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Ab stractAs part ofastudy todemonstratethe suitabil-it y of anX-rayorgammaray probe formonitoringthe quantity and flowrateofslushhydrogen, massat tenuationcoefficientsforCd109X-andgammara-diationin ve chemic al compoundshavebeen mea-sured .TheAg109KXrayswere usedfor waterandaceticacid, whereasE3 transitionfrom th e rstex-citedstate at87.7keVinAg109providedtheproberadiationfor bromobenzene,ff2-chloroisodurene, andcetylbromide.Measurements weremadeforasinglephase (gas,liquid,andsoli d)aswellasmixedphases(liquidplus solid)in allcases. It has beenshownthatt he massattenuationcoeffici entfor theselectedra diationsisindependent of thephaseofthetestflu-idsorphaseratiosinthe case ofmixed-phase uids.Th isreport describes the procedureandresults forthefive uidsystemsinvestigated.Introductio nSlu sh hydrogen isbeing considered(refs.1 and 2)as apropellantforthe NationalAero-SpacePlane(NASP).Ithaslow mass, highenergycontent,andhigherdensitythanliquid hydrogen.However,nosuitable techniquei scurrentlyavailable formonitor-in gthequantityandflowrateofafuelwhosephaseconfigurationiscontinuouslychanging.We havein-vestigated thefeasibilityofalow-energygammaraysystemforgaugingthe quantityandflowrate forsuch fuels. Itisbased onthe principlethatthe massat tenuationcoefficient ofachemicalcompoundforgammaradiationdoes not dependon the phaseofthech emical.Thisreportdescribes theresults forfivemixe d-phase uidsystems.TheoreticalBackgroundWhen anarrow beam of gammarays passesthro ugh matter, someof th erays are scattered orabsor bedinthemediumbecauseof interactionswiththe atoms of the medium.Forexample,I=Ioe??x(1)whereIintensityofgamma radi ationarriving atde tectorIointensityofgamma radi ationincidentontestmedium?attenuationperunitpath length in themedi um(i.e.,the linear attenuationcoefficient)xpathlengthin themediumThel inearattenuation co efficient(?, cm?1) foraneleme ntalmediumisgivenby?=oen=oeNaeA(2)whereoetotalinteraction crosssectionperatom,c m2/atomNAvogadro'snumberonphysicalscale,6:025?1023atoms/moleaedensity of themedium, g/cm3Aatomi cor molecularweight, gS inceoeisafunctionof gamma rayenergy,?isclearlyenergy dependent. Theexpression for? c anbegen eralizedforany medium containing diff erentkindsofatoms. It isthe basisof theconventionalnuclearradiationattenuation(NRA)monitors.Amoreuseful variationofequation(2)is?ae=oeNA(3)where?=aeisinunitsof centimeterssquaredpergramandcalledthemass attenuationcoefficientofthemediumforthe incident gammarays. Mass atten-u ationc oefficient isamorefundamental parameterthanthelinearattenuationcoefficientsince it is in-d ependentofthe actual densityandphysical stateofthe medium.Ifthemediumisamixture ofseverald ifferentcompound sor elements,its massattenua-tioncoe fficient isgivenby??ae?mixture=Xiwi??ae?i(4)wherewifraction byweightoftheithcomponent??ae?imassattenuation coefficient of thei thcomponentForamultiphase mixture ofanygivenchemicalcompound ,equation(4)can bewritten asfollows:1