The role of core excitations in the structure and decay of the 16(+) spin-gap isomer in Cd-96

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Physics Letters B

www.elsevier.com/locate/physletb

The role of core excitations in the structure and decay of the 16 ⁺ spin-gap isomer in ⁹⁶ Cd

P.J. Davies

^a^,^∗

, H. Grawe

^b

, K. Moschner

^c^,^g

, A. Blazhev

^c

, R. Wadsworth

^a

, P. Boutachkov

^d

, F. Ameil

^b

, A. Yagi

^e

, H. Baba

^g

, T. Bäck

^h

, M. Dewald

^c

, P. Doornenbal

^g

, T. Faestermann

^k

, A. Gengelbach

ⁱ

, J. Gerl

^b

, R. Gernhäeuser

^k

, S. Go

ⁿ

, M. Górska

^b

, E. Gregor

^d

, T. Isobe

^g

, D.G. Jenkins

^a

, H. Hotaka

ⁿ

, J. Jolie

^c

, I. Kojouharov

^b

, N. Kurz

^b

, M. Lewitowicz

^o

, G. Lorusso

^g

, L. Maier

^k

, E. Merchan

^d

, F. Naqvi

^j

, H. Nishibata

^f

, D. Nishimura

ⁿ

,

S. Nishimura

^g

, F. Nowacki

^p

, N. Pietralla

^d

, H. Schaffner

^b

, P.-A. Söderström

^g

, H.S. Jung

ⁿ

, K. Steiger

^k

, T. Sumikama

^l

, J. Taprogge

^m

, P. Thöle

^c

, N. Warr

^c

, H. Watanabe

^g

, V. Werner

^d^,^j

, Z.Y. Xu

^g

, K. Yoshinaga

^g

, Y. Zhu

ⁿ

aDepartmentofPhysics,UniversityofYork,YorkYO105DD,UnitedKingdom bGSIHelmholtzzentrumfürSchwerionenforschung,D-64291Darmstadt,Germany cInstitutfürKernphysik,UniversitätzuKöln,D-50937Köln,Germany

dIKP,TechnischeUniversitätDarmstadt,64289Darmstadt,Germany

eResearchCenterforNuclearPhysics,OsakaUniversity,Ibaraki,Osaka567-0047,Japan fDepartmentofPhysics,OsakaUniversity,Toyonaka,Osaka560-0043,Japan gRIKENNishinaCenter,Wako,Saitama351-0198,Japan

hDepartmentofPhysics,RoyalInstituteofTechnology,SE-10691Stockholm,Sweden iDepartmentofPhysicsandAstronomy,UppsalaUniversity,SE-75121Uppsala,Sweden jWrightNuclearStructureLaboratory,YaleUniversity,NewHaven,CT 06511,USA kPhysikDepartmentE12,TechnischeUniversitätMünchen,D-85748Garching,Germany

lDepartmentofPhysics,TohokuUniversity,6-3Aramaki-Aoba,Aoba,Sendai,Miyagi980-8578,Japan mInstitutodeEstructuradelaMateria,CSIC,E-28006Madrid,Spain

nDepartmentofPhysics,UniversityofTokyo,Bunkyo,Tokyo113-0033,Japan

oGrandAccélérateurNationald’IonsLourds(GANIL),CEA/DSM–CNRS/IN2P3,F-14076CaenCedex5,France pIPHC,IN2p3-CNRSetUniversite´deStrasbourg,F-67037Strasbourg,France

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received19October2016

Receivedinrevisedform11January2017 Accepted9February2017

Availableonline14February2017 Editor: V.Metag

Keywords:

β-decay βp-decay γ-rayspectroscopy Half-life Shell-model WKB

Theﬁrstevidenceforβ-delayedprotonemissionfromthe16⁺spingapisomerin⁹⁶Cdispresented.The datawereobtainedfromtheRareIsotopeBeamFactory,attheRIKENNishinaCenter,usingtheBigRIPS spectrometerand theEURICAdecaystation.βp branchingratiosforthegroundstateand16⁺ isomer havebeenextracted alongwithmorepreciselifetimesforthesestatesandthelifetime fortheground statedecayof⁹⁵Cd.Largescaleshellmodel(LSSM)calculationshavebeenperformedandWKBestimates madefor=⁰,2,4 protonemissionfromthreeresonance-likestatesin⁹⁶Ag,thatarepopulatedbytheβ decayoftheisomer,andtheresultscomparedtothenewdata.Thecalculationssuggestthat=^{2 proton} emissionfromtheresonancestates,whichreside∼5 MeV abovetheprotonseparationenergy,dominates theprotondecay.Theresultshighlighttheimportanceofcore-excitedwavefunctioncomponentsforthe 16⁺state.

©²⁰¹⁷^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/4.0/).FundedbySCOAP³.

*

Correspondingauthor.

E-mailaddress:paul.john.davies@gmail.com(P.J. Davies).

http://dx.doi.org/10.1016/j.physletb.2017.02.013

0370-2693/©²⁰¹⁷^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/4.0/).Fundedby SCOAP³.

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1. Introduction

The region around ¹⁰⁰Snis the location of a fascinating vari- etyofphysicalphenomena[1–5].Forexample,recentexperimental work on¹⁰⁰Snmeasured the largest Gamow Teller(GT) strength foran allowed β-decay[6], thelow lying levels of ⁹²Pd [7] and thehighspin isomericstate in⁹⁶Cd[2]highlightthe importance oftheisoscalar(T=⁰⁾ proton–neutron(pn)interactiononenergy levelsinself-conjugatenucleiinthe region,andtheproximityof theN=^Z=^{50 shell}^closure^leads^toâ^significant^numberôf^both spin-gapandseniorityisomers,e.g.,see[2,4,5,8–13].Theserather differentnuclearstructurefeaturesprovideanidealtestingground for shell model effective interactions and model spaces. In par- ticular,theexperimental andtheoretical investigation ofisomeric stateshascontributedsignificantlytoourunderstandingofthenu- clearstructureaswellasourabilitiestopredictnuclearproperties inthisregionofthenuclearchart.

Thekeyactive orbitals(p1/2,g9/2)forthe N≈^{Z nuclei}ⁱⁿ^the mass90region areexpectedtobewell isolated,whichmadethe empirical shell model (ESM) a particularly attractive tool forin- terpreting thestructure ofexcited states [14–16].Away fromthe N=^{Z line} ît ^has ^been demonstrated that such calculations can provideagooddescriptionoflow energystatesandthata domi- nant g9/2 orbitalleads toboth spin-gapandseniorityisomers[2, 8–13].The spin-gap isomers arise from thestrong attraction be- tweenneutronsandprotonsoccupyingthisorbital.Insomenuclei thiscanproducestateswithlifetimeslongenoughtogivesignifi- cantβ-decaybranches[17–19].Furthermore,thelarge Q_β+ values forthesestatesoftenallows asignificant partofthe GTstrength tobe probed. Sincethe GTstrength isrelatedto theoverlap be- tweentheinitialandfinal wavefunctions, studiesofsuchisomer decay properties can provide a sensitive test of shell model interactions.An area ofparticularinterest inrecentyearshasbeen the high-spin isomeric states in N≈^{Z nuclei,} ^where ^recent êx- perimentalworkhastestedthesuitability ofdifferentshellmodel spaces. Examples include: ⁹⁴Pd [5], ⁹⁴Ag [9,13], ⁹⁶Ag [4], ⁹⁸Cd [3],wherethepg (p1/2,g9/2), f pg ( f5/2,p3/2,p1/2,g9/2),andgds (g9/2,g7/2,d5/2,d3/2, and s1/2) model spaces have been used to explaintheobservedisomers.

96Cd is the last even–even N =^{Z nucleus} ^before ¹⁰⁰^Sn. ^The most recent work on this nucleus identiﬁed the 16⁺ spin gap isomer,measuring thelifetimeandB(GT)strength [2,20].By per- formingshell modelcalculationsusing theGFinteraction [16], it wasdemonstratedthattheT=0 pn interactionplaysanimportant role in explaining the isomerism ofthis state. LSSM calculations presentedin the samestudy, which employed an effectiveinter- actionfor the gds modelspace, predictedthe existence of three high-lyingresonance-likestatesthatreside∼5 MeV abovethepro- tonseparationenergyinthedaughternucleus⁹⁶Ag[2].Thiswork suggestedthat∼^{30% of}^theβ decay strengthfromthe16⁺isomer shouldpopulatetheseresonance-likestatesandhencethe observation ofβ-delayedprotons might be expected. However, dueto limitedstatistics no protons(or γ rays) fromthe decayofthese resonance-likestatesweredetectedinthepreviousexperiment[2].

Additionally, thededucedB(GT) strengthforthe β decay of ⁹⁶Cd was found to be consistent with that calculated using both interactions.Identiﬁcationofa β-delayedprotonbranchalongwith detailed γ ray spectroscopy wouldyield valuable informationon thevalidity ofthe LSSM calculationsaswell asprovidingimpor- tantinformationonthewavefunctionofthe16⁺ isomer.

In this letter we report evidence for the existence of the β-delayedprotonbranchfromthedecayofthe16⁺isomerin⁹⁶Cd alongwiththeobservationthatthispredominantlypopulatesthe 25/2⁺and29/2⁺statesin⁹⁵Pd.LSSMcalculations,usingthegds modelspace,incombinationwithnewWKBestimatesforthepro-

Fig. 1. TheparticleIDforionstransmittedthroughBigRIPSandidentiﬁedasbeing implantedintheactivestopper(AS).

tonemissionprobability,revealtheimportanceofthecoreexcited orbitalsinthewavefunctionofthe16⁺ spin-gap isomer.Alsore- ported are improvedlifetime measurements forthe groundstate and16⁺isomerin⁹⁶Cd,aswellasthelifetimeofthegroundstate of⁹⁵Cd.

2. Experiment

Asecondarycocktailbeam,whichincludedthenuclideofinter- est⁹⁶Cd,wasproducedbytheRadioactiveIonBeamFactory(RIBF) attheRIKENNishinaCentre,byinﬂightfragmentationofaprimary beam of ¹²⁴Xe. The primary beam, at an energy of 345 A MeV, was incident on a ⁹Be target of areal density 740 mg cm⁻² and BigRIPS[21,22]wasemployedtoseparatetheresultingfragments.

Identificationoffragmentsinthesecondarybeamwasdoneevent- by-event;particletrackingcombinedwiththemagneticrigidityof BigRIPS isused toobtain time-of-flight measurementsanddeter- mine A/Q, whilst the energy loss in an ionisation chamber was used for Z identification (see Fig. 1). Ions of interest were implanted in theactive stopper(AS) SIMBA [6],located at the end ofthezerodegreespectrometer[22].

SIMBA was constructed from three double sided silicon strip detectors (DSSSD) arranged in a stack along the direction of the beam.Eachdetectorhad40stripsof1 mmpitchontheupstream sideand60stripsofthesamepitchonthedownstreamside,pro- viding a total of 7200 pixels. The three DSSSD’s in the AS were 1 mm thick, these DSSSD’s provided good β-proton discrimina- tion forthedecays ofinterest.A thinner DSSSD, readout witha XY resistor chain, located upstream of the AS was employed to trackthepositionofincomingheavyions,andusedinconjunction withthe DSSSDto determinethe implantation pixel.In addition, a β-particlecalorimeter was located downstream ofthe AS. This was composed ofa stackof 16silicondetectors, sandwichedbe- tween4singlesidedsiliconstrip detectors(SSSSD), 2arrangedto providexandystripsupstreamofthestackof16silicondetectors andthe other 2 located in thesame conﬁguration down stream.

TheSSSSDallowedβ particlestobetrackedthroughthecalorime- ter,inadditiontheyalsoactedasavetoonheavyionswhichwere not stopped by the AS. Surrounding the AS was the EUROBALL- RIKEN Cluster Array (EURICA), which contained 84 large volume HPGedetectors[23,24].

All events were time stamped andin the offline analysis decays were correlated,in time and position,with eitherthe most recentimplantinthesamepixelandwithinafixedperiodoftime orall implants thatoccurredwithin theprevious 60 s, chosen to coverthedaughterandgranddaughterdecays.Thefirstcorrelation procedureproduces cleanβ andβp delayed γ ray spectraatthe

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Fig. 2. (a)γ-raysemittedinpromptcoincidence(0–200 ns)followingthedetection ofβparticlesand(inset)γ-raysemittedbetween0.2and4 μsafterdetectionof theβparticle.Theγ raymarkedwitha*773 keVisthe4⁺→2⁺in⁹²Mo andis attributedtocorrelationswiththetimerandombackground,the2⁺→0⁺(transi- tionnotlabelled) isalsopresentat1510 keV, the**1415 keVisthedecayfrom the 2⁺→⁰⁺ ⁱⁿ^the granddaughter⁹⁶Pd. (b)γ-raysemittedinprompt coinci- dencefollowingthe detectionofaβ delayedproton (insetsleftandrightshow the821 keV and1375 keV transitions,respectively).Inbothcasesthedecayevents areuniquelycorrelatedtotheprevious⁹⁶Cdionimplantedinthesamepixel,using themethoddiscussedinsection2,andusingaβcorrelationtimeof1.35 s,which isthreehalflivesofthe16⁺isomerdeducedinthecurrentwork.

expenseofcorrelationefficiency.Inthelattercorrelationprocedure theefficiencyisimprovedandallows fora goodmeasurement of the time random background, the background introduced by the daughter and granddaughter decays can be accounted for using valuesforthehalf-livesalreadypublished.Thegeometricefficiency forthedetectionofβ particlesandprotonswasdeterminedusing aGeant4simulation[25].Thesewerefoundtobe∼³⁰(5)% inthe caseofβ-ioncorrelationsand∼⁹²(10)% forβp-ioncorrelations.

3. Results

Intotal 17,000ions of⁹⁶Cd were transmittedthrough BigRIPS andimplantedintotheASofthedecaystation.Anenergythresh- oldof2 MeV wasusedtodifferentiatebetweenβ andβp decays, whilst a lower energythreshold of 150 keVwas used for β decays. The 2 MeV threshold represents a suitable lower limit for summed βp eventsandwas determined using theGeant4simu- lationdiscussedinsection 2 [25].Fig. 2(a)showsbothprompt-γ

anddelayed-γ spectrafollowing⁹⁶Cdion-β detection.Inthemain partofFig. 2(a)prompt γ rayeventsbetween0and200 ns after a β-decay eventin theAS are presented, whilst theinset shows

γ rays emitted between 0.2 and4 μs after the β decay. The γ

rays observed in the inset spectrum at 470, 630,667, 1249 and 1506 keVarefromtheknowndecayofthe15⁺isomerin⁹⁶Ag[2, 4],andthe421 keVγ rayinFig. 2(a)hasbeenpreviouslyassigned asbeingduetothedecayofalow-lying1⁺statein⁹⁶Ag[2].

Fig. 2(b)shows γ raysemittedbetween0and200 nsafterthe identiﬁcationof β delayed proton eventsin the AS that are cor- relatedwithan implanted⁹⁶Cd ion.The γ raysobserved at130, 691,821,and1375 keVareknowntoformadecaysequencewhich populatesthe21/2⁺ isomerin⁹⁵Pd[11].A further γ-ray canbe seen inFig. 2(b)at 681 keV, γ rays ofthisenergy exist inboth 95Rh [20] and⁹⁵Pd [11,12], whichare the βp daughters of ⁹⁶Ag and ⁹⁶Cd, respectively. The time distribution of this γ ray, and an estimateoftheintensityexpectedfortheknown βp decay of 96Ag within the β correlation window (1.35 s) used to construct

Table 1

Observed,eﬃciency corrected,γ rayenergies and in- tensitiesfortransitionsin⁹⁵Pdfollowingtheβp decay of⁹⁶Cd.Theintensitiesarenormalisedtothe130 keV transitionintensity,whichhasbeen setat 100%(elec- tronconversioncoeﬃcienthasnotbeenincludedinthe calculationoftheseintensities).

Eγ Ii→^If Iγ

691 ²³₂⁺→²¹2

+ 91(28)

130 ²⁵₂⁺→²³2

+ 100(19)

821 ²⁵₂⁺→²¹2

+ 36(19)

1375 ²⁹₂⁺→²⁵2

+ 81(36)

Fig. 3. Proposeddecayschemefortheβdecay andβp decay ofthe16⁺isomerin 96Cd,thetransitionsmarkedwithdashedlineswerenotobservedbutarewithin theavailableangularmomentumoftheprotondecay.Thecalculatedbranchingra- tiostotheGTresonancestatesin⁹⁶Ag,extractedfromLSSMcalculations,arealso presented.Thepartialdecayschemeof⁹⁵Pdwastakenfrom[11].

the spectrum, indicate that all the events forthis transitioncor- respond to theβp delayed γ ray from2⁺ state in ⁹⁶Ag [20,26].

These events are presentin the spectrum because of the low β correlation eﬃciency,whichresults inareasonable probability of missing theﬁrst β eventand detecting asubsequent eventfrom thedaughter.

The intensities of the γ rays identiﬁed as being in ⁹⁵Pd are presentedinTable 1andadecayschemeshowingthestatespop- ulatedinthecurrentworkispresentedinFig. 3.Alsoincludedin this ﬁgure are the 27/2⁺, 31/2⁺ and 33/2⁺ states in ⁹⁵Pd, the non-observation of γ-rays fromthesestateseliminates some po- tentialdecaypathways.

A time distribution of events between the implantation of a ⁹⁶Cd ion and its decay (dT) was obtained for the 16⁺ spin gap isomer by gating on the β and βp delayed γ rays. The Schmidt methodwas then employed to ﬁt the time distribution, see Fig. 4(a),this resultispresented inTable 2whereit iscom- pared to the previously measured value. The implantation rate precluded performing the same analysis for the ground state. In thiscasethehalf-lifewasextractedbyallowingittobeafreepa- rameterinthe ﬁtsofthe β-decaytime distributions discussedin the next paragraph andpresented in Fig. 4(d). Alsopresented in