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RADIoactiveDecay

Isotopesofagivenelementhavenucleiwiththesamenumberofprotonsbutdifferentnumbersofneutrons.Someisotopesarestable,howeverradioisotopesareunstableanddisintegrate,withtheemissionofthreemaintypesofradiation.

Alphaemittersreleaseaparticlecomposedof2neutronsand2protons.Theatomicnumberisthereforereducedby2,andtheatomicmassby4.Alphaparticlesaresoheavythatevenwithlowvelocitytheirmomentumishigh.Theydon"ttravelfar,butwhentheycollidewithothermoleculestheydoalotofdamage,thereforealphaemittersareconsideredtobequitehazardous.

Whenabetaparticleemitterdecays,oneofitsextraneutronsisconvertedtoaproton,increasingitsatomicnumberby1withoutchangingitsatomicmass.Thebreakdownisaccompaniedbytheemissionofanegativelychargedparticleoflowmass,calledthebetaparticle,andanunchargedparticleoflowmass,calledaneutrino.Forexample,hydrogenconsistsofjustoneprotonandoneelectron.Deuterium(2-H),acomponentof"heavywater,"consistsofaproton,anelectron,andoneneutron,andisastableisotope.Tritium(3-H)isanunstableisotopeofhydrogen,consistingofaproton,anelectron,andtwoneutrons.Whenanatomoftritiumdecays,oneoftheneutronsisconvertedtoaproton,onebetaparticleandoneneutrinoarereleased,andaheliumisotope(3-He)remains.Tritiumiscalleda"soft"betaemitter,becauseitsbetaparticleshaverelativelylowvelocities.Ahardbetaemittersuchas32-P(phosphorous)ismoredangerousbecauseitsbetaparticlescarrymorekineticenergy(howeveritiseasiertodetect-readon).

GammaraysconsistofelectromagneticradiationresemblingX-rays.Anexampleofagammaemitteris131-I(iodine).Gammaradiationmayaccompanyeitheralphaorbetaparticleemission.

AtraditionalunitofradioactivityistheCurie(Ci),whichisdefinedasthatquantityofanyradioisotopeundergoing2.22x10^12atomicdisintegrationsperminute(DPM).AmilliCurie(mCi)ofaradioisotopeundergoes2.22x109DPM,andamicroCurieproduces2.22x10^6DPM.Since1975,thebecquerel(Bq)hasreplacedtheCurieasthepreferredinternationalunitofradioactivity.OneBqisdefinedasoneatomicdisintegrationpersecond,or2.703x10^-11dpm.WorkingamountsinalaboratorymightbedescribedinmicroCuriesormilliCuries,kiloBecqerelsormegaBecquerels.

Whentheproductofanatomicdisintegrationisastableisotope,atomicdecayleaveslessradioactivematerialbehind.Therefore,astimepasses,theamountofactivitydeclineslogarithmically.Thehalf-lifeofaradioisotopeisthetimeittakesforone-halfoftheunstableatomstodisintegrate.Eachradioisotopehasacharacteristicrateofdecayandpatternofradiation.Forexample,14-Cisalowenergybetaemitterwithahalflifeof5500years.

Aradioisotope,oranycompoundthatcontainsaradioisotope,issaidtoberadiolabeledandiscalledaradionuclide.Thespecificactivityofaradionuclideistheamountofradioactivity,BqorCi,permoleofthecompound.Clearly,asradioactivedecayproceeds,thespecificactivityofanyradionuclidedeclines.

Methodsofdetection

Themethodemployedtodetectradiationdependsonthetypeofemitterandtheintendedpurposeofdetection.Themostwellknownmethodofdetectingradiationiswithanionizationchamber.Ahighenergyparticlecandislodgeelectronsfromtheatomsitstrikes,producingpairsofions.Particlesareallowedtopassbetweenparallelplates,onewithapositivechargeandonewithanegativecharge.Asionizationtakesplacetheionseachmovetotheplatewiththeoppositecharge,producingacurrent.Thecurrentisreadonameter.TheGeiger-Muellercounterisbasedontheionizationdetectionprinciple.

Photographicfilmcanbeexposedbyalltypesofradiation,andisusedtomonitorexposureofpersonnelworkingwithhighenergyemitters.AvisIBLetrackinacloudorbubblechambercanpickupradioactivity,ascanacalorimeteriftheenergyemittedisquitehigh.

Aproblemwithmanymethodsofdetectionisthattheenergyoftheemittermustbehighenoughtotravelsomedistancethroughair.Theparticlesemittedbymanyradionuclides,especially14-Cand3-Hlabeledcompounds,donottravelasignificantdistanceinair,butposeadangerifinternalizedbecauseoftheirproximitytomoleculessuchasDNA.Forexample,abetaparticleemittedbytritiumcannotpenetrateasheetofpaper,yettritiuminthebodyfluidscanposeasignificanthazard.Aliquidscintillationdetectorcanpickupradiationfrom"soft"betaemittersaswellasfromotherradioisotopes.Anionizingparticleisallowedtopassthroughacrystalorliquidphosphor,whichabsorbsitsenergyandre-emitstheenergyasflashesoflight.Usuallytheemittermustbedissolvedinliquidcontainingtheluminescentcompound,sothatthedistancetraveledbytheparticleisveryshort.

Liquidscintillationcounting

Theamountofkineticenergyinabetaparticlediffersfromonedecaytothenext.However,eachradioisotopehasatypicalenergyspectrum,thatis,apredictablerangeofenergies.Typicalenergyspectrafordifferentradioisotopescanberadicallydifferentinshapeandmagnitude,aswiththecommonlyused14-Cand3-H.

Inliquidscintillationcounting,thematerialcontainingradioisotopesisdissolvedinanorganicsolventcontaininganaromaticsolute(thescintillant).Whenradioactivedecaytakesplace,theenergyofabetaparticleistransferredbycollisiontoanelectronintheshellofthescintillant,excitingthatelectron.Theelectronthenreturnstoitsgroundstate,releasingaphoton.Theenergytransfercanbefrombetaparticletosolventtoscintillant,ordirectlytothescintillant,andusuallytherearemultiplecollisionsperbparticle.Thenumberofphotonsemittedfollowingeachatomicdisintegrationisproportionaltotheenergyofthereleasedbetaparticle.

Nowhereishowthedetectionsystemworks.Thevialisloweredintoadarkchamberwithphotoelectricdetectorsoneachside.Each"flash"receivedbythedetectorscorrespondstooneatomicdisintegration.Thedetectorsareconnected,viaphotomultipliertubes,toamicroprocessorunitthatrecordsnotonlyeachevent,butalsothenumberofphotonsdetectedduringeachevent(brightnessoftheflash).Atbriefintervals,suchas1/100second,theinstrumentcalculatesthenumberofflashesperunittime,displayingthemascountsperminute,orCPM.

BackgroundandQuenching

Nodetectionsystemperfect,ofcourse.Inliquidscintillationcounting,cosmicrays,betaparticlesfromdecayingpotassiumintheglassvial,spontaneousdischargesfromtheverysensitivephotodetectors,andchemicalsdissolvedinthescintillationfluidallcancontributetospontaneousflashesoflightthatarerecordedascounts.TheCPMattributabletosuchsourcesarecalledbackground.Backgroundcountsareoftensolowrelativetotheactivitybeingmeasuredthattheyareignored.Howeverifthenumberof"real"countsislow,backgroundcountscancontributetoexperimentalerror.Itisagoodpracticetoincludeavialcontainingeverythingexceptaddedradioactivityasacontroltodeterminethebackgroundlevel.BackgroundCPMarethensubtracteddirectlyfromtheCPMfortheexperimentalsamples.

Unfortunately,therearetwomorelittleproblems.Noinstrumentiscapableofrecordingalloftheatomicdisintegrationswithinascintillationvial.Becauseofthegeometryofthevialandphotoelectricdetectorssomeeventsgoundetected.Themaximumefficiencywithwhichalowenergyemittersuchastritiumcanbedetectedisabout70%.Worseyet,theenergyofsomephotonsisabsorbedbychemicalsinthesolventbeforethephotoncanreachthedetector.Thelatterphenomenonisknownasquenching.Withthechemicalquenchingthatistypicalofmostexperiments,theusualcountingefficiencyfortritiumis30to40%,andsometimesmuchless.Theamountofquenchingcanvaryfromsampletosample,thereforeitisoftennecessarytoestimatetheefficiencyofcountingforeachindividualsample.

Rememberthattheamountoflightdetectedisproportionaltotheenergyofthebetaparticlethatwasreleasedbythedisintegratingnucleus.Whenyoupreparetocountsamples,youselectappropriate"windows",thatis,rangesoflightintensitythattheinstrumentwillrecordascounts.Theinstrumentrecordstheamountoflightdetectedfollowinganevent,andifthatamountiswithintheenergyrangeforaparticularwindow,theeventisrecordedasonecount.Itisignoredifitfallsoutsidetheselectedrange.Eachwindowisgivenachannelnumber,andthecountforeachwindowisgivenasCPMforthecorrespondingchannel.

Asquenchingtakesplace,theenergyrecordedforeacheventislessthanitwouldbeforanunquenchedsample,sinceforeacheventtheenergyofsomephotonsisabsorbedbeforedetectionispossible.Theenergyspectrumisshiftedtotheleft,andthegreaterthequenchingthegreatertheshift.Astheshifttakesplace,theratioofcountsinchannel2tocountsinchannel1becomessmaller.Thatratioisknownasthesamplechannelsratio,orSCR.

CountingefficiencyispositivelyrelatedtotheSCR.TogetaconversionfactorbetweenSCRandefficiency,equalknownamountsoftheisotopeareaddedtoaseriesofvials.Progressivelygreateramountsofaquenchingagent,suchascarbontetrachloride,areaddedtoeachvial.ThevialsarecountedandCPMisdividedbyknownDPMtogetthefractionalefficiencyofcounting.Forexample,ifavialwith20,000DPMoftritiumyields5,000CPM,thefractionalefficiencyofcountingwas5,000/20,000=0.25,thatis,25%oftheatomicdisintegrationsweredetected.FractionalefficiencyisplottedversusSCRtoyieldaquenchcurve.TheinstrumentprintsoutCPMandSCRforeachsample,thereforetogetactualDPMinasampletheinvestigatormust(1)subtractbackgroundCPMfromtheCPMforthesample,(2)determinefractionalefficiencyfromtheSCRforthesample,and(3)dividenetCPMbythefractionalefficiency.

Example.Youarecountingsamplescontaining14-C.Sample#1givesyouchannel1CPMof1323.Thebackgroundcountswere23CPMandtheSCRwas0.5.Fromthequenchcurve,anSCRof0.5correspondstoanefficiencyof0.93.Thentheestimatedamountofradioactivityinthesampleis(1323-23)/0.93=1398DPM.

Thereareotherproblemsassociatedwiththemeasurementofradioactivitythatarenotsoeasilysolved.However,forsinglelabelexperimentsthequenchcorrectionisallyouneed.Reviewthespecialsafetycautionsforradioisotopework,andyouarereadytogo.

Thescintillationcounterprintout

Modernscintillationcountershaveaconveyorsystemthatautomaticallyfeedssamplesinorderintothecountingchamber.Aseachsampleiscountedtherelevantinformationisprinted.Atypicalprintoutincludespreliminaryinformationfollowedbyspecificinformationonasamplebysamplebasis.Everymanufacturerusesadifferentsystemoftermsandabbreviations,soeitheryouwillneedaccesstotheinstrumentmanualortheprintoutfromyourinstrumentwillhavetobetranslatedbyanexperiencedindividual.

Preliminaryinformationmayincludethedate,time,useri.d.,andprogramselected.Commoncountingparametersareusuallylisted.Theparametersmayinclude:numberoftimeseachsampleiscounted;timeperiodofcountingeachsample;numberoftimestheentiresetofsamplesiscounted;typeofquenchcorrection;windowsselected;specialfeaturessuchascriteriaforcuttingshortacountifcountsareveryhigh,orautomaticbackgroundsubtract.Thepreliminaryinformationmustindicatethatthecountingparameterswereappropriateforyourtypeofsamples.

Theinformationsuppliedforeachsampletypicallyincludes:samplei.d.,whichmaybepositionnumberinarack,positioninorderofcounting,orboth;channelnumbers(windows)andcorrespondingcountsperminute;timeofcounting;elapsedtimesincetherunwasstarted(thisisimportantforradioisotopeswithveryshorthalflives);samplechannelsratioorothermeasureofquenching.

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