Alpha indirect conversion radioisotope power source(2)

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