of Polonium Mendeleev s eka-boron (scandium) had been discovered in 1879, eka-aluminum (gallium) in 1875, and eka-silicon (germanium) in In his book Principles of Chemistry, 1891, the Russian chemist continued his prophetic activity and anticipated an element with properties similar to those of tellurium and with an atomic weight of about 212. The element then would be in a position to the right of bismuth in the periodic table. In their hunt for new elements, Marie and Pierre Curie treated a batch of material with strong hydrochloric acid. Both the solution and the insoluble residue were radioactive. From the hydrochloric acid solution, sulfides were precipitated with hydrogen sulfide. The radioactivity followed bismuth. André Debierne introduced a modified technique. An iron foil was placed in the acid solution. Metals nobler than iron, thus copper, lead and bismuth, precipitated, at least partially. The radioactivity followed this metallic fraction. The precipitate was again dissolved in hydrochloric acid. Now a copper foil was introduced. A precipitate was obtained even now, but small and almost invisible small, but the carrier of the whole activity. This precipitate was again dissolved in hydrochloric acid and something unknown in the solution was precipitated by addition of stannous chloride SnCl 2. When filtered off, it was shown that its radioactivity was 3500 times greater than that of pure uranium. A further purification was made by hydrogen sulfide precipitation, dissolution and finally electrolytic precipitation. Some tonnes of raw material had been treated. The amount precipitated by electrolysis was g, with an estimated quantity g of a new element. This new element was given the name polonium (Po) after Marie s native country. Polonium was the first element whose discovery was based on radioactive measurements. The isotope found in Curie s and Debierne s investigations was 210 Po with a half-life of 138 days. In nature polonium exists only in equilibrium with its mother substances in different decay series. If separated from its environment, it disintegrates to lead The Discovery of Radium In their systematic investigation of how the radioactivity appeared in the different fractions, the Curies discovered that a radioactive element followed barium. After fractional crystallization of (a contaminated) barium chloride, they observed that the

46 Radioactive Elements radioactivity increased in fractions with lower solubility. In the course of this investigation, they contacted Eugène-Anatole Demarçay. He worked in Paris with rare earth metals and discovered the element europium (see Chapter 17 Rare earths). He had great experience of fractional crystallization and he had also developed a special spectral analytical technique for identification of rare earth metals. With his new spectroscope, lines of a previously unknown element were observed in the less soluble barium chloride fractions. The crystallization was continued and the crystals were examined with spectral analysis and radioactivity measurements. Finally a substance was obtained with a spectrum clearly of the same type as that of the alkaline earth metals. Barium lines had disappeared and no lines from other known alkaline earth metals could be found. A new element had been discovered. Its chloride showed such strong radiation that it shone in darkness. Because of that, Marie and Pierre Curie named it radium (Ra). They calculated the atomic weight of the new element from the weight ratio between the radioactive chloride RaCl 2 and an equivalent amount silver chloride AgCl. From this ratio they got the atomic weight 225 for radium. In 1907 Marie Curie made new determinations, utilizing a greater sample quantity. Then she got the result Today the accepted value is Marie Curie s Later Life and Death Rewarded for Her Genius In June, 1903, Marie Curie defended the thesis for her doctorate [52.8], based on the results she had obtained. It was a remarkable occasion, partly because she was the first woman to receive a doctorate in France, but above all for the content of the dissertation. It has been said that it certainly is the most important doctoral thesis ever. Already in the same year, together with Pierre Curie and Henri Becquerel, she got the Nobel Prize for physics. The description of the further success and worldwide public attention is not a subject of this book. Few have been so exhaustively portrayed as Marie Curie (Figure 52.5). Her own daughter wrote a well-known biography [52.9]. Not surprisingly the Nobel Prize inf luenced Pierre and Marie s situation in France. Pierre Curie was appointed Professor of Physics at the Sorbonne and Marie, for the first time in her career, got both a title Chief of Laboratory and a university salary. Marie was satisfied but expressed that: it was not without regret that we left the School of Physics, where we had known such happy work days, despite their attendant difficulties.

47 52.7 Marie Curie s Later Life and Death 1187 Figure 52.5 Marie and Pierre Curie, portrayed on the cover of a French magazine. The year was 1904 and the scientists were already renowned for their discoveries. (Reprinted from ref. [52.5].) The Tragedy of Death On April 19, 1906, Pierre was on his way home from a meeting. Hurrying to cross a street, he was run over by a horse-drawn wagon with a heavy load. He was killed instantly. After this tragic event, Marie had to solve, alone, without her life companion, the many new, waiting assignments. Less than a month after the accident, the university offered her the chance to take up Pierre s academic post. By accepting, she hoped to honor the memory of Pierre. She started to create a scientific institution with better laboratory facilities than had been at Pierre s disposal Curie and Debierne Prepare Radium Metal Metallic radium was prepared in 1910 by Marie Curie and André Debierne. They used 0.1 g of radium chloride and carried out an electrolytic reduction on a mercury cathode. When the liquid amalgam that had been formed was heated, mercury distilled off and elementary radium was isolated.

48 Radioactive Elements An Unprecedented Second Nobel Prize In 1911 Marie Curie was awarded a second Nobel Prize, this time in chemistry. At the ceremony in Stockholm on December 10, 1911, the President of the Royal Swedish Academy of Sciences expressed the opinion that Marie Curie s achievements deserved an additional, and chemical, recognition: for her services in the advancement of chemistry by the discovery of the elements radium and polonium, by the isolation of radium and the study of the nature and compounds of this remarkable element. It was also a remarkable honor, as no other person, before or after, has been awarded the Nobel Prize in both physics and chemistry A Victim of Radioactivity Radium metal has a bright luster and has many similarities with barium. And yet it is completely different. Its radioactivity is strong and dangerous. During Marie Curie s whole adult life, she was exposed to large radiation doses. When she died in 1934, at the age of 67, the cause of death was leukemia Radium its Importance and Use Radium has played a great role in the exploration of knowledge about the atom. By combination of the alpha radiator radium with beryllium, the first neutron source was created. During the first decades of the 20 th century, radium had a large medical use for combating tumors and for treatment of rheumatic complaints. Radium institutes and radium homes were established in different parts of the world. They still existed in the 1950s. Today new medical techniques have been developed. Radium was also utilized in self-luminous paints for watch, clock and instrument dials and for emission in automatic control systems. Safer radioisotopes for technical properties, such as cobalt-60 and cesium-137, can nowadays be tailored in nuclear reactors and have entirely replaced radium. This has released us from the need for radium, which is a great advantage, as radium is so difficult to handle from an environmental point of view. It forms gaseous radon, affecting its surroundings. And the problem remains for a long time, as the most usual radidum isotope, 226 Ra, has a halflife of 1600 years. Nowadays the use of radium has ceased. The annual amount manufactured is only round 100 g.

49 52.9 Two Other Key Radioactive Elements Two Other Key Radioactive Elements Actinium The Discovery of Actinium It was mentioned above that the two Curies had called upon André Debierne for chemical support in the separation of the pitchblende residues. In that connection he made his own element discovery in From a solution containing iron and some rare earth metals, Debierne precipitated a mixture of hydroxides. It was radioactive, an activity that could not have its origin in uranium, radium or polonium. A new element could be isolated by fractional crystallization of magnesium lanthanum nitrate. The element was named actinium after the Greek word aktinos, meaning ray. Actinium metal has been prepared by the reduction of actinium f luoride with lithium vapor at about 1100 to 1300 C The Actinides Element number 89, actinium, is chemically similar to number 57, lanthanum, which makes the placing of the element 89 in group 3 logical. In the 6 th period we have met (Chapter 17) the element lanthanum and the 14 following lanthanides (58 71). In a corresponding way the followers of actinium in period 7 constitute the actinides (elements ). The actinides among the natural elements up to number 92 are thorium (Th), protactinium (Pa) and uranium (U). The synthesizing of the actinides after uranium is treated in section Radon Radioactive Infection Marie and Pierre Curie examined the radiation emitted by radium. They found that all substances near the radiation source became radioactive themselves. The activity even remained some time after the removal of the emitting radium. It was questioned whether there is some radioactive force that was transferred with the radiation and was induced in the receiving substance. In 1900 F. Dorn in Halle, Germany, reported on a study of an isotope emanating from radium and, because of that, certainly incorrectly, was credited with the discovery of radon. In fact Ernest Rutherford ( ) and Frederick Soddy ( ) were the first to isolate radon and also the first to really understand the nature of radon. In the first decade of 1900 they worked at McGill University in Montreal, Canada, and carried out very comprehensive work on radioactivity [52.10]. They discovered that the emanation from radium was a gas of the same type as the noble gases. Ramsay at University College in London completed the spectral work. He showed that the spectrum of radon resembled that of xenon. He also showed that the formation of radon was accompanied by the simultaneous production of helium, as he also observed its spectrum. This observa-

50 Radioactive Elements tion by Ramsay was a key investigation in deciphering the structure of the atom. Rutherford and Soddy clearly realized how radium disintegrates into both radon and helium ( particles). They became the first to conclude that radioactivity is a phenomenon of natural and spontaneous transmutation of elements, involving atomic disintegration with the formation of new kinds of matter Emanation Niton Radon The scientific world accepted the term emanation. However, in 1910 Ramsay suggested the name niton from a Greek word meaning shining. The reason was the appearance of the condensed, liquid element. However, the International Committee on Chemical Elements decided in 1923 in favor of the name radon, expressing its daughter relationship to radium Radon Is Unstable In 1923 F. Aston searched for the new gas in common air. He let as much as 400 tonnes of air pass over a bed of charcoal for adsorption of radon. After that a mass spectrographic analysis was carried out. The result was negative, indicating that stable or very long-lived isotopes of radon do not exist. Considerations of nuclear physical character also suggest that radon should not have stable isotopes Radon in Nature, Buildings and Mines 4) Radon is part of both the uranium and thorium decay series (see section ). In the uranium series: 238 U 226 Ra 222 Rn 218 Po radon is formed from 226 Ra and disintegrates to 218 Po. The half-life of the main isotope 222 Rn is 3.8 days. Radon, as a gas, carries a radioactive infection from the original radiation source (a uranium ore, for instance) and may place it in the lungs of a man. There the disintegration process continues, now locked in the tissue. It may result in the formation of cancerous tumors. The effects of radon were observed long before knowledge of radioactivity was established. Miners in the south of Germany died in the 16 th century of Bergkrankheit (mine disease), which later became known as lung cancer. Radon in houses began to be observed in the 1950s. A great Swedish epidemiological investigation showed that the injurious effect of radon depends on whether a person is a smoker or not. For smokers, the number of additional lung cancer cases, due to radon, was about 400 per year. For non-smokers, no significant increase could be detected. The radon content we get in a house depends on the type and composition of the ground material and how permeable it is. If clay, the radon cannot move very far before it disintegrates. The house will not be a radon house. If the ground material instead is gravel or sand, the radon can migrate a long distance before it disintegrates. 4) The information in this section is taken mainly from ref. [52.2].

51 52.10 The Periodic Table Becomes Complete 1191 It can penetrate into the house and be a risk. If the building material itself contains uranium, it may also be a source of radon. For dwellings and factory premises, radon contents are expressed in terms of the radiation it causes. A recommendation is to restrict the radiation to 200 Bq/m 2 while the maximum allowed value is 400 Bq/m 2. The situation for a radon house can be improved rather simply by installation of a radon fan that ventilates the earth below the foundations. Minimizing indoor radon content should be a conscious measure to take in connection with the construction and building of a new house. The radon content of drinking water ought also to be observed. Surface water normally has a low radon content, corresponding to round 20 Bq/l. In water from deepbored wells the radon content is higher, often corresponding to Bq/l. Water with more than 1000 Bq/l is not allowed as drinking water The Periodic Table Becomes Complete Mendeleev noted in 1871 that there is a gap between thorium and uranium in the element system. Investigations by Henry G. J. Moseley, before his too early decease in 1915, confirmed this and furthermore that still undiscovered elements might be expected between polonium and radon and between radon and radium. The situation around 1915 is described by Table Table 52.1 Known and unknown elements in 1915 a) Number Element Polonium Unknown Radon Unknown Radium Actinium Thorium Unknown Uranium a) Further unknown elements were numbers 43 (became known as technetium) and 61 (became known as promethium). The discoveries of these unknown elements are described by Wahl and Bonner [52.11] Number 91, Protactinium Discovered in It was a mystery that actinium exists in nature. Its most long-lived isotope, 227 Ac, has a half-life of 22 years. It must have a long-lived precursor, from which it is continuously formed. Otto Hahn and Lise Meitner, working at the Kaiser-Wilhelm Institute for Chemistry, Berlin, were aware of that and started an examination of the siliceous residues of pitchblende. While Otto Hahn was absent due to service in the army, in 1918 Lise Meitner succeeded, by utilizing tantalum pentoxide as a carrier, in detecting a long-lived alpha emitter that could be the mother of actinium. Although she had not separated the element itself, she (and Otto Hahn) had discovered the new element number 91.

52 Radioactive Elements Five years earlier, in 1913, K. Fajans and O. Göhring in Germany studied the decay of 234 Th and found a short-lived isotope. They named the nuclide brevium, a name later changed to UX 2. It is now known as a short-lived isotope of element number 91. In 1918, Fajans published a new report, indicating the discovery of element number 91. In Great Britain, F. Soddy, J. Cranston and A Fleck independently discovered the element at about the same time as Hahn and Meitner. All three groups of scientists are credited for the discovery. The name protactinium Pa (from the Greek protos, meaning first ) was selected because the element was recognized as the originator of actinium. Pitchblende ores contain about 200 mg per tonne of both radium and protactinium. Natural protactinium is almost entirely the isotope 231 Pa with a half-life of years. The American chemist Aristid Grosse first isolated 2 mg of protactinium in In about 1960 a large quantity of 125 g of protactinium with 99.9% purity was prepared in Great Britain by the Atomic Energy Authority. The extraction was made from 60 tonnes of waste material Number 87, Francium Discovered in 1939 Marguerite Perey worked on the investigation of very pure actinium in France in She found particles and could show that these did not come from protactinium. Moreover, she could follow an element growing in the actinium sample, an element with activity. After dissolving the sample she found that the element with activity did not follow barium in precipitation of barium carbonate. Nor did it follow lead in lead sulfide or cerium in cerium(iv) hydroxide precipitation. However, it was carried by cesium when cesium perchlorate was precipitated. Element number 87 ought to be an alkali metal, a new element that Margeruite Perey had discovered. She named it francium after her native country Number 85, Astatine Discovered in 1940 Moseley demonstrated clearly that there was a gap and a missed element between polonium and radon. A discovery of this element, number 85, was announced in 1931 by a group of scientists at Alabama Polytechnic Institute. They had dissolved monazite in aqua regia and claimed, after examination with a magneto-optic method [52.12], that the sought element was present in the solution at a concentration corresponding to % of the monazite weight. The element was named alabamium Ab after Alabama. The claim could not be verified and the discovery was not approved.

53 52.10 The Periodic Table Becomes Complete 1193 In Bern, Walter Minder examined polonium in He found radiation. If it originated from polonium, this element had been transformed to element number 85: 84 Po 85 (Unknown) + 1 Although the element was not directly detected, he gave it the name Helvetium Hv after Switzerland. Nor could this discovery be verified, and no element named after Switzerland exists. In the USA the problem was solved at the same time, D. Corson, K. MacKenzie and E. Segrè used the Berkeley 60-inch cyclotron and bombarded bismuth 209 Bi with energetic helium ions. The scientists expected the reaction: 83 Bi + 2 He 85 (Unknown) n A balance for the atomic masses: 209(Bi) + 4(He) = X(unknown element) + 2 1(n) gives X=211 for the mass of the unknown element. Thus an isotope with the mass number 211 ought to be present in the surface region of the target material. The new element could also be distilled from the target by heating in air. The discoverers named the new element astatine At from the Greek word astatos, meaning unstable. Astatine is the only member of the halogen family without stable isotopes. This highly radioactive halogen behaves chemically very much like iodine. It is however said to be more metallic and, like iodine, it probably accumulates in the thyroid gland. Astatine belongs to the most rare elements of all. Its total amount in the earth s crust is estimated to some grams. The isotope 211 At, which Segrè and co-workers discovered, has a half-life of 7.2 hours. A little more long-lived, 8.1 hours, is the 210 At isotope A Complete Periodic Table The actinide elements missing in 1915 were all discovered in the period Number 43 technetium was also discovered in 1940, so now only one of the 92 elements on earth was missing. It was number 61, promethium Pm, one of the rare earth elements. Its discovery in 1945 was described in Chapter 17 Rare earths.

54 Radioactive Elements Thorium as a Technological Metal Occurrence In the earth s crust thorium is almost four times more common than uranium, and the radioactive decay of thorium generates a considerable part of the geothermal energy. The primary source of thorium is monazite (Figure M25), a yellow or reddishbrown rare-earth phosphate. Seashore deposits at Travancore in India contain very big quantities of monazite. The mineral formula is (Ce,La,Nd,Th)PO 4 and the thorium content in monazite ores is 5 10%. In 2001 the production of monazite concentrate in the world was 5710 tonnes [52.13], of which India accounted for 5000 tonnes or 88%. Other producers were Brazil and Malaysia. Monazite is recovered mainly for its content of rare earth metals, and thorium has to be separated. This has led to an overproduction of thorium oxide. The excess is stored for potential use or disposed of as a radioactive waste. Most producers of rare earths have however switched to thorium-free raw materials due to the problems (and costs) of dealing with the radioactivity. This may change the situation for thorium supply. Other thorium minerals, besides monazite, are thorite ThSiO 4 (Figure M24) and thorianite ThO Manufacture On the treatment of monazite with sulfuric acid, thorium and many other elements are dissolved. The separation occurs with liquid liquid extraction. The f luoride ThF 4 is prepared from the thorium phase and reduced with magnesium. Fused salt electrolysis is also applied. The electrolyte is a mixture of equal parts of fused sodium and potassium chlorides, to which the double f luoride ThF 4 KF is added. The process temperature is 800 C Uses Practical interest in thorium metal and oxide lagged until 1884, when Auer von Welsbach developed and patented the incandescent gaslight mantle, in which thorium oxide was the essential ingredient (see Chapter 17 Rare Earths). Mantle production decreased and by 1925 thorium was relatively unimportant to commerce. Interest came back when thorium turned out to have a possible role in nuclear power plants (see section ). Thorium oxide ThO 2 has the highest melting point (3300 C) of all the binary oxides and has found some uses in special refractory applications.

55 52.12 Uranium as a Technological Metal Uranium as a Technological Metal Occurrence Minerals Uranium occurs as oxide and vanadate. The uranium oxygen system is complicated. During geological processes the oxide UO 2 is partly oxidized to a composition between UO 2.6 and UO Uranium oxide is present in the brown to black mineral pitchblende (Figure M76). It may be expressed so that the oxide essentially is UO 2, but usually it is partly oxidized to U 3 O 8. As mentioned earlier, it was in this mineral that radium and polonium were discovered. Besides the original locality, pitchblende was also found in Port Radium at Great Bear Lake in Canada. The deposit there was discovered in 1930 and was expropriated by the Canadian Government during World War II due to the importance of its uranium oxide for atomic energy. The mines were closed in Uraninite (Figure M76) is a black or brown mineral with a composition similar to pitchblende. It is the chief mineral in uranium ores. It is strongly radioactive, and often contains impurities such as thorium and radium. When heated it emits helium. Carnotite K 2 (UO 2 ) 2 (VO 4 ) 2 (H 2 O) 3 is a yellowish mineral, strongly radioactive. It is a constituent of uranium and vanadium ores and a source of radium. Coffinite U(SiO 4 ) 1 x (OH) 4x is a black mineral, important as a uranium ore. It occurs in sandstone deposits and hydrothermal veins Some Geological Notes Acid magmas from the interior of the earth react with basic rocks and start to solidify. Uranium is enriched in the remaining melts. When these finally solidify, the uranium content may be 100 g/tonne (100 ppm, 0.01%). Compared to the mean content in the earth s crust, 2.7 ppm, this is a considerable geochemical enrichment. In addition, uranium is concentrated into phosphate minerals and coal, especially brown coal. This is the background to the fact that coal-fired power stations emit more uranium than nuclear power stations! By hydrothermal leaching 5) of granitic rocks containing uranium and subsequent deposition, veins with uranium contents of several percent can be formed. This is the case in deposits at Joachimsthal in Bohemia, at Port Radium in Canada, and at Chingolobwe in Zaire (Congo Kinshasa). On the weathering of pegmatites containing uranium, secondary concentrations are formed, in which uranium is oxidized to the hexavalent stage. Water-soluble uranyl complexes are enriched in clays and phosphates. In sandstones, uranyl-containing solutions may again be precipitated under reducing conditions and form 5) In hydrothermal processes, water, sometimes superheated, dissolves minerals at high pressure. Several minerals that normally are almost insoluble in water may be dissolved in the extreme conditions of hydrothermal processes.

56 Radioactive Elements minerals. One example is carnotite, found in the red and yellow sandstones of Arizona and Colorado, USA. If the oxygen supply is limited, the weathered grains of uranium minerals are not oxidized and the uranium is not leached out. The grains are transported by the streaming water and are deposited in positions determined by their high density. The deposits in the Lake Superior region of Canada and at Witwatersrand in South Africa have been formed in that way. The two deposits are similar but the Canadian ore contains % uranium oxide, which is about 10 times more than the South African ore. On the other hand, the South African ore contains gold to such a great extent that uranium is obtained as a byproduct from gold winning. Sweden has considerable uranium reserves, with low metal content, in the Cambrian schists in the middle part of the country. Canada stands out as the most important uranium country of the world. One of the richest uranium deposits known is situated at Cigar Lake in the Canadian province of Saskatchewan. High-grade uranium ores are mined there in underground work, using water jets to cut the rock. The annual production is more than 8000 tonnes of U 3 O 8 from the rich eastern part of the ore body Production The world production, counted as uranium, was about tonnes in How the production was divided by country is shown in Table 52.2 [52.14]. Table 52.2 World production of uranium (tonne) in 2001 Country Production Percentage Country Production Percentage tonnes of total tonnes of total Canada South Africa Australia Ukraine Niger China Russia Czech Republic Namibia India Kazakhstan France Uzbekistan Other countries USA Source: taken from ref. [52.14]. Total

57 52.12 Uranium as a Technological Metal Manufacture of Uranium Metal and Isotope Separation Preparation of Uranium Fluoride From Uranium Ore The mined uranium ore is crushed and ground into a fine powder. After ore dressing, the concentrate is leached with sulfuric acid. The solution is treated in a liquid liquid extraction, in which uranium is transferred to an organic phase. It is extracted from that with ammonia, and ammonium uranate is precipitated. At 1000 C it is decomposed to yellow uranium oxide UO 2. Uranium hexaf luoride is prepared by treating the oxide with hydrogen f luoride to make uranium tetraf luoride. This in turn is treated with elemental f luorine to prepare the gaseous hexaf luoride UF 6 (sublimation point 56 C) Isotope Separation It is the isotope 235 U, not 238 U, that can be utilized in conventional processes for fission by neutrons, and thus for energy generation (and for manufacturing atomic bombs). In natural uranium, only 0.7% of the nuclei are of the lighter isotope. The rest, 99.3%, consists of the heavier isotope 238 U. When uranium is to be used as a nuclear fuel, it generally needs to be enriched. In conventional reactors, a nuclear fuel with 3% 235 U is utilized 6). That enriched uranium is obtained by diffusion processes originally developed in the so-called Manhattan Project. The classic method for isotope separation utilizes a gas diffusion method in cells. Each cell is divided into two parts, separated by a barrier, pierced with a lot of tiny holes. On one side of the barrier an under-pressure is maintained, on the other side an over-pressure. The f luoride is fed in to the part with overpressure and diffuses through the barrier. The mean speed of the molecules of a gas is lower if the molecular weight is higher. It can be calculated that the diffusion speed for 235 UF 6 is times higher than that for 238 UF 6. When half of the gas volume has passed into the low-pressure part, a certain, very small, enrichment of the light isotope has occurred. That gas volume now is pumped to the high-pressure part of the next cell. The process is repeated with a lot of cells connected in a cascade. The light isotope moves forward in the process, the heavier backward. When the 235 U content of the enriched f luoride has increased from 0.7% to 3.25%, the depleted f luoride has decreased its content to 0.2%. The process has fundamentally great similarities with fractional crystallization, described by figure in chapter 17. The diffusers work at a temperature of 80 C. The enriched hexaf luoride is transformed to uranium oxide. The hydrof luoric acid is set free and re-used. The uranium oxide is pressed into small cylindrical pellets, 10 mm high and 10 mm in diameter. The pellets are filled into 4 5 m long tubes, made of zircaloy 7). The tubes are filled with helium and welded. They are the fuel of the nuclear power stations. 6) An atom bomb needs to contain % fissionable material, since the chain reaction otherwise proceeds too slowly. Consequently, there is no possibility of nuclear fuel exploding. 7) For more information about zircaloy, see Chapter 19 Zirconium.

58 Radioactive Elements A modern variant of isotope separation is the gas centrifuge. In a very rapidly rotating cylinder, the heavier isotope is forced closer to the outer wall, while the lighter one stays nearer to the center. It is necessary to have many centrifuges, operating in cascade, to process large amounts of material. However the plant is more economic than diffusion equipment. A potential problem is that smaller and/or poorer nations may utilize the technique in order to produce their own nuclear weapons. A fundamentally different technique is laser isotope separation (LIS) or atomic vapor laser isotope separation (AVLIS). A laser beam is tuned to a wavelength that excites only one isotope of the material and ionizes those atoms preferentially. After the atom is ionized, it can be removed from the sample by applying an electric field. As uranium has a density almost 70% higher than that of lead, ammunition made from this metal is an effective anti-tank weapon. When used in combat, the uranium in the bullet ignites upon impact and a cloud of uranium oxide dust is formed. To reduce the radiation risk, depleted uranium (DU) is used in weapon systems of this type. It is obtained as a residue when natural uranium has been enriched in respect of uranium-235. DU is a substance that is only about half as radioactive as natural uranium. But due to its radioactivity even if it is low the dust can cause internal injuries if it is inhaled or ingested Nuclear Fission In 1939 the German scientists Otto Hahn ( ), Lise Meitner ( ) and Fritz Strassman ( ) found that nuclear fission is possible. It was demonstrated in a paper in Die Naturwissenschaften in Lise Meitner, being both a woman and a Jew, did not have her name on the important paper. She had f led Berlin in July 1938 to escape Nazi persecution. However, in another paper in 1939, in Nature, together with Otto Frisch ( ), she described the disintegration of uranium by neutrons and explained the theory of uranium fission. The fission process will be described here for 235 U. If the nucleus of this atom captures a neutron, a very unstable nucleus 236 U is formed. Fission into two big fragments occurs spontaneously (Figure 52.6). Many different reactions are possible, one example of which is: 235 U + 1 n 139 Ba + 94 Kr n + energy The sum of the masses of the fragments is less than the original mass. This missing mass (about 0.1% of the original mass) has been converted into energy according to Einstein s equation. Fission of 1 g uranium per day corresponds to a power of 1MW. It is important and noteworthy that one neutron is used and two or more free neutrons are generated. This makes a chain reaction possible. In the practical utilization

59 52.14 The Nuclear Reactor 1199 Figure 52.6 Fission of a uranium atom. of nuclear fission for energy generation, the mean speed of the free neutrons has to be suitable for the capture process with new uranium atoms. A moderator reduces the speed to this level The Nuclear Reactor With Enriched Uranium as the Fuel Uranium is almost entirely used for energy production in nuclear power plants. The commercial nuclear era began with graphite-moderated reactors. The first plant for production of electricity (5 MW) was built at Obninsk near Moscow. In England, at Calder Hall, four reactors of this type, each with a power of 45 MW, were built in The Chernobyl reactors also were graphite-moderated. New types of reactors utilize common water as moderator. Commercial nuclear power plants have pressurized water reactors (PWR) and boiling water reactors (BWR). A typical Swedish boiling water reactor is described in Figure In a modern reactor, almost 100 tubes with uranium oxide pellets are assembled into a fuel cluster, which in turn is placed in a casing, known as a fuel rod, prior to insertion into the reactor. A modern reactor contains several hundred of these fuel assemblies. Approximately one-fifth of the fuel is replaced each year. Control rods are used to start and stop the reaction and to regulate the power output. They contain substances, e.g. boron carbide, that have the ability to absorb neutrons. In 1998, there were 434 power-generating nuclear reactors in operation in 31 countries and the total production amounted to about 2300 TWh 8), or approximately 16% of total electricity production. 8) 1 TWh (terawatt hour) = 1 billion kwh (kilowatt hours).

60 Radioactive Elements The water starts to boil, forming steam. The steam is transported onward to the turbine. The steam strikes the turbine blades at high speed. The turbine shaft rotates at 3000 rpm. The turbine drives the generator, which produces electricity. 7. Power lines transport the electricity to the various consumers When fission occurs in the uranium, energy is released that heats the surrounding water. The reactor contains uranium fuel and water. Like a gigantic kettle A nuclear power plant can be likened to a gigantic kettle. As with oilor coal-fired power plants, steam is produced to drive the turbines. However, in oil or coal-fired power plants the water is boiled by direct heat, while in a nuclear power plant it is heat resulting from nuclear fission that boils the water to produce the steam. The reactors at both the Barsebäck and Oskarshamn plants are of the same type, known as a Boiling Water Reactor (BWR). 9. The water is pumped back to the reactor, where it is heated again to begin a new cycle. 8. After the steam has dissipated its energy in the turbine, it is transported onward to the condenser, which consists of numerous narrow tubes through which seawater is pumped. When the steam comes into contact with the outer surfaces of the tubes, it condenses back to water. Twenty-five cubic meters of seawater per second The water used in the condensers to cool the steam is drawn from the sea. At Barsebäck, each reactor unit uses 25 cubic meters of seawater per second, which is pumped into a channel leading into the power plant. When the cooling water is returned to the sea in the Öresund Strait, it is 10 C warmer than when it entered the plant. The seawater is pumped through the condenser in a special system of tubes. It never comes into contact with the steam produced by the reactor. Figure 52.7 A boiling water reactor. (Reproduced by permission of Sydkraft, Sweden, and L+J AB.)

61 52.14 The Nuclear Reactor Breeder Reactors A conventional nuclear reactor has a very low efficiency regarding uranium utilization. Unlike the lighter isotope, the heavy isotope (which is the major component) will never be utilized. A breeder reactor produces more fuel than it consumes. With molten sodium as a coolant, the main isotope in natural uranium, 238 U, absorbs neutrons and reacts according to the formulas given in Figure 52.9 (section ). The plutonium isotope is fissile and can produce energy. However, the handling of plutonium is a risk, as it can be used weapons. Super-Phenix, the first large-scale breeder reactor, was put into service in France in The isotope 232 Th can also be used in a breeder reactor. A fissile substance 233 U is obtained in a chain reaction: Th 233Th 233Pa U These reactions can utilize the normal moderating and cooling system, and thus the reactions can occur in a conventional reactor, to which thorium is added Oklo Nature s Own Reactor Natural uranium has a content of 0.72% of the 235 U isotope. In Oklo, an African uranium deposit in Gabon, the ore has a lower content, only 0.44%. This indicates that a part of the lighter isotope has been consumed in a nuclear reactor, started and run by nature itself. Of course, there are discussions about this fantastic story. However this supposition is strongly confirmed by the results of mass spectroscopic examinations of disintegration products in the region of the deposit. The rare earth metal neodymium has been found to contain 2% of the isotope 142 Nd. In natural neodymium its content is 27% [52.15]. This is in agreement with the fact that neodymium, formed during fission of uranium in modern reactors, contains 1% of 142 Nd. So nature s own reactor was perhaps active some 2000 million years ago. Water is thought to have been the moderator. Fission of 235 U started the process. In the actual period the content of 235 U (counted on total uranium) is estimated to have been 3%. This is one reason why the reactor started. With the 235 U content 0.72% of today a spontaneous reactor of Oklotype is not possible. The abundant access to 238 U made the formation of 239 Pu possible. Thus this natural reactor was a true breeder reactor.

62 Radioactive Elements More Than Just 92 Elements The Transuranium Actinides On earth there are 92 elements designated as naturally occurring. Hydrogen with atomic number 1 is the lightest, uranium with atomic number 92 the heaviest. The elements 43 technetium and 61 promethium are special and do not occur naturally. As described in Chapter 28 Technetium and Chapter 17 Rare earths, they are not present in detectable amounts on earth. They were discovered in fission products from atomic reactors. The 14 elements following actinium (Z=89) are called the actinides. Up to 1940 only three of them, thorium, protactinium and uranium, were known. The question then was, would it be possible to find or synthesize the others? The Situation in 1940 In the lanthanide series, the missing element number 61 was discovered in 1945, which made the lanthanide subgroup complete. The corresponding actinide group was almost empty, as shown in Figure The squares attracted great interest, typical for science, which in all times has endeavored to fill empty spaces Ce (a) 59 Pr 60 Nd Sm 63 Eu 64 Gd 65 Tb 66 Dy 67 Ho 68 Er 69 Tm 70 Yb 71 Lu 7 90 Th (b) 91 Pa 92 U Figure 52.8 The a) lanthanides and b) actinides known in Synthesis and Discovery In 1940 E. McMillan and P. H. Abelson at the University of California, Berkeley, irradiated 238 U with neutrons; this led to the formation of element number 93, which they called neptunium [52.18]. What they did can be described by the first formulas in Table Their successful result was the start of intensive work that led to the discovery of all the actinide elements. In fact neptunium and also plutonium occur in nature. Minute amounts of these elements are produced in the radioactive decay of uranium. In the famous laboratory at Berkeley, E. O. Lawrence had developed the cyclotron. By irradiation with particles heavier than neutrons, accelerated in the cyclotron, several new transuranium elements could be produced, although in very small quanti-

63 52.16 More Than Just 92 Elements The Transuranium Actinides 1203 ties. In 1940 G. T. Seaborg ( ), E. M. McMillan, J. W. Kennedy and A. C. Wahl succeeded in preparing element 94, plutonium, by bombardment of uranium with deuterons, according to Table Seaborg was co-discoverer of all the nine elements Together with R. A. James, L. O. Morgan and A. Ghiorso, he prepared americium Am at the University of Chicago in They irradiated plutonium with neutrons. The element 95 was formed in a three-stage process. Curium, element 96, was also prepared by Seaborg s group at the University of Chicago. In 1944 they bombarded plutonium with helium ions and obtained the isotope 242 Cm. Back in California, Seaborg synthesized the element 97 in a cooperation in 1949 with S. G. Thompson and A. Ghiorso. They used a method with bombardment of americium with helium ions. The new element got its name berkelium after its birthplace, the University of California, Berkeley. Still six actinides remained to be discovered. On bombardment of curium in 1950 with helium ions, Seaborg s group obtained element 98 and called it californium. After the thermonuclear explosion of 1952 at Enewetok Atoll in the Marshall Islands, in the Pacific Ocean, successive neutron captures and subsequent decay was found to have changed 238 U to an isotope of the element 99. This was a big cooperative project between scientists at Berkeley, Argonne National Laboratory (Illinois) and Los Alamos Scientific Laboratory (New Mexico). The new element 99 got the name einsteinium. In the debris from the Enewetok Atoll the combined scientific group also found, in 1953, the element 100 that was named fermium. The new discoveries gave Seaborg and his colleague new targets for bombardment. Einsteinium was reacted with helium ions. The new element 101 Md, mendelevium, was formed, in 1955 at the University of California. For the synthesis of the transfermium elements, thus elements with atomic number 101 or greater, it became usual to utilize projectile particles heavier than neutrons, deuterons or helium ions. Carbon, boron and oxygen were used. In Berkeley in 1958, J. R. Walton and G. T. Seaborg bombarded curium with carbon ions and got the element 102, nobelium. From the 1960s the Russian Joint Institute for Nuclear Research at Dubna became a new, important center for research regarding the transuranium elements. In 1964 E. D. Donets, V. A. Schegolev and V. A. Ermakov verified the formation of nobelium by bombardment of curium with carbon (see Table 52.3). After nobelium had been discovered, only one actinide remained to be found, element number 103. A. E. Larsh and R. M. Latimer at Berkeley in 1961 and E. D. Donets, V. A. Schegolev and V. A. Ermakov at Dubna in 1965 bombarded californium isotopes with a mixture of boron-10 and boron-11 ions and were able to identify element 103. It was named lawrencium. Bombardment of americium with oxygen ions created another lawrencium isotope. The properties of some isotopes of the transuranium actinides are collected in Table 52.4.

64 Radioactive Elements Table 52.3 The synthesis and discovery of the transuranium actinides Synthesis/ Reaction(s) Description discovery and year Neptunium U + 1 0n U Neptunium-93 is synthesized from uranium U Np + by neutron bombardment. The neptunium isotope obtained has a half-life of 2.36 days Plutonium U + 2 1H Np n Plutonium-94 is synthesized from uranium Np Pu + by deuteron bombardment. The isotope obtained has a half-life of 87.7 years Americium Pu + 1 0n Pu Americium-95 is synthesized from plutonium Pu + 1 0n Pu 94 by neutron bombardment. The isotope Pu Am + obtained has a half-life of 432 years Curium Pu + 4 2He Cm + 1 0n Curium-96 is obtained by bombardment of 1944 plutonium with helium ions. The isotope has a half-life of 163 days Berkelium Am + 4 2He Bk n Berkelium-97 is obtained by bombardment of 1949 americium with helium ions. The isotope obtained has a half-life of 4.5 hours Californium Cm + 4 2He Cf + 1 0n Californium-98 is synthesized from curium by bombardment with helium ions. The californium isotope obtained has a half-life of 45 min Einsteinium Successive neutron captures Einsteinium-99 was found in the debris from 1952 by uranium-238 and decay the first thermonuclear explosion on November of the capture products 1, 1952, at Enewetok Atoll. The einsteinium resulted in Es isotope obtained has a half-life of 20.5 days Fermium Successive neutron captures Fermium-100 was found in the debris from the 1953 by uranium-238 and decay first thermonuclear explosion at Enewetok of the capture products Atoll on November 1, The fermium resulted in Fm isotope obtained has a half-life of 20.1 hours Mendelevium Es + 4 2He Md + 1 0n Mendelevium-101 is obtained by bombardment 1955 of einsteinium with helium ions. The mendelevium isotope obtained has a half-life of 1.3 hours Nobelium Cm C No n Nobelium-102 is synthesized from curium and 1964 by bombardment with carbon ions. The isotope obtained has a half-life of 55 seconds Lawrencium Cf B Lr n Lawrencium-103 is synthesized from califor Am O Lr n nium-98 by bombardment with boron and from americium by bombardment with oxygen. The isotopes 258 Lr and 256 Lr have halflives of 3.9 and 28 seconds respectively Source: Information from ref. [52.18]

65 52.16 More Than Just 92 Elements The Transuranium Actinides 1205 Table 52.4 Properties of the transuranium actinides Element Atomic Yearly Isotope Isotope examples weight production a) range Nuclide Half-life Decay Mode Energy Q (MeV) 93 Np 237 kilograms Np yr Neptunium 94 Pu 244 tonnes Pu 87.7 yr Plutonium 239 Pu yr Pu yr Pu yr Am 243 grams Am 432 yr Americium 243 Am 7370 yr Cm 247 grams Cm 163 day Curium 244 Cm 18.1 yr Cm yr Cm yr Bk 247 milligrams Bk 1380 yr Berkelium 249 Bk 320 day Cf 251 grams Cf 357 yr Californium 251 Cf 898 yr Cf yr Es 252 milligrams Es 1.29 yr and Einsteinium EC Es 20.5 day Es 276 day Es 39.8 day and Fm 257 micrograms Fm 20.1 h Fermium 257 Fm 101 day Md 258 zero Md 51.5 day Mendelevium 102 No 259 zero No 58 min and 7.89 Nobelium EC Lr 260 zero Lr 180 s and 8.31 Lawrencium EC 2.74 Source: Information mainly from ref. [52.15]. a) According to ref. [52.16].

66 Radioactive Elements Uses of the Transuranium Actinides Plutonium Plutonium is the most toxic substance known. It is a very dangerous radiological hazard and is specifically absorbed in bones and collected in the liver. For those reasons the element must be handled extremely carefully. Plutonium is produced in all uranium reactors according to the process in Figure U n U Np + (t 1/2 = 24 min) Np Pu + (t 1/2 = 2.36 day) 239 U Figure 52.9 Formation of plutonium in a uranium reactor. With more neutrons available than in an ordinary reactor, the production of 239 Pu would increase and become greater than the consumption of 235 U. This may be achieved by extra enrichment of the fuel and removal of the moderator. A fast breeder reactor is then obtained. The advantage of such a reactor is, as already mentioned in section , that both 235 U and 238 U are used to produce heat. Up to 60 times more energy can be obtained compared with a conventional reactor [52.17]. There are however many serious political objections to the use of the breeder technology, as plutonium may be used to produce nuclear weapons. The originally discovered plutonium isotope 238 Pu is not fissile and has been used in quite other applications than nuclear power and weapons. It is an intensive particle emitter and thus an energy producer. 238 Pu is used in kilogram quantities as the power source for satellites. In the Apollo lunar missions, it was used to power scientific equipment on the lunar surface. Plutonium in irradiated fuel elements may be chemically separated from uranium. On treatment of a mixture of hexavalent uranium and tetravalent plutonium with divalent iron, plutonium is reduced to the trivalent stage. The stable uranyl nitrate is unaffected. In liquid liquid extraction with tri-n-butyl phosphate (TBP) and kerosene, uranium goes into the organic phase, while plutonium stays in the water phase Americium Americium (the isotope 241 Am) is the vital ingredient of household smoke detectors. The detector is a small chamber in which a low-level electrical voltage is applied between two electrodes. In an ordinary situation, without smoke in the room, alpha particles from the isotope collide with the oxygen and nitrogen molecules in the detector and ions are produced. As a consequence a steady small electric current f lows between the electrodes. When smoke enters the space between the electrodes, the alpha radiation from the isotope is absorbed by smoke particles. This causes the rate of ionization of the air to fall, the current is changed and this change sets off an alarm. The

67 52.17 The Elements After the Actinides The Transactinides 1207 alpha particles from the smoke detector do not themselves pose a health hazard, as they are absorbed in a few centimeters of air Curium Curium may be used as a thermoelectric power source (a never-ending battery) for space f lights. Curium was also the alpha particle source for the Alpha Proton X-Ray Spectrometer on Mars Physical Properties For obvious reasons, the physical properties are fragmentarily known. Information about densities and melting points are given in Table Table 52.5 Density and melting points for the actinides a) Element Density Melting point Element Density Melting point g/cm 3 g/cm 3 K C K C Actinium Curium Thorium Berkelium Protactinium Californium Uranium Einsteinium Neptunium Fermium Plutonium Mendelevium Americium Nobelium Lawrencium a) The values for neptunium and the subsequent elements are taken from ref. [52.19] The Elements After the Actinides The Transactinides The Situation at the Beginning of the 1960s When all the actinides had been synthesized and/or discovered, was the periodic table then complete? No! In Figure the situation in 1965 is described for the two periods 6 and 7. After the actinides, a lot of empty squares were waiting for new discoveries or syntheses.

68 Radioactive Elements Cs 56 Ba 57 La Lanthanides 72 Hf 73 Ta 74 W 75 Re 76 Os 77 Ir 78 Pt 79 Au 80 Hg 81 Tl 82 Pb 83 Bi 84 Po 85 At 86 Rn 7 87 Fr 88 Ra 89 Ac Actinides Figure The situation for the transuranium elements after 103, at the beginning of the 1960s New Discoveries In fact all the discoveries of the transactinide elements have been made at three big institutes: The Lawrence Berkeley National Laboratory in California, active since 1940 The Russian Joint Institute for Nuclear Research at Dubna near Moscow, active since the 1960s The Institute for Heavy Ion Research (Gesellschaft für Schwerionenforschung) GSI in Darmstadt, Germany, active since the 1970s Two new techniques were gradually introduced: Linear accelerators as is shown in Figure 52.11, instead of cyclotrons Figure Heavy ion research at GSI in Darmstadt: a linear accelerator.