onAugust 2, 1939, just BEForE thE BEginning oF worlD wAr ii,AlBErtEinstEin wrotE to thEn prEsiDEnt FrAnklinD、 roosEvElt.EinstEin AnD sEvErAl othEr sCiEntists tolD roosEvElt oF EFForts in nAzi gErmAny to puriFy urAnium-235, whiCh CoulD BE usED to BuilD An AtomiC Bom
B、it wAs shortly thErEAFtEr thAt thE unitED stAtEs govErnmEnt BEgAn thE sErious unDErtAking known thEn only As "thE mAnhAttAn projECt". simply put, thE mAnhAttAn projECt wAs CommittED to ExpEDiting rEsEArCh thAt woulD proDuCE A viABlE AtomiC Bom
B、 thE most CompliCAtED issuE to BE ADDrEssED in mAking oF An AtomiC BomB wAs thE proDuCtion oF AmplE Amounts oF "EnriChED" urAnium to sustAin A ChAin rEACtion.At thE timE, urAnium-235 wAs vEry hArD to ExtrACt. in FACt, thE rAtio oF ConvErsion From urAnium orE to urAnium mEtAl is 500:1.CompounDing this, thE onE pArt oF urAnium thAt is FinAlly rEFinED From thE orE is ovEr 99% urAnium-238, whiCh is prACtiCAlly usElEss For An AtomiC Bom
B、to mAkE thE tAsk EvEn morE DiFFiCult, thE usEFul u-235 AnD nEArly usElEssu-238 ArE isotopEs(同位素), nEArly iDEntiCAl in thEir ChEmiCAl mAkEup. no orDinAry ChEmiCAl ExtrACtion mEthoD CoulD sEpArAtE thEm; only mEChAniCAl mEthoDs CoulD work.
A、mAssivE EnriChmEnt lABorAtory/plAnt wAs ConstruCtED At oAk riDgE, tEnnEssEE. hArolDC、urEy AnD his CollEAguEs AtColumBiA univErsity DEvisED An ExtrACtion systEm thAt workED on thE prinCiplE oF gAsEous DiFFusion, AnDErnEst o. lAwrEnCE (invEntor oF thECyClotron) At thE univErsity oFCAliForniA inBErkElEy implEmEntED A proCEss involving mAgnEtiC sEpArAtion oF thE two isotopEs. nExt, A gAs CEntriFugE wAs usED to FurthEr sEpArAtE thE lightEr u-235 From thE hEAviEr, non-FissionABlE u-238. onCE All oF thEsE proCEDurEs hAD BEEn ComplEtED, All thAt nEEDED to BE DonE wAs to put to thE tEst thE EntirE ConCEpt BEhinD AtomiC Fission ("splitting thE Atom", in lAymAn’s tErms). ovEr thE CoursE oF six yEArs, From 1939 to 1945, morE thAn $2 Billion wAs spEnt During thE history oF thE mAnhAttAn projECt. thE FormulAs For rEFining urAnium AnD putting togEthEr A working AtomiC BomB wErE CrEAtED AnD sEEn to thEir logiCAl EnDs By somE oF thE grEAtEst minDs oF our timE.ChiEF Among thE pEoplE who unlEAshED thE powEr oF thE Atom wAs j. roBErt oppEnhEimEr, who ovErsAw thE projECt From ConCEption to ComplEtion. FinAlly, thE DAy CAmE whEn All At losAlAmos woulD FinD out iF "thE gADgEt" (CoDE-nAmED As suCh During its DEvElopmEnt) wAs going to BE thE ColossAl DuD oF thE CEntury or pErhAps An EnD to thE wAr. it All CAmE Down to A FAtEFul morning in miDsummEr, 1945. ACCorDing to thE pAssAgE, iF thErE wErE 500 tons oF urAnium orE, how mAny tons oF usEFul urAnium-235 CoulD BE ExtrACtED
A、1 kilogrAm.
B、10 kilogrAm.
C．50 kilogrAm.
D．500 kilogrAm.