NEUTRON ACTIVATION ANALYSIS FUNDAMENTAL CONCEPTS Nucleus – Nuclear Radiations Neutrons Classification Nuclear Reactions Neutron Reactions Neutron Sources Nuclear Reactor Schematic Neutron Flux Neutron Capture Cross-Sections Neutron Activation Radioactivity 1Nucleus Material Compounds Elements Atoms (Neutrons + Protons) + Electrons {Nucleus} Element X can be depicted by A A = Mass Number X N = Neutron Number Z N Z = Atomic Number (Proton Number) A = Z + N 2Nucleus … … Mass Charge Neutron - 1.008665 u – No electrical charge Proton - 1.007277 u – Positive charge Electron - 0.000548 u – Negative charge Ref: Nuclide Idotopes Chart of the Nuclides 16th Edition Lockheed Martin 2002 Nuclides Isomer – Same N, Z, A but exists in an excited state for a period of time. 60 Co33(T12=5.26 y) and 60mCo (12T =10.48m)27 27 33 Isotope – Same Z number, but different N 59 Co32, 60Co 27 27 33 Isobar – Same A number, but different Z 60 Co33, 60Ni 27 28 32 Isotone – Same N number 59 61Fe33, 60Co33, 28Ni26 27 33 3Nuclides Radioactive Stable Artificial Natural (p) �+ (p) �+ (p) �+ (p) �+ (p) (p) �+ �+ �+ α α αα �+ �+ �+ �+�+ (p) �+ βα�+� α � α �+βα β� βα βα βα βα β� βα βα βα βα ( ), ( ) �+ α β� βp α α αα �−�− �− �− α � �+ α + α �−�− α +e -βαβα α α ε, e -α � Ni Cont.... 28 27 Ni48 Ni50 Ni51 Ni52 Ni53 Ni54 Ni55 58.6934 Ni49 12 ms ~0.04 s 0.05 s 0.10 s 202 ms Nickel Co 58.933200 Cobalt 2.73, 1.99 1.9 7.66 937.1 849.4, 1535.3, 1329.0, 1941.7 1407, 1130, 411 1328.2 Co50 44 ms Co52 0.12 s p 1.55 Co51 � ε � ε � ε � ε Co53 0.25 s 0.24 s Co54 1.46 m 4.5 7.220 193.2 ms Ni56 Ni57 Ni59 Ni60 Ni61 Ni62 Ni63 Ni64 5.9 d 35.6 h 158.4, 811.8, ..... Ni58 68.0769 7.6E4 a 26.2231 1.1399 3.6345 101. a 0.9256 1377.6, .... �, .85, ... 4.6, 2.2 2.9, 1.5 < .03 mb ~2.5, ~1.8 14.5, 6.6 1.6, 1.2 24 0.0669 < .03 mb 78, 1.2E2 1.4E5, 21+16 39+353E5 ε, no 14, 20 2, 3 no 67.4, .... 1173.0, .... 1173.0, 1163.6, ...... Co59 100 77.3 d Co55 17.53 h Co61 1.650 h 931.2, 477.2, 1408.4, ... Co62 13.9 m 1.50 m 810.8,.. ..... 1.9E3, 7E3 �, �Co58 9.1 h 70.88 d 1.22, .... 87.3D, .... Co63 27.5 s 3.6, .... 2.9, .... 4.1, 2.9, .. 1.50, 1.03, ..... Co56 846.8, 1238.3, ... �, �1.459, ..... .474, IT 24.9, 1332.5, 1173.2, ..... 2.0, 4 6E1, 2.3E2 1332.5 Co60 10.47 m 5.271 a .318, .... 1.6 .... ε, ... IT 58.6, 271.8 d Co57 122.1, 136.5, 14.4, .... http://ie.lbl.gov/toi.htm 4 URL for Table of Isotopes:Nuclear Radiations Radioactivity is produced when unstable nuclei decay. For example 1) The slow decay rate of primordial heavy elements such as U and Th 2) The radioactive daughter products which form during natural radioactive decay series of U and Th 3) Irradiation of stable isotopes with particles generate unstable isotopes, which decay to stable isotopes by emitting radiation. Neutron Activation is important in this regard. 4) Particle bombardment of fissionable element leads to unstable fission fragments The disintegration of radionuclides releases excess energy in the form of nuclear radiations. Radioactive decay takes place in several ways emitting radiation such as: • Alpha rays • Beta (negative and positive) rays • Gamma rays • Neutrons 5• Neutrinos • Proton decay • Internal conversion electrons • Characteristic x-rays • Fission fragments Gamma-rays and β− play important role in neutron activation analysis. Gamma rays are emitted when an excited nucleus de-excites, by the transition from an excited energy state to a lower energy state. Gamma-rays have well defined energies and their emission often is accompanied by nuclear reactions and nuclear decays. Negative Beta particles (β−) or negatrons are emitted when neutron is transformed into a proton during the nuclear transformation. Negative beta particles are electrons formed during nuclear transformation, hence are of nuclear origin. The atomic number (Z) of the resultant nucleus is one unit greater, but the mass number is unchanged. 6Neutrons Classification Neutron classification according to kinetic energy Type Energy 1. Thermal 0.025 eV 2. Epithermal 0.025 eV – 0.2 eV 3. Resonance 1 eV – 1000 eV 4. Intermediate 1 keV – 500 keV 5. Fast > 0.5 MeV Ref. Ch.2 General principles of neutron activation analysis, J. Dostal and C. Elson p 28 Figure 2.3, Mineralogical Association of Canada Short Course in Neutron Activation Analysis in the Geosciences, Halifax May 1980, Ed: G. K. Muecke Nuclear Reaction Nuclear reaction occurs when target nuclei are bombarded with nuclear particles, depicted pictorially 7 The pictorial representation of the above table is given in the reference:X + a Y + b + Q Or X(a,b)Y Target X is bombarded by particle “a”, Y is the product nuclei with resulting particle “b” . Q is the energy of the nuclear reaction, which is the difference between the masses of the reactants and the products. Ex: 59 60 27Co32 + 01n1 →27Co33 + γ or 59 60 Co32(n, γ)Co 27 27 33 8Neutron Reactions When target nuclei are bombarded with neutrons, of the many possible nuclear reactions that can take place, the four major reactions are 1) Neutron capture 2) Transmutation 3) Fission reaction 4) Inelastic Scattering 1)Neutron capture: The target nucleus absorbs (captures) a neutron resulting in a product isotope, the mass number of which is incremented by one. If the product nucleus is unstable, it usually de-excites by emission of gamma rays and/or β− . Ex: 58 59 Fe32 (n,γ)Fe33.26 26 92)Transmutation Target nucleus absorbs a neutron, emitting charged/non-charged particles like alpha, proton, 2 neutrons, deutron. The unstable product nucleus generally de-excites through β− emission back to the target nucleus 55 25Mn30 (n,p) 55Cr30 .24 55 β-55 24Cr30 ⎯⎯→ 25Mn30 Transmutation neutron reactions are caused by neutrons of high energies (fast or intermediate neutrons). 3) Fission Reaction: Fissionable target nucleus (usually Z >90) absorbs a neutron, triggering the fission 10 .process, splitting into two large segments and simultaneously releasing 2 to 3 neutrons. The fission process can become a chain reaction producing large amount of neutrons which become source to a NUCLEAR REACTOR. 4)Inelastic Scattering: Target nucleus does not absorb the incident neutron,