STUDIES IN NUCLEAR STRUCTURE AND BIG BANG NUCLEOSYNTHESIS USING PROTON BEAMS

Abstract

Proton inducedcrosssectionmeasurementshavetheirownimportanceinthefield of experimentalnuclearphysics.Capturereactionoflowenergyprotonsondiffer- enttargetsisoneofthemajortoolsforstudiesinnuclearastrophysics.Inelastic scattering ofprotonshasbeenofparamountimportanceinstudyingstructuresof low-lyingexcitedstatesinnuclei.Light-ioninducedreactionsplayasignificantrole in studyingnucleon-nucleoninteraction.Inthisregardthestudieswithspin-polarized light-ionsandtargetsareoffundamentalimportance.Thephenomenaladvancements in modernradiationdetectorsandtheirassociatedtechniquesmakeitimperativeto revisit light-ioninducedreactionscarriedoutinthepast.Studyofprotoninduced capture crosssectionmeasurementsarecrucialintheformationofelementsinthe universe[1],toprobenuclearstructure,tounderstandnuclearreactiondynamics,in medical applicationfortheproductionofradioisotopes,inthefieldofradiobiology etc. Anothermajoraspectsofprotoninducedreactionsisscatteringreaction.This includes bothelasticandinelasticprocesses.Inthisthesiswork,wehavestudied bothoftheseproton-inducedreactions.Theyare,radiativecaptureofprotonby deuteron andinelasticscatteringofprotonsonCarbon-12nucleuswhichareoffun- damentalimportancefornuclearastrophysicsandalsonuclearstructure.Studies of inelasticscatteringofprotonsandtheunderstandingof 12C(p,p0γ)12C reactionin i ii terms ofscatteringtheoryinvolvestwosteps.First,theexcitation,followedbythe de-excitation oftheCarbon-12nucleus.Thesestepsarenotfullyunderstood.The deexcitation isthereleaseofenergyviaagammaraywithaknownenergyandangu- lar momentum.Theexcitationmechanismhasuncertainties.Avarietyofquestions are requiredtobeaddressedandanswered.Namely,isthestateunderconsider- ation excitedintoacollectivemodeorisitbetterunderstoodbyasinglenucleon excitation, ordobothrepresentationshavevalidity?Howimportantistheexchange processforidenticalparticles,thealphaparticleclusteringoftheCarbon-12nucleus? Howsignificantarethegiantresonanceeffectsof 12C andtheenergylevelsof 13N in theexcitation?Themeasurementof 12C(p,p0γ)12C playsaveryessentialrolein answeringthesequestions.Inaddition,thescatteringofprotonintheenergyrange of 10to30MeVposeschallengetotheoristseventhoughtheopticalmodelformalism workswellathigherenergies.Thisprovidesenoughjustificationforthemeasurement of scatteringcrosssectionsof 12C(p,p0γ)12C reactionuptoabout30MeV.Themea- surementswerecarriedoutwithprotonbeamenergiesfrom8to22MeVusing14 MV BARC-TIFRpelletronacceleratorfacility.Wehavemeasuredthegamma-rays correspondingtothedecayfromtheexcitedstatesofthe 12C nucleus,namely,4.43, 9.64, 12.7and15.1MeV.Theangulardistributionofthegammaraysweremeasured for sixdifferentangles,namely,60◦, 75◦, 90◦, 105◦, 120◦ and 135◦. Foralltheabove mentionedstates,thedifferentialcrosssectionshavebeenmeasured.Eventually,the excitation functionofthestateswereextracted.Thegammabranchingratioofin- dividual stateswascalculatedfromthemeasureddata.Theopticalmodelanalysis wascarriedouttoget 12C(p,p0γ)12C crosssectiondataforeachstate.Thecross section obtainedfromexperimentalmeasurementsandtheoreticalcalculationswere compared andfoundtobeingoodagreementwitheachother.Theoreticaldifferen- tial gammacrosssectionwasalsocalculatedusingphenomenologicalopticalmodel potential.Thecalculateddifferentialgammacrosssectionforeachstatewascom- pared withthemeasuredgammacrosssection.Forthefirsttime,wehavereported the crosssectionandbranchingratioof9.64MeVstateof 12C nucleus.Another iii importantbranchofprotoninducedreactionsisradiativecaptureprocess.Thera- diativecaptureofprotonbydeuterond(p,γ)3He isareactionofgreatsignificance bothinnuclearastrophysicsandfew-bodynuclearphysics.Therearemainlythree differentscenariosinwhichthedepletionorproductionofheliumcantakeplace via d(p,γ)3He reaction,namely,theBigBangNucleosynthesis(BBN),productionin low-massprotostars,andproductioninlowtomediummassstarslikeoursun[1,2]. Fromexperimentalstandpoint,thebeamenergyforthisreactionisvariedoverawide range dependingupontheexactnatureoftheproblemoneisinterestedin.Invery lowenergyregion,say,from2.5keVto30keVsufficientdataareavailablecovering the Gamowpeakregion[3].ButthesituationisratherunclearforBBNenergyregion that rangesfrom30keVto300keV.Veryfewgroupshavedonemeasurementsin the BBNregion[4].Therefore,themeasurementsofcrosssectionandS-factorfor d(p,γ)3He reactionatBBNrelevantenergiesareofgreatfundamentalimportance. The experimentwascarriedusingECRionsourceavailableinTIFR,Mumbai.For the firsttime,thecrosssectionsandS-factorsweremeasuredford(p,γ)3He capture reaction atthreenewbeamenergiesnamely,100keV,175keV,250keVwhichare relevantinBBNregionusingalargevolumeLaBr3:Ce detector.RealisticGEANT4 simulationswerecarriedoutinordertoestimatetheenergydependentefficiencyof large volumecylindrical3.5 00×6 00 LaBr3:Ce detector[5].Thecrosssectionsmeasured byusareinexcellentagreementwiththelatestadvancedtheoreticalcalculationsfor this reaction[4]. In ordertomeasurethegamma-raysproducedin 12C(p,p0γ)12C reactionandd(p,γ)3He reaction, wehaveusedalargevolume(1246cm3) LaBr3:Ce scintillationdetector.For the firsttime,efficiencycalibrationandcoincidencesummingcorrectionwasdonefor a largevolumeLaBr3:Ce detectorusingpositronemitter(22Na) togenerateenergy dependentefficiencycurve. Contentsofallthechaptersofthisthesisarepresentedbelowinconciseform. Chapter 1: This chaptergivesageneraloverviewofprotoninducedreactions,namely, inelastic scatteringandradiativecaptureprocessesandtheiruseinnuclearstructure iv and nuclearastrophysics.Italsogivesaglimpseofexperimentalfacilitiesusedto carry outthesereactionsemphasizingtheroleofscintillationdetectors.Thischapter also touchesuponourworkinvolvingthecharacterizationoflargevolumeLaBr3:Ce detectors usedinthemeasurements.Theoreticalformalismstoanalyseinelasticscat- tering data,inparticulartheopticalmodelanalysisarealsopresentedbrieflyinthis chapter. Chapter 2: This chapterisdevotedtoourstudieswithscintillationdetectors,pri- marily,lanthanumbromidedetectorsusedforourmeasurements.Wediscussthe results ofexperimentalmeasurementsandGEANT4simulationsforefficiencycali- bration andcoincidencesummingcorrectionofalargevolumecylindrical3.500× 600 LaBr3:Ce detector.WehaveappliedamethodproposedbyVidmar et al ., [6] forthe first time,forcorrectingthesimulatedandthemeasuredefficienciesextractedusing 22Na sourcethatemitscoincidentgamma-rayswithdifferentdecayintensities. In addition,wealsoreportourin-depthstudiesofpropertiesandresponseoflarge volumesquarebars(200×200×800 ) ofLaBr3:Ce detectors,individually,andinacom- pact arrayoffoursquarebarswithgamma-raysupto22.5MeV.Theproperties studied includeuniformityofthecrystal,internalradioactivity,energyresolution, timing resolution,linearityoftheresponseanddetectionefficiencies.Theresponse of thearrayfor22.5MeVgamma-raysproducedfrom 11B(p,γ)12C capturereaction and for15.1MeVgamma-raysproducedfrominelasticscatteringof 12C(p,p0γ)12C are studiedindetail.Themeasuredabsoluteefficiencies(bothtotaldetectionand photo-peak)for 137Cs arecomparedtothoseobtainedusingrealisticGEANT4[7] simulations. Chapter 3: This chapterdiscussesthemeasurementsofcross-sectionandastrophys- ical S(E)factorforp-dcapturereactionusingprotonbeamenergies,namely,100keV, 175 keVand250keV.RealisticGEANT4simulationswerecarriedoutinordertoes- timate theenergydependentefficiencyoflargevolumecylindrical3.500× 600 LaBr3:Ce detector. ThemeasuredcrosssectionandastrophysicalS(E)factorforthreenew proton energiesarefoundtobeconsistent,withintheerrors,withthetrendreported v in theliterature.Thecrosssectionsmeasuredbyusareinexcellentagreementwith the latestadvancedcalculationsforthisreaction[4]. Chapter 4: This chapterpresentstheinelasticscatteringcross-sectionsof 12C(p,p0γ)12C reaction measuredusingprotonbeamswithenergiesfrom8to22MeV.Wehave measured thegamma-rayscorrespondingtothedecayfromtheexcitedstatesofthe 12C nucleus,namely,4.43,9.64,12.7and15.1MeV.Theangulardistributionofthe gamma-raysweremeasuredforsixdifferentangles,namely,45◦, 60◦, 75◦, 90◦, 105◦ and 135◦. Foralltheabovementionedstates,thedifferentialcrosssectionshave beenmeasured.Eventually,theexcitationfunctionofthestatesweredetermined. The gammabranchingratioofeachofthesestateswascalculatedfromthemeasured data. Detailed opticalmodelanalysiswascarriedouttocalculate 12C(p,p0γ)12C crosssec- tion foreachofthefourexcitedstates.Thecrosssectionsobtainedfromexperimental measurementsandtheoreticalcalculationswerecomparedandfoundtobeingood agreementwitheachother.Theoreticaldifferentialgammacrosssectionwasalso established usingphenomenologicalopticalmodelpotentialforthefirsttime.The calculated differentialgammacrosssectionforeachstatewascomparedwithmea- sured gammacrosssection. Chapter 5: All thesalientpointsofourmeasurements,bothcaptureandinelastic scattering, aresummarizedinthischapter.Theconclusionsarrivedatfromthese studies arepresentedandfuturescopeofworkarediscussed.