Nuclear Spectroscopy: From Natural Radioactivity to Studies of the Most Exotic Isotopes.

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1 Nuclear Spectroscopy: From Natural Radioactivity to Studies of the Most Exotic Isotopes. Paddy Regan Department of Physics University of Surrey Guildford, GU2 7XH & Radioactivity Group National Physical Laboratory Teddington, TW11 0LW

2 Outline of talk Elements, Isotopes and Isotones Alpha, beta and gamma decay Primordial radionuclides..why so long? Internal structures, gamma rays and shells. How big is the nuclear chart? What could this tell us about nucleosynthesis?

3 The Microscopic World ATOMS ~ m NUCLEI ~ m NUCLEONS m QUARKS ~?

4 Darmstadtium Roentgenium Copernicium

5 Nuclear Isotopes Not all atoms of the same chemical element have the same mass (A) Frederick Soddy (1911) gave the name isotopes. (iso = same ; topos = place). Results for natural terrestrial krypton Krypton, Z=36 Mass Spectrograph (Francis Aston 1919) Atoms of a given element are ionized. The charged ions go into a velocity selector which has orthogonal electric (E) and magnetic fields (B) set to exert equal and opposite forces on ions of a particular velocity (v/b) = cont. The magnet then separates the ions according to mass since the bending radius is r = (A/Q) x (v/b) Q = charge of ion & N = A is the mass of the isotope 0.4%

6 Some current nuclear physics questions 286 combinations of protons and neutrons are either stable or have decay half-lives of more than 500 million years. What are the limits of nuclear existence i.e. how many different nuclear species can exist? N/Z ratio changes for stable nuclei from ~1:1 for light nuclei (e.g., 16 O, 40 Ca) to ~1.5 for 208 Pb (126/82 ~ 1.5) How does nuclear structure change when the N/Z ratio differs from stable nuclear matter?

8 Rapid-neutron capture path discussed, but only theoretical; nuclei through which the path ran were completely out of experimental reach do they even exist?

9 How do you make radioactive nuclei? Heavy-ion fusion evaporation makes neutron-deficient nuclei at high angular momentum. High-energy projectile fragmentation: makes neutron-deficient and neutron rich nuclei at medium spins. (Neutron-induced) fission of heavy nuclei (such as 235 U) Makes neutron-rich nuclei at medium angular momentum.

12 Atomic Masses and Nuclear Binding Energies M(Z,A) = mass of neutral atom of element Z and isotope A M(Z,A) Ζm ( 1 1H ) + Nm n - B nuclear The binding energy is the energy needed to take a nucleus of Z protons and N neutrons apart into A separate nucleons energy Mass of Z protons + Z electrons + N neutrons (N=A-Z) Mass of neutral atom = binding energy (nuclear + atomic) MeV ev 12

13 Nuclear chart

14 increasing binding energy = smaller mass increasing Z A=125, odd-a even-z, odd-n or odd-z, even N 125 Sn, Z=50, N= Xe, Z=54, N=71 increasing Z A=128, even-a even-z, even-n or odd-z, odd- N ISOBARS have different combinations of protons (Z) and neutrons (N) but same total nucleon number, A A = N + Z. (Beta) decays occur along ISOBARIC CHAINS to reach the most energetically favoured Z,N combination. This is the stable isobar. This (usually) gives the stable element for this isobaric chain. A=125, stable isobar is 125 Te (Z=52, N=73); Even-A usually have 2 long-lived. 14

15 Note, the number of 40 K decays would then be equal to the number of 1461 kev gamma rays emitted, divided by the branching ratio which is in this case. signature 1461 kev 1461 gamma Some (odd-odd) nuclei can decay by competing types of beta decay (a) p n + β + ν ; (b) n p + β + + ν ; (c) p + e - n+ v ). Decay rate depends on energy released (Q β value) and CONSERVATION OF ANGULAR MOMENTUM. Big change in angular momentum and small Q β long half-life.

16 (Heavy) nuclei can decay by α emission.. ejection of a 4 He nucleus. Depends (again) on binding energies & masses Before After 232 Th, Z = 90 N = Ra, Z = 88 N =140 α 4 He, Z=2 N=2

17 Radioactive decays occur as a result of conservation of mass/energy E= mc 2 M( 204 Hg) = u M( 4 He) = u M( 200 Pt) = u 1 u = 1 atomic mass unit = MeV/c 2 mc 2 = M(204Hg) [ M( 200 Pt) + M( 4 He)])c 2 mc 2 = ( ) uc 2 mc 2 = uc 2 = MeV i.e. it requires an additional 511 kev of energy to release an alpha particle from 204 Hg (which is stable). Alpha decay of 204 Hg is energetically forbidden.

18 Alpha decay masses show shell effect at N= Th 204 Hg

19 Alpha decay can also leave daughter in excited states which can then decay by (characteristic) gamma emission. α

20 What is NORM? Naturally Occurring Radioactive Materials Two main sub-groups Cosmogenic (from cosmic ray interactions) 14 C (from 14 N(n,p) 14 C), 7 Be, 26 Al Primordial (i.e. very old, here when the earth formed) Single nuclei (e.g., 40 K, 138 La, ) Decay chains ( 232 Th, 235 U, 238 U/ 226 Ra)

21 Natural decay chains. Sequences of α and β decaying radioisotopes from Uranium (Z=92) or Thorium (Z=90) to Lead (Z=82). On earth since formation.isotopic ratios (e.g. for 235/238 U) used to age the earth.

22 Radiation occurs in nature the earth is bathed in radiation from a variety of sources. Humans have evolved with these levels of radiation in the environment. Naturally Occurring Radioactive Materials These include Uranium-238, which has radioactive half-life of 4.47 billion years. 238 U decays via a series of alpha and beta decays (some of which also emit gamma rays). These create radionuclides including: Radium-226 Radon-222 Polonium-210 (all of which are α emitters). Other NORM includes 40 K (in bones!)

24 238 Pu

25 Bateman equations, for secular equilibrium, The activity (decays per second) of cascade nuclide equals the activity of the parent.

26 Relevance to nuclear astrophysics? 210 Po terminate the s-process (α decay T 1/2 =138 days) Existence of 235,238 U, 232 Th evidence for explosive r- process nucleosynthesis. Abundance ratios and half-lives used to estimate age of earth/solar system (~5,0000 million years). Presence of 244 Pu (T 1/2 ~80 million years) could be used to infer presence of nearby supernovae / r- process events?

27 Other nuclides in the background Man-made ( anthropogenic ) radionuclides in the environment. Nuclear weapons tests / Chernobyl / Fukushima Fission fragment daughters such as 137 Cs, 90 Sr, 131 I 241 Am, decays to 237 Np, which has a very long halflife; can be used as an erosion tracer etc. 239 Pu from neutron capture on 238 U in fuel Neutron capture products (e.g., 134 Cs)

28 Anthropogenic (= man made) Radiation. Mostly 137 Cs and 90 Sr (fission fragments)

31 How do you measure the gammas? i.e., How do you see inside the nucleus?

32 Little ones single hyper-pure germanium detector, CNRP labs, U. of Surrey

33 Bigger ones the RISING array at GSI-Darmstadt, Germany, 105 Germanium detectors (see later)

34 How do you know how much radioactive material is present? Activity (A) = number of decays per second The activity (A) is also equal to the number of (radioactive) nuclei present (N), multiplied by the characteristic decay probability per second for that particular nuclear species (λ). A = λ N λ is related to the half-life of the radioactive species by λ = / T 1/2 One signature that a radioactive decay has taken place is the emission of gamma rays from excited states in the daughter nuclei. If we can measure these, we can obtain an accurate measure of the activities of the different radionuclides present in a sample.

36 Not all the gamma rays observed have to originate from the same radionuclide. 226 Ra Different radionuclides are identified by their characteristic gamma-ray energies. 228 Ac 40 K

38 Fukushima? Big change in the background gammaray spectra expect other radionuclides present, specifically 134 Cs (and briefly 131 I) etc.

39 Z=51 Z=52 Evidence of 131 I decay, formed either directly and/or from decays of A=131 precursors, is the 364 kev gamma-ray from an excited state in the 131 Xe daughter nucleus. If production stops, activity should decay away with 8 day half-life. Z=53

40 Look for signature gamma ray of 131 I decay (365 kev) in various samples... such as Vancouver rainwater. Obvious effect of 8 day half-life of this particular activity as the 131 I decays to form the (stable) 131 Xe.

41 134 Cs a smoking gun

42 134 Cs (T 1/2 ~2 years) can not be created by β - decay of heavier A=134 fission fragments since 134 Xe is stable. Presence of 134 Cs is evidence for nuclear reactor waste. 134 Cs is made in reactors via (n,γ) capture on stable 133 Cs. 134 Cs is not present in nuclear weapons fallout. X

Brown rice grown in Fukushima after the nuclear

NPL using low-background HPGe dets 100 ml samples

Saegusa, Fukushima Environmental Safety Center,

45 Brown rice grown in Fukushima after the nuclear accident; measured in radioactivity department at NPL using low-background HPGe dets 100 ml samples in U8 container, net weight 81 gram/sample Jun Saegusa, Fukushima Environmental Safety Center, Japan Atomic Energy Agency & Visiting Scientist at NPL

46 From Jun Saegusa, Fukushima Environmental Safety Center, Japan Atomic Energy Agency & Visiting Scientist at NPL, Teddington. 604 kev: 5702 counts, 661 kev ( 137 Cs): counts, 795 kev: 3987 counts, 1461 kev (from 40K): 602 counts. 80,000 sec measurements on 05 Sep Evaluations underway

47 Radioactivity in Fukushima Rice? Japanese AEA inspected 10 million bags of rice at 160 Inspection centres last year. 71 bags showed radiocesium values which exceeded the reference level. (i.e % were below this) Rice above reference level not shipped out. Source: Jun Saegusa, Fukushima, Japan Atomic Energy Agency & Visiting Scientist at NPL

48 A few new physics examples.

52 isospin impurity in beta decay of 0+ decaying ground state of N=30 ; Z=32 nucleus 62 Ge 30 to N=31 ; Z=31 nucleus 62 Ga 31 is an important Input (correction) into the standard is ap ure Fermi superallowed beta Decay, decays are allowed Gamow-Teller decays which the transition rate has to be corrected for, Check unitary of the CKM matrix (in the conserved vector current hypothesis).

53 How are the heavy elements made? 205 Au 126 T1/2 = 10.4 s Is it via the Rapid Neutron Capture (R-) Process? K-electrons L-electrons 202 Pt Many of the nuclei which lie on the r-process predicted path have yet to be studied. Do these radioactive nuclei act as we expect?

54 SN1987a before and after!!

56 A (big!) problem, can t reproduce the observed elemental abundances. We can fix the result by changing the shell structure (i.e. changing the magic numbers).but is this scientifically valid? N=82 N=126 Need to look at N=82 and 126 exotic nuclei in detail.

57 Even-Even Nuclei Excitation energy (kev) Excited states spin/parities depend on the nucleon configurations. i.e., which specific orbits the protons and neutrons occupy. Result is a complex energy level scheme. First excited state in (most) even-n AND even-z has I π =2 + ~2 = pair gap Ground state (E x =0) config has I π =0 + ;

58 Evidence for nuclear shell structure.. energy of 1 st excited state in even-even nuclei.e(2 + ).

Excitation energy (kev) 2 + 0 + PHR, Physics World, Nov.

59 Excitation energy (kev) PHR, Physics World, Nov. 2011, p37 Ground state Configuration. Spin/parity I π =0 + ; E x = 0 kev

60 Protons and neutrons are Fermion particles (i.e., have intrinsic angular momentum projection of ½ ħ). The Pauli exclusion principle means that these are then arranged into discrete, quantised energy levels. Basic (1950s) model of nuclear structure assumes a central, spherical mean -field with (empirical) correction terms for spin-orbit splitting etc.

61 82 1i Independent particle model (from 1950s) 13/2 1h 9/2 2f Protons & neutron (fermions) fill orbits by PEP. 7/2 1h 11/2 2d 3/2 3s 1/2 1g 7/2 Each j=l+s level has 2j+1 projections of m j e.g., g 9/2 orbit can have 50 (40) V= SHO + l 2.+ l.s. 2d 5/2 1g 9/2 2p 1/2 2p 3/2 1f 5/2 1f 7/2 1d 3/2 2s 1/2 1d 5/2 1p 1/2 1p 3/2 1s 1/2 [(2 x 9/2)+1 ] = 10 protons with mj from -9/2, -7/2,..,,+7/2, +9/2. Clustering of levels causes energy gaps leading to MAGIC NUMBERS. Can approximate the average field experienced by each nucleon by e.g., as: H = HO + al.l + bl.s Changes in the Hamiltonian alters the level ordering. The magic numbers can change for exotic N/Z ratios

62 Evidence for a N=82,126 shell quenching? r-process abundances exp. pronounced shell gap shell structure quenched mass number A Assumption of a N=82, 126 shell quenching leads to a considerable improvement in the global abundance fit in r-process calculations!

65 Proton-hole neutron particle interactions around 208 Pb? Virtually no data on excited states in Z<82, N>126 nuclei. R(4/2) = 3.33 for ideal rotor; 2.0 for ideal vibrator <2.0 for seniority

68 Evidence for three-body forces in nuclei?

Darmstadt, Germany Will bring currently theoretical

69 Facility for Anti-Proton and Ion Research (FAIR) To be constructed at the current GSI site, near Darmstadt, Germany Will bring currently theoretical nuclear species into experimental reach for the first time.

70 Plenty more to look at.

72 Summary Radionuclides (e.g. 235 U, 238 U, 232 Th, 40 K) are everywhere. Radioactive decays arise from energy conservation and other (quantum) conservation laws. Characteristic gamma ray energies tell us structural info. The limits for proton-richness in nuclei has been reached. Neutron-rich nuclei are harder to make at the extremes, but we are starting to be able to reach r-process radionuclides. Does the nuclear shell model remain valid for heavy nuclei with diffuse neutron skins? FAIR will increase dramatically our reach of nuclear species for experimental study

SOURCES of RADIOACTIVITY

Section 9: SOURCES of RADIOACTIVITY This section briefly describes various sources of radioactive nuclei, both naturally occurring and those produced artificially (man-made) in, for example, reactors or