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Radionuclides Physics of Novel Radiation Modalities: Radionuclides James S. Welsh Stritch School of Medicine Loyola University Chicago Disclosure • Member of the Advisory Committee on the Medical Uses of Isotopes (ACMUI) for the United States Nuclear Regulatory Commission (NRC) • Board of directors: – Coqui Radioisotopes – Colossal Fossils Learning Objectives • Understand the basic physics of alpha, beta, gamma and other types of radioactivity • Gain some familiarity with the various sealed and unsealed radionuclides commonly used in radiation oncology Types of radioactivity • Alpha • Beta – Beta minus – beta plus (positron emission) – electron capture • Gamma – Isomeric transitions – Internal conversion – Internal pair production • Cluster radioactivity • Spontaneous fission – Binary or ternary • Rare types: – Proton radioactivity – b+ delayed proton emission – b- delayed neutron emission – b+ delayed deuteron or triton emission – Beta delayed fission Fun with Isotopes Radioactive decay supposedly follows a mathematically precise exponential function • Supposedly unaffected by temperature, pressure, chemical environment • First declared by Rutherford, Chadwick and Ellis Generally true but… …well-known exceptions do exist …well-known exceptions do exist • Electron Capture (e.g. 7Be, 109In, 110Sn) – If chemical environment make K-shell electrons less accessible, decay rate might be altered …well-known exceptions do exist • Electron Capture (e.g. 7Be, 109In, 110Sn) – If chemical environment make K-shell electrons less accessible, decay rate might be altered • Isomeric Transitions – 99mTc: observable half-life changes due to chemical environment – T1/2 difference ~0.3% when in Tc2S7 vs NaTcO4 (sodium pertechnetate) in physiological saline Is it stable??? • Z > 83 (bismuth)???? – If so, the isotope is unstable – Every (natural) element from 84 (Po) upwards is radioactive Is it stable??? • Z > 83 (bismuth)???? – If so, the isotope is unstable – Every (natural) element from 84 (Po) upwards is radioactive – Even Bi-209 might be unstable… – with an α-emission half-life of 1.9×1019 years Is it stable??? • Recall: • Z = number of protons • N = number of neutrons • A = number of protons + neutrons (i.e. total number of nucleons) • Are both Z and N even? – If so, the isotope is probably stable (e.g. C-12, O-16) • Are both Z and N odd? – If so, the isotope is probably unstable (e.g. F-18) • Oddness of both Z and N tends to lower the nuclear binding energy Odds of being stable Protons Neutrons Number of Stable Nuclides Stability Odd Odd 4 least Odd Even 50 less Even Odd 57 more Even Even 168 most Is it stable??? • Is there a “magic number” of nucleons? – If so, the isotope is stable – Results in complete nuclear shells – High average binding energy per nucleon • Protons: 2, 8, 20, 28, 50, 82, 114 • Neutrons: 2, 8, 20, 28, 50, 82, 126, 184 Double the magic • Nuclei with both N and Z each being one of the magic numbers are “double magic” • Only 10 of ~2500 nuclides • Unusually stable against decay (note: this does NOT mean they are absolutely stable!) • Some double magic isotopes include – helium-4 – oxygen-16 – calcium-40 – nickel-48 – nickel-78 – lead-208 Is it stable??? • What is the N:Z ratio? • Where is the isotope in relationship to the “zone of stability”? • In other words - Is it in the zone? Regarding the zone • As Z increases, A must increase disproportionately for stability – Number of neutrons needed increases as the number of protons increases • Fe-56 is the most stable isotope (lowest mass per nucleon) – Below Fe-56 fusion can generate energy – Above Fe-56 fission can generate energy • No natural elements with Z > 83 (bismuth) are stable Regarding the zone • Stable light nuclides contain about equal protons and neutrons • Stable heavy elements contain up to 1.6x more neutrons than protons • Nuclides above (to the left of) the band of stability are neutron-rich • Nuclides below (to the right of) the band are neutron deficient Neutron-rich nuclides • To the left of the zone: Need more protons – Want to rid the excess n and produce more p • Below Z=83, neutron-rich radioisotopes decay via beta minus emission – (i.e. conversion of a neutron into a proton) • Above Z=83, neutron-rich nuclei also decay via alpha emission • Note: alpha decay actually increases the n:p ratio 238 234 4 – e.g. U92 Th90 + He2 – 146n and 92p (n:p = 1.587) vs 144n and 90p (n:p = 1.6) – Daughters tend to be more n-rich than the parents Some more definitions Examples 131 125 Isotopes Same Z, different A I53 I53 39 40 Isotones Same N, different A and/or Z Ar18 K19 228 228 Isobars Same A, different Z Ra88 Th90 235 231 Isodiaphers Excess mass (N-Z) is the same U92 Th90 Isomers Same Z, same A (different energy) 99mTc 99Tc • On this particular diagram style: • Isotopes on horizontal line • Isobars on NE line (beta decay) • Alpha decay on vertical line Alpha decay • Ejection of a Helium nucleus A A-4 4 • Xz Yz-2 + He2 • Requires: • Mx > My + MHe 210 206 4 – Poz Pb + He2 – (209.9829u) (205.9745u) + (4.0026u) • 209.9829u > 209.9771u Therefore a allowed • Cu-64 cannot alpha decay Polonium-210 • T1/2 = 138 days • 5.3 MeV • 166,500 TBq/kg (4500 Ci/g) • Extremely toxic: 1 mg can kill an average adult – ~250,000x more toxic than HCN by weight • Used to kill Russian dissident Alexander Litvinenko in 2006 Americium-241 • A trans-uranium actinide • Ordinary household smoke detectors contain ~0.29 mg of americium dioxide • Am-241 alpha decays to Np-237 – T1/2 = 432.2 years • a collide with O and N molecules in the air • Generates ions in the ionization chamber – Ions produce an electric current between electrodes • Ions are neutralized upon contact with smoke – Decreasing the electric current – Activates the detector's alarm Plutonium-238 • Half-life of 87.7 years • Powerful alpha emitter – Does not emit significant g • Radioisotope Thermoelectric Generators (RTGs) – Converts heat into electricity via Seebeck effect – 1g Pu-238 generates approximately 0.5W – Voyager 1 and 2, Cassini–Huygens, New Horizons and the Mars Science Laboratory • 250 plutonium-powered cardiac pacemakers made: – 22 were still in service more than 25 years later – No battery-powered pacemaker could achieve that! Radium-226 • T1/2 = 1600 years • Alpha decay to Rn-222 • 6th Member of the Uranium Series - ultimately ending in Pb-206 • 78 g rays from Ra-226 and decay products • Energy ranging from 0.184 MeV - 2.45 MeV (these photons are what were clinically useful) – Average 0.83 MeV • HVL 14 mm Pb • 0.5 mm Pt encapsulation for beta particle filtering Primordial radionuclide decay series • Thorium series (n) • Neptunium series (4n+1) • Uranium series (4n+2) • Actinium series (4n+3) • Thorium series • 4n series • "Decay Chain Thorium" by http://commons.wiki media.org/wiki/User: BatesIsBack - http://commons.wiki media.org/wiki/File:D ecay_Chain_of_Thoriu m.svg. Licensed under CC BY-SA 3.0 via Wikimedia Commons • Neptunium series • 4n+1 series • Extinct • "Decay Chain(4n+1, Neptunium Series)" by BatesIsBack - http://commons.wikim edia.org/wiki/File:Deca y_chain(4n%2B1,Neptu nium_series).PNG. Licensed under CC BY 3.0 via Wikimedia Commons • Uranium series • 4n+2 series • "Decay chain(4n+2, Uranium series)" by User:Tosaka - File:Decay chain(4n+2, Uranium series).PNG. Licensed under CC BY 3.0 via Wikimedia Commons - • Actinium series • 4n+3 series • "Decay Chain of Actinium" by Edgar Bonet - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - Radium Basics • One gram of radium-226 undergoes 3.7 × 1010 disintegrations per second • Thirty-three isotopes of radium – All radioactive • Half-lives (generally) short: – less than a few weeks – with the exceptions of radium-226 (1,600 years) and radium-228 (5.8 years) 40 Biological effects • Radium dermatitis: • Only 2 years after its discovery, A. Henri Becquerel developed a skin ulcer after carrying an ampule in his pocket for six hours • Marie Curie developed a skin ulcer after a few days following 10 hrs of direct contact with a tiny sample 41 “The Radium Craze” • 1903 - numerous commercially available products became available – Cosmos Bag for arthritis – Liquid Sunshine – Radiathor • The sad case of Eben Byers ended this era upon his death in 1932 – He consumed an estimated 1400 bottles of Radiathor – This Wall Street Journal line said it all: – "The Radium Water Worked Fine… 42 “The Radium Craze” • 1903 - numerous commercially available products became available – Cosmos Bag for arthritis – Liquid Sunshine – Radiathor • The sad case of Eben Byers ended this era upon his death in 1932 – He consumed an estimated 1400 bottles of Radiathor – This Wall Street Journal line said it all: – "The Radium Water Worked Fine until his jaw came off” 43 The Radium Girls • U.S. Radium Corporation • Watch dial luminous paint containing 70 mg/g of paint • Contained RaBr and ZnS (which glows upon alpha irradiation) • Of 800 employees from 1917 to 1924, 48 developed radiation sickness (including mandibular necrosis) and 18 died (including cases of osteosarcoma) The Great Radium Scandal. Roger Macklis. Scientific American 1993 44 So why is there possibly any interest in Radium today??? • Radium-223 is the isotope of interest presently • Part of the actinium series (4n + 3 series) • Radiologically well-suited for radiopharmaceuticals • 11.4-day half-life • 5.99 MeV alpha emission • First FDA-approved unsealed source alpha-emitting radiopharmaceutical • Some compelling clinical data has emerged recently 45 Radium-223 Decay Chain • Of the total decay energy 223Ra – 95.3% emitted as 11.43 d a particles α 219Rn – 3.6% emitted as 3.96 s b particles α 215 211 – 1.1% emitted as Po β− Po 1.78 ms 516 ms (0.27%) 211Bi g or x-rays α β− α 2.17 m • Easily measured on 211 207 Pb α (99.73%) β− Pb standard dose calibrators 36.1 m stable 207TI α 4.77 m Henriksen et al.
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