Alpha indirect conversion radioisotope power source(3)

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

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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.