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 decay Identify which modes of radioactive decay have application in radiation medicine Radioactivity Some nuclei are unstable Become stable by ejecting excess energy and often a particle in the process Types of radiation particle - particle + particle ray 1
Some definitions Parent unstable atom that decays Daughter resulting product of decay Daughter may also be unstable Isotope atoms with same Z but different N Same number of electrons, so same chemistry Natural radioactivity Produced long ago long half-life or part of chain 238 U 226 Ra 206 Pb Generally these tend to be high Z Produced by cosmic radiation 14 C Artificial radioactivity Produce by Insert nucleon into nucleus to make it unstable n 0 from a reactor p +. d +, ++, from an accelerator Introducing photon energy into nucleus Photonuclear processes occur in accelerators Typical threshold is > 6MeV 2
Radioactive nuclides ~1100 known nuclides 272 stable 800+ radioactive Almost all elements with Z < 82 have at least one stable isotope All elements with Z > 82 have no stable isotopes Most decay ultimately to Pb (Z=82) Mathematical treatment Previous treatment of exponential behavior spoke of number of atoms whose decay exhibits exponential behavior Mathematical treatment Difficult to measure number of atoms Easier to measure number decaying at any time Activity 3
Activity Same rules apply to activity half life, mean life, etc, as apply to number Activity Recall unit of activity 1 Bq = 1 sec -1 Traditional unit of activity 1 Ci = 3.7 10 10 Bq Typical activities 1 kci (10 14 Bq) teletherapy sources, e.g. 60 Co 1 mci (10 8 Bq) brachytherapy sources; diagnostic procedures 4
Types of radioactive decay 1. decay 2. decay 3. Electron capture 4. Internal conversion 5. Isomeric transition Alpha particle is 4 2He nucleus Occurs mainly in heavy nuclei electrostatic repulsive forces overcome cohesive nuclear forces Alpha decay Alpha decay Note: Both Z and A conserved in decay process Z = -2; A = -4 Mass of products < mass of Ra Mass difference is 4.78 MeV Transition energy goes primarily into kinetic energy of particle (see problem set) 5
Decay is stochastic process 94% of nuclei decay by giving off 4.78 MeV 6% of nuclei decay by giving off 4.59 MeV and 0.19 MeV Branching ratio for given path is that fraction of nuclei decaying by that path Alpha decay Alpha decay Alpha particle is densely ionizing (many ionizations per unit track length), stops in a short distance Alpha decay not clinically relevant, but presence of alpha decay required filtration on Ra sources Cloud chamber image Alpha decay What is dose rate deposited per g of water from decay of 1 mci 226 Ra? Average kinetic energy given to charged particles per disintegration given by E = 0.946 4.78 MeV + 0.054 4.60 MeV = 4.77 MeV 6
Alpha decay 1 mci 226 Ra is 3.7 10 7 disintegrations/sec Energy deposited in 1 g target given by E= 4.77 MeV g -1 3.7 10 7 s -1 = 1.76 10 8 MeV g -1 s -1 1.602 10-13 J MeV -1 = 2.82 10-5 J g -1 s -1 10 3 g kg -1 = 2.82 10-2 J kg -1 s -1 = 2.82 10-2 Gy s -1 Beta decay Emission of electron from nucleus Occurs when n/p ratio too large for stability (recall stability curve) n decreases by 1 p increases by 1 Beta decay Note that for 137 Cs the ray from decay of 137 Ba is used for radiation therapy, not the from the decay of 137 Cs 7
Example of beta decay Pure beta emitter no gamma rays 32 P has 15p + 17n (odd-odd) Decays to 16p + 16n (even-even) Beta decay Energy considerations: mass of P nucleus = 31.98403-15 m 0 mass of S nucleus = 31.98220-16 m 0 mass of beta = 1 m 0 mass of neutrino = 0 m 0 energy released in mass units = 0.00183 amu Beta decay Energy considerations: energy released in mass units = 0.00183 amu energy released in MeV = 0.00183 931.2 = 1.70 MeV 8
Beta decay Not all particles have kinetic energy equal to transition energy. 's have energy spectrum with mean energy approx 1/3 max energy. Mean energy is important quantity from point of view of determining dose deposition Beta decay Must postulate the existence of an additional particle to carry away excess energy. Particle called a neutrino (actually an antineutrino) Zero mass and zero charge Negligible interactions hard to detect Mean free path ~ earth diameter Beta decay Almost all kinetic energy goes to particle or antineutrino Very small amount of kinetic energy goes to recoil nucleus Insufficient energy to cause ionization 9
Beta decay Beta particle is less densely ionizing than alpha, more straggling than alpha Cloud chamber image Example of beta decay Gamma rays emitted as well as betas Beta decay What is the dose rate at the center of a sphere of water 1 cm in diameter, irradiated with a 32 P source placed at the center of the sphere, with 6 10 5 disintegrations per sec per g of water? 10
Beta decay How far from the source will the s deposit energy? Maximum energy of rays is 1.71 MeV, and corresponding range of electrons is 0.8 cm, so s produced in center of sphere will deposit all their energy in sphere If the s had much higher energy, some energy would be deposited outside the sphere and problem would be messier Beta decay Mean energy is approx 0.33 1.71 MeV (actually 0.694 MeV) Dose rate = 6 10 5 dis g -1 s -1 3600 s hr -1 0.694 MeV dis -1 1.602 10-10 Gy MeV -1 g -1 = 0.24 Gy hr -1 Gamma emission After radioactive decay, nuclide generally left in excited state Nuclide decays to ground state by emission of ray 11
Gamma emission Positron decay Positron is anti-electron same mass, charge of opposite sign Process occurs when n/p ratio too low for stability Interacts with target via Coulomb forces creates ionization Anti-matter interacts with matter, giving rise to annihilation complete conversion of matter into energy Produce two 0.511 MeV photons emitted at 180 Application in PET imaging Positron decay Look at energy relationship: mass of 12 N nucleus = 12.02278-7 m 0 mass of 12 C nucleus = 12.003803-6 m 0 mass of positron = 1 m 0 mass of neutrino = 0 m 0 energy released in mass units = 0.018977-2 m 0 12
Positron decay energy released in mass units = 0.018977-2 m 0 note threshold of 1.02 MeV transition energy Positron decay Odd-odd decays to even-even Neutrino emitted to conserve energy Beta decay Beta minus decay occurs when nucleus has too many neutrons reactor-produced isotopes Beta plus decay occurs when nucleus has too many protons accelerator-produced isotopes In rare cases, odd-odd nuclei with stable neighboring isobars at Z=±1 can decay via both routes, e.g., 84 Rb 13
Electron capture Competing process with positron decay Excited nucleus interacts with and captures inner shell (generally K shell) electron Virtually all excess energy taken off by neutrino Electron capture Hole left behind 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% 14
Electron capture Major mode of decay for 125 I and 103 Pd Produce low energy, characteristic x-rays used in brachytherapy Internal conversion Excited nucleus imparts energy directly to inner shell electron escapes atom with net kinetic energy of E ex -E b Hole left behind characteristic radiation and Auger electrons Competes with emission Internal conversion Note analogy with Auger electrons Internal conversion electrons Excitation energy originates in nucleus Energies characteristic of nuclear energy levels Auger electrons Excitation energy originates in atom Energies characteristic of atomic energy levels 15
Internal conversion Internal conversion coefficient Fluorescent yield Internal conversion Coefficient increases with increasing Z Coefficient increases with increasing lifetime of excited nucleus Complex decay scheme 16
Isomeric transitions Metastable states decay with isomeric transitions Parent decays to excited state of daughter Most excited states decay to ground state simultaneous with decay of parent (t ½ < 10-6 s) If daughter stays in excited state long enough to do something about it Called metastable state Ultimate decay to ground state is isomeric transition Isomeric transitions Isomeric transitions 99m Tc has different Z from 99 Mo so it is chemically different soluble in saline ( 99 Mo is toxic!) Use resin column to separate 99m Tc from 99 Mo 99m Tc bonds to many biologically relevant compounds Tag compound Give to patient 140 kev very good for imaging studies 17
Isomeric transitions Another example: 137 Cs decays to metastable state of barium 137m Ba has half-life of 2.55 min Not exploited leave Cs and Ba together Summary of processes Mode of decay Alpha Beta minus Beta plus Electron capture Energy absorbed (How much? Where?) Total energy of alpha is locally absorbed. Mean energy of beta is locally absorbed. Typically E avg ~ E max /3. Mean energy of beta is locally absorbed. Two annihilation gammas are emitted, deposit energy outside immediate vicinity. Characteristic x-rays deposit energy away from immediate vicinity but are low energy so energy absorbed somewhat locally. Auger electrons locally absorbed. Summary of processes Mode of decay Gamma rays Internal conversion Energy absorbed (How much? Where?) Some gammas interact locally so some energy absorbed locally, but many travel far from vicinity so are not locally absorbed High-energy electron locally absorbed. Characteristic x-rays deposit energy away from immediate vicinity but are low energy so energy absorbed somewhat locally. Auger electrons locally absorbed 18