Single-neutron orbits near $^{78}$Ni: Spectroscopy of the N=49 isotope $^{79}$Zn

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Single-neutron orbits near

Ni: Spectroscopy of the

N=49 isotope

79

_Zn

R. Orlandi, D. Mücher, R. Raabe, A. Jungclaus, S.D. Pain, V. Bildstein, R.

Chapman, G. De Angelis, J.G. Johansen, P. Van Duppen, et al.

To cite this version:

R. Orlandi, D. Mücher, R. Raabe, A. Jungclaus, S.D. Pain, et al..

Single-neutron orbits near

78

_{Ni: Spectroscopy of the N=49 isotope}

79

_{Zn. Physics Letters B, Elsevier, 2014, 740, pp.298-302.}

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Contents lists available atScienceDirect

Physics

Letters

B

www.elsevier.com/locate/physletb

Single-neutron

orbits

near

78 Ni:

Spectroscopy

of

the

N

=

49 isotope

79 Zn

R. Orlandi

a

,

b

,

c

,

d

,

e

,

∗

,

D. Mücher

f

,

R. Raabe

b

,

A. Jungclaus

a

,

S.D. Pain

g

,

V. Bildstein

f

,

R. Chapman

c

,

d

,

G. de Angelis

h

,

J.G. Johansen

i

,

P. Van Duppen

b

,

A.N. Andreyev

c

,

d

,

j

,

e

,

S. Bottoni

b

,

k

,

T.E. Cocolios

m

,

H. De Witte

b

,

J. Diriken

b

,

J. Elseviers

b

,

F. Flavigny

b

,

L.P. Gaffney

n

,

b

,

R. Gernhäuser

f

,

A. Gottardo

h

,

M. Huyse

b

,

A. Illana

a

,

J. Konki

l

,

o

,

p

,

T. Kröll

q

,

R. Krücken

f

,

J.F.W. Lane

c

,

d

,

V. Liberati

c

,

d

,

B. Marsh

r

,

K. Nowak

f

,

F. Nowacki

s

,

J. Pakarinen

l

,

o

,

p

,

E. Rapisarda

b

,

F. Recchia

t

,

P. Reiter

u

,

T. Roger

b

,

v

,

E. Sahin

h

,

M. Seidlitz

u

,

K. Sieja

s

,

J.F. Smith

c

,

d

,

J.J. Valiente Dobón

h

,

M. von

Schmid

q

,

D. Voulot

r

,

N. Warr

u

,

F.K. Wenander

r

,

K. Wimmer

f

a_Instituto_de_Estructura_de_la_Materia,_IEM-CSIC,_Madrid,_E-28006,_Spain b_KU_Leuven,_Instituut_voor_{Kern- en}_{Stralingsfysica,}_B-3001_Heverlee,_Belgium

c_School_of_Engineering,_University_of_the_West_of_Scotland,_Paisley,_PA1_2BE,_United_Kingdom d_Scottish_Universities_Physics_Alliance_(SUPA),_United_Kingdom

e_Advanced_Science_Research_Center,_Japan_Atomic_Energy_Agency,_Tokai,_{Ibaraki, 319-1195,}_Japan f_Physik_Department_E12,_Technische_Universität_München,_{D-85748 Garching,}_Germany g_Physics_Division,_Oak_Ridge_National_Laboratory,_{Oak Ridge,}_{TN 37831,}_USA

h_Istituto_Nazionale_di_Fisica_Nucleare,_Laboratori_Nazionali_di_Legnaro,_Legnaro,_I-35020,_Italy i_Department_of_Physics_and_Astronomy,_Aarhus_University,_DK-8000_Aarhus,_Denmark j_Department_of_Physics,_University_of_York,_Heslington,_{YO10 5DD,}_United_Kingdom k_Dipartimento_di_Fisica,_Università_di_Milano_and_INFN_Sezione_di_Milano,_I-20133,_Italy l_PH_Department,_{CERN 1211,}_{Geneva 23,}_Switzerland

m_School_of_Physics_and_Astronomy,_University_of_Manchester,_Manchester,_{M13 9PL,}_United_Kingdom n_Oliver_Lodge_Laboratory,_University_of_Liverpool,_Liverpool_{L69 9ZE,}_United_Kingdom

o_Helsinki_Institute_of_Physics,_University_of_Helsinki,_{P.O. Box 64,}_{FIN-00014 Helsinki,}_Finland p_Department_of_Physics,_University_of_Jyväskylä,_{P.O. Box 35,}_{FIN-40014 Jyväskylä,}_Finland q_Institut_für_Kernphysik,_Technische_Universität_Darmstadt,_D-64289_Darmstadt,_Germany r_AB_Department,_{CERN 1211,}_{Geneva 23,}_Switzerland

s_IPHC,_CNRS/IN2P3,_Université_de_Strasbourg,_F-67037_Strasbourg,_France t_Università_degli_studi_di_Padova_and_INFN_Sezione_di_Padova,_{Padova I-35131,}_Italy u_Institut_für_Kernphysik,_Universität_zu_Köln,_D-50937_Köln,_Germany

v_GANIL,_{CEA/DSM-CNRS/IN2P3,}_F-14076_Caen,_France

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received5October2014

Receivedinrevisedform18November2014 Accepted3December2014

Availableonline9December2014 Editor: D.F.Geesaman Keywords: Nuclearstructure γ-Raytransitions Transferreactions N=50 shellclosure

Single-neutronstatesinthe Z=30,N=49 isotope79_Zn_have_been_populated_using_the78_{Zn(d, p)}79_Zn transfer reaction at REX-ISOLDE, CERN. The experimental setup allowed the combined detection of protons ejected inthe reaction, and of

γ

rays emitted by79_Zn._The _analysis _reveals _that_the _lowest excitedstatespopulatedinthereactionlie atapproximately1 MeVofexcitation,and involveneutron orbitsabovetheN₌50 shellgap.Fromtheanalysisof

γ

-raydataandofprotonangulardistributions, characteristicoftheamountofangularmomentumtransferred,a 5/2+ conﬁgurationwasassignedtoa stateat983 keV.Comparisonwithlarge-scale-shell-modelcalculationssupportsarobustneutronN=50 shell-closurefor78Ni.Thesedataconstituteanimportantsteptowardstheunderstandingofthemagicity of78_Ni_and_of_the_structure_of_nuclei_in_the_region.

*

Correspondingauthorat:AdvancedScienceResearchCenter,JapanAtomicEnergyAgency,Tokai,Ibaraki,319-1195,Japan.

E-mailaddress:orlandi.riccardo@jaea.go.jp(R. Orlandi).

http://dx.doi.org/10.1016/j.physletb.2014.12.006

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Shell structure characterizes several many-body systems of fermions moving in a common potential, such as atomic elec-trons,metal clustersandnuclei.Angular momentumquantization inducesabunchingofthesingle-particlestates,resultinginshells separatedbyenergygaps.Inthenuclearmedium,suchshellgaps are revealed by nuclei withneutron andproton numbers corre-sponding to closed-shell conﬁgurations. The properties of these so-calledmagic nucleiandof their neighbors,which were cardi-naltothedevelopment ofthenuclearshellmodel,couldonly be reproducedwhentherole played bythenuclear spin–orbit inter-actionwasrecognized[1].

Inrecent years,experiments with radioactiveion beamshave shownthatinsomeneutron-richnucleiwell-establishedshell clo-surescan vanish,andnewmagicnumbersappear[2,3].The chal-lengetoexplainandpredictthesizeofshellgapsawayfrombeta stabilityhas ledtoconsiderable progressinnuclearphysics, both experimentally and theoretically. Despite some remarkable steps forwardindescribingtheevolution ofshellstructure,e.g. the in-clusion of the tensor interaction [4] and three-body forces [5,6], rare-isotope data are still essential to test and guide theoretical advances.Nucleiawayfromthevalleyofbetastabilitywithmagic numbers of neutrons and protons, and isotopes in their vicinity, havebecomenewcornerstones forthedevelopment ofa reliable theoreticalpictureofallnuclei.

Theregionofisotopes near78Niisthefocusofintense exper-imentalandtheoreticalresearch (cf., forexample,[7–12] and ref-erences therein).Whether78_Ni_can_be _considered_a_doubly-magic

sphericalnucleusdependsultimatelyonthesizeoftheZ

=

28 and N

=

50 shellgaps.Todate,however,scarceinformationisavailable on78_Ni _and_on _its _immediate_neighbors,_and _contrasting

predic-tionshavebeenmade[12,13]aboutitsmagicity.

Thepropertiesofnucleilyingcloseto78Nialsoimpactstrongly on astrophysical models ofstellar nucleosynthesis andevolution. A recentexampleisrelatedtothemeasurementofthe82Zn bind-ing energy and its implications on the composition of neutron-star crust [14]. Furthermore, a sensitivity studyon the effect of neutron-captureratesontheA

∼

80 andA

∼

130 r-processpeaks

[15] revealed that 78Zn and 79Zn are among the few isotopes which can causethe largest change (

>

15%) in theoverall abun-dance pattern, affecting the abundances of masses as high as A

∼

195.

Single-nucleontransferreactionsareaverysensitivetechnique topopulatesingle-particlestatesandtoinvestigatethestructureof theisotopesproduced[16–19].Performingsuchreactionson78Ni willrevealtheenergiesofthesingle-particleorbitsgoverningthe properties of 78_Ni _and _its _neighbors. _The _necessary _experiments

however still require years of developments in radioactive-ion-beam production. Revealing insights about the structure of 78Ni cannonethelessbegainedbystudyingclose-lyingisotopes. More-over,anaccurate descriptionoftheevolutionofnuclearstructure acrossneighboringnucleiisanimplicittestfortheoretical predic-tionsofthepropertiesof78_Ni.

In this Letter, the ﬁrst spectroscopic study of the Z

=

30, N

=

49 isotope79Znispresented.Inthisnucleus,neutronscan oc-cupyorbitswhichliebothbelowandabovethe N

=

50 shellgap. Priortothiswork, theavailable informationabout79_Zn_was

lim-itedtoitsground-statehalflife,0.995(19) s[20].Thebetadecayof

79_Zn_[8]_supports _a _{J π}

₌

₉

_/

₂+ _ground-state_{conﬁguration,}_in_line

withthe shell-modelexpectationthat theoddneutron (hole) oc-cupiestheg9/2 orbit,andwithN

=

49 systematics.Inthepresent

work,the9

/

2+assignmentforthegroundstatehasbeenadopted. Inthiswork,excited statesin79_Zn_have_been_populated_using

the 78Zn(d, p)79Zn reaction in inverse kinematics at REX-ISOLDE, CERN (Q value

=

1

.

796 MeV [21]). 78Zn (T1/2

=

1

.

47

(

15

)

s) was

producedincollisionsof1.4 GeVprotonsfromtheCERNPSBooster

Fig. 1. (Coloronline.) a)79_Zn_excitation_energy_deduced_from_proton _kinematics

forallthetransferprotons(blacksolidline)andfromtheprotonsincoincidence withanyγray(green,solidﬁll).b)79_Zn_excitation_energy_in_coincidence_with_the

983-keVγ ray,correctedfor γ-rayeﬃciency.c) Sameasb),butincoincidence eitherwiththe236-keV(blue)orthe1859-keVγ transitions(red).

witha UCx target. 78Znatomswere laser ionizedusingthe RILIS

set up [22], mass separated, and post-accelerated by the REX-LINAC to 2.83 MeV per nucleon. The 78Zn beam impinged on a thin(105

(

10

)

μg

/

cm2₎_deuterated_polyethylene_(DPE)_target._In

ad-dition to 78Zn, which made up

∼

75% of the total intensity, the beamalsocontained78_Rb₍

_∼

_20%)_and78_Ga₍

_∼

_5%)._Exploiting _the

fact thatwithoutlaser ionizationonly 78Zndisappeared fromthe beam cocktail, the contribution from the contaminants could be identiﬁed andsubtractedoﬄine by collecting datawiththelaser periodicallyturned on andoff(in total, approximately100hours withand35hours withoutlaser ionization).From theanalysisof elastically scattereddeuterons, the estimatedaverage 78Zn beam intensitywas7

.

8

(

7

)

·

105 _particles_per_second._Additional_data

(ap-proximately20hours),collectedusingathick(

∼

1

.

7 mg

/

cm2)DPE target, permitted to conﬁrm weak coincidences observed in the thin-targetdata.

ThereactionwasstudiedusingthesegmentedT-REXarrayofSi telescopes[23],andeighttriple-clusterHPGedetectorsofMiniball

[24],which surrounded the T-REX scatteringchamber. The coin-cident detection oflight chargedparticles and

γ

rays led to the identiﬁcationofstateswhichcouldnotberesolvedusingonlythe proton data. Furthermore, the charged-particle data constrained the placement of states in the level scheme which would have beenambiguousfromthe

γ

-raydataalone.

InFig. 1(a),the79_Zn_excitation_energy_deduced_from_reaction

kinematicsisshownforprotonsinglesandprotonsincoincidence with all detected

γ

rays. Three main peaks can be seen, cen-teredrespectively around1.2, 2.5and3.3 MeV. Dueto kinematic compressionandtothedetectionthreshold,onlytransferprotons corresponding to the lowest-energy peak could be detected both at forward and backward angles, and meaningfully compared to DWBAcalculations.Atthisbeamenergy,thetransferredneutrons shouldpopulatemainlystatesorgroupsofstatescorrespondingto low-

orbits,mostlyabovethe N

=

50 gap,namelyd5/2,s1/2 and

d3/2.The g7/2 orbitabove the gap (

=

4) andthe neutron–hole

states based on p or f conﬁguration are likely to be populated veryweakly. Asanillustration, the79_Zn_case_can_be_compared_to

thestudyofthe N

=

81 isotone131Snvia the130Sn(d, p)131Sn re-action [17], inwhich only neutron orbits above the N

=

82 gap werepopulated.

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Fig. 2. a)Doppler-corrected79_Zn_γ_-ray_spectrum,_gated_by_all_transfer_protons._The

strongestpeaksarelabeledbytheirenergy.Thepeaklabeled“c”isduetoasmall amountof78_Ga_in_the _beam,_due_to_in-ﬂight 78_Zn

β decay. b) Samespectrum gatedon79_Zn_excitation _energy_in_the _{0.8–1.7 MeV}_range,_where_only_the

441-and983-keVtransitionscanbeseen.Seetextfordetails.

Fig. 3. Coincidenceγ-rayspectragated,respectively, onthe 983-keV (a)or the 1859-keV (b)transitions,obtainedusingthick-targetdata.

discussed below, the analysis of proton angulardistributions are indeedconsistentwithtransferofneutronstothed5/2ands1/2

or-bits.Thegreencurveinthesameﬁgureshowsthat,incoincidence with

γ

-rays,theintensityofthefirstpeak decreasessignificantly more than that of the two higher-lying peaks. This observation suggeststhe presence, in theexcitation-energy range of thefirst peak, ofone ormorestateswhich aretoo longlived(more than fewnanoseconds)fortheirdecay(s)tobedetectedinflight.

The

γ

rays emitted by 79Zn were identiﬁed by requiring a coincidence with the protons ejected in the neutron-transfer re-actions.TheDoppler-corrected

γ

-rayenergyspectrumisshownin

Fig. 2(a).Fig. 1(b) and (c) areexamplesofexcitation-energy spec-tra of79Zngated byindividual

γ

rays. Similarly,Fig. 2 (b) shows thatonlythe441- and983-keVlinesarestillclearlyvisibleifthe

79_Zn_excitation_energy_is_restricted_to_the_lowest_peak_of_{Fig. 1}_(a)

(from 0.8 to 1.7 MeV). Such spectra, together with observed

γ

coincidences, such as those presented in Fig. 3, proved essen-tial to place the main transitions in the level scheme. No other

γ

ray was observed below 0.8-MeV excitation energy. It should alsobenotedthatthe

γ

-raydetectioneﬃciencydropsdrastically below120 keV.Fig. 3 showsthat boththe 441–983-keVandthe 236–1859-keVpairswereeachfoundtobecoincident.

Thepartial levelscheme of79Zn deducedinthiswork is pre-sentedinFig. 4.Accordingtomeasuredintensitiesandcoincidence relations, the 983-keV state decays directly to the ground state, andit isfed by the 441-keV transition. Since the 983-keV tran-sition is prompt, despite a low energy tail,its multipolarity can only be dipole or quadrupole. If the tail is due to the lifetime of the983-keV state, andnot to unobserved feeding, it suggests a lifetime of the order of few hundred picoseconds, with mul-tipolarity E2 or M2. A slow E2 (0

.

05

<

B(E2)

<

0

.

2 W.u.) seems muchmorelikely.AnM2characterwouldinfactrequirethis low-lyingandstronglypopulatedstatetobea5

/

2− state,

correspond-Fig. 4. Partiallevelschemeof79_Zn_deduced_in_this_work._The_width_of_the_arrows_is

proportionaltotherelativetransitionintensities.Withtheexceptionoftheisomeric state‘X’at1.10(15) MeV,allenergiesareinkeV,withtheerrorgiveninbrackets. Seetextfordetails.

ing to aneutron–hole inthe f5/2 orbit. Interestingly,comparable

B

(

E2

;

5

/

2+

→

9

/

2+

)

strengths have been measured in neighbor-ing N

=

49 isotones 81_Ge _and 83_Se _{(respectively, 0.0383(20)}_and

≈

0

.

13 W.u.[25]).

The direct and indirect feeding to the 983-keV state, which makes its ground-state decay the most intense transition in the spectrum, supports a 5

/

2+ assignment. The DWBA analysis dis-cussed below conﬁrms the 5

/

2+ assignment deduced from the

γ

-rayanalysis.

Inthecaseofthe1424-keVstate, thepromptcharacterofthe 441-keVtransition,thecoincidencebetweenthe983- and441-keV

γ

raysandtheunobservedcrossoverground-statetransitionfavor aspin3

/

2+or5

/

2+.Fewadditionalweaktransitions,of888,1774 and2321 keV,weretentatively placedin thelevel scheme:their

γ

-ray-gatedexcitationenergyspectrainfactexhibitapeak respec-tively at 2.35(15), 3.30(15) and3.45(15) MeV, compatiblewith a direct feeding toeither the 1424- orthe 983-keV states. Forthe 888-keV transition, this placement is also supported by the ob-servedcoincidencewiththe441-keVtransition.

Theanalysisofthe236- and1859-keVsingle-

γ

-ray-gated exci-tationenergyspectra,showninFig. 1(c),iskeytopositioningthe 236-keV transitioninthelevelscheme.A peakat

∼

3

.

20

(

15

)

MeV canbe seeninbothspectra(blueandredintheﬁgure),butonly the 236-keV gate showsalso apeak at 2.65(15) MeV,where the large uncertainty is due to the Si-detector resolution (the thin red peak appearing at2.6 MeV in the 1859-keV gated spectrum is notstatisticallysigniﬁcant). Theirobservedcoincidence implies that the 236-keV lies below the 1859-keV

γ

ray (otherwise no peak could appear also at 2.65 MeV excitation energy). Hence, the 236-keV transitionmustbe the decayofa state withenergy at leastaslow asthe differencebetween the3.2 MeV excitation energy and the 1859-keV

γ

ray, i.e.1.34(15) MeV. In Fig. 1 (c), however,nopeakappearsaroundorbelow1.3 MeV,whichmeans thatthestateat

∼

1

.

34 MeV isnot(ortooweakly)populatedinthe direct reactionandonlyfed fromhigher-lyingstates.The prompt character of the 236-keV transitionimpliesthat it has E1or M1 multipolarity.Thepositionofthe1185-keVtransitionwasdeduced from its

γ

-gated excitation energyand thecoincidence with the 236-keV

γ

ray.

From the unobserved coincidence withthe983-keV transition it follows that the 236-keV

γ

ray feeds an isomeric state lying approximatelyat1.10(15) MeV.Protonangulardistributions,which are characteristicoftheangular momentumtransfer,

,provide a strongargumenttoidentifythis1.10(15)-keVstatewitha1

/

2+ state.

The measured proton angular distributions were compared to DWBA calculations performed with the codes FRESCO [26]

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Fig. 5. (Coloronline.)a) Protonangulardistributionsforthe79_Zn_excitation_energy

range0.85–1.55 MeV andscaledDWBA calculationsfor =0,2 andasumof thetwo.b) Sameprotondata,gatedalsoonthe983-keVγ-rayline,andDWBA calculationsfor=0 or 2.

Refs.[28,29].Fortheneutronbound-statepotential,theradiusand diffusenessparameterswere1.25and0.65 fm,respectively. Exper-imentalspectroscopicfactors(SF) weredeterminedfromtheratio ofexperimental andDWBAcrosssections,whichwere calculated forSF

=

1.Thelowbeamenergyleadstoappreciabledifferencesin the DWBAdistributions calculated using different optical-model-potentialparameterizations.Additionalﬁtswerethereforealso per-formedusingdifferentparametersfromRefs.[30–33],andvarying thebound-state radiusparameter from 1.20 to 1.30. The system-aticuncertaintiesamounttoapproximately20–25%variationinthe calculatedSFs.Itshouldberemarkedhoweverthatthearguments basedon the analysis ofangular distributions do not depend on thechosenparameterization.

IllustrativeangulardistributionsareshowninFig. 5(a) and (b). Inboth cases,theexcitation energyof79Zn wasrestricted tothe lowest peak of Fig. 1 (a). The data in Fig. 5 (a) were not gated onany

γ

-ray line, andare poorly ﬁtted by anysingle

trans-fer. These data are instead well described by a sum of

=

0 and

=

2 distributions. The simultaneous ﬁt of two distribu-tionsyieldsasigniﬁcantlysmallerreduced

χ

2 _(2.1)_than_obtained

byﬁtting asingle

(

χ

2

₌

₁₇

_.

_{8 and} _18,_{respectively).} _The

scal-ing factors were left as free parameters in the ﬁt and found to be,respectively,0.41(3)(10)and0.51(4)(12)(wheretheﬁrsterror isstatistical,andthesecondsystematic).If,inadditionto

=

0 and 2,alsoathird,

=

1 distributionisaddedtothe simultane-ousﬁt,thelatterhasa scalingfactorcompatiblewithzerowhile the formerare substantially unchanged, indicating that mostly d ands orbitswerepopulatedinthisenergyrange.

Theangulardistributionchangesinsteadconsiderablyby requir-ingthe coincidentdetectionofthe983-keV

γ

ray. Thisisshown inFig. 5 (b),together withDWBAcalculationsfortransfer to, re-spectively,pure

=

0 or

=

2 states. A pure

=

2 distribution (transfer to a d5/2 or d3/2 neutron state) yields a better ﬁt and

conﬁrmsthe5

/

2+assignmentofthe983-keV statededucedfrom

γ

-raydata. J π

=

3

/

2+caninfactbeexcludedsinceitwouldlead toanisomericM3ground-statetransition.

Asimilardistributionisobservedwhengatingthesame excita-tionenergyregionbythe441-keVtransition:thedata,notshown here, are best ﬁtted by

=

2, and yield SF

=

0

.

05

(

1

)(

2

)

. The comparisonbetween Fig. 1 (b) and the intensity strength of the 441-keV transitionsuggeststhat the983-keV state isalso fed by additionallow-energyundetected

γ

-ray transition(s)fromoneor morestatesbetween1.0 and1.4 MeV. Themeasuredangular dis-tributions imply however that these states have

=

2 character

Fig. 6. (Coloronline.)ExperimentalandcalculatedN=50 gapsizes(filledandopen diamonds)andfirst-excited5/2+-stateenergies(filledandopencircles)forN=49 isotones. Themeasurementfromthiswork ishighlighted inblue. Thegapsizes weredeterminedfromtwo-neutronseparationenergiescalculatedfrommass ex-cessesfrom[21](and[14]for82_Zn).

andfeedthe983-keVstate.Insummary,approximately75%ofthe measured

=

2 strengthinthisenergyrangeresultsinthedirect orindirectfeedingofthestateat983 keV,againconsistentwitha 5

/

2+assignment.

The

=

0 strengthrevealedbytheprotondata(notgatedby any

γ

ray)ofFig. 5(a) testiﬁesthepresenceofan isomeric1

/

2+ state near1 MeV,withSF

=

0

.

41

(

3

)(

10

)

.The proton-singlesdata were also split in three smaller ranges in excitation energy and comparedtoDWBAcalculationssimilar tothoseinFig. 5(a).The weightedaverageofthe

=

0 strengthsmeasuredinthesub-sets indicates that the1

/

2+ statelies atapproximately1.05(15) MeV. It seemslikely thatthis s1/2 neutronstate corresponds infact to

the1.10(15)-MeV statededuced fromgamma-ray gatedexcitation energyspectra(labeledXinFig. 4).Ifthestronglypopulated1

/

2+ statecoincides indeedwiththelevelat1.1 MeV,thentheprompt observation ofthe 236-keV

γ

-ray limitsthe possible spin of the 1.34(15)-MeVstateto

(

1

/

2

)

or

(

3

/

2

)

(X

+

236inFig. 4).Thelarge amountoffeedingfromhigherlyingstatesfavorsapositiveparity, butanegativeparitycannotbeexcluded.

It is instructive to compare the 5

/

2+-state energy, 983 keV, withstate-of-the-artshell-modelcalculations[12].Thelarge shell-model spaceincludes the proton orbits p1/2, p3/2, f72 and f5/2,

andthe neutron orbits p1/2, p3/2, f5/2, g92 andd5/2, which lies

above the N

=

50 shellgap.Details aboutthe interactioncan be found in Refs. [12,34]. As it can be seen in Fig. 6, these calcu-lations reproduce very well the measured energies of thelowest lying5

/

2+-state N

=

49 isotones.For79Zntheypredictanenergy of1029 keV,strikinglyclosetothemeasured983 keV.Thislowest calculated 5

/

2+ state (SF

=

0

.

53) is formed by the promotion of one neutronfromthe g9/2 to thed5/2 orbit.The calculatedB(E2)

transitionstrengthtothegroundstateisonly4 e2_fm4 _{(0.2 W.u.).}

The equivalent lifetime, 240 ps, is compatiblewith the observed tailofthe983-keVtransition.

Fig. 6 shows that the calculations also reproduce reasonably welltheevolutionoftheN

=

50 gapdeducedfrom2-neutron sep-aration energies[14,21],witha minimumat Z

=

32 anda larger gapinzincthaningermanium,althoughtheexperimentalgapsize inthesetwoisotones is450 keVsmaller. Thecalculationspredict adoubly-magic78_Ni_with_shell_gaps_of_{4.7 MeV}_for_neutrons_and

5.0 MeVforprotons,andaﬁrst2+-statelyingatnearly4 MeV.If atleasttherelativeincreaseingapsizebetweenzincandnickelis correct, the N

=

50 gapin78Ni canbe expectedto beatleastas largeas4.2 MeV.

In conclusion, this spectroscopic study of 79_Zn _has _permitted

(6)

basedontheoccupancyofsingle-particlestatesabovetheN

=

50 shell gap.The agreement betweenthe currentmeasurement and recentlarge-scaleshell-modelcalculationssupportsthepictureof arobust N

=

50 shellclosurefor78_Ni._This_newly_acquired

knowl-edgeaboutneutronsingle-particle statesin 79_Zn_will_also_be

im-portanttoconstrainingneutron-capturerateson78_Zn,_which_have

beenshowntoimpacttheﬁnal r-process abundancepattern dur-ingfreeze-outperiods.

Acknowledgements

ThisworkwassupportedbytheEuropeanCommissionthrough theMarie Curie ActionsContracts Nos.PIEFGA-2011-30096 (R.O.) and PIEFGA-2008-219175 (J.P.), by the Spanish Ministerio de Ciencia e Innovación under contracts FPA2009-13377-C02 and FPA2011-29854-C04, by the Spanish MEC Consolider – Ingenio 2010, Project No. CDS2007-00042 (CPAN), by FWO-Vlaanderen (Belgium), by GOA/2010/010 (BOF KU Leuven), by the Interuni-versity Attraction Poles Programme initiated by the Belgian Sci-ence Policy Oﬃce (BriX network P7/12), by the European Union Seventh Framework Programme through ENSAR, contract no. RII3-CT-2010-262010, andby the German BMBF under contracts 05P09PKCI5,05P12PKFNE, 05P12RDCIA and06DA9036I.R.O., R.C., J.F.W.L.,V.L.andJ.F.S.alsoacknowledge supportfromSTFC,Grant Nos. PP/F000944/1, ST/F007590/1, and ST/J000183/2. The authors thankA.M. Moroforvaluablediscussions.

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