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Session I.2.6Session I.2.6

Part I Review of Fundamentals

Module 2Basic Physics and MathematicsUsed in Radiation Protection

Session 6Modes of Radioactive Modes of Radioactive Decay and Types of Radiation Decay and Types of Radiation

IAEA Post Graduate Educational CourseIAEA Post Graduate Educational CourseRadiation Protection and Safe Use of Radiation SourcesRadiation Protection and Safe Use of Radiation Sources

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IntroductionIntroduction

Modes of radioactive decay and types of Modes of radioactive decay and types of radiation emitted will be discussedradiation emitted will be discussed

Students will learn about basic atomic Students will learn about basic atomic structure; alpha, beta, and gamma decay; structure; alpha, beta, and gamma decay; positron emission; differences between positron emission; differences between gamma rays and x-rays; orbital electron gamma rays and x-rays; orbital electron capture; and internal conversioncapture; and internal conversion

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ContentContent

Basic atomic structure and isotopesBasic atomic structure and isotopes

Alpha, beta, and gamma decayAlpha, beta, and gamma decay

Decay spectraDecay spectra

Differences between gamma rays and x-raysDifferences between gamma rays and x-rays

Positron emissionPositron emission

Orbital electron captureOrbital electron capture

Internal conversionInternal conversion

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OverviewOverview

Fundamental atomic structure will be Fundamental atomic structure will be describeddescribed

Modes of radioactive disintegration Modes of radioactive disintegration and types of radiation emitted will be and types of radiation emitted will be discusseddiscussed

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proton neutron electron

Atomic StructureAtomic Structure

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Atomic Number (Z)Atomic Number (Z)

HydrogenHydrogen 11

CarbonCarbon 66

CobaltCobalt 2727

SeleniumSelenium 3434

IridiumIridium 7777

UraniumUranium 9292

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IsotopesIsotopes

An isotope of an element has:An isotope of an element has:

the same number of the same number of protonsprotons

a different number of a different number of neutronsneutrons

1H 2H 3H

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IsotopesIsotopes

The number of The number of protons determines protons determines the element. the element. Elements with the Elements with the same number of same number of protons but different protons but different numbers of neutrons numbers of neutrons are called isotopes. are called isotopes. Some isotopes are Some isotopes are radioactive.radioactive.

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Radioactive DecayRadioactive Decay

Spontaneous changes in the Spontaneous changes in the nucleusnucleus of an of an unstableunstable atom atom

Results in formation of new elementsResults in formation of new elements

Accompanied by a release of energy, either Accompanied by a release of energy, either particulate or electromagnetic or bothparticulate or electromagnetic or both

Nuclear instability is related to whether the Nuclear instability is related to whether the neutron to proton ratio is too high or too lowneutron to proton ratio is too high or too low

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The Line of StabilityThe Line of Stability

N > Z

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Alpha EmissionAlpha Emission

Emission of a highly energetic helium nucleus Emission of a highly energetic helium nucleus from the nucleus of a radioactive atomfrom the nucleus of a radioactive atom

Occurs when neutron to proton ratio is too Occurs when neutron to proton ratio is too lowlow

Results in a decay product whose atomic Results in a decay product whose atomic number is 2 less than the parent and whose number is 2 less than the parent and whose atomic mass is 4 less than the parentatomic mass is 4 less than the parent

Alpha particles are monoenergeticAlpha particles are monoenergetic

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Alpha particleAlpha particle

charge +2charge +2

Alpha Particle DecayAlpha Particle Decay

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Alpha Particle DecayAlpha Particle Decay

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Alpha Decay ExampleAlpha Decay Example

226226Ra decays by alpha emissionRa decays by alpha emission

When When 226226Ra decays, the atomic mass decreases Ra decays, the atomic mass decreases by 4 and the atomic number decreases by 2by 4 and the atomic number decreases by 2

The atomic number defines the element, so the The atomic number defines the element, so the element changes from radium to radonelement changes from radium to radon

226226Ra Ra 222222Rn + Rn + 44HeHe2286868888

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Beta EmissionBeta Emission

Emission of an electron from the nucleus Emission of an electron from the nucleus of a radioactive atom ( n of a radioactive atom ( n p p++ + e + e-1-1 ) )

Occurs when neutron to proton ratio is too Occurs when neutron to proton ratio is too high (i.e., a surplus of neutrons)high (i.e., a surplus of neutrons)

Beta particles are emitted with a whole Beta particles are emitted with a whole spectrum of energies (unlike alpha spectrum of energies (unlike alpha particles)particles)

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Beta particleBeta particlecharge -1charge -1

Beta Particle DecayBeta Particle Decay

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Beta Particle DecayBeta Particle Decay

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Beta Decay of Beta Decay of 9999MoMo

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Beta SpectrumBeta Spectrum

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Rule of ThumbRule of Thumb

Average energy of a beta spectrum is about Average energy of a beta spectrum is about one-third of its maximum energy or:one-third of its maximum energy or:

EEavav = E = Emaxmax1133

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Positron (BetaPositron (Beta++) Emission) Emission

Occurs when neutron to proton ratio is Occurs when neutron to proton ratio is too low ( ptoo low ( p++ n + e n + e++ ) )

Emits a positron (beta particle whose Emits a positron (beta particle whose charge is positive)charge is positive)

Results in emission of 2 gamma rays Results in emission of 2 gamma rays (more on this later)(more on this later)

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Positron (BetaPositron (Beta++) Emission) Emission

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Positron DecayPositron Decay

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Positron DecayPositron Decay

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Positron DecayPositron Decay

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Positron AnnihilationPositron Annihilation

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Orbital Electron CaptureOrbital Electron Capture

Also called K CaptureAlso called K Capture

Occurs when neutron to proton ratio is too Occurs when neutron to proton ratio is too lowlow

Form of decay competing with positron Form of decay competing with positron emissionemission

One of the orbital electrons is captured by One of the orbital electrons is captured by the nucleus: ethe nucleus: e-1-1 + p + p+1+1 n n

Results in emission of characteristic x-raysResults in emission of characteristic x-rays

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Orbital Electron CaptureOrbital Electron Capture

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Orbital Electron CaptureOrbital Electron Capture

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radiationpath

-1

ejectedelectron+1

ionizedatom

IonizationIonization

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characteristicx-rays

X-Ray ProductionX-Ray Production

electronelectronejectedejected

electron fillselectron fillsvacancyvacancy

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Electromagnetic SpectrumElectromagnetic Spectrum

x- and x- and -rays-raysInfra-Infra-redred

Ultra-Ultra-violetviolet VisibleVisible

Increase in wavelength : decrease in frequency and energyIncrease in wavelength : decrease in frequency and energy

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Gamma Ray EmissionGamma Ray Emission

Monoenergetic radiations emitted from Monoenergetic radiations emitted from nucleus of an excited atom following nucleus of an excited atom following radioactive decayradioactive decay

Rid nucleus of excess energyRid nucleus of excess energy

Have characteristic energies which can be Have characteristic energies which can be used to identify the radionuclideused to identify the radionuclide

Excited forms of radionuclides often Excited forms of radionuclides often referred to as “metastable”, e.g., referred to as “metastable”, e.g., 99m99mTc. Tc. Also called “isomers”Also called “isomers”

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Gamma Radiation

Gamma Ray EmissionGamma Ray Emission

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Gamma Ray EmissionGamma Ray Emission

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Photon EmissionPhoton Emission

DifferenceDifferenceBetween Between

X-Rays andX-Rays andGamma RaysGamma Rays

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Internal ConversionInternal Conversion

Alternative process by which excited Alternative process by which excited nucleus of a gamma emitting isotope rids nucleus of a gamma emitting isotope rids itself of excitation energyitself of excitation energy

The nucleus emits a gamma ray which The nucleus emits a gamma ray which interacts with an orbital electron, ejecting interacts with an orbital electron, ejecting the electron from the atomthe electron from the atom

Characteristic x-rays are emitted as outer Characteristic x-rays are emitted as outer orbital electrons fill the vacancies left by the orbital electrons fill the vacancies left by the conversion electronsconversion electrons

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Internal ConversionInternal Conversion

These characteristic x-rays can themselves These characteristic x-rays can themselves be absorbed by orbital electrons, ejecting be absorbed by orbital electrons, ejecting them.them.

These ejected electrons are called These ejected electrons are called AugerAuger electronselectrons and have very little kinetic energy and have very little kinetic energy

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Internal ConversionInternal Conversion

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Internal Internal Conversion Conversion

Electron Electron emitted emitted

about 10%about 10%

Internal ConversionInternal Conversion

137Cs Emits Betas

0.946 x 0.898 = 0.85

Gamma Gamma Ray emitted Ray emitted during 85% during 85%

of of 137137Cs Cs transitionstransitions

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Summary of Radioactive Decay MechanismsSummary of Radioactive Decay Mechanisms

DecayDecay

ModeMode

CharacteristicsCharacteristics

of Parent of Parent RadionuclideRadionuclide

Change in Change in

Atomic NumberAtomic Number

(Z)(Z)

Change inChange in

Atomic Atomic MassMass CommentsComments

AlphaAlpha Neutron PoorNeutron Poor -2-2 -4-4 Alphas MonoenergeticAlphas Monoenergetic

BetaBeta Neutron RichNeutron Rich +1+1 00 Beta Energy SpectrumBeta Energy Spectrum

PositronPositron Neutron PoorNeutron Poor -1-1 00 Positron Energy SpectrumPositron Energy Spectrum

ElectronElectron

CaptureCaptureNeutron PoorNeutron Poor -1-1 00

K-Capture; Characteristic K-Capture; Characteristic X-rays EmittedX-rays Emitted

GammaGammaExcited Excited

Energy StateEnergy StateNoneNone NoneNone Gammas MonoenergeticGammas Monoenergetic

Internal Internal ConversionConversion

Excited Excited

Energy StateEnergy StateNoneNone NoneNone

Ejects Orbital Electrons; Ejects Orbital Electrons; characteristic x-rays and characteristic x-rays and Auger electrons emittedAuger electrons emitted

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SummarySummary

Basic atomic structure was describedBasic atomic structure was described Isotopes were definedIsotopes were defined Modes of radioactive disintegration were Modes of radioactive disintegration were

discussed (including alpha, beta, gamma, discussed (including alpha, beta, gamma, positron emission, orbital electron capture, positron emission, orbital electron capture, and internal conversion)and internal conversion)

Ionization was definedIonization was defined X-ray production and the differences X-ray production and the differences

between gamma rays and x-rays were between gamma rays and x-rays were describeddescribed

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Where to Get More InformationWhere to Get More Information

Cember, H., Introduction to Health Physics, 3Cember, H., Introduction to Health Physics, 3rdrd Edition, McGraw-Hill, New York (2000)Edition, McGraw-Hill, New York (2000)

Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8Table of Isotopes (8thth Edition, 1999 update), Wiley, Edition, 1999 update), Wiley, New York (1999)New York (1999)

International Atomic Energy Agency, The Safe Use International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)IAEA, Vienna (1995)