Decay Modes.pptx

Radioactive isotope decay modes

1. Advanced Medical Physics Decay Modes, Radiation Properties Dr. Munir Ahmad Postdoc Medical imaging phd medical physics, UCL, UK

2. Nuclear Stability 2

3. Nuclear InStability Sea Stable nuclides Naturally occurring radioactive nuclides Other known nuclides

4. a decay b decay 140 130 120 110 100 90 80 70 60 50 40 30 20 10 10 20 30 40 50 60 70 80 90 Protons (Z) Neutrons (N) 184 74 W 107 47 Ag 56 26 Fe 20 10 Ne 209 83 Bi positron emission and/or electron capture Nuclear Stability Decay will occur in such a way as to return a nucleus to the band (line) of stability.

5. Effects of Radioactive Emissions on Proton and Neutrons Number of protons Number of protons Loss of Loss of or electron capture Loss of

6. moves into band of stability by beta decay. moves into band of stability by positron emission. Electron capture would also move into the band of stability. Band of Nuclear Stability

7. Uranium Radioactive Decay U-238 206 210 214 218 222 226 230 234 238 Mass number 81 82 83 84 85 86 87 88 89 90 91 92 Atomic number Th-230 a Th-234 a Ra-226 a Rn-222 a Po-218 a Pb-206 a Pb-214 a Pb-210 a Pa-234 b Bi-214 b Po-214 b Bi-210 b Po-210 b U-234 b 4.5 x 10 9 y 24 d 1.2 m 2.5 x 10 5 y 8.0 x 10 4 y 1600 y 3.8 d 3.0 m 27 m 160 m s 5.0 d 138 d stable

8. Half-Lives of Some Isotopes of Carbon Nuclide Half-Life Carbon-9 0.127 s Carbon-10 19.3 s Carbon-11 10.3 m Carbon-12 Stable Carbon-13 Stable Carbon-14 5715 y Carbon-15 2.45 s Carbon-16 0.75 s

9. Nuclear Stability Analysis & Decay Modes The repulsive electrostatic forces between the protons have an impact on nuclear stability. The number of neutrons must increase more rapidly than the number of protons to provide ‘ dilution ’ and to add additional nuclear forces. If the nuclear (attractive) and electrostatic (repulsive) forces do not balance , the atom will not be stable. An unstable nucleus will eventually achieve stability by changing its nuclear configuration, changing neutrons to protons, or vice versa, and then ejecting the surplus mass or energy from the nucleus. This emitted mass or energy is called radiation . 9

10. When an atom transforms to become more stable it is said to disintegrate or decay . T his property of certain nuclides to spontaneously disintegrate and emit radiation is called radioactivity . The time required for half of a sample of atoms to decay is known as the half-life The atom before the decay is the parent and the resulting atom is called the daughter 10 Nuclear Stability Analysis & Decay Modes

11. Nuclear Decay Modes 11

12. Alpha Decay Alphas are large particles ejected by the heavier nuclides and is primarily limited to nuclides with Z > 82 where Source is mainly from fuel-related materials . Alpha contains two protons and two neutrons (no electrons) and is, in effect, a helium nucleus, thus, the atomic number decreases by two and the mass number decreases by four 12

13. Alpha Decay 13 Parent U-235 Daughter Th-231

14. Alpha Decay Since nothing else is emitted, all energy of decay goes to the alpha particle (except for a small amount towards recoil of nucleus). Alphas, therefore, are mono-energetic or with discrete energy . For alpha, energy of decay reaction (Q) is, 14

15. Alpha Decay Calculate Q for the  st decay of Rn-222. 15 Mass of Rn-222 is 222.017610 amu Mass of Po-218 is 218.009009 amu

16. Assignment III The binding energy of 214 84 Po is 1.66601 GeV , the binding energy of 210 82 Pb (lead) is 1.64555 GeV and the binding energy of 4 2 He is 28.296 MeV . The Q-value for the decay? 16

17. Beta Decay Betas are physically the same as electrons , but may be positively or negatively charged. Negative beta is a beta minus or negatron. Positive beta is a beta plus or positron. Betas are ejected from the nucleus, not from the electron orbital In all beta decays the atomic number changes by one while the atomic mass is unchanged. 17

18. Beta (β - ) Minus Decay Occurs in neutron-rich nuclides . The nucleus converts a neutron into a proton and a beta minus (which is ejected from the nucleus with an anti-neutrino). Mass and charge are conserved . 18

19. Beta (β - ) Minus Decay For beta minus decays, 19

20. Beta (β - ) Minus Decay 20 Parent K-40 Beta Particle Anti-neutrino Daughter Ca-40

21. Beta (β - ) Minus Decay During radioactive decay energy is released Source of this energy is from the conversion of mass Since energy is conserved , energy equivalent of the parent must equal energy equivalent of daughter, particles, and any energy released Energy is released as kinetic energy of beta minus particle and an anti-neutrino 21

22. Beta (β - ) Minus Decay For beta minus, energy of decay reaction (Q) is, 22 Mass of beta minus particle is not included since an additional electron is gained due to increase of Z

23. Beta (β - ) Minus Decay Calculate Q for β - decay of Co-60. 23 Mass of Co-60 is 59.933813 amu Mass of Ni-60 is 59.930787 amu

24. Beta (β - ) Minus Decay (Energies) The Q value for beta minus decay of Co-60, for example, is always the same However, negatrons rarely are emitted with the same energies Their energies can range from 0 MeV to the calculated maximum, E max The anti-neutrino carries energy difference between actual and calculated values 24

25. Beta (β - ) Minus Decay Energies 25 # of betas with energy E Energy

26. 26 99+% 0.013% 0.12% 1.173 2.158 0.83 1.332 Co-60 Decay Scheme Q

27. Beta (β + ) Plus Decay Occurs in proton-rich nuclides The nucleus converts a proton into a neutron and a beta plus (which is ejected from the nucleus with a neutrino) As with negatrons, the positron can have a range of energies from 0 to E Max MeV Positron is the negatron’s anti-particle A positron and a negatron will annihilate one another and release two 0.511 MeV photons 27

28. Beta (β + ) Plus Decay For beta plus decays, 28

29. Beta (β + ) Plus Decay 29 Parent F-18 Beta Particle Neutrino Daughter O-18

30. Beta (β + ) Plus Decay For beta plus, energy of decay reaction (Q) is, 30 Since the energy equivalent of two electron masses is 1.022 MeV , the equation can be rewritten as,

31. Beta (β + ) Plus Decay 31 • • • • • • • • • • • • • • •  + e -

32. Beta (β + ) Plus Decay Calculate Q for β + decay of F-18. 32 Mass of F-18 is 18.000937 amu Mass of O-18 is 17.999160 amu

33. Electron Capture Proton-rich nuclides may also decay via orbital electron capture (EC) Usually an innermost K shell electron is captured and often referred to as K-capture The electron and a proton are converted into a neutron and a neutrino is emitted Electrons from higher orbitals will fill vacancy and usually emit characteristic x-rays 33

34. Electron Capture For electron capture decays, 34

35. Electron Capture For electron capture, energy of decay reaction (Q) is, 35 Since the electron was absorbed into the nucleus and not removed, there is no need to account for electron mass

36. Auger Electrons When electrons change shells , x-rays are usually emitted In some instances, the excess energy is transferred to another orbital electron, which is then ejected from the atom This ejected electron is known as an Auger electron Another orbital vacancy now exists and x-rays may be emitted if they are filled 36

37. Auger Electrons 37 • • • • • • • • • •

38. Nuclear De-excitation Daughter nuclei from radioactive decays are often ‘born’ with excess energy Occasionally the excited nucleus will emit additional alphas or betas Usually the excited nucleus reaches ground state via nuclear de-excitation The excited nucleus and the final ground state nucleus have the same Z and A and are called isomers If the excited state has a half-life >1 sec, it is said to be a metastable state The metastable state is denoted by the use of a lowercase ‘m’, such as Ba-137m, Tc-99m The longest known excited state is Bi-210m with a half-life of 3.5 x 10 6 years During de-excitation no nuclear transformation occurs, so no ‘new’ element is formed 38

39. Nuclear De-excitation Internal Conversion The excess nuclear energy is transferred to an inner orbital (usually K or L) electron. This electron is then ejected from the atom with a distinct energy and X-ray emission may follow as electrons shift orbitals to fill vacancies. Gamma emission Most frequently the excess energy is relieved via the emission of one or more gamma rays. Gammas have no mass or electric charge and if gammas are emitted by an isomer in the metastable state, the emission is known as an isomeric transition (IT) Photon Energy (E) = hf , where h is Planck’s Constant (4.14 x 10 -15 eV -sec), f is frequency (sec -1 ) 39

40. Gamma Ray Radiation 40 Parent Co-60 Gamma Rays Daughter Ni-60 Anti-neutrino

41. 41 99+% 0.013% 0.12% 1.173 2.158 0.83 1.332 Co-60 Decay Scheme Q

42. Decay Schemes Vertical lines represent energy Horizontal lines indicate atomic number (Z) Beta minus points down to the right Alpha and EC point down to the left Beta plus points down to the left with a 1.022 MeV offset Parent half-lives are shown 42

43. Decay Schemes Ground states are bold horizontal lines Excited states are light horizontal lines Isomeric states are medium horizontal lines Total amount of energy for the reaction is shown (Q) Abundances (probabilities) of transitions are shown 43

44. Radioactive Decay Series 44

45. Uranium Series 45

46. Neptunium Series 46

47. Thorium Series 47

48. Actinium Series 48