Effect of the intermediate velocity emissions on t

arXiv:nucl-ex/0009004v16Sep2000Effectoftheintermediatevelocityemissionsonthequasi-projectilepropertiesfortheAr+Nisystemat95A.MeVD.Dor´ea,d1,Ph.Bucheta,J.L.Charveta,R.Dayrasa,L.Nalpasa,D.Cussolb,T.Lefortb,R.Legraina,C.Volanta,G.Augerc,Ch.O.Bacrid,N.Bellaizeb,F.Bocageb,R.Bougaultb,B.Bouriquetc,R.Broub,A.Chbihic,J.Colinb,A.Demeyere,D.Durandb,J.D.Franklandc,E.Galichetd,h,E.Genouin-Duhamelb,E.Gerlice,D.Guinete,S.Hudanc,P.Lautessee,F.Lavaudd,J.L.Lavillec,J.F.Lecolleyb,C.Leduce,N.LeNeindreb,O.Lopezb,M.Louvelb,A.M.Maskaye,J.Normandb,M.Parlogf,P.Pawlowskid,E.Plagnold,M.F.Rivetd,E.Rosatog,F.Saint-Laurentc2,J.C.Steckmeyerb,M.Sterne,G.Tabacarud,B.Tamainb,L.Tassan-Gotd,O.Tirelc,E.Vientb,J.P.Wieleczkoc[INDRACollaboration]aDAPNIA/SPhN,CEA/Saclay,91191Gif-sur-YvetteCedex,FrancebLPCCaen(IN2P3-CNRS/ISMRAetUniversit´e),14050CaenCedex,FrancecGANIL(DSM-CEA/IN2P3-CNRS),B.P.5027,14076CaenCedex5,FrancedIPNOrsay(IN2P3-CNRS),91406OrsayCedex,FranceeIPNLyon(IN2P3-CNRS/Universit´e),69622VilleurbanneCedex,FrancefNuclearInstituteforPhysicsandNuclearEngineering,Bucharest,RomaniagDipartimentodiScienzeFisiche,Univ.diNapoli,180126Napoli,ItalyhConservatoireNationaldesArtsetM´etiers,75141ParisCedex03,France.1DAPNIA/SPhN,CEA/Saclay,91191Gif-sur-YvetteCedex,France,e-mail:[e-mail],FAX:[phone]presentaddress:CEA,DRFC/STEP,CECadarache,13108Saint-Paul-lez-Durance,FrancePreprintsubmittedtoElsevierPreprint8February2008AbstractThequasi-projectile(QP)propertiesareinvestigatedintheAr+Nicollisionsat95A.MeVtakingintoaccounttheintermediatevelocityemission.Indeed,inthisre-action,between52and95A.MeVbombardingenergies,thenumberofparticlesemittedintheintermediatevelocityregionisrelatedtotheoverlapvolumebe-tweenprojectileandtarget.Meantransverseenergiesoftheseparticlesarefoundparticularlyhigh.Inthiscontext,themassoftheQPdecreaseslinearlywiththeimpactparameterfromperipheraltocentralcollisionswhereasitsexcitationenergyincreasesupto8A.MeV.Theseresultsarecomparedtopreviousanalysesassumingapurebinaryscenario.Keywords:PACSNumbers;24.10.-i,25.70.-zTheQuasi-Projectiledeexcitationhasbeenstudiedthroughawidevarietyofsystemsatintermediateenergies[1]-[10].Inthisenergydomainatransitionfromabinaryprocess,leadingtotwomainexcitedfragments(thequasi-projectile(QP)andthequasi-target(QT))intheexitchannel,towardsaparticipant-spectatormechanism,isexpected.Frominclusiveorsemiexclu-sivemeasurementsithasnotbeenpossibletodistinguishbetweenthesetwomechanisms.Insomecases,theexperimentaldatacouldbedescribedequallywelleitherassumingapurebinarymechanismorageometricalprocess[3],[11].Withtheimprovementofexperimentalsetups,namelytheadventof4πmulti-detectorsallowingfullyexclusivemeasurements,itshouldbecomepossibletoreconstructtheQPandtheQTfromtheirdecayproductsonaneventbyeventbasis.However,thisreconstructionprocessdependsgreatlyonourabilitytoidentifyunambiguouslytheoriginofthedetectedproducts.Unfortunately,intheintermediateenergyrange,thevarioussourcesofemissionstronglyover-lapinthevelocityspace.Thusonehastorelyonsomeassumptionsontheunderlyingmechanismsinordertounfoldthevarioussourcesofemission.Al-thoughitwasgenerallyadmittedthatbelow100AMeV,heavyioncollisionshaveessentiallyabinarycharacter,ithasbeenshownsinceseveralyearsthatthedecayproductscouldnotbefullyimputedtothedecayofexcitedquasi-projectileandquasi-target[9]-[10],[12]-[18].Besidespreequilibriumanddirectemissionsalreadyobservedatlowenergies,processeslikeneckemissionandalignedfissionhadtobetakenintoaccountinordertoexplaintheexperi-mentaldata.Indeedanexcessofparticlesandfragments,notexplainedbythestatisticaldeexcitationoffullyequilibratedQPandQT,isobservedatintermediatevelocitywithunusualkinematicalproperties.FromrecentexperimentaldataontheAr+Nireactionsbetween52and95A.MeVobtainedatGANILwiththe4πmultidetectorINDRAitwasshown2[18]thatitwasnotpossibletoreproducethelightparticlerapidityspectrabyassumingonlystatisticalemissionsfromexcitedQPandQTandthattherewasanexcessofhighenergyparticlesatmid-rapiditywhichincreasesastheimpactparameterdecreases.Inthepresentpaper,wewillconcentrateontheAr+Nireactionat95AMeVandwewillshowhowthepropertiesoftheQPthatonecanextractfromthedataarestronglyaffectedbyparticleemissionaroundmid-rapidity.Firsttheimpactparameterclassificationandtheeventselectionwillbepresented.Then,theQPproperties,massandexcitationen-ergy,willbeestablishedaccordingtotwobasicassumptions.i)Neglectingmid-rapidityemission,followingpreviousanalysis[19]-[20],atwosourcere-constructionwillbeperformedintheframeofapurelybinaryscenario.ii)Inanattempttotakeintoaccountmid-rapidityemission(MRE)asevidencedin[18]wewillunfoldtheexperimentallightchargedparticlerapidityspectraassumingthreesourcesofemission,theQP,theQTandathirdsourceofemis-siontosimulatethemid-rapiditycontribution.Thermalandshapeequilibriumareassumedineachsource.ThenthepropertiesoftheQPareextractedfromitsdecayproducts.Inbothcases,themassandtheexcitationenergyoftheQPthusobtainedwillbepresentedasafunctionofanexperimentalimpactparameter.Finally,resultsofbothreconstructionmethodswillbecomparedan