The 8q isomer in 78Zn and the doubly magic character of 78Ni

16 March 2000Ž .Physics Letters B 476 2000 213–218The 8 q isomer in 78 Zn and the doubly magic character of 78 NiJ.M. Daugas a , R. Grzywacz b,c , M. Lewitowicz a , L. Achouri d , J.C. Angelique d ,D. Baiborodin a,e , K. Bennaceur a , R. Bentida f , R. Beraud ´f , C. Borcea g ,C. Bingham b , W.N. Catford h , A. Emsallem f , G. de France a , H. Grawe i ,K.L. Jones h , R.C. Lemmon h , M.J. Lopez Jimenez a , F. Nowacki j ,F. de Oliveira Santos a , M. Pfutzner ¨c , P.H. Regan h , K. Rykaczewski k,c ,J.E. Sauvestre l , M. Sawicka c , G. Sletten m , M. Stanoiu a,ga GANIL BP 5027, 14076 Caen Cedex 5, Franceb UniÕersity of Tennessee, KnoxÕille, TN 37996, USAc IFD, Warsaw UniÕersity, Hoza ˙ 69, PL-00681 Warsaw, Polandd LPC Caen, 14021 Caen Cedex, Francee Nuclear Physics Institute, 250 68 Rez, Czech Republicf IPN Lyon, 69622 Villeurbane Cedex, Franceg IAP, P.O. Box MG6, Bucharest-Magurele, Romaniah Department of Physics, UniÕersity of Surrey, Guildford, GU2 5XH, UKiGSI, Postfach 110552, D-64220, Darmstadt, GermanyjLPT, UniÕersite´Louis Pasteur, 67084 Strasbourg Cedex, Francek ORNL, Physics DiÕision, Oak Ridge, TN 37830, USAlCEA Bruyeres ` le Chatel, ˆ BP 12, 91680 Bruyeres ` le Chatel, ˆ Francem Niels Bohr Institute, UniÕersity of Copenhagen, Copenhagen, DenmarkReceived 15 September 1999; received in revised form 1 February 2000; accepted 1 February 2000Editor: J.P. Schiffer´AbstractA new isomeric state in 78 Zn has been identified and studied in the fragmentation of a 60.5 AMeV 86 Kr beam on a nat Nitarget. The measured energies of the transitions and the half-life, T s319Ž.1r2 9 ns, of the isomeric decay suggest a spin andp qŽqparity assignment I s8 . The deduced B E2,8 6 q. value equals 1.21Ž 5.W.u. is well reproduced in large scale shellmodel calculations using realistic interactions in the proton-neutron 2p 3r 2, 1f 5r 2, 2p 1r 2, 1g9r 2 model space. Standard E2polarisation charges of 0.5e for both protons and neutrons were used. This result consists the first experimental evidence ofthe persistance of the Ns50 shell gap in the vicinity of 78 Ni . q 2000 Elsevier Science B.V. All rights reserved.28 50Nuclei in the vicinity of the recently observed78doubly magic nucleus Ni w1,2x28 50 are amongst thebest candidates to study the evolution of nuclearstructure far from the valley of stability. The largeneutrons to protons ratio in this region might allowthe observation of unusual shell effects. In particular,the question of whether the Ns50 shell gap persistsso far away from stability might be studied in theŽ .78two-hole particle structure neighbours of Ni,namely 76 Ni, 80 Zn orrand 78 Zn. As proven in ourwx78recent studies 3 we can progress towards Ni usingisomer spectroscopy following a projectile fragmen-0370-2693r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0370-2693 00 00177-5

214( )J.M. Daugas et al.rPhysics Letters B 476 2000 213–218tation reaction. This method provides information onthe energy levels and lifetimes of excited states. Thevalues of transition strengths obtained for the highspin isomers with simple and pure configurations Žasobserved near shell closures.are essential to extractthe nucleon effective charge. In a large scale shellmodel calculation the effective charge is a measureof the polarisation effects on the core induced by thevalence particle undergoing an electromagnetic transition.In the 100 Sn region the effective charges forproton, neutron and proton-neutron configurationshave been studied. The results were discussed controversiallybut imply that a large isovector charge isrequired, i.e. a large difference in proton and neutronpolarisation charge w5–7 x. For neutron rich systems,with relatively small valence particle binding, due tothe reduced overlap of valence neutrons and protoncore, a decreasing role of polarisation effects andthus a smaller effective neutron charge is expected.On the other hand a diminishing shell gap couldenable core excitations and even deformation as inthe well known Mg example 8 . The single particlestructure at the Fermi surface for proton holes inNs50 nuclei below 100 Sn is identical to that ofneutrons in Zs28 Ni isotopes below 78 Ni, i.e. theseare valence mirror states. The respective neutron andproton particle states differing by one major shell32wxexhibit again very similar features. Therefore 78 Zn 30 48is related to 78 Ni and 76 Ni , as is 100 28 28 48 48 Cd52to100 Sn and 98 50 50 48Cd 50, if we exploit isospin symmetryof mirror nuclei in the latter case. The importantdifference though, is that in contrast to the highlysymmetrical 100 Sn system, where protons and neutronsoccupy the same orbitals, 78 Ni has a largeneutron excess. This makes the 8 q isomer in 78 Zn avery good model state for the 8 q level in 76 Ni,which is a two neutron-hole excitation in 78 Ni. Themere existence of an isomer in 78 Zn implies ‘‘afortiori’’ its existence in 76 Ni and would be a directproof for the persistence of the Ns50 shell closurein 78 Ni. By comparison with a shell-model calculationone can, in principle, extract the neutron polarisationcharge of 78 Ni. A similar method was previouslyused to extract the effective E2 operator forprotons and neutrons in the vicinity of the NsZ100w xdoubly magic nucleus Sn 5,9,10 .The present work was performed in order tosearch and study new isomeric states in neutron richnuclei in the potassium to germanium region producedby the fragmentation reaction of a 86 Kr beam.It is a continuation of the previous survey experimentwhere new spectroscopic information has beenobtained for 13 isomeric states wx 3 . In that work,evidence for the existence of an isomeric state in78 Zn was shown, but due to a very low statistics,both the half-life and the energy of the isomerictransition could not be accurately deduced.The experiment has been performed at GANILusing the LISE spectrometer w11 x. The neutron rich86 Kr 34q beam with an energy of 60.5 AMeV andŽ11mean intensity of 1.6 mAe 3=10 pps.impingedon a rotating 100 mm thick nat Ni target. A 500 mmthick Be backing placed behind the target allowed areduction of the width of the charge states distributionof produced fragments. A 50 mm thick Be wedgewas used in the first dispersive plane of the spectrometerto remove light fragments and to furthersuppress non-fully-stripped ions. In order to reducethe time of flight of fragments to less than 200 nsŽcompared with 1.2 ms in our previous experimentwx. 3 the experimental set-up was placed in the firstachromatic focal point of LISE. The heavy ions weredetected by a three-element Si-detector telescope.The silicon detectors were 300 mm, 300 mm and500 mm thick, respectively. The last detector was xand y position sensitive and it was mounted at anangle of 458 with respect to the beam axis.The last Si-detector was surrounded by 4 highpuritygermanium detectors: one four-crystal cloverof 120% relative efficiency Ž add-back including .,two coaxial-type of 90% and 80% efficiency respectivelyand one low energy photon spectrometerŽ LEPS .. In order to increase the geometrical efficiencyof the setup, two detectors with the highestefficiency, were placed at about 1.5 cm from theimplantation Si-detector. The distance between theother two Ge-detectors and the beam axis was about5 cm. The sensitivity range for g-ray energies wasbetween 30 keV and 4 MeV. The total photopeakefficiency was measured to be 6.2Ž.1 % for 1.3 MeVg-rays. All detectors were mounted perpendicularlyto the beam axis. Three large volume BaF2crystalswere placed at about 0 o with respect to the beamaxis. The lifetimes of isomeric states were measuredby storing the time difference between the heavy ionimplantation signal and delayed g-rays. Two separate

( )J.M. Daugas et al.rPhysics Letters B 476 2000 213–218 215time ranges of up to 500 ns and 40 ms respectivelywere recorded for each Ge crystal using standardtime-to-digital converter Ž TDC.and time-to-amplitudeconverter Ž TAC.modules respectively.The identification of fragments in mass, atomicnumber and atomic charge was achieved by meansof energy-loss, total-kinetic-energy and time-of-flightmeasurements. The independent confirmation of thefragment identification comes from the observationof characteristic g-rays corresponding to the previ-67,68ously identified isomeric decays of Ni w12,13 x.All isomeric states reported in Ref. wx 3 were observedalso in the present experiment, in most cases, withmuch higher statistics. For example, due to the reducedtime of flight, increased primary beam intensityand improved g-detection efficiency, for 70m Nithe number of counts in the corresponding g-raylines was about 400 times higher than in the previouswork.In the present letter only results concerning theisomeric state in 78 Zn are presented. A summary ofthe measured gamma rays and half-lives for othernew isomeric states observed in this experiment maybe found in Ref. wx 4 .The measured g-ray spectrum gated by the eventscorresponding to 78 Zn is shown in Fig. 1. Thisspectrum represents the sum of spectra measured byall Ge-detectors. Four pronounced peaks at the energiesof 144.7Ž. 5 , 729.6Ž. 5 , 889.9Ž. 5 and 908.3Ž.5keV are clearly observed. When corrected for thedetection efficiency and internal conversiona Ž E2;144.7keV.tots0.165, all the peaks have thesame intensity and half-life and the correspondingg-rays are found to be in coincidence. From theseexperimental observables the E2 multipolarity wasdeduced for the 144.7 keV transition. The assignmentof the E2 multipolarity to three other transitionscomes from the intensity balance and systematicsdiscussed below. The time decay pattern of theisomeric transition, summed over four g-rays isshown in the inset of Fig. 1. An exponential x 2 fitto the experimental histogram gives a half-life valueof 319Ž.9 ns. The g-ray energies and the half-lifemeasured by means of the much faster BaF2crystalsare also in agreement with those found using thegermanium detectors.The low energy g-ray at 144.7Ž.5 keV was assumedto correspond to the primary isomeric transi-Fig. 1. The g-ray energy spectrum of the isomeric state in 78 Zn.The decay curve summed over the four observed transitions isshown in the inset.78tion in Zn. The assignment of the 729.6Ž.5 keVg-ray to the 2 q 0 q transition as well as I p s8 qassignment to the isomeric state comes from thesystematics of the 8 q isomers in the neighbouringnuclei as shown in Fig. 2. Though going from 80 Ge78towards Zn an increase of EŽ2 q . is expected, avalue of about 890 or 908 keV seems to be too high.From the EŽ2q . ratios in the above mentioned valencemirrors 98 Cd – 100 48 50 48 Cd52and corresponding50,52 52,54 132,134 208,210pairs Ti, Fe, Te, and Po w5,14xthe latter assignment would imply EŽ2 q , 76 Ni .,1250keV, EŽ2 q , 70 Ni. wx 3 . This appears too high, as70 76from Ni towards Ni an appreciable drop of EŽ2q .due to reduced pairing in the ground state is exwx5 and Ns126 w14xisotones.pected, as observed in Ns50The only information previously available on excitedlevels of 78 Zn comes from the b-decay of78Cuw15 x. However, the 737 keV g-ray, assignedw xq qtentatively in Ref. 15 to the 2 0 transition in

216( )J.M. Daugas et al.rPhysics Letters B 476 2000 213–218Fig. 2. Systematics of the 8 q isomeric states in the neutron-richNs48 isotones. The proposed level scheme of 78 Zn is from the80 82present work, those of Ge and Se from Ref. w22xand theothers from Ref. w14 x. The level energies are in keV. The assignmentof the 2 q and 4 q energies in 78 Zn is based on systematics.See text for details.78 Zn, has an energy about 7 keV higher than thatmeasured in the present work.It should be mentioned that as the above assign-Ž.q qment of the 729.6 5 keV g-ray to the 2 0transition is based on the systematics, other valuesq 78for the 2 energy in Zn, namely 889.9Ž.5 keV or908.3Ž.5 keV, can not be completely ruled out. Also,the order of the 889.9Ž. 5 keV and 908.3Ž.5 keVtransitions in 78 Zn remains uncertain. However, theseuncertainties do not change final conclusions of thepresent work.An I p s8 q assignment to the isomeric state is themost probable for reasons similar to those explainedwxq 98in Ref. 5 for the case of the 8 isomer in Cd.That implies that the 6 q and 8 q states are predominantlyneutron hole states of configuration g y2 9r2,q qwhile the 0 –4 states are mixed with Ž fp. 20y4 configurations.Thus the 6 q and 8 q states are fairly pureneutron hole states. There are of course other possibilitiesof coupling neutrons together with protons to6 q and 8 q states, but these configurations are notexpected to be mixed in strongly as particles andholes do not overlap much in space and thereforetheir interaction remains small.Assuming configurations n g y29r2 the experimentalŽq q. Ž .2value of B E2;8 6 s 24.0 11 e fm 4Ž1.21Ž 5..78W.u. for Zn, corresponds to the neutron effectivecharge of e s1.25Ž.n 2 e, if harmonic oscillator waveŽ .1r6functions with bs " rMv s1.01 A s2.09 fmare used. This is rather large in comparison to e n sŽq1.0e needed to reproduce the corresponding B E2;8q.70Žy 6 in Ni and the B E2;17r2 13r2 y. in69 Ni in a shell model calculation in the 2p 3r 2, 1f 5r 2 ,2p , 1g neutron space w16 x1r 2 9r 2. Therefore we haveextended these calculations into the proton spaceusing the realistic interaction deduced by Sinatkas etal. w17 x. Starting from experimental single particleenergies in the 56 Ni neighbors 57 Ni and 57 Cu, wehave adjusted them to reproduce the neutron andproton single particle states at and beyond the Ns4068 69wx69 – 73subshell in Ni, namely Ni 3 and Cu w18 x.This is justified by the well known deficiency ofrealistic interactions with respect to the monopoleterms, which can be absorbed in a shift of singleparticle energies with increasing shell occupancy.This results in the relative single particle energiesrelative to a 78 Ni core given in the Table 1. Theresults of the calculation Ž A.are shown in Fig. 3.For comparison and to illuminate the influence ofdifferent shell model approaches to the monopoleproblem, Fig. 3 also shows the results of a shellmodel calculation derived from an overall fit of themonopole terms along the Zs28 and Ns50 chainsw19,20 x. The starting point is a realistic interactionfrom the Oslo group. The interaction was derived fora 56 Ni core and the details of the method are disw21x. The single particle energies arecussed in Ref.taken from 57 Ni to be 0.0, 0.77, 1.11 and 3.0 MeVfor the p 3r 2, f 5r 2, p1r 2 and g9r 2 orbitals. Theevolution of the mean field through the monopoleTable 1Single proton particle and neutron hole energies of the 78 Ni coreŽ . Ž .79 77used in the shell model calculations A and B for Cu and Ni2p3r 2 1f5r 2 2p1r 2 1g9r 2Calculation A B A B A B A B79 Cu 1.34 0.30 0 0 3.33 1.77 3.61 3.0177 Ni 4.52 1.54 5.42 4.84 2.23 1.06 0 0

( )J.M. Daugas et al.rPhysics Letters B 476 2000 213–218 21778 mFig. 3. Deduced decay scheme of Zn compared to the predictions of the shell model Ž A and B .. The levels and transition energies are inkeV. The relative g-ray intensities, were normalized to the 729.6 keV transition. The energy of the 2 q and 4 q levels is based onsystematics. See text for details.terms of the interaction is directly connected to thesingle particle and single hole energies in Table 1.The spectra A and B are rather similar and bothcalculations reproduce the isomerism of the 8 q6 q transition. Using a polarisation charge of des0.5e for both protons and neutrons BŽ E2.values of1.47 W.u. and 1.06 W.u. are calculated for models Aand B, respectively, in good agreement with theexperimental value 1.21Ž.5 W.u. Both interactionsalso reproduce the experimental level schemes andŽ .qB E2 values for other known 8 Ns48 isomersw22xproving the adequacy of the monopole correctionapplied to the realistic interaction. Thus theeffective polarisation charges in the vicinity of 78 Niseem to be close to standard ones.In conclusion we have observed for the first timean 8 q isomeric state in 78 Zn. This result representsthe closest approach to the doubly-magic nucleus78 Ni made so far in g-spectroscopy studies. From thecomparison of the measured transition energies andthe BŽ E2.value with shell model calculations thefirst spectroscopic information in this region of thechart of nuclei has been gained. The shell modelcalculation seems to describe correctly the observedisomeric state with the standard neutron and protonpolarisation charge of 0.5e. Thus, in the presentwork, the first experimental evidence for the persistenceof the Ns50 shell gap the vicinity of the 78 Nihas been obtained.AcknowledgementsWe are grateful to the technical support providedto us by staff of the GANIL facility. This work hasbeen partially supported by the POLONIUM projectN.98275, KBN grant N.2P03B03615, DOE contractDE-AC05-96OR22464, Alliance grant N.98029 betweenUniversity of Surrey and GANIL and byEPSRCŽ UK ..Referenceswx 1 Ch. Engelmann et al., Z. Phys. A 352 Ž 1995.351.wx 2 M. Bernas et al., Nucl. Phys. A 616 Ž 1997.352c.wx 3 R. Grzywacz et al., Phys. Rev. Lett. 81 Ž 1998.766.