Accurate-gammaray-spectrometry-of-environmental-sa

Accurategamma-rayspectrometryofenvironmentalsamples:achallengeO.Sima-BucharestUniversityD.Arnold-PTBBraunschweigC.Dovlete-ERLBucharestAccurategamma-rayspectrometryofenvironmentalsamples:achallenge•Introduction•Problemsinefficiencycalibrationofthespectrometer•Coincidencesummingeffects•Matrixeffects•Geometryeffects•GESPECOR•SummaryandconclusionsIntroduction•Assessmentofradioactivityofenvironmentalsamples:–carefullydesignedsamplingprocedures–appropriatesamplepreparation–accuratesamplemeasurement–rigorousanalysisoftheresults•Modernrequirementsandconditions:–lowdetectionlimits–accurateevaluationofuncertainty–highnumberofsamples,varioustypes,matrices,availablequantities–highefficiencydetectorsavailableProblemsinefficiencycalibration•Lowlevelactivity+lowdetectionlimit:–highefficiencymeasurementconditions–volumesources•Detectionefficiencyforvolumicsamples:–dependentuponsamplematrixanddensity=Directexperimentalcalibration:-limitednumberofmatrices-specificnuclides-expensive,problemswiththemanagementofradioactivematerial=Additionalproceduresforacompletecalibrationrequired•Detectionefficiencyforhighefficiencymeasurements:–nuclidespecificcoincidencesummingeffectsCoincidencesummingeffects•Areencounteredinthecaseofmeasurementofnuclideswhichdecaythroughtheemissionofcoincidentradiation(cascadingphotons,X-rays,annihilationphotonsetc)•Dependonthedetailsofthedecayscheme:–Nuclideandpeakspecificeffects•Aremuchenhancedinhighefficiencymeasurementconditions•Effects:–summingout(coincidencelossesfromthepeak)=decreaseoftheapparentefficiency–summingin(additionalcountsinthesumpeak)=increaseoftheapparentefficiencySPECTRUMOF22Na(WELL-TYPEDETECTOR)1.E+001.E+011.E+021.E+031.E+041.E+051.E+061.E+07050010001500200025003000EnergyinkeVCountsperchannel511keV1275keV1022keV1786keV2297keVCoincidencesumming:-sampleanalysis-efficiencycalibration:ex:1lMarinellibeakerCo-60=1173keV0.926,1332keV0.924Y-88=898keV0.932,1836keV0.920ex:Well-typedetector:Co-60=1173keV0.445,1332keV0.424Y-88=898keV0.472,1836keV0.390=AccurateproceduresfortheevaluationoftheeffectsrequiredMatrixeffects•Matrixeffectsareencounteredwhenthecalibrationsourcehasadifferentcompositionanddensitythanthesampleofinterest•Dependon:–samplegeometry–linearattenuationcoefficient–photonenergy–detectorparameters•Linearattenuationcoefficientobtainedfrom:–samplecompositionanddensity;–transmissionexperimentsdSampleDetectordEmptycontainerDetector-Transmissionfactorapproximatedbyexp(-d)?Transmissionfactors(logscale).Sample:R=3.5,H=2cmLinearattenuationcoefficient(1/cm)10.10.0100.20.40.60.81.01.21.4exp(-d)Relativeerror(%).Soilsample,R=3.5cm,H=2cm.Energy(keV)1010010003020100-10-20Geometryeffects•Forsamplesmeasuredclosetotheendcapofaclosedendcoaxialdetectorefficiencyverysensitivetogeometrydetails•Forsomesamples(e.g.powder)itisdifficulttoassureexactlythestandardgeometry•Detectorsparametersmayvaryintime(e.g.theentrancewindowoftheendcap)Relativeerrorofactivity(A)(%).Soilsample,R=3.5cmRelativeerrorofactivity(A)(%).Soilsample,R=3.5cmEnergy(keV)1010010000-2-4-6-8-10GESPECOR•MonteCarlobasedsoftwarededicatedtosolveproblemsingammaspectrometry:-computationofcoincidencesummingcorrections-computationofself-attenuationeffects(matrixeffects)-computationoftheefficiency•Typicalapplications:environmentalspectrometry-detectors:HPGe(closedendorwell-type),Ge(Li)-sources:cylinder,Marinelli,point,parallelepiped,ring-matrix:any(knowncomposition)orknown-nuclides:~100(forcoincidencesummingeffects)•Extensionforverylargesamples:variancereductiontechniques(focusedphotonemission,weightedemissionpoint)Directefficiencycalibration:-typicalsources(cylinder,Marinelli,pointsources)-specialgeometries:Parallelepiped(Al-26inmeteoritesamples)Sphericalsource(Rn-222sources)InsitumeasurementsDrumwastecontainersEfficiencytransfer–lesssensitivetodetectordetailsSummaryandconclusions•Accurateassessmentoftheradioactivityofenvironmentalsamples–achallenge–Coincidencesummingeffects–Matrixeffects–Geometryeffects•TheGESPECORsoftwarecansolvetypicalproblemsrequiredbyanaccurateassessmentoftheradioactivityofenvironmentalsamplesxiexie!谢谢！