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

• Supposedly unaffected by temperature, pressure, chemical environment

• First declared by Rutherford, Chadwick and Ellis

Radioactive decay supposedly follows a mathematically precise exponential function

Generally true but…

…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

…well-known exceptions do exist

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– e.g. 238U92

234Th90 + 4He2

– 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

Isotopes Same Z, different A 131I53 125I53

Isotones Same N, different A and/or Z 39Ar1840K19

Isobars Same A, different Z 228Ra88228Th90

Isodiaphers Excess mass (N-Z) is the same 235U92231Th90

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

• AXz A-4Yz-2 + 4He2

• Requires:

• Mx > My + MHe

– 210Poz 206Pb + 4He2

– (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.wikimedia.org/wiki/User:BatesIsBack -http://commons.wikimedia.org/wiki/File:Decay_Chain_of_Thorium.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.wikimedia.org/wiki/File:Decay_chain(4n%2B1,Neptunium_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)

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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

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“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…

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“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”

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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)

44

The Great Radium Scandal. Roger Macklis. Scientific American 1993

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

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Radium-223 Decay Chain

• Of the total decay energy

– 95.3% emitted as a particles

– 3.6% emitted as b particles

– 1.1% emitted as g or x-rays

• Easily measured on standard dose calibrators

223Ra11.43 d

219Rn3.96 s

α

α

α

α

β−

β−

β−

α

215Po1.78 ms

α

211Pb36.1 m

207TI4.77 m

211Bi2.17 m

211Po516 ms

207Pbstable

(0.27%)

(99.73%)

Henriksen et al. Cancer Res. 2002;62:3120-3125. 46

Nilsson et al Clin Cancer Res 2005

47

Radium-223

• Bone-seeking like the beta emitters Sr-89 and Sm-153 EDTMP

• But Ra-223 is a high-LET alpha emitter

• α-particles cause double-strand DNA– Limited penetration of α particles (~ 2-10 cell diameters)

• In principle: – potentially more effective at killing tumor cells

– less myelosuppressive due to range <100 mm of alpha particles

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Spontaneous fission

• Although possible, not prevalent in nature

• U-238 decays via spontaneous fission 2 million x slower than its already slow alpha decay (4.5 Ga vs10 Pa)

• For artificial radionuclides with Z>90, this does occur

• Typically with emission of one or more neutrons (up to 10)

An Isotopic Source of Neutrons: Cf-252

• Alpha decay (97%) and Fission (3%):

– 252Cf 248Cm + 4a (96.9%)

– 252Cf fission + 1n (3.1%)

• Average of 3.7 neutrons per fission

• Neutron energy range of 0 to 13 MeV

• Mean value of 2.1 MeV and most probable energy 0.7 MeV

• T1/2 = 2.64 years

• Average photon energy 0.8 MeV

Cluster Decay and Ternary Fission

• AKA Cluster Radioactivity or heavy particle radioactivity

• Nucleus emits a small "cluster” of neutrons and protons

– Larger than alpha particles

– Smaller than a normal binary fission fragment

• 223Ra → 209Pb + 14C

• Ternary fission into three fragments can also produce products in this size range

– Although 3rd fission product most often is a He nucleus

Beta decay

• Parent and daughter are isobars

• Electron emission: “Beta minus”

• Positron emission: “Beta plus”

• Electron capture

• Can be considered “inverse beta decay”

• But inverse beta decay also refers to:

Beta minus decay• Converts a neutron into a proton (Z increases)• An electron is emitted from the nucleus• A SPECTRUM of energies• Neutron becomes a proton plus an electron plus an (electron)

antineutrino

• Recall that neutrinos (n) exist in 3 flavors:– ne, nm, nt

• Equations must balance:– Mass (baryons)– Charge– Matter/anti-matter– Lepton number– Energy– Momentum

Welsh JS. Am J Clin Oncol 2007;30: 437–439)

At a more granular level…

• A neutron (composed of 2 down quarks and 1 up quark) is converted into a proton (composed of 2 up quarks and 1 down quark)

• In other words a down quark is converted into an up quark

• This “weak” interaction is mediated by a W-

intermediate vector boson• Recall the 4 fundamental forces:

– Gravity– Electromagnetism– Strong nuclear– Weak nuclear

Positron emission

• A way to deal with excess protons• Competes with electron capture• Proton converted into a neutron plus a positron plus an

(electron) neutrino• Recall that neutrinos exist in 3 flavors:

– ne, nm, nt

• Same conservation rules:– Mass (baryons)– Charge– Matter/anti-matter– Lepton number– Energy– Momentum

Welsh JS. Am J Clin Oncol 2007;30: 437–439)

-

At a more fundamental level

• A proton (uud) is converted into a neutron (ddu)

• An up is converted into a down

• Recall that up quarks carry +2/3 charge while down quarks carry -1/3

• Thus a +1 baryon (proton or uud) is converted into a charge zero baryon (neutron or udd)

• Mediated by W+ boson

Electron capture

• AXz + 0e-1 AyZ-1 + ne

• 1p1 + 0e-1 1n0 + ne

• EC can only happen if:

• MA - MB > W/c2

– (MA - MB)c2 > W

• For 2 neighboring isobars on the periodic table, EC can occur only when atomic mass difference between parent and daughter exceeds mass-energy equivalence of lowest electron binding energy of parent

Electron capture

• Electron capture and positron emission both solve the problem of excess protons

• Competing nuclear mechanisms

• Positron emission wins out in low-Z elements– e.g. 11C, 15O, 18F

• EC wins out in high-Z elements– e.g. 131Cs, 125I, 103Pd

– Due to Coulombic attraction pulling electron cloud closer to nucleus

– Some can do both

A variety of radiation can follow electron capture

• Loss of an electron leaves a vacancy that is filled by cascading electrons from higher energy shells leading to characteristic x-rays

• Instead of characteristic x-rays, Auger and Coster-Kronig electrons can be emitted

• Capture of an electron leaves the nucleus in an excited state

• (Prompt) gamma photons or conversion electrons (via internal conversion) can be emitted from the nucleus

Coster-Kronig and Auger electrons follow electron capture

(compete with characteristic photons)

Conversion electrons can also follow electron capture

(compete with gamma photons)

Conversion electrons can also follow electron capture

(compete with gamma photons)

Isomeric transition

• Excited nuclear state decays to ground level• No change in Z, N or A• Typically refers to metastable states transitioning

to lower energy (as opposed to “prompt” gammas)

• Results in emission of a gamma ray• Example: 99mTc 99Tc + g (~140keV)

– Note: reason for metastability is difference in parent (+1/2) and daughter (+9/2) spin states

• Competing with gamma photon production is internal conversion

Internal conversion: “Conversion electrons”

• Instead of a g emanating from the metastableisomer nucleus, an e- is ejected from the electron cloud

• Can be thought of as an “internal photoelectric effect” – a virtual g interacts with and ejects an electron

– More precisely, a 1s (or 2s or 3s) orbital e-wavefunction interacts with the nucleus and the excitation energy is directly transferred to the electron

Internal conversion: “Conversion electrons”

• Explains how half-life of Tc-99m can differ based on chemical environment…

• If electrons are less available (because of chemical bonds pulling them away) conversion is less likely and the branching ratio and half life are affected!

Internal conversion: “Conversion electrons”

• More likely with high Z:

– internal conversion ~Z3

– Conversion coefficient: (# of de-excitations via e) / (# of de-excitations via g)

• Technically NOT beta decay since the electron originates from the orbital cloud rather than the nucleus

– Also, conversion electrons are monoenergetic!

Internal pair production

• Also competes with gamma emission

• An electron/positron pair emitted instead of a gamma photon or a conversion electron

• Can happen if energy of the decay >2x the rest mass of the electron:

• Eg > 2mec2 (i.e. 0.511MeV x 2 = 1.02 MeV)

Internal pair production

• “…although 90Y has been traditionally considered as a pure β– emitter, the decay of this radionuclide has a minor branch to the 0+ first excited state of stable 90Zr at 1.76 MeV, which is followed by a β+/β– emission...”

• …it was proposed to use this pair production in radiation therapy in order to assess 90Y biodistribution by (PET)…

• D'Arienzo M. Emission of β+ Particles Via Internal Pair Production in the 0+ – 0+

Transition of 90Zr: Historical Background and Current Applications in Nuclear Medicine Imaging. Atoms. 2013; 1(1):2-12.

• Selwyn, R.G.; Nickles, R.J.; Thomadsen, B.R.; DeWerd, L.A.; Micka, J.A. A new internal pair production branching ratio of 90Y: the development of a non-destructive assay for 90Y and 90Sr. Appl. Radiat. Isot. 2006, 65, 318–327.

Double beta decay

• 35 naturally occurring isotopes are capable of double beta decay

– 2 neutrons in the nucleus are converted into 2 protons

– 2 electrons (and two electron antineutrinos) are emitted

• For double (or single) beta decay to occur, the final nucleus must have a larger binding energy than the original nucleus

• For some nuclei, (e.g. Ge-76), the nucleus one atomic number higher (As-76) has a smaller binding energy, preventing single beta decay

• However, the nucleus with two greater protons (Se-76) does have a higher binding energy

• so double beta decay of Ge-76 is allowed

Double beta decay

Double electron capture

• 35 naturally occurring isotopes are theoretically capable of double electron capture– 2 protons in the nucleus are converted into 2

neutrons by capturing two orbital electrons (and forming two electron neutrinos)

• Z drops by 2 but A remains the same

• Only experimentally confirmed for Ba-130– (by detection of predicted daughter product Xe-130

in geological samples)

– T1/2…. 1021 years!

Postassium-40 decay• A primordial radionuclide• T1/2 ~1.248 × 109 years• Major endogenous radionuclide• 0.012% (120 ppm) of all potassium• 70 kg body contains ~160 total grams K and ~19mg 40K

– 0.00012 x 160g = 0.0192 g of 40K

• Decay continuously produces about 4,900 Bq• Quite unusual• THREE modes of decay

– 88.8% beta minus– 12.2% electron capture– Tiny fraction (~0.001%) via positron emission

Beta emitters for bone metastases

• Strontium-89

• Phosphorus-32

• Samarium-153

• Holmium-166

• Rhenium-188

• Tin-117m

Phosphorus-32

• Historical use dates back to 1940’s

• Half-life = 14.3 days

• Max beta energy = 1.71 MeV

• Avg beta particle energy = 0.693 MeV

• Significant marrow toxicity

• Half-life = 50.5 days

• Ebmax= 1.463 MeV (100%)

• Max range in tissue: 8 mm

• Average soft-tissue range 2.4 mm

• Decays to 89Y (a stable isotope) with emission of a

negative beta and an electron antineutrino

Strontium-89

Silberstein, et al. Society of Nuclear Medicine Procedure Guideline for Palliative Treatment of Painful Bone Metastasesversion 3.0 Jan, 2003

Strontium-89

• Biochemically acts as a calcium analogue

• Used as a chloride salt (89SrCl2)

• Can be produced via:

– 88Sr (n,g) 89Sr

–89Y (n,p) 89Sr

Possibly via nuclear transformations of the fission products in the decay chain 89Se→89Br→89Kr→89Rb→89Sr

Samarium-153

• Beta and Gamma emitter

• Beta: 640 keV (30%)

710 keV (50%)

810 keV (20%)

• Gamma: 103 keV (29%)

70 keV (5.2%)

97 keV (1.3%)

Samarium-153

• Produced in high yield and purity by neutron irradiation of isotopically enriched samarium oxide (152Sm2O3)• 152Sm (n, g) 153Sm• 152Sm2O3 + 1n ------> 153Sm + g

• (specific activity might be hindered by this approach?)

• Physical half-life = 46.3 hours (1.93 days)• Complexed with ethylenediamine

tetramethylene phosphonate (EDTMP or lexidronam)

Holmium-166

165Ho (n,g) 166Ho

Chelated to a phosphonate with skeletal uptake similar to Tc-99m-MDP

Primarily a beta emitter with a relatively high energy (Emax = 1.85 MeV) Eβavg = 0.67 MeV

May be useful for larger tumorsHalf-life of 26.8 hours

Relatively high dose-rate

Minor gamma component (81 keV) suitable for imaging

Re-188

Physical half life 17.00 h

Maximum beta energy (abundance)

2120.4 keV (71.1%)

1965.4 keV (25.6%)

Gamma energy (abundance) 155.0 keV (15%)

Maximum penetration in tissue

10 mm (average 3.1 mm)

Penetration of g-rays, β Particles and α Particles into Bone and Marrow

Figure from Brady D, Parker C, O’Sullivan J. Bone-Targeting Radiopharmaceuticals Including Radium-223. The Cancer Journal. 2013;19:71-78. Copyright © 2013 The Cancer Journal. Reprinted with permission from Lippincott Williams and Wilkins/Wolters Kluwer Health.

93

Comments• Some gamma photons are low energy

• EC agents like Pd-103– Therapeutic radiation = gamma photons and

characteristic x-rays

– Dose distributions not much different from hi-energy betas

• Some electrons are VERY short range – shorter than alpha particles

• Auger and Coster-Kronig electrons

• High-LET/RBE!

Beta-Emitting Radionuclides Used in Brachytherapy

• Strontium-90

• Phosphorus-32

• Ytrium-90

Strontium-90

• T1/2 = 29 years (28.78y)

• Beta decay to Y-90 and Y-90m with a maximum energy of about 0.5 MeV

• Classic fission byproduct

• Therapeutic radiation is primarily from 2.27 MeV betas from Y-90

• Pterygium eye applicators and coronary brachytherapy

Ytrium-90

• T1/2 = 64.1 hours

• Beta decay into Zr-90 with a maximum energy of 2.28 MeV

• Range: 1.1 cm

• Used in microspheres (resin and glass) for liver microsphere brachytherapy (“radioembolization”)

Phosphorus-32

• T1/2 = 14 days (14.262d)

• Beta decay to Sulfur-32 with a maximum energy of 1.71 MeV

• 32P15 ----> 0e-1 + 32S16

• Average beta particle energy = 0.693 MeV

• Intracavitary applications (colloidal)

• Slightly more limited penetration than Sr-90

Electron Capture Radionuclides Used in Brachytherapy

• Palladium-103

• Iodine-125

• Cesium-131

Pd-103

• Half life = 17.0 days

• Avg 0.021 MeV (21 keV) x-rays

• 103Pd + e-

103Rh* + ne

• Excited 103Rh emits characteristic X-rays, gamma photons, conversion electrons and Auger electrons

• In the encapsulated “seed” form only the photons are of clinical relevance

• Photon energy range: 20-23 keV

• Average ~21 keV

• HVL 0.004 Pb

I-125

• T1/2 = 60.1 days

• 125I + e-

125Te* + ne

• Internal conversion 93% of time (yielding 27.0 keV and 31.0 keV x-rays; avg 28.5 keV) and produces a prompt gamma ray (35.5 keV) 7% of time

• Average 0.028 MeV

• HVL 0.025 mm Pb

Cs-131

• T1/2 = 9.689 days

• 131Cs + e-

131Xe* + ne

• Excited Xe-131 emits characteristic x-rays

• 4-34 keV photons

• Most prominent peaks in 29-34keV range

• Average 30.4 keV

Photon-Emitting Radionuclides Used in Brachytherapy and Teletherapy

• Cesium-137

• Iridium-192

• Gold-198

• Radium-226

• Cobalt-60

Co-60

• Half life 5.263 yrs

• Beta decay 60Co 60Ni + b- + g

• Principal gamma rays produced: 1.17 MeV, 1.33 MeV

• Average gamma energy = 1.25 MeV

• Beta: 0.32 MeV (99%) and 1.48 MeV (1%) Emax

Co-60 decay scheme

Radium-226

• T1/2 = 1600 years

• Alpha decay to Radon-222 and down to Pb-206 but the photons are what are used clinically

• 78 g rays from Ra-226 and decay products

• Energy ranging from 0.184 MeV - 2.45 MeV

– Average 0.83 MeV

• HVL 14 mm Pb

• 0.5 mm Pt encapsulation for beta particle filtering

Gold-198

• T1/2 = 2.7 days

• Beta decays to Hg-198

• 0.412 MeV photons

• Nearly monoenergetic

• Also emits beta particles (maximum energy 0.96 MeV)

• These electrons are absorbed by the 0.1mm thick platinum wall of the seed)

• HVL = 2.5 mm Pb

Cesium-137

• T1/2 = 30.07 years

• Beta decay to Ba-137m

• 662 keV photons

• Another classic fission product

• HVL 5.5 mm Pb

• Stainless steel encapsulation

• Less shielding than Ra-226

• Typically needs replacement after 7 years

Iridium-192

• T1/2 = 74 days (73.831d)

• Beta decay to excited states of Pt-192

– AND

• Electron capture to Os-192

• Complex energy spectrum

• Average photon energy ~0.38 MeV

• HVL 2.5 mm Pb

Conclusions

• Isotopes are fun