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LifetimeMeasurementsoftheNeutron-RichCrIsotopesJ.
J.
Valiente-Dobon1,A.
Algora2,D.
Bazzaco3,A.
B¨urger4,L.
Corradi1,G.
deAngelis1,R.
Depalo3,A.
Dewald5,M.
N.
Erduran6,E.
Farnea3,E.
Fioretto1,A.
Gadea2,K.
Geibel5,A.
Gottardo1,2,M.
Hackstein5,T.
H¨uy¨uk2,R.
Kempley7,S.
Lenzi7,S.
Lunardi3,B.
Melon8,R.
Menegazzo3,D.
Mengoni3,9,C.
Michelagnoli3,V.
Modamio1,O.
M¨oller10,G.
Montagnoli3,D.
Montanari1,A.
Nannini8,D.
R.
Napoli1,P.
Reiter5,F.
Recchia3,W.
Rother5,P.
A.
S¨oderstr¨om11,E.
Sahin1,F.
Scarlassara3,A.
M.
Stefanini1,S.
Szilner12,C.
Ur3andtheAGATAcollaboration.
1INFN,LaboratoriNazionalidiLegnaro,Legnaro(Padova),Italy.
2IFICValencia,Valencia,Spain.
3DipartimentodiFisica,Universit`adiPadovaandINFN,SezionediPadova,Padova,Italy.
4DepartmentofPhysics,UniversityofOslo,Oslo,Norway.
5IKP,Universit¨attzuK¨oln,K¨oln,Germany.
6DepartmentofPhysics,IstanbulUniversity,Istanbul,Turkey.
7DepartmentofPhysics,UniversityofSurrey,Guildford,U.
K.
8DipartimentodiFisica,UniversitadiFirenzeandINFN,SezionediFirenze,Firenze,Italy.
9UniversityoftheWestofScotland,Paisley,Scotland.
10TechnischeUniversit¨atDarmstadt,Darmstadt,Germany.
11DepartmentofPhysicsandAstronomy,UppsalaUniversity,Uppsala,Sweden.
12RuderBoskovicInstitute,Zagreb,Croatia.
INTRODUCTIONThemotivationoftheexperimentdescribedbelowwastostudythedeformationintheCrisotopicchaindrivenbyparticle-holeexcitationstotheg9/2andd5/2orbitals.
Inparticular,ouraimwastoinvestigatethe58Crisotope,whichhasbeenidentiedasanucleusatthecriticalpointoftheshapephasetransitionoftheE(5)dynamicalsymmetry,andthecollectivecharacterofthelowspinstatesin60Cr.
Inordertoaddressthisissue,theRecoilDistanceDopplerShiftmethod(RDDS)withtheAGATADemostrator-PRISMAsetupwasusedtomeasurethelifetimesoftherstexcitedstatesintheneutron-rich58Crand60Cr.
EXPERIMENTIntheregionrightbelowtheneutron-richNiisotopes,thereisanincreaseofcollectivityproducedmainlybythemonopolepartofthetensorinteractionofthenuclearforce:theinteractionbetweenf7/2protonsandbothf5/2neutrons(attractive)andg9/2neutrons(repulsive)weakenswhentheprotonf7/2orbitalisnotfullylled,andthereforethegapbetweentheneutronorbitalsf5/2andtheg9/2decreases.
Thismechanismwillfavorparticle-holeexcitationsacrossN=40subshellclosureandthereforetheg9/2orbitalwillplayanimportantroleindrivingthenucleustowardsdeformation.
Infact,theremovaloftwoorfourprotonsfromtheN=4068NiisenoughtomaketheN≈40Fe,MnandCrisotopesdeformed,developinganewregionofdeformation[1–5].
Aclearexperimentalobservationisthatforneutron-richCrisotopesheavierthanN=32,theenergyoftherstexcited2+statedropsoffconsiderably.
The56Crisotopepresentsa2+energy(1007keV[6])andaB(E2)value(8.
7(3.
0)W.
u.
[7])consistentwithasubshellclosureatN=32.
From58Crto62Crtheenergyoftherst2+statedecreasesfrom880keV[8](58Cr),646keV[9](60Cr)to446keV[9](62Cr),pointingtoanincreaseincollectivitytowardsN=40.
ThecromiumisotopesfromN=28uptoN=38undergoachangeofshapefromspherical(N=28,52Cr)todeformedin60,62Cr,witha4+/2+typicalofaγ-softrotor.
ThiskindofshapetransitionhasbeendescribedwithintheframeworkofIBMbyIachello[10,11]andthecritical-pointwherethephasetransitionhappensisdescribedbytheE(5)symmetry(fromsphericalvibratorU(5)toaγ-softrotorO(6)).
Theexpectedbehaviourintheexcitationenergies(energyspectrum)oftheE(5)symmetrywasobservedintheN=34Crisotope(58Cr)[12].
InthiscaseonlytheB(E2)(14.
8(4.
2)W.
u.
[7])oftherstexcited2+stateisknown,whichwasmeasuredusingCoulombexcitationatrelativisticenergies.
Theparameter-freeanalyticalexpressionfortheenergyratiospredictedbytheE(5)dynamicalsymmetryforanucleusatthecriticalpointare:E(4+)/E(2+)=2.
20,E(6+)/E(2+)=3.
59,E(8+)/E(2+)=5.
17;thecorrespondingvaluesin58Crare2.
20,3.
65and5.
31,respectively[12,13].
ThisproposalispartofaprojecttomeasuretheB(E2)valuesoftheyraststates(thisproposal)andnon-yraststateswhichwillbemeasuredinfutureexperimentsinRIBfacilities.
ThesecomplementarymeasurementsareessentialtoelucidatetheE(5)characterof58Cr.
Thenucleiofinterest,58Crand60Cr,werepopulatedasproductsofamultinucleon-transferreactionfollowingthecollisionofa64Nibeamontoa238Utarget.
The64Nibeam,atabombardingenergyof460MeV,wasdeliveredbytheLNLTandem-ALPIacceleratorcomplex.
Thebeamintensitywaslimitedto2.
5pnAtoavoidthermalstressoftheplunger-targetdeviceandtokeeptheγcountingrateofAGATAataround60.
0KHz.
TheprocedureforstretchingsimultaneouslythetargetandthedegraderfoilsaswellasthededicatedtargetholderhavebeendevelopedattheInstituteofNuclearPhysicsattheUniversityofK¨oln[14].
Thetargetconsistedof1.
35mg/cm2ofenriched238Uevaporatedontoa1.
2mg/cm2Tasupporttoaccomplishthestretchingofthetarget.
Athick4.
13mg/cm2natNbfoilusedasanenergydegraderoftherecoilingejectileswaspositionedafterthetarget.
Differenttarget-degraderdistances,rangingfrom20mto150mwereemployedduringtheexperiment.
InthisexperimentAGATAconsistedof4AGATATripleClusters.
Theprojectile-likeproductswereselectedwiththemagneticspectrometerPRISMAplacedatthegrazingangleθLAB=60.
Energy(keV)CountsFig.
1.
Doppler-correctedγ-rayspectrashowingthe2+→0+824-keVtransitionin60Feforatarget-degraderdistanceof45m.
ThemostchallengingCrisotopes(fourprotonstripping)havenotbeenfullyanalysed,howevertheFeisotopes(twoprotonstripping)areshowninFig.
1thatpresentstheDoppler-correctedγ-rayspectraassociatedwith60Fe,foradistanceof45m,fortheselectedenergyregionincludingthe2+→0+824-keVtransition.
TheDopplercorrectionisperformedonanevent-by-eventbasisusingthevelocityobtainedbythereconstructionoftherecoiltrajectoriesinPRISMA.
Furtheranalysisisongoingforthemorechallengingchannels58Crand60Cr.
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