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AirFilterAlphaSpectrometryReport
AirFilterAlphaSpectrometryReport
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Posted:7/31/201121:05
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BRAWMData
(2011-2013)
Summary
BRAWMtookanairfiltersampleinmid-Aprilandperformedalphaspectrometryontheairfilter.We
didnotdetectanyUranium,Plutonium,orAmericium-241,andourcalculateddetectionlimitsindicate
safelevels.OurlimitsaremuchhigherthanthelimitssetbytheEPAintheirtestingforUraniumand
Plutonium.Ifyouwouldliketoskipdirectlytoourlimits,pleaseclickhere.
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Introduction
FrequentlyAsked
Questions/AskA
Question
Radioactivitycomesinthreeprimaryforms:alpha,beta,andgammadecay.Aradioactivenucleuswill
undergooneormoreofthesedecays,releasingparticleswithcharacteristicenergies.
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ThefissionproductnucleithatarethesubjectofBRAWM'smeasurementsarebetaemitterswith
associatedgammarays.Betaparticlesarehighenergyelectronsthatdonothavediscreteenergies
—theyarereleasedinacontinuumwithan"endpoint"energythatdependsonthenucleus.Gamma
raysarephotons("particlesoflight")andhavediscreteenergies.Thesediscreteenergiesare
fingerprintsforaspecificnucleus,anddetectinggamma-raysiswhatBRAWMspecializesin.For
example,Cesium-137hasastronggamma-ray"line"at662keV,andthatiswhatwelookforinour
spectra.
Whataboutalphadecay?Alphaparticlesareheliumnuclei(twoprotonsandtwoneutrons),andthey
areverysimilartogammaraysinthattheyarereleasedatspecificenergiesthatarefingerprintsofa
givennucleus.Forexample,Uranium-238hastwostrongalphalinesat4.151and4.198MeV.Since
manynucleimightbealphaemittersbutnotstronggammaemitters(e.g.,U-238andPu-239),one
mustuseanalphadetectortodetectthem.
Alphaspectroscopyisdifficult,evenwhencomparedtogammaspectroscopy.Theprimarydifference
isthatsincealphaparticleshaveelectriccharge(+2),eventinyamountsofmaterialbetweenthe
nucleusandthedetectorwillcauseenergylossorcompletestoppingofthealphaparticle.Soinstead
ofnice,discretelinesyouwillfindsmearedoutblobsinyourspectrum—orevennothingatall.The
morethealphalinesaresmearedandblocked,theworseyourabilitytodetectandidentifyalpha
emittersis.Gamma-raysdonotsufferfromthisproblem.
ExperimentalSetup
Weplacedapieceoffilterpaper(3MFiltrete)overthenozzleoftheshopvacusedforourair
measurementsforthreedays,April16–19,2011.Wefilteredatotalofapproximately5,500,000liters
ofair(5,500cubicmeters).Attheendofthecollectiontime,thefilterpaperhadturnedblackfromthe
airparticlesitfilteredfromtheair.
Withinminutes,wetookthefilterpaperandplaceditinfrontofandnearlyincontactwithasiliconPIN
alphadetector.Thedetectorandsamplewerehousedinasealedchamberthatweevacuatedsothat
theairwouldnotblockthealphaparticles.Thechamberwascoveredwithanopaqueblackclothso
thattheambientlightintheroomwouldnotcreatenoiseinthesilicondetector.
NaturalAlphaBackgroundfromRadondecayproducts
Withinminutes,therewerefourprominentalphalinesbetween5and9MeV.Weidentifiedtheseas
comingfromPolonium-210(5.3MeV),Polonium-212(8.8MeV),Polonium-214(7.7MeV),and
Bismuth-212(6.1MeV),allnaturally-occurringisotopesthataredecayproductsofRadongas
(specifically,thetwoisotopesRn-222andRn-220).Itisnotsurprisingtoseethem.Polonium-214was
thebrightestline,butitwasonlyvisibleinthespectrumduringthefirstfewhours.Thisiseasily
explainedbyitspositioninthedecaychainofRadon-222—itis"fed"bythedecaysofthebetaemittingnuclidesLead-214(27minutes)andBismuth-214(20minutes),andithasanextremelyshort
half-lifeitself(164microseconds).SoPo-214isverybrightforthefirsthourbutdisappearswhenthe
decaychainisexhausted.
Thisplotshowsthespectrumfromthefirstthreehours,overlaidwithaspectralmodelforthefour
isotopes:
Thisnextplotshowsthespectrumfor3daysafterthePo-214hasdecayedaway.Thethreeisotopes
Po-210,Bi-212,andPo-212remain:
Afterthefirst3days,theBi-212andPo-212entirelydisappear.Thismakessensebecausetheyare
ontheRn-220decaychainafterthebeta-emittingisotopeLead-212(10.6hours),soafterafewhalflivesofPb-212thisdecaychainisexhausted.
WhatremainsisPolonium-210(138days),whichis"fed"byanotherRadondecayproduct,Lead-210
(22.3years).SoPo-210remainsforaverylongtime.Infact,itsactivityhasbeensteadilyincreasing
duringourtestasmoreandmorePb-210decaysintoPo-210.Hereiswhatthespectrumlookslike
fromthreedaysafterthestartuntilpresently:
SearchingforUraniumandPlutoniumSignatures
Plutonium,Uranium,andAmericiumisotopesemitalphaparticlesatwell-knownenergies,allinthe
vicinityofthe5.3MeVlinefromPo-210:
Isotope
AlphaEnergies
U-238
4.1–4.2MeV
U-234
4.7–4.8MeV
U-235
4.3–4.4MeV
Pu-238
5.4–5.5MeV
Pu-239
5.1–5.2MeV
Am-241
5.4–5.5MeV
Sothebasicideaistolookatthealphaspectrumatthoselocationsandseeifthereisanyexcess
fromthoseisotopes.Aline,ifpresent,shouldhavethesameshapeasthealphalinesfromthe
naturally-occurringisotopes—i.e.,"triangle"shapeswiththerightmostedgeatthealphalineenergy.
IfitthePo-210spectrumwithamodel(redlineinthepreviousfigure),andsubtractedthemodelfrom
theobservedspectrum.Itisinthissubtractedspectrumthatthesignaturesoftheisotopesofinterest
wouldlurk.Hereisthesubtractedspectrum,withblacklinesdenotingthe"1sigma"uncertaintiesfor
eachbin:
Thesubtractedspectrumhasnoclearsignaturesofanyexcesscounts.Thesedataappeartobe
randomfluctuationsaroundzero.Sonowwemustgoaboutdeterminingwhatthedetectionlimit
(MDA)ofthistestis.
CalculatingtheLimits
Toinferwhatthisnon-detectionmeans,wecalculatedanupperlimitonthetotalactivityconcentration
fortheseisotopes.
Oneveryimportantconceptisefficiency.Efficiencyisthefractionofallemittedalphaparticlesthatare
detectedbythedetector.Therearevariousindependentcomponentstoit,whichIhave
conservativelyestimatedasfollows:
Efficiency Estimate Reasoning
Geometric
3.9%
Thisistheratiooftheareaofthedetectortotheareaofthefilterpaper.
Left/Right
50%
Halfofallalphaswilltravelawayfromthedetector,andhalfwilltravel
towardsit.
Escape
fromfilter
0.33%
Thisiswherethelargestuncertaintylies.Thiswasestimatedbyassuming
a1micronrangeforalphaparticlesinorganicmaterial,andnotingthatthe
totalthicknessofthefilterpaperis150microns,withacoursemesh
coveringabout50%ofthefinefilterpaper.
Detection
100%
Essentiallyallalphasthatstrikethedetectorshouldbedetected.
Total
0.0064% SimilartotheefficiencyiswhatIwillcallthe"spectralformfactor."Tocalculatethedetectionlimits,
we'llselectacertainregionofthespectrum,suchas3.2–4.2MeVforU-238.Sinceanalphalinewill
spreadoutbeyondthisrange,weneedtoknowwhatfractionshouldfallinthatselectedrange.Based
onthePo-210lineshape,awindow1MeVwideshouldyieldgoodsensitivity,butonlyabout50%of
countswouldberegistered.Wewilluse1MeVwindowsandaspectralformfactorof50%.
Hereissomemathshowinghowminimumdetectableactivity(MDA)iscalculated:
ActivityConcentration(Bq/L)
×AirVolume(L)
Numberof
Alphasfrom
Source
=
×CountingTime(sec)
×Efficiency(%)
×Spectralformfactor(%)
NumberofAlphasfromSource
Activity
Concentration
(Bq/L)
=
Background
Noise
=
Minimum
Detectable
Activity(Bq/L)
AirVolume(L)×CountingTime(sec)×Efficiency(%)×Spectralform
factor(%)
SquareRoot[NumberofAlphasinBackground]
2×BackgroundNoise
≈
AirVolume(L)×CountingTime(sec)×Efficiency(%)×Spectralform
factor(%)
Forourtest,herearetherelevantdata:
AirVolume:
5,500,000L
CountingTime:
7,259,912sec
Efficiency:
0.0064%(estimated)
HereareourestimatedMinimumDetectableActivitiesforeachisotopeofinterest:
Isotope
Rangeof
spectrum
investigated
Numberof
alphasin
background
MDA(Bq/L)
MDA(aCi/m3)
Yearsof
breathingthe
airtoequal
dosefrom
oneplane
flight(5
millirem)
U-238
3.2–4.2MeV
3,036
<8.6E-8Bq/L
<2,300aCi/m 3
>2.6years
U-234
3.8–4.8MeV
4,354
<1.0E-7Bq/L
<2,800aCi/m 3
>1.9years
U-235
3.4–4.4MeV
3,445
<9.2E-8Bq/L
<2,500aCi/m 3
>2.4years
Pu-238
4.5–5.5MeV
5,047
<1.1E-7Bq/L
<3,000aCi/m 3
>0.7years
Pu-239
4.2–5.2MeV
5,588
<1.2E-7Bq/L
<3,200aCi/m 3
>0.6years
Am-241
4.5–5.5MeV
5,047
<1.1E-7Bq/L
<3,000aCi/m 3
>0.7years
ComparisonwithEPAresults
WhileourlimitsarecomparabletothelimitswehavesetforfissionproductisotopesfromJapan,the
EPAreachedmuchlowerlimits.TheEPAperformedseveraltestsforPuandUinthefirstfewweeks
aftertheFukushimadisaster.Theyissuedareportontheirfindings.InSanFrancisco,therewasa
detectionofU-238,butnodetectionsofU-234,U-235,Pu-238,orPu-239.Herearethedata,
accessedviatheEPAquerysearch.MDAwasestimatedbytakingtwicetheuncertainty(CSU):
Isotope
Result
(aCi/m3)
MDA(aCi/m3)
U-238
14.1±6.3
12.6
U-234
ND
10.8
U-235
ND
8.8
Pu-238
ND
7.0
Pu-239
ND
11.6
Soourlimitsareabout200–300timeshigherthantheEPAlimits.Eventhoughwespentalongertime
collectingdata,thesignal-to-noiseisjusttoopoorforustoreachsimilarlimits.
Becauseofthedifficultiesinherentinmeasuringalphaparticlesfromanairfilter,thestandard
procedureistoperformsomechemistryonthefiltertoextractandconcentratetheactinides
(Uranium,Plutonium,Americium,andothers).Theseelementsarethenelectroplatedontoametalfilm
andplacedinfrontofadetector.Becausethereisnointerveningfiltermaterial,thesesampleshave
muchsharperlinesandgreatersensitivity(i.e.,lowerlimits)canbeachieved.
Sinceuraniumandplutoniumhavelonghalf-lives,wemayconsiderperformingotheranalysesonour
filtersamplethatcouldbemoresensitive.
MoreInformation
Ifyouwouldliketoknowmoreaboutalphaspectroscopy,hereareafewonlinereferences:
EPAAlphaSpectrometryTutorial
Canberra:APracticalGuidetoSuccessfulAlphaSpectroscopy
Canberra:SamplePreparationforAlphaSpectroscopy
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