Isotopes: atoms with the same Z but different A s (number of neutrons varies)

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Martha Leonard
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2 All atomic nuclei have protons and neutrons, except hydrogen. Z = atomic number = number of protons in the nucleus A = mass number = Z + number of neutrons A Z X Isotopes: atoms with the same Z but different A s (number of neutrons varies) Stable isotopes: do not decay spontanously. Radioactive isotopes (radionuclides): undergo disintegrations stable isotopes Disintegration or radioactive decay: emission of gamma rays, formation of electrons, positrons, alpha particles, fission.

4 Neutron activation analysis: Activity is induced in samples by neutron irradiation. Resulting radioactivity (gamma rays) is measured. About 70 elements can be determined. Tracer methods: Radioactivity is induced by a measured amount of a radioactive species. e.g. nuclear medicine (PET scan). Isotope dilution: A known quantity of radioactively-labelled analyte is added to the sample. The analyte is purified, and its radioactivity is measured. About 30 elements + biochemical compounds.

5 Radioactive decay Product Symbol Charge A (u) Alpha α 2+ 4 Electron β x 10-4* Positron β x 10-4* Gamma ray γ 0 0 X-Ray Х 0 0 Neutron n 0 1 Neutrino ν 0 0 * (45) kg (23) 10 4 u

6 Alpha decay: occurs most often with heavier nuclei (A > 150, Z > 60) Alpha particle: He 2+ 4 MeV (K.E.)

7 Beta decay: smaller atoms Beta energies: expressed as the thickness of a material (Al) needed to stop them.

9 γ-ray emission: Some α and β emission processes lead to excited nuclei. These nuclei return to the ground state by releasing quantized γ-rays. γ-rays often have the same energies as X-rays, but are from a different source. γ -Rays: nuclear relaxations. X-Rays: electronic relaxations. γ -Ray emission spectrum: characteristic to each nucleus and useful for identification.

10 Radioactive decay rates Rates must be looked at statistically as each nucleus originated at a different time, and behaves slightly differently. For a large distribution of nuclei of one type: dn dt λn N = number of radioactive nuclei of one kind λ = decay constant

11 Integrating over the interval t = 0 and t = t (N o to N): ln N N 0 t or N N 0 e t

12 Half life of nuclides: time when half the nuclei have decayed.

13 Activity A of a radionuclide, in s -1 : A dn dt λn 1 Becquerel = 1 decay/s 1 Curie = 3.70 x Becquerels = Activity of 1 g of radium 226 (α).

14 In the lab, the counting rate R of a radionuclide is measured in s -1 : R ca c N C = detection coefficient (constant) R 0 R e t

15 1. A sample containing 10 g of 53 Mn (half-life 312 days) is irradiated with neutrons. a) How many grams of 54 Mn are left in the sample after 90 days? b) 54 Mn is a beta emitter. What is the product of radioactive decay? 2. An unstable nucleus decays by beta emission, then two nuclei of the product are combined in a reactor to produce 24 Mg plus an alpha particle. Determine the nature of the original isotope. a) Be b) Li c) C d) N e) Al

17 Main Purpose of NAA: Targets an element for quantitative analysis by transforming it into an unstable nucleus which emits measurable quantized gamma radiation. e.g Na 1 0 n Na

18 How do neutrons interact with matter? -They have no charge and can thus approach other particles without coulombic repelling forces. -Most important reaction: neutron capture. Product has same Z, A+1, and is highly energetic from binding with the neutron (8-10 MeV). -Result: prompt gamma-ray emission (among others).

19 Sources of neutrons: - Radionuclides - Reactors - Accelerators

20 From radionuclides Flux: neutrons cm -2 s -1 NAA detection limits: not as good Commonly used: 238 Cf, t 1/2 = 2.6 y. Decay is by fission (3.8 n/fission). Fast neutrons: 14 MeV, for lighter elements such as N, O, F, Si.

22 From nuclear reactors Reactors have a neutron-producing fuel (uranium, plutonium) immerged in water, heavy water or paraffin oil Thermal neutrons: Have been slowed down to < 0.1 ev K.E. (from MeV) by a moderating material (H 2 O, D 2 O, paraffin). Used analytically for most elements. Flux: neutrons cm -2 s -1 NAA detection limits: 10-3 to 10 1 μg

24 From accelerators (for lighter elements) Ion source generates and delivers deuterium ions (D + ) which are then accelerated to 150 kev to a target with adsorbed tritium: 2 3 H H He o n Fast neutrons: 14 MeV, for lighter elements such as N, O, F, Si. 14 MeV

25 Neutron activation methods Non-destructive route Irradiation of sample and standard Cooling period Counting (R x, R s, R x + R s ) Readout, data Processing

26 Features of neutron activation analysis Irradiation: duration is generally [3-5 x t 1/2 ] of the analyte product Cooling: let the products decay to get rid of unwanted shorter lived species and reduce health hazards. Counting: immediately after cooling.

27 Activation methods Irradiation of sample and standard Cooling period Counting (R x, R s, R x + R s ) Readout, data Processing Extraction, isolation separation Destructive route

28 Nuclear research in Canada There are currently eight research reactors in Canada: two at AECL's Chalk River Laboratories and six at universities. The Chalk River reactors are based on CANDU technology. The university reactors include five 20 kwth SLOWPOKE-2 reactors (at the University of Alberta (Edmonton), Saskatchewan Research Council (Saskatoon), Royal Military College (Kingston), Dalhousie University (Halifax), and L'Ecole Polytechnique (Montreal)), and a 5 MWth MTR-type reactor at McMaster University. A Canadian-supplied SLOWPOKE-2 is also operated at the Centre for Nuclear Sciences, Kingston, Jamaica, and two SLOWPOKE-2 units - the original prototype at the University of Toronto and one at MDS Nordon's facility in Kanata - have been shut down.

29 The SLOWPOKE-2 is a low-energy, pool-type research reactor designed by AECL. It is licensed to run unattended for short periods of time (e.g. overnight). AECL also designed a scaled-up version (2-10 MWth) of SLOWPOKE for district heating.

30 Polytechnique, Montréal

31 Research Program The Laboratory is used primarily for NAA, which is a type of non-destructive chemical analysis used for measuring the concentrations of chemical elements in solids et liquids. When a substance is irradiated with neutrons it becomes radioactive and after it is removed from the reactor, atoms of the different elements emit gamma-rays which are detected. Geochemistry - for the determination of rare-earths and platinum group elements which yield information on the formation of rocks and mineral deposits. Materials science - for verifying the composition of new materials. Archaeology - the chemical composition of ancient metal, ceramic and lithic objects is used to determine their provenance. Medicine - studies on the toxicity of trace metals. Environment - studies of heavy metal pollution in air and water, emissions of metals from the combustion of petroleum products. The results of our major study on atmospheric emissions from automobiles were instrumental in convincing the government to approve a new gasoline additive, less polluting than previous additives, which will significantly improve the quality of the air in our cities.

33 The latest research-reactor technology developed by AECL is called MAPLE. AECL currently has two MAPLE reactors undergoing commissioning -- both are owned by radioisotope supplier MDS Nordion. AECL has also proposed a MAPLE reactor replacement for its NRU reactor, in a facility to be called the "Canadian Neutron Facility" or CNF.

36 Marie Sklodowska-Curie Henri Becquerel Pierre Curie Nobel Prize in Physics, 1903 for discovery of spontaneous radioactivity Marie Sklodowska-Curie Nobel Prize in Chemistry, 1911 for discovering radium and polonium Irène Curie Frédéric Joliot Nobel Prize in Chemistry, 1935 for synthesis of new radioactive elements

37 Questions 3. A 0.25 M solution of radioactive 41 CaC 2 O 4 (50 ml) obtained by NAA is measured for its gamma ray radiation intensity and the value obtained is 5823 units. This solution is poured into a large container of unknown volume and the well water is stirred mechanically. A 50-mL sample of the mixed water is then collected and measured for radiation at an intensity of 48 units. a) Determine the volume of the container. b) Would it matter if rate measurements were made a few minutes apart? Justify. 4. A 1-g sample of pure neutron activated 28 Al emits a beta radiation rate of 2500 decays/min. What would be the radiation rate of 28 Al in 15 g of Al 2 (SO 4 ) 3? 5950/min b) 3053/min c) 6150/min d) 2959/min

1.1 ALPHA DECAY 1.2 BETA MINUS DECAY 1.3 GAMMA EMISSION 1.4 ELECTRON CAPTURE/BETA PLUS DECAY 1.5 NEUTRON EMISSION 1.6 SPONTANEOUS FISSION

Chapter NP-3 Nuclear Physics Decay Modes and Decay Rates TABLE OF CONTENTS INTRODUCTION OBJECTIVES 1.0 RADIOACTIVE DECAY 1.1 ALPHA DECAY 1.2 BETA MINUS DECAY 1.3 GAMMA EMISSION 1.4 ELECTRON CAPTURE/BETA