THEORETICAL INVESTIGATIONS IN THE BREAKUP OF EXOTIC NUCLEI

Abstract

“Imaginefourblindmeninaroomwithanelephant.Wheneachofthemis askedto describetheelephant,theytrytodosowiththehelpoftheirsense oftouch.Butas eachofthemlayshishandsonadifferentbodypartofthehugemammal,eachman hasadifferentexplanation.Themanwhotouchesthestronglegssaysitisapillar, theonetouchingtheearsrecommendsittobeacurtain.Anotherthinksitisarope ashehappenedtogetholdoftheelephant’stail,whereasthefourth man,feelingthe animal’sbigtempleexplainsthatitisarock.Eachwithanoriginalandtrue,yet incompleteversion.Ourfieldissimilartothatelephantandweallarenothingbut blindmentryingtofigureoutwhatexactlyitisforalmostacenturynow.” Thus,withthesewordsbyProf.AshokKumarJainduringtheintroductory lectureoftheAdvancedNuclearPhysicscourseatthepostgraduatelevel,began myfirstseriousforayinnuclearphysics.Indeed,itisnocoincidence thatunclear is ananagramofnuclear,forthefieldisstillfullofdoubtsandassumptions.Itis unclearwhatistheexactformofthenuclearpotential.Thereisnolucidjustification fortheappearanceanddisappearanceofmagicnumbersandthepresenceoftheso called‘islandsofinversion’.Nooneknowspreciselywhystablenucleiwith mass numbers5and8arenotfoundinnature.Theexplanationoftheabundancecurve andnucleosynthesisisnotabsolutelycertain.Itisbelievedthatmeasuringnuclear i ii propertiesshould,ideally,helptouncoverthetruthabouttheorigin ofradiationand thus,shedlightontheuniverseformation.Buthowexactly,wehave noidea.The occurrenceofresonancesinthecontinuumofsomenucleiisanenigma.Nodistinct answerisknownastowhycompoundnucleihavenomemoryoftheinitialchannel, anditisstillnotclearhowdifferentreactionchannelsmixamongthemselves.Vaguely put,thesearesomeofthequestionsthathaveexistedinthefieldforfartoolong, andyet,satisfactoryanswerstothemarerare.Therehasbeen considerableprogress nonetheless,andwehavecomealongwayfromthediscoveryofthe nucleustoitsuse asapowersourcetorunourcomputers.Truly,eachstephasbeen equallysignificant. Thistoo,isbutasmallandfeebleattempttounderstandandexplore thebranch whichhaspuzzledmanyforsolong. Wenowknowthatnucleiclosetothedriplinesarequeeranduncanny in their behaviourascomparedtotheirsiblingsfoundinthevalleyofstability, andare, therefore,calledexotic.Wedealwithasubclassoftheseexoticnuclei(callednu- clearhalos)andtheirCoulombbreakup.Halonucleihavetheirvalence nucleon(s)so looselyboundthattheyhavealmostnoexcitedstatebelowthedissociationthreshold anduponenteringanexternalCoulombornuclearfieldofatargetduringexperi- ments,theyeasilybreakuptogiveoffacoreandusuallyone(two)nucleon(s).We haveusedafullyquantummechanicaltheoryoffiniterangedistortedwaveBorn approximationforournuclearreactionstudies.Thetheorycancompetentlytake theprojectiledeformationintoaccountandonlyrequirestheprojectilegroundstate wavefunctionasaninput. Inthisthesis,weinvestigateandtreat 34Naasthenucleusofourinterest.Lyingin the‘islandofinversion’,andapartfromusesinthenuclearfield,itissignificantasit haspotentialastrophysicalapplicationstoo.Afterbrieflyintroducingthesubjectin chapter1,webringtoviewinchapter2asurveyofthemodelsthatarethestrongest contenderstodescribebreakupreactionsoccurringundertheaegisofCoulombdis- sociation.Chapter3showcasestheaspectsofthetheoreticalformalismthatgoes in themakingofthisthesis.Inchapter4,weinvestigatethetheoreticalbreakupof iii 34Naona 208Pbtargetatabeamenergyof100MeV/u.Asapartoftheprocess, wecalculatevariousreactionobservables,likethetotalcross-sections,relativeenergy spectra,angularandmomentumdistributions,etc.,whichhelpusdefinelimitsonits uncertainstructuralparametersinamoreaffirmativemanner.Chapter5presents theresultswhenweextendtheutilityofourtheoryasanindirecttooltostudyradia- tivecapturereactionsusingtheprincipleofdetailedbalance.Westudytheprospects ofthe 33Na(n,γ)34NaradiativecapturereactionaffectingNaisotopeproductionnear theneutrondriplineanditsroleintheproductionof r -processseednuclei.We concludeinchapter6withaneyeonfutureprospects.Intheappendix,wepresent someadditionalmathematicalandcomputationaldetailsaboutthe formalism.