Alpha indirect conversion radioisotope power source

AppliedRadiationandIsotopes66(2008)173–177

/locate/apradiso

Alphaindirectconversionradioisotopepowersource

MaximSychovÃ,AlexandrKavetsky,GalinaYakubova,GabrielWalter,ShahidYousaf,

QianLin,DorisChan,HeatherSocarras,KennethBower

TRACEPhotonicsInc.,1680WestPolk,Charleston,IL61920,USA

Received1October2006;receivedinrevisedform5August2007;accepted3September2007

Abstract

Advantagesofradioisotope-poweredelectricgeneratorsincludelongservicelife,widetemperaturerangeoperationandhigh-energydensity.Wereportdevelopmentofalong-lifegeneratorbasedonindirectconversionofalphadecayenergy.Prototypingused300mCiPu-238alphaemitterandAlGaAsphotovoltaiccellsdesignedforlowlightintensityconditions.Thealphaemitter,phosphorscreens,andvoltaicarrayswereassembledintoapowersourcewiththefollowingcharacteristics:Isc¼14mA;Uoc¼2.3V;poweroutputÀingthisprototypewehavepoweredaneight-digitelectroniccalculatorandwristwatch.r2007ElsevierLtd.Allrightsreserved.

Keywords:Microbatteries;Plutonium;Phosphor;Indirectconversion;Powergeneration

1.Introduction

Anewgenerationofultralowpowerdevicesneedsminiaturelong-lastingelectricsources.Oneoptionistheutilizationofradioisotope-poweredmicrogeneratorsasshownbyOlsen(1992)andSimsetal.(1995).Advantagesofradioisotope-poweredenergysourcesarelongservicelife(over10yearsdependingonisotope),lowweight,smallsize,wideoperatingtemperaturerangeandhighreliability(Boweretal.,2002;Sychovetal.,2002).

Amongthemanyradioisotopedecayenergyconversiontechniques,weheredescribealphaindirectconversion.Thekineticenergyofalphaparticlesisconvertedtolightinaphosphorscreen.Thematchedphotonwavelengthsarethenguidedtothephotovoltaic.Thephosphorscreenisopticallycoupledtothephotovoltaicdevice,asshowninFig.1.

Indirectconversionusinganalphasourcehasseveraladvantagestobothdirectconversionofalphasandutilizationofbetaemitters:

speci cpower(powerperunitsurface)comparedtobetaparticles.Therefore,lessphotovoltaicsurfaceareaisneeded,makingthedevicesmallerandcheaper.

Forthesamereason,alpharadioluminescence(RL)isbrightercomparedtobeta-excitationandphotovoltaicconversionismoreef cientatbrighterlightlevels.

Comparedtodirectconversionofalphas,initialoverallef ciencyislower,butthestabilityismuchhighersincephosphorsaremorestablethanvoltaics.Overthelongterm,theindirectpoweroutputsurpassesthatofdirectalphavoltaics.

InthispaperwedescribefabricationandtestingofpowersourcesusingPu-238.2.Phosphorscreenfabrication

Phosphorwasdepositedonglasssubstratesusingthefollowingprocedure:glassslides2.5Â2.5cm2werewashedwithalkalinesolutionandthenwithasolutionofpotassiumdichromateinsulfuricacid.Phosphorpowderwasmixedwitha5%solutionofphosphoricacidinacetoneandthatmixturewaspouredintoavialinwhichwasplacedtheweighedglassslides.Afterseveralhoursofsedimentation,theslidesweretakenoutofthevial,bakedat2501Candweighedagain.Thedifferencebetween

Thehighenergyofalphaparticlesprovideshigher

Correspondingauthor.Tel.:+12173486703;fax:+12173486394/

6713.

E-mailaddress:msychov@(M.Sychov).

0969-8043/$-seefrontmatterr2007ElsevierLtd.Allrightsreserved.doi:10.1016/j.apradiso.2007.09.004

Fig.1.Alphaindirectconversion

setup.

startingand nalglassweightdividedbysurfaceareaisthesurfacedensityofthephosphorlayer,H(mg/cm2).

LayersofdifferentsurfacedensitywerefabricatedfromZnSphosphorandtestedagainstPu-238andSr-90sources.Radioluminescentintensitywasmeasuredwithaphoto-multipliertubeandresultsareshowninFig.2.

Curvesdonotcrosstheoriginduetobinderweight.Withalphaexcitation,theoptimumthicknessofZnSphosphorisfoundtobe7–10mg/cm2,whichcorrespondsto17–24mm.Theoptimumlayerthicknessshouldbeapproximatelyequaltothealpharangeinthephosphorsincethickerlayersabsorbandscatterthelight.Thealpharangemaybecalculatedusingtheempiricalformula:

10À4

q RAxE30

x¼

r,(1)

x

whereRxistherange(cm),rxisthedensity(g/cm3),andE0

istheinitialenergy(MeV).FortheZnSAAx¼

AZnþS

2

,(2)

whereAZnandASareatomicweightsofzincandsulfur,respectively.IfE0¼5.5MeV,andrx¼4.1g/cm3,thenR$0.0022cm¼22mm,or9mg/cm2insurfacedensityunits.Thatvaluecorrespondsnicelytotheexperimentallydeterminedoptimum.

Forcomparison,datausingSr-90excitationisalsoshowninFig.2.Sr-90anditsY-90daughter,emithigh-energyelectrons.Itspenetrationinphosphorlayersismuchdeepercomparedtoalphaparticles.Therefore,thickerphosphorlayersareneededtocaptureallthekineticenergy,asisclearinFig.2.However,thethickerlayersself-absorbandscattertheproducedlight.There-fore,mostofphotonicenergyislost,showingtheadvantageofalphaemittingisotopeshavinghighenergyandshortrangeofparticlesinphosphorscreen.3.Batteryprototype

Foralphaindirectconversion,weuseda10mWplutonium-238alphasourcewithexternal uxof2.15mWandactivearea2.5cmÂ6cm¼15cm2,soenergy uxwasabout0.143mW/cm2.Forthelight-to-electricityconversion,weusedAlxGa1ÀxAs/GaAsphotovoltaicsfabricatedinthelaboratoryofProf.V.Andreev(IoffeInstitute).DetailsmaybefoundinthepaperofAndreevetal.(2001).Photovoltaicswereusedforthefabricationofassembliesofapproximately2.5Â6cm2sizeeachtomatchtheactiveareaofavailablePu-238.Voltaicswereexposedtotheradioluminescentlightofphosphor-coatedslidescoupledwiththeplutoniumsource,asshowninFig.1,andtheirI–Vcurvesweremeasured.

Toincreasetheef ciencyandpoweroutputofindirectconversionmodels,wefoundthinre ectivemetallayersbetweenthealphasourceandphosphorlayertobeuseful.Theideawastore ectbacklightemittedfromthephosphorscreentowardsourcetowardthevoltaic,andthusincreaselight uxonthephotovoltaics.Approxi-mately,1mmthickaluminumandgoldfoilswereused.PerformanceoftheindirectconversioncellcomposedofoneplutoniumsourcewithandwithoutfoilsispresentedinFig.3.

Analuminumre ectorincreasedpoweroutput60%,namelyfrom6.3to10mW.Sincelightemissionisisotropicinthephosphorscreen,theintensityoflightemittedtowardthesourceisthesameastowardthevoltaic.Then

M.Sychovetal./AppliedRadiationandIsotopes66(2008)173–177

175

some90%ofthatadditionallightisre ectedbythealuminummirror.Accordingtoourmeasurements,only75%ofthere ectedlightisdiffusivelytransmittedbythephosphorlayer.Thatshouldgiveabouta68%improvementoflight uxtowardthevoltaics.However,there ectorabsorbs$5%ofthealphaenergy,sothetotallight uxwasreducedtothesamedegreeandweobtain64%improvementasobservedexperimentally.Goldfoilhaslowerre ectivity,whileitsdensity,andhenceabsorbedalphaenergy,ishigher.Therefore,goldfoilgivesonly20%improvement.

Toshowtheoperationalcapabilitiesoftheindirectconversionprototype,wepoweredaneight-digitelectroniccalculatorwiththeindirectconversioncell.PhotosgiveninFig.4showthesequenceofcellassembly.Photo#6ofFig.4showsthesameassemblyasphoto#5exceptthecelliscoatedwithblackclothtoexcludetheeffectofambientlightonthegenerator.Thecellfunctions neinbothsituations.Anelectronicwatchwasalsopoweredinthesamemanner.Itshouldbenotedthatradioluminescencebrightnessof15–20cd/m2achievedinourexperimentsisadequateforbacklightingofliquidcrystaldisplays.

When veplutoniumcellswereconnectedinonepowersource,weachievedshortcircuitcurrentof14mA,opencircuitvoltageof2.3Vandpoweroutputof21mW.

4.Phosphorstability

Weconductedstabilityinvestigationsofvariousphosphorsincludingsomeexperimentalsamples.Allstabilitymeasurementsweredoneinvacuumtopreventcorrosionofthealphasource.2.4mWPu-238(300mCi)wasusedfortheirradiationofphosphors.Radiolumines-cenceintensitywasmeasuredasthecurrentofaphoto-multipliertube.

VariousphosphorsweretestedincludingZnSsamples,oxides,oxysul des,yttriumaluminumgarnetaswellasSiAlON:Euandtiogallates.Themoststablesamplesamongtestedoneswereidenti edandsubjectedtolonger-termtests.AdescriptionofthesesamplesisgiveninTable1.Fig.5showsradioluminescentemissionspectraunderalphaexcitation(measuredusingKSVU-23/MDR-23spectro uorimeter).Thesephosphorsemitlightmostlyintheyellow-redregionofthespectrum,wherethesephotovoltaicshavethehighestef ciency.

Alpharadiationstabilitywascharacterizedbythenormalizedradioluminescentintensityaswellasbyaradiationstabilitycoef cient,K.Thiscoef cientre ectsthepercentageofradioluminescentintensitychangeperkGyofabsorbeddoseandiscalculatedaccordingtotheformula

Fig.4.Calculatorisworkingwhenpoweredbytheindirectconversioncell.

Table1

PropertiesofphosphorstestedunderalphaexcitationPhosphorChemicalcompositionKfor26,300kGy(%perkGy)B-3g(Zn,Cd)S:Ag,Cl3.80EÀ03Mih-Y

(Zn,Mg)F2:Mn7.36EÀ04RST-612RGd2O3:Eu1.93EÀ03K-78RY2O3:Eu

2.65EÀ03K-78B-W

Y2O2S:Tb,Dy

2.68EÀ

03

takenfromMikhalchenko(1988)K¼

I0ÀI

ID

Â100%,(3)

0whereI0isinitialRLintensity,IistheRLintensityatcertainabsorbeddoseDinkGy.TocalculateabsorbeddoseinGy(J/kg),weusedD¼

Pat

H

,(4)

wherePaistheenergy uxfromthealphasourceinW/cm2,ttheisexposuretimeinseconds,andHisthesurfacedensityofphosphorlayerabsorbingallalphaenergy,kg/cm2.

ResultsofradiationstabilitytestsarepresentedinFig.6.SomeofthecurvesinFig.6a,showperiodicincreasesinRLintensity.Thiswasobservedwhenstabilityexperimentswereinterruptedforaweekend.Therefore,thesephos-phorsshowself-repairofradiationdamageatroomtemperature.ThecurvesofFig.6bshowchangeintheradiationstabilitycoef cient,K,withincreaseddose.Initialdegradation(highKvalues)leveloutforoxideandoxysul dephosphors.Forthesul de-typephosphor,Kstabilizesatahighervalueofabsorbeddoses.The uoride-typephosphorshowinitialimprovementofRL(negativevaluesofK),andthenKlevelsatthelowestvalueamongtestedphosphors.Valuesoftheradiationstabilitycoef cientfor26.3MGydosearepresentedinthelastcolumnofTable1.

Electricgeneratorsbasedonplutoniumemitters,phos-phorscreensandvoltaicarraysweresuccessfullyfabricatedandtested.Optimizedgeneratorsproducedshortcircuitcurrentof14mA,opencircuitvoltageof2.3Vandmaximumpoweroutputof21mWat0.11%overallef ciency.Thisbatterypoweredelectroniccalculatorsandanelectronicwatchclearlyshowingtheabilityofradio-isotope-basedpowersourcestoproduceenoughenergyformoderncircuits.StabilitytestsallowedustoidentifythemoststablephosphorunderthePu-238exposure:(Zn,Mg)F2:Mnlostonly19%ofinitialRLintensityafterthe26.3Â107J/kgabsorbeddosewhichcorrespondsto411hofcontinuousirradiationwithhigh-activityPu-238.Acknowledgments

Authorsgratefullyacknowledgeprojectsupportpro-videdbyDARPAandUSArmy,PicatinnyArsenal,underContractW15QKN-04C-1123.LowlightarraysprovidedbyProf.V.M.Andreevweregreatlyappreciated.References

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