Physics of Radioactive Decay. Purpose. Return to our patient

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Peregrine Hubbard
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1 Physics of Radioactive Decay George Starkschall, Ph.D. Department of Radiation Physics U.T. M.D. Anderson Cancer Center Purpose To demonstrate qualitatively the various processes by which unstable nuclides may undergo radioactive decay Emphasis will be placed on those decay processes that produce radiation used in either radiation therapy or medical imaging Return to our patient Treated with implant of 125 I sources What process does the 125 I undergo that delivers radiation? Why are we not as concerned about isolating the patient as we are for an HDR treatment? 1

2 Radioactive decay modes Many nuclei unstable emit particles and energy as they go to more stable state Classify mode of decay by identifying radiation emitted Alpha particle is 4 2He 2 nucleus Process occurs mainly in heavy nuclei Alpha decay 88Ra Rn He Alpha decay Ra Rn + He Note: Both Z and A conserved in decay process Mass of products < mass of Ra Energy equivalence of mass difference is 4.78 MeV Transition energy of process Goes primarily into kinetic energy of α particle 4 2 2

3 94% of nuclei decay by giving off 4.78 MeV α 6% of nuclei decay by giving off 4.59 MeV α and 0.19 MeV γ Branching ratio for given path is that fraction of nuclei which decay by that path Alpha decay Beta decay Emission of electron from nucleus Occurs when n/p ratio too large for stability n decreases by 1 p increases by 1 Beta decay Note that for 137 Cs the gamma ray from decay of 137 Ba is used for radiation therapy, not the β 3

4 Beta decay Energy considerations: mass of P nucleus = m 0 mass of S nucleus = m 0 mass of beta = 1 m 0 energy released in mass units = Beta decay Energy considerations: energy released in mass units = energy released in MeV = = 1.70 MeV which is transition energy of beta decay of 32 P Beta decay Not all β particles have kinetic energy equal to transition energy. β's have energy spectrum with mean energy approx 1/3 max energy 4

5 Beta decay Must postulate the existence of an additional particle to carry away excess energy Particle called a neutrino Zero mass and zero charge Gamma emission After radioactive decay, nuclide generally left in excited state Nuclide decays to ground state by emission of gamma ray Gamma rays are penetrating and are used for radiation therapy as well as for imaging Gamma emission 5

6 Positron decay Positron is anti-electron same mass, charge of opposite sign Process occurs when n/p ratio too low for stability Anti-matter interacts with matter, giving rise to annihilation complete conversion of matter into energy Positron decay Look at energy relationship: mass of N nucleus = m 0 mass of C nucleus = m 0 mass of positron = 1 m 0 mass of neutrino = 0 m 0 energy released in mass units = m 0 Positron decay energy released in mass units = m 0 note threshold of 2m 0 =1.02 MeV transition energy 6

7 Positron decay Electron capture Competing process with positron decay Nucleus captures inner shell (generally K shell) electron Hole left behind resulting in emission of characteristic radiation and Auger electrons Can define branching ratio for electron capture Electron capture e.g., for 22 Na, branching ratio for electron capture is 10%, for positron emission is 90% 7

8 Electron capture Examples of nuclides that decay via electron capture: 125 I 103 Pd Emitted radiation is characteristic radiation Low energy Non-penetrating Internal conversion Process competes with gamma emission Energy of emitted gamma ray sufficient to ionize inner shell electron Hole left behind characteristic radiation and Auger electrons Local energy deposition Internal conversion Coefficient increases with increasing Z Coefficient increases with increasing lifetime of excited nucleus 8

9 Complex decay scheme Summary Process Alpha Beta-minus Gamma Beta-plus Electron capture Internal conversion Energy Deposition Immediate vicinity of nucleus Immediate vicinity of nucleus, but gammas are penetrating Penetrating radiation Immediate vicinity of nucleus, but annihilation gammas are penetrating Non-penetrating Non-penetrating 9

Some nuclei are unstable Become stable by ejecting excess energy and often a particle in the process Types of radiation particle - particle

Some nuclei are unstable Become stable by ejecting excess energy and often a particle in the process Types of radiation particle - particle Radioactivity George Starkschall, Ph.D. Lecture Objectives Identify methods for making radioactive isotopes Recognize the various types of radioactive decay Interpret an energy level diagram for radioactive