1. Dr. Mohammed Alnafea alnafea@ksu.edu.sa Methods of Radioactive Decay

2. ATOMIC STRUCTURE Atomic number (Z): number of protons in nucleus Mass number (A): Number of protons + neutrons Neutron number (N): Nuclear forces: "Strong" attractive force "Strong" attractive force electrostatic repulsive force electrostatic repulsive force Radioactive decay caused by nuclear instability Due to p-p electrostatic repulsion 2 08/01/2016 4th lecture RAD 311

3. Nuclear Transformation When the atomic nucleus undergoes spontaneous transformation, called radioactive decay, radiation is emitted If the daughter nucleus is stable, this spontaneous transformation ends If the daughter is unstable, the process continues until a stable nuclide is reached Most radionuclides decay in one or more of the following ways: (a) alpha decay, (b) beta-minus emission, (c) beta-plus (positron) emission, (d) electron capture, or (e) isomeric transition. 08/01/2016 3 4th lecture RAD 311

4. Methods of Radioactive Decay 4 Consider a hypothetical nucleus that can undergo many of the major forms of radioactive decay. This hypothetical nucleus is shown below: 08/01/2016 4th lecture RAD 311

5. RADIONUCLIDE DECAY MODES No stable nuclei when Z > 83 or N > 126 Stable nuclei Unstable – radioactive : half-life < 1ms Unstable – radioactive : half-life > 1000 years Number of protons (Z) Number of neutrons (A-Z) 5 08/01/2016 4th lecture RAD 311

6. Alpha Decay Alpha (  ) decay is the spontaneous emission of an alpha particle (identical to a helium nucleus) from the nucleus. Typically occurs with heavy nuclides (A > 150) and is often followed by gamma and characteristic x-ray emission. 08/01/2016 6 4th lecture RAD 311

7. No stable nuclei when Z > 83 or N > 126 RADIONUCLIDE DECAY MODES 7 08/01/2016 4th lecture RAD 311

8. Nuclei with Z > 83 Nuclei with Z > 83 RADIONUCLIDE DECAY MODES  decay 8 08/01/2016 4th lecture RAD 311

9. Beta-Minus (Negatron) Decay Beta-minus (  - ) decay characteristically occurs with radionuclides that have an excess number of neutrons compared with the number of protons (i.e., high N/Z ratio) Any excess energy in the nucleus after beta decay is emitted as gamma rays, internal conversion electrons or other associated radiations 08/01/2016 9 4th lecture RAD 311

10. Occurs in nuclei with high neutron:proton ratio Occurs in nuclei with high neutron:proton ratio RADIONUCLIDE DECAY MODES  - decay 10 08/01/2016 4th lecture RAD 311

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12. Beta-Plus Decay (Positron Emission) Beta-plus (  + ) decay characteristically occurs with radionuclides that are “neutron poor” (i.e., low N/Z ratio). Eventual fate of positron is to annihilate with its antiparticle (an electron), yielding two 511-keV photons emitted in opposite directions. 08/01/2016 12 4th lecture RAD 311

13. Occurs in nuclei with a low neutron:proton ratio Occurs in nuclei with a low neutron:proton ratio RADIONUCLIDE DECAY MODES  + decay 13 08/01/2016 4th lecture RAD 311

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15. Electron Capture Decay Alternative to positron decay for neutron-deficient radionuclides Nucleus captures an orbital (usually K- or L-shell) electron Electron capture radionuclides used in medical imaging decay to atoms in excited states that subsequently emit detectable gamma rays 08/01/2016 15 4th lecture RAD 311

16. RADIONUCLIDE DECAY MODES Electron capture Occurs in nuclei with a low neutron:proton ratio Occurs in nuclei with a low neutron:proton ratio 16 08/01/2016 4th lecture RAD 311

17. Isomeric Transition During radioactive decay, a daughter may be formed in an excited state Gamma rays are emitted as the daughter nucleus transitions from the excited state to a lower- energy state Some excited states may have a half-lives ranging up to more than 600 years 08/01/2016 17 4th lecture RAD 311

18. Generally accompanies other radioactive decay Generally accompanies other radioactive decay associated with energy loss from changes in nuclear energy states associated with energy loss from changes in nuclear energy states RADIONUCLIDE DECAY MODES  emission 18 08/01/2016 4th lecture RAD 311

19. Used by high Z nuclei Used by high Z nuclei 2 nuclei of approximately equal mass produced 2 nuclei of approximately equal mass produced Accompanied by release of energy and neutrons Accompanied by release of energy and neutrons RADIONUCLIDE DECAY MODES Spontaneous fission 19 08/01/2016 4th lecture RAD 311

20. Decay Schemes Each radionuclide’s decay process is a unique characteristic of that radionuclide. Majority of pertinent information about the decay process and its associated radiation can be summarized in a line diagram called a decay scheme Decay schemes identify the parent, daughter, mode of decay, intermediate excited states, energy levels, radiation emissions, and sometimes physical half-life. 08/01/2016 20 4th lecture RAD 311

21. Generalized Decay Scheme 08/01/2016 21 4th lecture RAD 311

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27. Summary: Radioactive Decay 27 Fission: Some heavy nuclei decay by splitting into 2 or 3 fragments plus some neutrons. These fragments form new nuclei which are usually radioactive; Alpha Decay: Two protons and two neutrons leave the nucleus together in an assembly known as an alpha-particle; An alpha-particle is a He-4 nucleus; Beta Decay - Electron Emission: Certain nuclei with an excess of neutrons may reach stability by converting a neutron into a proton with the emission of a beta-minus particle; A beta-minus particle is an electron; 08/01/2016 4th lecture RAD 311

28. Summary: Radioactive Decay 28 Beta Decay - Positron Emission: When the number of protons in a nucleus is in excess, the nucleus may reach stability by converting a proton into a neutron with the emission of a beta- plus particle; A beta-plus particle is a positron; Positrons annihilate with electrons to produce two back-to-back gamma-rays; Beta Decay - Electron Capture: An inner orbital electron is attracted into the nucleus where it combines with a proton to form a neutron; 08/01/2016 4th lecture RAD 311

29. Summary: Radioactive Decay 29 Electron capture is also known as K-capture; Following electron capture, the excited nucleus may give off some gamma-rays. In addition, as the vacant electron site is filled, an X-ray is emitted; Gamma Decay - Isomeric Transition: A nucleus in an excited state may reach its ground state by the emission of a gamma-ray; A gamma-ray is an electromagnetic photon of high energy; Gamma Decay - Internal Conversion: the excitation energy of an excited nucleus is given to an atomic electron. 08/01/2016 4th lecture RAD 311