Radiations Interaction with Matter

OBJ 1 Radioactivity & Radioactive Decay

##Chart of the NuclidesGeneral LayoutEach nuclide occupies a square in a grid whereAtomic number (Z) is plotted verticallyNumber of neutrons (N) is plotted horizontallyHeavily bordered square at left side of each row givesNameChemical symbolElemental MassThermal neutron absorptions cross sectionResonance integral#Chart of the NuclidesNuclides on diagonal running from upper left to lower right have same mass numbers, called isobarsColors and shading used to indicate in chart squares used to indicate relative magnitude of Half-livesNeutron absorption propertiesFour different colors usedBlueGreenYellowOrange#Chart of the NuclidesBackground color of upper half of square represents T1/2Background color of lower half of square represents greater of the thermal neutron cross section or resonance integralWhen nuclide is stable and thermal neutron cross section is small or unknown, entire square is shaded greyGray shading also used for unstable nuclides having T1/2 sufficiently long (>5E8 yrs) to have survived from the time they were formed#Chart of the NuclidesSome squares, such as 60Co, 115In, and 116In are dividedOccurs when nuclide has one or more isomeric or metastable statesHas same A and Z, but different nuclear and radioactive properties due to different energy states of the same nucleus

#Chart of the NuclidesNuclide Properties Displayed on the ChartChemical Element Names and SymbolsSame element names and symbols as used on the Periodic Table of the ElementsAtomic Weights and AbundancesIsotopic masses in AMUs are given forStable isotopesCertain long-lived, naturally occurring radioactive isotopesNuclides particle decay becomes a prominent mode (>10%)#Chart of the NuclidesIsotopic AbundanceValues on chart given in atom percentSpecified for 288 nuclides (266 stable and 22 radioactive)Half-livesHalf-life listed below nuclide symbol and mass numberUnits usedpspicoseconds (1E-12 s)nsnanoseconds (1E-9 s)smicroseconds (1E-6 s)msmilliseconds (1E-3 s)ssecondsmminuteshhoursddaysayears#Chart of the NuclidesBackground Color of Chart Square Upper Half1 day to 10 day orange>10 days to 100 days yellow>100 days to 10 years green>10 years to 5E8 years blueBackground Color of Chart Square Lower HalfRefers to thermal neutron cross section or resonance integral#Chart of the NuclidesMajor Modes of Decay and Decay Energiesalpha particle-beta minus (negatron)+beta plus (positron)gamma raynneutronpprotonddeuteronttritonelectron captureITisomeric transitione-conversion electron--double beta decay cluster decayDdelayed radiation#Chart of the NuclidesTo understand decay schemes and energies, look at chart square for 38Cl- energies listed on 1st line in order of abundance energies listed on 2nd line in order of abundanceParticle energies always given in MeV energies always given in keV

#Chart of the NuclidesWhen more than one decay mode possible, modes listed on chart in order of abundance or intensityDifferent modes of beta decay (, +, -) appear on separate lines if intensity of one of the decay modes is 10% absolute intensity with most abundant listed first.

#Chart of the NuclidesWhen branching decay occurs by both - and + and/or , and each decay is accompanied by emission, format shown in 146Pm square is used

Metastable (or isomeric) state frequently decays to ground by IT emission, followed by one or more in cascade

Internal conversion is process resulting from interaction between nucleus and extra-nuclear electrons. Nuclear excitation energy xfrd to orbital electron (usually K shell) and is indicated by e-

#Chart of the NuclidesDelayed emission indicated by symbol D. When daughter product has too short of a half-life to have its own spot on the chart or half-life is much shorter than that of the parent nuclide, energy is listed with the parent

#Chart of the Nuclides

#Chart of the Nuclides

#Radiation ClassificationsIntroductionAll radiation possesses energyInherent electromagneticKinetic particulateInteraction results in some or all of energy being transferred to surrounding mediumScatteringAbsorption#Radiation ClassificationsIonizing or Non-IonizingNon-IonizingVisible lightRadio and TVIonizingParticulate or PhotonicParticulatenElectromagneticx

#Radiation ClassificationsDirectly or Indirectly IonizingDirectly IonizingPossesses chargeDoes not need physical contactIndirectly IonizingDoes not have chargeNeeds physical contact#Radiation CharacteristicsAlpha ()Charge +2 Range 2-4 in. (5 10 cm.)ShieldingPaperDead skinHazard InternalTarget Organ Anything internal (living tissue)#Radiation CharacteristicsBeta ()ChargeNegatron (-) -1Positron (+) +1RangeAverage 10 ft.Energy Specific 10 12 ft./MeVShieldingPlasticWoodAl, CuHazard InternalTarget OrganExternal eye (lens)Living tissueLow Z#Radiation CharacteristicsGamma () and X-Ray (x)Charge 0Range Infinite ShieldingPbDUHazard InternalTarget Organ Living tissueHigh Z#Radiation CharacteristicsNeutron (n)Charge 0Range Infinite ShieldingH20ConcretePlasticHazard InternalTarget Organ Living tissueHydrogenous#Energy Transfer MechanismsIonizationRemoving bound e- from electrically neutral atom or molecule by adding sufficient energy to allow it to overcome its BEAtom has net positive chargeCreates ion pair consisting of negatively charged electron and positively charged atom or molecule#Energy Transfer MechanismsNP+e-Ionizing ParticleNegative IonNPositive IonP+e-e-#Energy Transfer MechanismsExcitationProcess that adds sufficient energy to e- such that it occupies higher energy state than lowest bound energy stateElectron remains bound to atomNo ions produced, atom remains neutralAfter excitation, excited atom eventually loses excess energy when e- in higher energy shell falls into lower energy vacancyExcess energy liberated as X-ray, which may escape from the material, but usually undergoes other absorptive processes#Energy Transfer MechanismsNNNNN++++e-e-e-NP+P+Ne-e-#Energy Transfer MechanismsBremsstrahlungRadiative energy loss of moving charged particle as it interacts with matter through which it is movingResults from interaction of high-speed, charged particle with nucleus of atom via electric force fieldWith negatively charged electron, attractive force slows it down, deflecting from original pathKE particle loses emitted as x-rayProduction enhanced with high-Z materials (larger coulomb forces) and high-energy e- (more interactions occur before all energy is lost)#Energy Transfer MechanismsNNNNN++++e-e-e-e-e-e-#Directly Ionizing RadiationCharged particles dont need physical contact with atom to interactCoulombic forces act over a distance to cause ionization and excitationStrength of these forces depends on: Particle energy (speed)Particle chargeAbsorber density and atomic numberCoulombic forces significant over distances > atomic dimensionsFor all but very low physical density materials, KE loss for e- continuous because of Coulombic force#Directly Ionizing RadiationAlpha InteractionsMass approximately 8K times > electronTravels approximately 1/20th speed of lightBecause of mass, charge, and speed, has high probability of interactionDoes not require particles touchingjust sufficiently close for Coulombic forces to interactEnergy gradually dissipated until captures two e- and becomes a He atom from given nuclide emitted with same energy, consequently will have approximately same range in a given material#Directly Ionizing RadiationBeta InteractionsInteraction between - or + and an orbital e- is interaction between 2 charged particles of similar masss of either charge lose energy in large number of ionization and/or excitation events, similar to Due to smaller size/charge, lower probability of interaction in given medium; consequently, range is >> of comparable energyBecause s mass is small compared with that of nucleusLarge deflections can occur, particularly when low-energy s scattered by high-Z elements (high positive charge on the nucleus)Consequently, usually travels tortuous, winding path in an absorbing medium may have Bremsstrahlung interaction resulting in X-rays#Indirectly Ionizing RadiationNo charge and nNo Coulomb force fieldMust come sufficiently close for physical dimensions to contact particles to interact#Indirectly Ionizing RadiationSmall probability of interacting with matter Why?Doesnt continuously lose energy by constantly interacting with absorberMay move through many atoms or molecules before contacting electron or nucleusProbability of interaction depends on its energy and absorbers density and atomic numberWhen interactions occur, produces directly ionizing particles that cause secondary ionizations#Indirectly Ionizing RadiationGamma absorption and x-rays differ only in originName used to indicate different sources originate in nucleusX-rays are extra-nuclear (electron cloud)Both have 0 rest mass, 0 net electrical charge, and travel at speed of lightBoth lose energy by interacting with matter via one of three major mechanisms#Indirectly Ionizing RadiationPhotoelectric EffectAll energy is lost happens or doesntPhoton imparts all its energy to orbital e-Because pure energy, photon vanishesProbable only for photon energies < 1 MeVEnergy imparted to orbital e- in form of KE, overcoming attractive force of nucleus, usually causing e- to leave orbit with great velocityMost photoelectrons are inner-shell e-#Indirectly Ionizing RadiationHigh-velocity e-, called photoelectronDirectly ionizing particleTypically has sufficient energy to cause secondary ionizationsMost photoelectrons are inner-shell electrons#Indirectly Ionizing RadiationNNNNN++++e-e-e-e-e-Gamma Photon(< 1 MeV)Photoelectron#Indirectly Ionizing RadiationCompton ScatteringPartial energy loss for incoming photonDominant interaction for most materials for photon energies 200 keV 5 MeVPhoton continues with less energy in different direction to conserve momentumProbability of Compton interaction with distance from nucleus most Compton electrons are valence electronsBeam of photons may be randomized in direction and energy, so that scattered radiation may appear around corners and behind shields where there is no direct line of sight to sourceProbability of Compton interaction with distance from nucleus most Compton electrons are valence electrons#Indirectly Ionizing RadiationPair ProductionOccurs when all photon energy is converted to mass (occurs only in presence of strong electric field, which can be viewed as catalyst)Strong electric fields found near nucleus and are stronger for high-Z materials disappears in vicinity of nucleus and -- + pair appearsWill not occur unless > 1.022MeVAny energy > 1.022 MeV shared between the --+ pair as KEProbability < photoelectric and Compton interactions because photon must be close to the nucleus#Indirectly Ionizing RadiationNNNNN++++e-e-e-e-e-e-e-Gamma Photon(E > 1.022 MeV)e+e-ElectronPositron0.511 MeV Photons

#Indirectly Ionizing RadiationNeutron InteractionsFree, unbound n unstable and disintegrates by - emission with half-life of 10.6 minutes Resultant decay product is p+, which eventually combines with free e- to become H atomn interactions energy dependent classified based on KECategoryEnergy RangeThermal~ 0.025 eV (< 0.5 eV)Intermediate0.5 eV10 keVFast10 keV20 MeVRelativistic> 20 MeV#Indirectly Ionizing RadiationClassifying according to KE important from two standpoints:Interaction with the nucleus differs with n energyMethod of detecting and shielding against various classes are differentn detection relatively difficult due to:Lack of ionization along their pathsNegligible response to externally applied electric, magnetic, or gravitational fieldsInteract primarily with atomic nuclei, which are extremely small#Indirectly Ionizing RadiationSlow Neutron InteractionsRadiative CaptureRadiative capture with emission most common for slow n Reaction often results in radioactive nuclei

Process is called neutron activation

#Indirectly Ionizing RadiationCharged Particle EmissionTarget atom absorbs a slow n, which its mass and internal energyCharged particle then emitted to release excess mass and energyTypical examples include (n,p), (n,d), and (n,). For example

#Indirectly Ionizing RadiationFissionTypically occurs following slow n absorption by several of the very heavy elementsNucleus splits into two smaller nuclei, called primary fission products or fission fragmentsFission fragments usually undergo radioactive decay to form secondary fission product nucleiThere are some 30 different ways fission may take place with the production of about 60 primary fission fragments#Indirectly Ionizing RadiationFast Neutron InteractionsScatteringFree n continues to be free n following interactionDominant process for fast nElastic ScatteringOccurs when n strikes nucleus of approx. same massNeutron can xfer much of its KE to that, which recoils off with energy lost by nNo emitted by nucleusRecoil nucleus can be knocked away from its e- and, being (+) charged, can cause ionization and excitation

#Indirectly Ionizing RadiationNNe-P+#Indirectly Ionizing RadiationInelastic ScatteringOccurs when n strikes large nucleusn penetrates nucleus for short period of timeXfers energy to nucleon in nucleusExits with small decrease in energyNucleus left in excited state, emitting radiation, which can cause ionization and/or excitation#Indirectly Ionizing Radiatione-e-NP+P+NP+Ne-e-NN#Indirectly Ionizing RadiationReactions in Biological Systems Fast n lose energy in soft tissue largely by repeated scattering interactions with H nucleiSlow 0n1 captured in soft tissue and release energy in one of two principal mechanisms:

(2.2 MeV)and

(0.66 MeV)#Radioactivity and Radioactive DecayFollowing a transformation, nucleus is usually more stable than it was, but not necessarily stableAnother transformation will take place by nucleus emitting radiationAmount of energy given off and emission type depends on nucleus configuration immediately before transformationAs nucleus energy , nucleus disintegrates or decaysCalled radioactive decayAtom before decayparentAtom after decaydaughterSteps from parent to daughter traced to stability called decay chain#Radioactivity and Radioactive DecayParent-Daughter Relationships and EquilibriumProduces daughter product and radiation is emittedDaughter also produces radioactivity when it decays, as does each successive daughter until stability is reachedActivity contributed by the parent vs. daughters varies based on half-life of both parent and daughtersWhen activity production rate is same as product decay rate, equilibrium is said to exist#Radioactivity and Radioactive DecaySecular Equilibrium1/2,P >> 1/2,D (Parent half-life infinitely > daughter)As parent activity , daughter proportionatelyDuring 10 half-lives of the daughter, essentially no parent decay takes place during secular equilibriumTwo conditions necessaryParent must have 1/2 much longer than any other nuclide in the seriesSufficiently long period of time must have elapsed to allow for in-growth of the decay productsRule of Thumb

Secular equilibrium is reached in 6 daughter half-lives.#Radioactivity and Radioactive Decay

#Radioactivity and Radioactive Decay

1/2 = 6.57 h1/2 = 15.3 m1/2 = 53 m#Radioactivity and Radioactive DecayTransient Equilibrium1/2,P > 1/2,D (Parent half-life > daughter, but not infinitely)Daughter activity decays at same approx. rate as parentDifferent way of saying daughter atom formation rate = daughter atom decay rateSame fractional decrease in parent and daughter activitiesRule of Thumb

Transient equilibrium is reached in 4 daughter half-lives.#Radioactivity and Radioactive Decay

#Radioactivity and Radioactive DecayTransient1/2,P > 1/2,D , but not very long

1/2 = 12.75 d1/2 = 1.68 d#Radioactivity and Radioactive DecayNo Equilibrium1/2,P < 1/2,DParent activity decays at faster rate than daughterEquilibrium is never reached#Radioactivity and Radioactive Decay

#Radioactivity and Radioactive Decay

1/2 = 3.1 m1/2 = 27 m1/2 = 19.9 m1/2 = 23.3 y#Decay Modes and EmissionsAlpha Decay ()With few exceptions, only relatively heavy nuclides decay by emissionEssentially a helium nucleus (2 p+, 2 n)Charge of +2

#Decay Modes and Emissions

pn#Decay Modes and EmissionsBeta (Negatron) Decay (-)High n:p ratio usually - decaysn changed into p n:p ratio, results in - emissionHave same mass as e-Because n has been replaced by p, Z 1, but A remains unchanged

#Decay Modes and EmissionsBecause n has been replaced by p, Z 1, but A remains unchangedStandard notation for - decay is:

For example, 210Pb - decays to produce 210Bi as follows:

#Decay Modes and Emissions

pn#Decay Modes and EmissionsNeutrinos and anti-neutrinosneutral particles with negligible rest massTravel at speed of light and are non-interactingAccount for energy distribution among + (positrons) and - (negatrons)#Decay Modes and EmissionsNuclide having low n:p ratio) tends to decay by positron emissionPositron often mistakenly thought of as positive electronIn reality, positron is anti-particle of electron (has charge of +1)+ used to designate positronsWith positron emitters, parent nucleus changes p+ into n and emits a +Because p+ replaced by n, Z 1 and A remains unchangedNeutrino also emitted during + emission#Decay Modes and EmissionsStandard notation for + decay is:

For example, 57Ni + decays to produce 57Co as follows:

#Decay Modes and Emissions

pn#Decay Modes and EmissionsElectron Capture (EC)For radionuclides with low n:p ratio, another decay mode, known as EC, can occurNucleus captures e- (usually from K shell)Could capture L-shell electron, but K-electron capture much more probableDecay frequently referred to as K-captureCan result in formation of Auger e-In lieu of characteristic X-ray being emittedAtom ejects bound e-Auger e- are monoenergetic

#Decay Modes and EmissionsTransmutation resembles positron emission

Electron combines with p+ to form a n, followed by neutrino emissionElectrons from higher energy levels fill vacancies left in inner, lower-energy shellsExcess energy emitted causes cascade of characteristic X-rays

#Decay Modes and EmissionsGamma Emission ()Decay resulting in transmutation generally leaves nucleus in excited stateNucleus can reach unexcited, or ground, state by emitting Gammas are type of electromagnetic radiationbehave as small bundles or packets of energy, called photons, and travel at speed of light

#Decay Modes and Emissions essentially the same as X-ray usually higher energy (MeV); whereas, X-rays usually in keV rangeBasic difference between and X-ray is origin originate in nucleus, Xrays originate in electron shellsGeneral decay equation slightly different from others

Most decay reactions have emissions associated with themSome decay by particulate emission with no emission

#Decay Modes and EmissionsIsomeric Transition (IT)Commonly occurs immediately after particle emissionNucleus may remain in excited state for measurable period of time before dropping to ground stateNucleus that remains excited known as isomer because it is in a metastable stateDiffers in energy and behavior from other nuclei with the same Z and AGenerally achieves ground state by emitting delayed (usually > 109s)#Decay Modes and EmissionsInternal ConversionAn alternative isomeric mechanism to radiative transitionExcited nucleus of -emitting atom gets rid of excitation energyTightly bound e- (K or L) interacts with nucleus by absorbing Eexcitation and is ejectedElectron known as conversion electronDistinguished from - by energyConversion e- monoenergetic- spectrum of energies#Decay Modes and EmissionsEach radionuclide, artificial or natural, has characteristic decay patternSeveral aspects associated with pattern:Decay modesEmission typesEmission energiesDecay rate#Decay Modes and EmissionsAll nuclei of given radionuclide seeking stability decay in specific manner226Ra decays by emission, accompanied by only decay mode open to 226RaSome nuclides may decay with branching, where a choice of decay modes existsSome nuclides may decay with branching, where a choice of decay modes exists57Ni, mentioned previously, decays 50%by EC (K capture) and 50% by + emissionNuclides decay in constant manner by emission types, and emissions from each nuclide exhibit distinct energy picture

#Decay Modes and EmissionsSingle Ra nucleus may disintegrate at once or wait 1000s of years before emitting an All that can be predicted with certainty is 1/2 of all 226Ra nuclei present will disintegrate in 1,622 yearsCalled the half-lifeHalf-lives vary greatly for naturally occurring radioisotopes#Natural Decay SeriesNatural Decay SeriesUranium, radium, and thorium occur in three natural decay series, headed by uranium-238, thorium-232, and uranium 235, respectivelyIn nature, in secular equilibrium#

Natural Decay SeriesUranium-238 (Radon-222) (Radon)#Natural Decay SeriesUranium-235 (Radon-219) (Actinon)

#Natural Decay SeriesThorium-232 (Radon-220) (Thoron)

#Radioactive Decay LawRadioactive Decay LawDecays at a fixed rate and is not a function of temperature, pressure, etc.Half-life defined as amount of time it takes for activity to be reduced to 1/2 the original valueOccurs at an exponential rate#Radioactive Decay Law

#Radioactive Decay LawCan be expressed as ()n or e-t.When calculating half-life, units of time (t) must be the same time units as the half-life ()Radioactive Decay Formula

or

Decay Constant ()Equivalent to natural log of 2 (ln 2) divided by half-life (1/2)

#