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Spectroscopy of neutron-rich Ni isotopes produced in 208Pb + 64Ni collisions
.f__ _ B
L3&
NUCLEAR
__ 4
PHYSICS
4._
ELSEVIER
Nuclear Physics A 574 (1994)
A
623-641
Ni isotopes produced Spectroscopy of_^^ neutron-rich _. in ‘08Pb + 64Ni collisions T.Pawlat ‘, R. Brodaa, W. Kr6las ‘, A. Maj a, M. Zi~bli~s~ a, H. Grawe b, R. Schubartb*‘, K.H. Maier b, J. Heese b, H. Kluge b, M. Schrammb a Niewodniczarfski Institute of Nuclear Physics, Radzikowskiego 152, PL-31-342 Cracow, Poland b Hahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin 39, Germany Received 23 November 1993
Abstract The neuron-~ch Ni isotopes from A = 64 to 67 have been studied by the multid~te~tor y-y coincidence technique using quasi- and deep-inel~tic reactions of 350 MeV 64Ni with a thick 2osPb target. Cross coincidences between y-rays of the two nuclei in the exit channel were used for isotopic identitication. The information on the level structure of Ni isotopes has been significantly extended particularly on high-spin states and the role of the ggj2 neutron orbital. The M2 g912 isomers have been identified in 65Ni T I/2 = 25(5) ns and in 67Ni Tr/2 > 0.3 ,GS as well as the 6- isomeric state in 66Ni Trj2 = 4.3(3) ns. The experimental levels are compared with shell model calculations performed using two different approaches with %Ni as core and various sets of residual interactions. Key words: Nuclear reaction 2’%b(64Ni, X), E = 350 MeV, measured ry. levels, J. ?r, Tip. Multidetector Ge array. Shell model.
n(t);
&1*65*a6*67Ni deduced
1. Introduction In the Ni isotopes with the closed 2 = 28 proton shell the neutron Fermi level is localized within the low j negative parity orbitals ~312, f5/2 and PI/~, and with increasing neutron number it moves towards the higher lying g9/2 orbital. In heavier Ni isotopes 1 Present address: 2. PhysikaIisches Gettingen, Germany.
Elsevier Science
B.\I:
~S~~O375-9474(94)00257-N
Institut der Universitit
Gdttingen.
Bunsen
Strasse 7-9,
D-37073
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Physics A 574 (1994) 623-641
this high j positive parity int~der g9/2 neutron level should feature pro~nently in the structure of high spin yrast states. Until recently this effect could not be studied in any systematic fashion since the heavy (A 2 64) Ni isotopes could not be reached in processes suitable for gamma spectroscopy. Only few results are available that with particle detection techniques attempted to detect high spin states in nickel isotopes heavier than 64Ni. A recent (cu,2p) reaction study [ 11 which led to important findings for the 64Ni and 66Ni isotopes represents an interesting example of such an attempt. Nevertheless, gamma spectroscopy data are limited to a few states populated in radioactive decays. In these studies [ 21 the neutron rich parents have been selected with a mass separator from the large number of products formed in reactions of a tungsten target with a 76Ge beam. The new and promising way to extend the study of Ni isotopes by involving in-beam gamma spectroscopy is offered by the superior resolving power of the multidet~tor gamma coincidence arrays. As has been demonstrated earlier [3] the high quality coincidence data obtained with such arrays in thick target experiments allow to resolve the discrete radiation from the individual nuclei present in a complex assembly of heavyion collision products. Moreover, the observation of cross coincidences between the gammas emitted by the two partner nuclei which arise simultaneously in the exit channel can provide means to make an isotopic identification of a specific nucleus. Exploiting this selectivity we analysed the y-y coincidence data collected during the bombardment of a ***Pb target with @Ni beam with the aim to extract the spectroscopic information on neutron-~ch Ni nuclei produced in damped reactions. In accord with a general trend to ~uilibrate mass and charge between heavy ions colliding at energies signific~tly above the Coulomb barrier one expects abundant production of heavy Ni isotopes; the already neutron-rich 64Ni will pick up additional neutrons from the *08Pb target nucleus which has a much higher N/Z ratio. The same 208Pb+64Ni experiment data have been already analysed in some other aspects and provided interesting spectroscopic information on *07Pb [4] and 208Pb [ .5] nuclei as well as preliminary results on the damped reaction mechanism [ 61. In the present work we report the results concerning the spectroscopy of heavy Ni isotopes.
2. Experiment,
analysis and isotopic ident~~tion
The 350 MeV 64Ni beam from the VICKSI accelerator at HMI Berlin bombarded a 30 mg/cm2 thick 208Pb target (98.7% enriched) located at the center of the OSIRIS y-spectrometer consisting of 11 Compton suppressed Ge detectors and an inner ball of 48 BGO elements. The recorded y-y coincidences involved for each detector the energy and time signals with respect to the beam pulse and optionally the multiplicity and sum energy from the BGO ball. The natural pulsing of the cyclotron beam gave 69 ns spacing between the in-beam events, which was enough to separate in a clean way the off-beam events and to allow the determination of short (< 30 ns) half-lives. In the off-line analysis also the time between the firing of two Ge detectors could be
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to inspect the delayed coincidences between gammas preceding and following the isomeric decays. Two trivial aspects have to be mentioned, which are a consequence of using a thick target in the experiment. First, the data are integrated with respect to the kinematics of processes taking place as well as over the beam energy, which varies from approximately 15% above the Coulomb barrier well down below it. Second, the experiment is sensitive primarily to gamma transitions which are emitted from the stopped nuclei and the Doppler broadening severely limits observation of states, with half-life and/or feeding time shorter than 2 ps. Nevertheless it turned out that most of the states populated in the studied Ni isotopes involved feeding times long enough to observe narrow discrete gamma lines. In the analysis initially the gates were set on few gamma transitions known to occur in various Ni isotopes. The resulting coincidence spectra contained usually severaI strong new y-lines belonging to the same nucleus as well as the lines which could be identified as the known gamma transitions emitted from the heavy reaction partners - Pb isotopes. In coincidence gates set on gamma transitions from a specific Pb isotope the corresponding Ni isotope lines could also be easily recognized in the spectra. Fig. 1 shows examples of coincidence spectra obtained for gates set on the (a) 1425 keV 2+ --+ O+ transition in 66Ni and (b) 803 keV 2* + O+ transition in *06Pb. The prominent lines belonging to the same nucleus as the gated transition are marked with black dots; other symbols are used to mark main transitions from the reaction partners. As expected for such integrated reaction data the coincidence spectra reflect the complexity of processes taking place. @Ni most naturally arising from the pick up of two neutrons appears in the exit channel not only together with 206Pb, but lighter load isotopes also show up as heavy reaction partners. In those cases clearly neutrons have been evaporated, but it remains unknown whether the excited light or heavy primary reaction product was the source of this secondary neutron emission. Fig. 2 displays another example of the coincidence spectrum with the gate set on the 1017 keV 9/2+ + 5/2- transition in 65Ni In this case the delayed coincidence condition was used since the 9/2+ state in 65Ni’was found to be isomeric. The spectrum shows gamma transitions of 65Ni located above the isomer and gammas from the lead partner nuclei, which also precede in time the (j5Ni isomeric decay. The quantitative analysis of coincidence spectra provided intensities of gamma transitions located in the level schemes of the studied Ni isotopes. At the same time the intensities of the corresponding cross coincidence Pb isotope lines were analysed and gave the dis~bution of heavy partner nuclei accomp~ying a specific Ni isotope in various processes. The distributions of Pb isotopes observed in coincidence with prominent lines from 63Ni to 67Ni are displayed in Fig. 3. It should be emphasized that each point in the figure represents the intensity of one selected transition (indicated in the figure caption) in a given Pb isotope and the value does not correspond to the total isotope yield. The reno~alis~ion of these results to extract the more complete yields is attempted in the other part of the analysis concerned with the reaction aspects [ 61. For the purpose of the present work we emphasize only the features of the results of Fig. 3 which help to reinforce the identification of various Ni nuclei. For the relatively used
626
gate 1425 keV X
600
1000
energy
1400
l
68N;
x
eoepb
1600
[keVJ
Fig. 1. Gamma spectra in coincidence with the 2’ -+ O+ transitions in (a) &Ni and (b) 206W. Lines in the same nucleus as the coincident transition and in the reaction partners are marked. The insert shows the low energy part with the isomeric 58 keV transition.
better known 63Ni and 64Ni nuclei the heaviest observed Fb isotope partners are correspondingly 2WPb and *e’Pb as expected for simple one neutron transfer and inelastic scattering_ The spectrum of lighter Pb isotopes corresponds to processes involving neutron evaporation e.g. up to 6 neutrons were evaporated in some reactions where 64Ni is present in the exit channel. The earlier identification of the 1017 keV transition with the @Ni nucleus [4] is now strongly reinforced by the co~~spo~ding Pb isotope spectrum of Fig. 3 which shows 207Pb as the main partner. Similarly for the 1425 keV line the identification with the known %G transition is evident by the strong yield of 206Pb and complete absence of heavier Pb isotopes. Somewhat less trivial is the identification of 67Ni since the observed cross coincidences were very weak, apparently due to the long lived isomer present in the nucleus. Nevertheless the 694 keV transition known in 67Ni from the radioactive decay f2] shows the expected partnership with the 205Pb and *04Pb nuclei and not with any heavier. The right hand side of Fig. 3 displays similar results for gates set on transitions from various Fb isotopes where the Ni isotope yields are again represented by the intensity of
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I
X
gate1017keV
0 65Ni + zO4Pb Cl zO5Pb A zO6Pb
X z’J7Pb
200
600
1000 1400 1800 2200 2600 energy [heV]
Fig. 2. prompt y-spectrum in coincidence with the delayed 1017 keV transition in 65Ni. The gamma lines preceding the isomeric decay in 65Ni and lines in tbe reaction partners are marked. Note that, along with the known strong 570 keV and Doppler broadened 898 keV transition, the recently identified [41 new line at 2485 keV is also marked as belonging to *07Pb.
one selected transition. These results confirm the previous identifications and show the much narrower dis~butions of Ni isotopes which practically do not extend beyond 63Ni. Within this systematic presentation we also show the results for the 2485 keV line that has been previously [4] identified with the 207Pb nucleus in a somewhat tentative way. The clear observation of the 65Ni partner and the absence of @jNithat is easily detected makes this assignment now firm. From the results discussed above it is apparent that each Ni isotope, including the 64Ni projectile, is produced in a variety of processes ranging from quasielastic to deep inelastic reactions which involve large excitation energies, that are frequently released by neutron evaporation. Consequently, the integrated production for each of them leads to the population of states in a broad range of excitation energy and spin. The results of the spectroscopic analysis of these excitations are presented in the next section.
3. Results and discussion In this section we review and briefly discuss the spectroscopic results obtained in the present experiment for Ni isotopes with A > 64. The 63Ni nucleus was populated with fairly low yield and no new information has been extracted beyond that existing already in the literature. We note that the 8.6( 8) ns half-life determined by us for the 63Ni ygg/2 isomer agrees well with the earlier measured [ ‘71 more accurate value of 9.4( 3) ns.
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20QPb 779keV
,
64Ni
208Pb 2615 keV
l
l
1346keV 104.
104: *
1017keV
*
l
104
207P b 570keV
.
102’
Pb partner
63
64
65
66
67
Ni partner
Fig. 3. Distributions of Pb isotopes observed in cross coincidence with the indicated gamma lines from 63Ni to
67Ni (left) and corresponding Ni isotopes distributions for gates set on transitions from the F’bpartner nuclei (right). The intensity of the strongest Iine in the partner nucleus coincident with the indicated transition is presented, the scale is arbitrary but identicat for all cases. The detection Iimit for the heaviest isotopes marked with an atrow is crucial for the identification procedure (see text). The results indicated for the 2485 keV transition of XJJFh give a clear evidence supporting the tentative identification presented in Ref. [4].
3.1. Even Ni isotopes
64Ni used in our experiment as a projectile is the heaviest stable isotope and naturally the best known in the presently discussed series. It has been studied using many reactions predominantly populating the low spin states. Recent (a,2p) experiments f I] provided essential information on the high spin states confirmed later in the ( t2C, *‘C) transfer reaction [ 81. All of these results compiled in Ref. [ 91 include only scarce information on the y-radiation in the @Ni. Thus the level scheme displayed in Fig. 4 and established in the present experiment fills this gap by showing the observed gamma de-excitation of many states which were mostly known from other studies. Except for five gamma
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r+
1280.4
4962.4 I 430.3 4532.1
7-
.
*+t
102u.4 2*
2276.8
1264.3
1346.0
*
*
t
I
nn
64 28
+
2+
Of
.
Nl 36
Fig. 4. The deduced level scheme for blNi. The width of the arrows indicates the gamma intensity.
transitions
for the first time. Their relative intensities account for the integrated population of a specific state in 64Ni produced in several processes as described in the previous section. The numerical values of relative intensities are given in Table 1. The spin-parity values come from the previous studies and the observed gamma
decay
all others are observed
nicely
supports
these assignments.
For two states at 3396 and 3750 keV the
direct population from the well established 5 state at 3849 keV strongly favors the 4+ ch~acte~~tjon thereby removing the existing (2+, 3,4+) ~biguity. The observed main yrast decay clearly indicates that the states at 5812 and 4532 keV are the highest spin 8+ and 7- states established in Ref. [ 1] with much less accurate excitation energies of 5810( 50) and 4600(50) keV. We leave the level at 4962 keV unassigned which apparently corresponds to the 4965(5) keV level previously seen in the (p,p’) and (t,p) reaction [ 91. At the same time we suggest a tentative 6- assignment for the 4173 keV level which is strongly populated in the yrast decay, and was also observed in the (p,p’) reaction. The strong feeding from the 7- state and depopulation through the .5- state
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Table 1 Energies, intensities and placement of gamma transitions observed in 62-67Ni nuclei Ei (keV)
.& (keV)
Ii *
If
64Ni
99.6(6) 189.0(4) 236.5(5) 323.4(2) 359.4(2) 430.3(6) 452.9(6) 583.4(6) 785.7(5) 930.8( 1) 1239.0(3) 1264.3( 1) 1284.0(6) 1346.0( 1) 1474.8(5) 1521.5(4) 1820.4(5) 2049.8(4) 2214.4(5)
0.3( 1) 1.2(2) 0.5(2) 6.6( 5) 3.1(3) 0.5( 1) 0.8( 1) 0.4( 1) 1.3(2) 29.5( 15) 7.2(7) 65.0( 20) 0.3( 1) 100.0 1.0(2) 1.6(3) 1.0(3) 1.6(4) 1.1(3)
3849.3 3749.7 4085.5 4172.7 4532.1 4962.4 3849.3 3749.7 3396.0 2276.8 3849.3 2610.3 3560.6 1346.0 4085.5 2867.5 3166.4 3396.0 3560.6
5- h 4+ 4 (6-) 4 7- ---t 5- + 4+ 4+ ---t 2+ 5- 4 4f ---t 3- + 2+ d --t o+ (2+, 4+) 4 4f + 3- h
4f 355(6-) 74+ (2+, 4f) 4f 2+ 4+ 2+ 2+ o+ 4f 2+ 2+ 2+ 2+
65Ni
310.4(2) 382.4(4) 488.3(4) 616.5(3) 629.4( 3) 692.7(5) 720.6(2) 1003.5(4) 1017.0( 1) 1168.7(2) 1491.6(6) 1502.3(2) 1610.3(2) 1889.5(5)
71.0(30) 2.2(4) 3.0(4) 4.6(5) 21.0(20) 2.7(3) 7.1(5) 1.4(3) 100.0 14.6( 12) 1.1(3) 18.6( 15) 18.4( 15) 1.9(5)
310.4 692.8 4011.1 3522.8 692.8 692.8 2906.3 3522.8 1017.0 2 185.7 4011.1 2519.3 1920.7 2906.3
3/23/2(17/2+) (15/2+) 3/23/2(13/2+) (13/2+) 9/2+ (11/2+) (17/2+) (13/2+) 5/2+ (13/2+)
---f --, --+ 4 ---t -+ --t --t + h --P -+
5/23/2(15/2+) (13/2+) l/25/2(11/2+) (13/2+) 5/29/2+ (13/2f) 9/2+ 3/29/2+
66Ni
58.0( 5) 354.3(5) 355.9( 1) 471.1(4) 490.1(2) 1085.5(3) 1245.7(3) 1404.8(6) 1425.1( 1) 1760.3( 1) 1945.8(3)
1.3(2) 2.9(3) 69.0( 30) 3.4(5) 22.0( 20) 10.7( 12) 8.9(7) 2.3(4) 100.0 82.0(30) 9.0(8)
3599.3 3725.2 3541.3 4070.4 4089.4 5174.9 2670.8 6579.7 1425.1 3185.4 3370.9
-
(5-)
(5_)
--+ ---) 7- h 8+ + (lo+) 2f + (4+) --f -
(4+) (6-) (6-) 72+ 8+ o+ 2+ 2+
100.0 100.0
1007.2 694.1
9/2+ 5/2- d
5/21/2-
67Ni
313.1( 1) 694.1( 1)
(6-)
Z twit
et al. /Nuclear Physics A 574 (1994) 623641
631
66 28 Nl 38 l
Fig. 5. Partial level scheme of @Ni from the present investigation.
favors strongly the spin value of I = 6. Whereas a 6+ state at this low energy is very unlikely a negative parity 6- state is expected; this assignment is further supported by the observation of a similar state in &Ni as discussed below. In the 66Ni nucleus only three gamma ~ansitions were known from the %Yo (3+) radioactive decay studies [ 21. On the other hand the (cr,2p) experiments [ 1 ] established four high spin states at 3.39 (5-), 4.05 (7->, 4.76 (5-,8”) and 5.17 MeV (8+). In the present experiment the 66Ni was populated in the 2n quasielastic transfer reaction as well as in deep inelastic processes. Apparently these complex reactions contribute significantly to the overall a6Ni production since the observed population of high spin states is much more pronounced than in the 64Ni case (see Table 1). Cons~uently, the established level scheme shown in Fig. 5 involves mostly yrast states. Based on
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104
rA % 3
103;
E
102
j
I
,,lL. 0
10
20
time
30
40
[ns]
Fig. 6. The decay curves of the 1017 keV line in 65Ni (upper part) and the 1425 keV line in 66Ni (lower part). The dashed line represents the prompt curve obtained from the fast 1346 keV 2+ - Of transition in @Ni.
the ((u,2p) results we assign the states at 5175 and 4089 keV as the 8+ and 7- states correspondingly. In analogy to the 64Ni levels the states at 3541 and 3 185 keV which are strongly populated in the yrast decay are the 5- and 4+ states respectively; the mismatch of the 5- energy with the (cq2p) result can be attributed to the large energy uncertainty of the latter one. We note that the nearly 600 keV higher energy of the 4+ excitation in 66Ni compared to the 64Ni case is a simple consequence of the filling of the f5/2 neutron orbital at the neutron number N = 38 which closes an easy way to form the 4+ excitation. The gamma transitions below the 5- state did show a delayed component and the analysis of the time spectra indicated the presence of a Tt/2 = 4.3(4) ns isomer. The high energy of the 1760 and 1425 keV transitions involved in the isomeric decay gave the time resolution which enabled not only the half-life determination shown in Fig. 6, but also allowed to establish that the 471, 490, 1085 and 1405 keV transitions precede the decay of the isomer. With the 356 keV transition showing clearly the prompt component the low energy 58 keV transition (see Fig. 5) is the only candidate to be considered as the isomeric transition. Thus in the scheme of Fig. 5 the 4.3 ns isomer was located at the 3599 keV level and assigned as the 6- state in analogy to the 64Ni nucleus scheme. We leave unassigned the weakly populated new states at 3371, 3725 and 4070 keV. A possible correspondence with the states from the shell model calculation will be considered in the last section of this work. Few words should be devoted to the 2671 keV level which we characterize as the (yf;Ikpt,2) 3+ basing on the interpretation of the known %Zo radioactive decay [ 21. The $‘Cos9 ground state assigned tentatively as the 3+ state of the 7rf;)2vpt~2 doublet decays almost exclusively
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to the 2671 keV level in @Ni. Considering possible ways of this decay it is clear that
the Gamow-Teller transition changing one of the fs/z neutrons into the fT,q proton must be strongly favoured over other possibilities. Thus the final state in @Ni should be the simple neutron particle-hole f$pr,z excitation which is expected around the observed 2.7 MeV energy. Moreover, the complete absence of any /3 strength to the lowest ‘j6Ni 2+, which almost certainly involves a similar f;;ikpl/z component would then indicate a 4+ rather than a 3+ ~signment for the @Co ground state. Summ~~ng the results for the even Ni isotope we note that along with the fairly complicate nature of states with spin values I 6 4 the most transp~ent inte~retation of the @Ni and %Ni level schemes can be given for the high spin states. The negative parity states involve one g9/2 neutron and, as expected, their excitation energies are significantly lower in 66Ni compared to the 64Ni isotope. Whereas the 5- is predominantly of vgg/zpt/z nature, the 7- and 6- states structure must involve the yg9,2f;;!2 excitations. The presence of the 6- isomer in @jNi confirms this simple inte~retation and yields the ret~dation factor of 50 for the forbidden f5/2 +p1/2 Ml position. The even more pronounced lowering of the 8+ state excitation energy with increasing neutron number is naturally related to its est~lish~d two~neu~on g$,z structure. It would be highly desirable to extend the present info~ation including the closed N = 40 @Ni isotope, for which the 5- isomer and the further lowering of the 8+ state is expected. Indirectly our data indicate that the 68Ni isotope is produced in the present experiment, but as yet we were not able to extract any specific info~ation on its gamma transitions.
3.2. Odd
Ni
isotopes
The excited states of @Ni populated in the present experiment are shown in the level scheme of Fig. 7 and nume~c~ data on gamma intensities are given in Table 1. Since 65Ni is predominantly produced in the qu~iel~tic In transfer one observes a strong ~pulation of non-yrast states known from the earlier (d,p), (n,r) and j3 decay studies [ 103 ; these states are distinguish~ in Fig. 7 by firm spin-parity ~signmen~. Among them the ygg/2 excitation ten~tively assigned at 1013f 10) keV from the (dq) data [ 111 is strongly supported by the present result. The 1017 keV t~sition, un~biguously assigned to the 65Ni isotope f see Fig. 2) shows a decay displayed in Fig. 6. The resuhing 25(5) ns isomeric half-life perfectly matches the B(M2) (8912 --+fs/z) derived from the identical known transition in the a3Ni isotope. In the analysis this 9/2+ isomer half-life was exploited to obtain clean spectra in coincidence with the delayed 1017 keV transition thereby signi~cantly enhancing the detection sensitivity for the generally much weaker populated high-spin states. Five new levels were establish~ as shown in the scheme of Fig. 7 with tentative spin-p~ity ~signments which are based on simple expectations and observed gamma decay. Whereas the shell model c~culations presented in the next section support in general these ~signments it is clear that the ~sumption of 17/2+ for the 4011 keV level was crucial in suggesting the spin-pity ch~cte~~tion of the four lower lying new states. At 4 MeV excitation energy 1712’ is naturally expected as the
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5l2'
65 28
.
NI37
1007.2
9/2+
>0.3ps
6w.qr
694.1 0.0
67 28
I
a-
.
NI39
Hg. 7. The level schemes of the 65Ni and 67Ni isotopes. The 9/2+ prior to this study are marked with firm spin-parity assignments.
highest possible
spin from the coupling
isomer and the low-spin
states known
of the g9/2 neutron with the 4+ core excitations
of 5$j2 or vf5/2p3/2 type. Much less complete results were obtained for the 67Ni isotope and the experimental difficulty arises from the presence of the long lived isomer which virtually excludes the possibility to establish coincidence relationships with any transition feeding the isomer. The scheme of Fig. 7 shows the simple decay of this isomer naturally interpreted as the vggl2 state in 67Ni. The 694 keV gamma transition de-exciting the fs/2 67Ni state known to be populated strongly in the Gamow-Teller /3 decay of 67Co [2] was seen in the coincidence projection. The 694 keV gate gave only one strong coincident gamma line of 3 13 keV, except for the weak cross coincidence 205,204Pb lines discussed earlier
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Physics A 574 (1994)
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635
and confirming the 67Ni nucleus identification. The resulting 1007 keV excitation energy coincides with the 1020( 22) keV level suggested from the (cr,%e) reaction study I:121. Apparently the 313 keV gamma line corresponds to the 9/2+ --+ 5,f2- M2 isomeric transition from the neutron gs/z state, which is expected in the N = 39 67Ni at much lower energy than in the 63Ni and 65Ni isotopes. The similar B(M2) values of about 2 ( e~/2~c)2 fm2 for the 63Ni and 65Ni M2 positions allow to predict the value of M 8 ,US for the 67Ni isomeric half-life. Restricted by the present expe~ment~ conditions we can give only the lower limit of 0.3 ,us.
4. Shell model caIculations for Ni isotopes Shell model calculations for low-lying, low-spin states in Ni and Cu isotopes have been performed by Koops and Glaudemans [ 131. Assuming an inert 56Ni core and a p3/2, f5/2, pi/z model space for protons and neutrons they used the modified surface delta interaction (MSDI) and the isospin formalism. In the spirit of this approach selected Ni high spin states were calculated in a highly truncated model space including the g9/2 neutron orbital [ I]. More recently a complete set of realistic two-body matrix elements (tbme) was derived for the full p3/2, f5/2, PI/Z, gg/2 model space [ 141, which enables a comparison to the schematic MSDI. The realistic interaction has been used successfully for the corresponding proton space in N = 50 nuclei [ 14,15]. Its application to the Z = 28 neutron space allows an investigation of the concept of valence mirror nuclei. Therefore in the present work the two different residual interactions were used and adapted to the full N = 28-50 neutron shell UP~,Q, f5/2, pi/z, gg/2. The core nucleus was 56Ni and free parameters of the various model interactions were fitted to 25 selected single-, two- and three-particle states in 64Ni to 68Ni. Occupation of the p, f orbitals was left free up to their m~imum values of 2j + 1, whereas occupation of the 8912 orbit was limited to < 4. This turned out to be important to reproduce the splitting of the (f,p)“-2 x (gs,2)2 and (f,p)“-’ xg9,~2states of opposite parity, which cannot be achieved with any further truncation of the gs/2 occupation. In a first approach following Ref. [ 131 the MSDI was used in a model space including the g9,q orbital, which requires a renormalization of the interaction strength as compared to Ref. 1131. As a first step the single particle energies for P~,Q, fs/z and pt,2 were taken from previous work [ 131 as they are essentially fixed by the respective single particle states in 57Ni. The T = 1 interaction strength A1 and the gs/2 single particle energy were treated as free parameters. To allow for modification of the single particle energies (spe) in the mid shell nuclei 64-68Ni due to proton ph-excitation, in a second step the spe were optimized with respect to the level schemes of these nuclei. Finally, the monopole term Bt was fitted to the ground state binding energies of the Ni isotopes. In the second approach we used the realistic set of tbme, further on referred to as S3V, which was derived by Sinatkas et al. [ 141 from the Sussex nucleon-nucleon interaction [ 161 employing the Kuo-Brown method [ 171. As the interaction was renormalized with respect to a ‘@%n core and the single particle energies were adjusted to fit the spectra
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Physics A 574 (1994) 623-641
Table 2 Shell model parameters Single particle (hole) energies [ MeV]
Interaction P3f2
MSDI
A1 = 0.38 MeV
-10.20
core
$2
PI/2
g9/2
-9.14
-8.97
-7.41
“Ni
Bt = 0.438 MeV s3v
see text
8.22
7.84
7.30
-3.41
68Ni
-9.42 8.78
-10.20 7.59
-8.30 7.07
-6.43 -3.21
56Ni @Ni
of N = 48-50 nuclei in the upper half of the proton and neutron shell, a new set of spe had to be found by a fit to the Ni isotopes. As in the MSDI approach this was performed with special emphasis on the 64-68Ni level schemes. To fit the ground state binding energies a general shift of 0.456 MeV was applied to all diagonal tbme and all single particle energies were reduced by 0.827 MeV relative to the 56Ni core. The final interaction parameters and single particle energies used in the two approaches are listed in Table 2. It should be noted that with the full model space the MSDI strength parameters A1 and B1 as compared to Ref. [ 131 are reduced to nearly their “canonic” values 25/A MeV [ 181. The single particle energies found for the two model approaches are quite different, note the p3/2-f5/2 inversion. For comparison energies relative to a 68Ni core are also listed in Table 2.
the single particle (hole)
5. Discussion of shell model results The results of the shell model calculations are compared to the experimental level schemes of 64Ni 66Ni 65Ni and 67Ni in Figs. 8-10. The experimental levels used in the fitting proiedure’ are marked by asterisks. In view of the large distance of 8-l 1 neutrons from the core nucleus 56Ni and the neglect of proton core excitations the overall agreement with experimental levels up to 6 MeV excitation energy is satisfactory. The mean level deviation (MLD) as defined in Ref. [ 191 is 0.317 MeV for both the MSDI and S3V sets of tbme. The main observations in comparison of experiment and shell model results can be summarized as follows.
5.1. Even isotopes
(9 In both calculations
(ii)
the first 2+ and 3- states, which presumably contain some collectivity, are predicted too high in excitation energy as a consequence of the limited configuration space (Figs. 8 and 9). The level order and the splitting of the two-particle states In = 5-, 6-, 7- with their main configurations g9/2p1/2, g9/2p3/2 and g9/2f5/2 is poorly reproduced in contrast to the well predicted (g9/2);+ state. In the SD1 approach this is mainly due to the vanishing interaction for states of unnatural parity as I” = 6-. The S3V
1: Pawlat et af./Nuclear Physics A 574 (1994) 623-641
637
+
-8
6248
-
8+
-
7-
8+
l 581.?
+
-6 -
10+ +
5615 --9 ---__6+
7-
-6+ -2 -
4962 4756 4693
5; _
7-
7-
l 4532
4431
6-
:.----$
-
-4
8' s-
+
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-4
D
4+
+
-2
3-
0+
-
4+
-2
4+ 2+ +
3166
(2+,4+)
-
4+
o+
s3v
-
4064 3935 -
6+ - s-
z+
*%A 3561
$
-
+
(6-l
iE
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0+
'2610
4+
2277
2+
'1346
2+
3100
3-
2724
0+
%9%
2+ 4+
1565
2+
2+
-0
+
MSDI
0
0+
ECCP
0
0+
EXP
64Ni,, Fig. 8. Experimental and shell model level schemes for @Niqe _ and the experimental scheme of the valence-mirrornucleus $Kr, interaction seems to overcompensate this effect. Inspection of the model wave functions for the I” = 5-, 6-, 7- multiplet shows that these states are mixed with predominant gg/zf;i: (> 70%) and g9/2p1/2 (> SO%) character for the I” = 6-, 7- and .5- states, respectively. Therefore their relative positions can hardly be influenced by reasonable variations of the spe, but are an inherent consequence of the residual interaction. This raises the problem of explaining the retarded 6- ---+ 5- Ml transition, giving rise to an isomer in 66Ni. In the case of pure g9/2f5/2 and g9/2p1/2 configurations, respectively, I-forbiddeness would be a straightforward explanation. We have calculated the Ml transition strength using effective single particle Ml matrix elements for p3i2, f5/2 and pl,~ 1203. The effective g9/2 Ml matrix element was taken from the g-factor of the (g/2+) level in 63Ni [ 71. The
638
I: Pawtat et al./Nuclear Physics A 574 (1994) 623-641
-
6560
lo+ -
-
10
$
(lo+)
+
'517%
8+
*go-
?-
+
-4
-
7-
-
g:
m
34: 6_
-
74' s-
-
x+ 3+ +
-0
-$ -
+
*
-0
SW
g3
*3185
(4*)
3'
-o+
-2
3725 a': 3371
-
2671
3+
'2445
o+
*142S
2+
2+
-0
MSDI
+
0
0+
EXP
~p~~~e~~ and shell model level schemes for @Ni. Experimental levels included in the fitting procedureare markedby asterisks. Fig. 9.
B(M1 ) values are 8.9 and 50 mWu for the MSDI and S3V approach, respectively, which agree well with the experimental value of 21(2) mWu. Also in 66Ni the I” = 3+ state with predominant configuration f;/\pt,z and the
f$g;,* * I” = lO+ state are well accounted for in both theoretical approaches. (iii) In Fig. 8 for imprison the valence mirror nucleus to 64Nis6 the N = 50 nucleus $Cr is shown. The data are from Ref. [ 15 f . A nice one to one correspondence is established, which is only distorted for states carrying collective strength that are subject to core excitation. This does, however, not imply, that the corresponding states are pure valence nucleon states and not affected by core excitations. It has been shown that the 8*, 9+ and lO+ states in %r are strongly influenced by
T: Pawht et aL/Nuclear Physics A 574 (1994) 623-641 -
17/2+
'4011
(17/2+)
3523
(15/2+)
2903
(13/2+)
'2519
(13/2+)
'2186
w/2+)
-s/2+ -
13/2+
-
11/2+
-
13/2+ 5t2+
-
17/2:
----cafe+
1921
-Q/2+
639
-3f2
5/2+
*+ l 1017
9/Z*
693
3/2-
* 310
3/2-
*
$$I
--3$2
-
1l2-
-s&c
-3l2--
s3v
5l2-
MSDI
l/2-
y
Es@
@Ni,,
0a -3/2-
-
-
s3v
w
-
Ql2+
-
3/2-
9/2+ 5/2-
-
5/2-
1/2-
-
l 1007
g/2+
694
512-
l
l/2-
0
MSDI
1/2-
EXP
67Ni3,
Fig. 10. Same as Fig. 9 for 6sNi (a) and 67Ni (b).
the yg$ds,2
core excitation
[ 151, and the co~sponding
7if;‘ifg,t2 con~guration
should be at least equally effective in @‘Ni. Neve~el~s the similarity of the valence mirror level schemes is su~rising, as the proton single particle level sequence for a 78Ni core differs considerably [ 141 from 56Ni. 5.2. Odd isotopes (i)
With the readjusted single particle energies (see Table 2) the Iz = 3,12-, 5/Z-, l/2- and Q/2+ single particle (hole) states in 65*67Niare well described. The fact
640
T: Pawiat et al./Nuclear
Physics A 574 (1994) 623-641
that the 56Ni spe have to be changed towards midshell can be taken as evidence for proton core excitations, which according to the Sn and Pb isotopes should be strongest in midshell. The treatment of lplh and 2p2h proton core excitation due to computer capacity limitations were beyond the scope of the present model approach. (ii) In 65Ni for the three-quasiparticle states of main configuration (f,p)$gg,z and spin/parity 5/2+-17/2+ similar deficiencies of the model calculations are observed as for the I* = 5--7- two-quasiparticle states in the even isotopes. While the centroids of the two multiplets are well accounted for, the splitting is too small in both model approaches. Therefore the level sequence of yrast and yrare high spin states is not well reproduced for similar reasons as in the even isotopes. In suck this first attempt to give a consistent shell model desc~ption of neutron rich Ni isotopes accounts reasonably well for the gross structural features of one-, twoand three-particle states. There are still open problems concerning the trend of single particle levels from 56Ni to the N = 40 subshell closure, which for the moment do not allow a meaningful extrapolation towards ‘*Ni. More experimental data, especially on 69,70Ni,would help to clear the situation. The role of proton core excitations has to be investigated, which is only feasible with severe truncation in the neutron configuration space. The concept of valence mirror nuclei works nicely, and core excitations seem to have only little influence on it. This is all the more surprising as the proton and neutron single particle level sequences are different. The realistic interaction (S3V), first used successfully for proton excitations in N = 50 nuclei [ 14,151, in the N = 28 neutron space gives only marginal improvement over the schematic MSDI interaction.
Acknowledgement
This work has been supported in part through the agreement on scientific cooperation between Poland and Germany and by the Polish Scientific Committee under grant no. 2243 19203.
References [ 11 U. Fister, R. Jahn, Pvon Neu~n-Cosel, i? Schenk, T.K. Trelle, D. Wenzel and U. Wienands, Phys. Rev. C 42 (1990) 2375. f21 U. Bosch, W.-D. Schmidt-Ott, E. Runte, P Tidemand-Pettersson, R. Koschel, E Meissner, R. Kirchner, 0. Klepper, E. Roeckl, K. Rykaczewski and D. Schardt, Nucl. Phys. A 477 (1988) 89. [3] R. Broda, M.A. Quader, P.J. Daly, R.V.F. Janssens, T.L. Khoo, WC. Ma and W.M. Drigert, Phys. L&t. B 251 (1990) 245. 141 M. Schramm, H. Gmwe, J. Heese, H. Kluge, K.H. Maier, R. Schubart, R. Broda, J. Grebosz, W. Kr6las, A. Maj and J. Blomquist, Z. Phys. A 344 (1992) 121. t51 M. Schramm, H. Grawe, J. Heese, H. Kluge, K.H. Maier, R. Schubart, R. Broda, J. Gqbosz, W. K.r6las, A. Maj and J. Blomquist, Z. Phys. A 344 (1993) 363. [61 W. Kr6las, R. Broda, J. Grebosz, A. Maj, T. Pawtat, M. Schramm, H. Grawe, J. Heese, H. Kluge, K.H. Maier and R. Schubart, IFJ Krak6w Ann. Rep. (1992) p. 57, HMI Berlin Ann. Rep. (1992) p. 243.
T. Pawlal et al. /~~c~~ar Physics A 574 (I 994) 623441
[7] t8l [9] [IO]
641
W. Mliller, H.H. Bert&at, H. Haas, B. Spellmeyer and W.-D. Z&z, whys. Rev. B 40 (1989) 7633. A. Boucenna, L. Kraus, I. Linck and T.U. Ghan, Phys. Rev. C 42 (1990) 1297. B. Singh, Nucf. Data Sheets 62 (1991) 614. M.R. Bhat, Nucl. Data Sheets 69 (19933 215. [Ill T.R. Anfinsen, K. Bjemdal, A. Graue, J.R. Lien, G.B. Sandvik, LG. Tveita, K. Ytterstad and E.R. Cosman, Nucl. Phys. A 157 (1970) 561. 1121 R.T. Kouzes, D. Mueller and C. Yu, Phys. Rev. C 18 (1978) 1587. [ 131 J.E. Koops and l?W.M. Glaudemans, Z. Phys. A 280 (1977) 181. [ 141 J. Sinatkas, L.D. Skouras, D. Strottman and J.D. Vergados, J. Phys. G 18 (1992) 1377, 1401. [ 151 G. Winter, R. Schwengner, J. Reif, H. Prade, L. Funke, R. Wirowski, N. Nicolay, A. Dewald, R von Brentano, H. Grawe and R. Schubart, Pttys. Rev. C 48 ( 1993) 1010. [ 161 JR Elliott, A.D. Jackson, H.A. Mavromatis, E.A. Sanderson and B. Singh, Nucl. Phys. A 121 (1968) 241. [ 171 T.T.S. Kuo and GE. Brown, Nuci. Phys. 85 (1966) 40; A 114 ( 1968) 241. I181 P.J. Brussaard and RW.M. Glaudemans, Shell model applications in nuclear spectroscopy (NortbHolland, Amsterdam, 1977) p. 116. [ 191 R. Gross and A. Frenkel, Nucl. whys. A 267 ( 1976) 85. [20] P.W.M. GIaudem~s, M.J.A. de Voigt and B.F.M. Steffens, Nucl. Phys. A 198 (1972) 609
L3&
NUCLEAR
__ 4
PHYSICS
4._
ELSEVIER
Nuclear Physics A 574 (1994)
A
623-641
Ni isotopes produced Spectroscopy of_^^ neutron-rich _. in ‘08Pb + 64Ni collisions T.Pawlat ‘, R. Brodaa, W. Kr6las ‘, A. Maj a, M. Zi~bli~s~ a, H. Grawe b, R. Schubartb*‘, K.H. Maier b, J. Heese b, H. Kluge b, M. Schrammb a Niewodniczarfski Institute of Nuclear Physics, Radzikowskiego 152, PL-31-342 Cracow, Poland b Hahn-Meitner-Institut, Glienicker Strasse 100, D-14109 Berlin 39, Germany Received 23 November 1993
Abstract The neuron-~ch Ni isotopes from A = 64 to 67 have been studied by the multid~te~tor y-y coincidence technique using quasi- and deep-inel~tic reactions of 350 MeV 64Ni with a thick 2osPb target. Cross coincidences between y-rays of the two nuclei in the exit channel were used for isotopic identitication. The information on the level structure of Ni isotopes has been significantly extended particularly on high-spin states and the role of the ggj2 neutron orbital. The M2 g912 isomers have been identified in 65Ni T I/2 = 25(5) ns and in 67Ni Tr/2 > 0.3 ,GS as well as the 6- isomeric state in 66Ni Trj2 = 4.3(3) ns. The experimental levels are compared with shell model calculations performed using two different approaches with %Ni as core and various sets of residual interactions. Key words: Nuclear reaction 2’%b(64Ni, X), E = 350 MeV, measured ry. levels, J. ?r, Tip. Multidetector Ge array. Shell model.
n(t);
&1*65*a6*67Ni deduced
1. Introduction In the Ni isotopes with the closed 2 = 28 proton shell the neutron Fermi level is localized within the low j negative parity orbitals ~312, f5/2 and PI/~, and with increasing neutron number it moves towards the higher lying g9/2 orbital. In heavier Ni isotopes 1 Present address: 2. PhysikaIisches Gettingen, Germany.
Elsevier Science
B.\I:
~S~~O375-9474(94)00257-N
Institut der Universitit
Gdttingen.
Bunsen
Strasse 7-9,
D-37073
624
If: Pawlat et ai.fNuclear
Physics A 574 (1994) 623-641
this high j positive parity int~der g9/2 neutron level should feature pro~nently in the structure of high spin yrast states. Until recently this effect could not be studied in any systematic fashion since the heavy (A 2 64) Ni isotopes could not be reached in processes suitable for gamma spectroscopy. Only few results are available that with particle detection techniques attempted to detect high spin states in nickel isotopes heavier than 64Ni. A recent (cu,2p) reaction study [ 11 which led to important findings for the 64Ni and 66Ni isotopes represents an interesting example of such an attempt. Nevertheless, gamma spectroscopy data are limited to a few states populated in radioactive decays. In these studies [ 21 the neutron rich parents have been selected with a mass separator from the large number of products formed in reactions of a tungsten target with a 76Ge beam. The new and promising way to extend the study of Ni isotopes by involving in-beam gamma spectroscopy is offered by the superior resolving power of the multidet~tor gamma coincidence arrays. As has been demonstrated earlier [3] the high quality coincidence data obtained with such arrays in thick target experiments allow to resolve the discrete radiation from the individual nuclei present in a complex assembly of heavyion collision products. Moreover, the observation of cross coincidences between the gammas emitted by the two partner nuclei which arise simultaneously in the exit channel can provide means to make an isotopic identification of a specific nucleus. Exploiting this selectivity we analysed the y-y coincidence data collected during the bombardment of a ***Pb target with @Ni beam with the aim to extract the spectroscopic information on neutron-~ch Ni nuclei produced in damped reactions. In accord with a general trend to ~uilibrate mass and charge between heavy ions colliding at energies signific~tly above the Coulomb barrier one expects abundant production of heavy Ni isotopes; the already neutron-rich 64Ni will pick up additional neutrons from the *08Pb target nucleus which has a much higher N/Z ratio. The same 208Pb+64Ni experiment data have been already analysed in some other aspects and provided interesting spectroscopic information on *07Pb [4] and 208Pb [ .5] nuclei as well as preliminary results on the damped reaction mechanism [ 61. In the present work we report the results concerning the spectroscopy of heavy Ni isotopes.
2. Experiment,
analysis and isotopic ident~~tion
The 350 MeV 64Ni beam from the VICKSI accelerator at HMI Berlin bombarded a 30 mg/cm2 thick 208Pb target (98.7% enriched) located at the center of the OSIRIS y-spectrometer consisting of 11 Compton suppressed Ge detectors and an inner ball of 48 BGO elements. The recorded y-y coincidences involved for each detector the energy and time signals with respect to the beam pulse and optionally the multiplicity and sum energy from the BGO ball. The natural pulsing of the cyclotron beam gave 69 ns spacing between the in-beam events, which was enough to separate in a clean way the off-beam events and to allow the determination of short (< 30 ns) half-lives. In the off-line analysis also the time between the firing of two Ge detectors could be
I: Rawkzt et nL/Nucleur
Physics A 574 (1994) 623-641
625
to inspect the delayed coincidences between gammas preceding and following the isomeric decays. Two trivial aspects have to be mentioned, which are a consequence of using a thick target in the experiment. First, the data are integrated with respect to the kinematics of processes taking place as well as over the beam energy, which varies from approximately 15% above the Coulomb barrier well down below it. Second, the experiment is sensitive primarily to gamma transitions which are emitted from the stopped nuclei and the Doppler broadening severely limits observation of states, with half-life and/or feeding time shorter than 2 ps. Nevertheless it turned out that most of the states populated in the studied Ni isotopes involved feeding times long enough to observe narrow discrete gamma lines. In the analysis initially the gates were set on few gamma transitions known to occur in various Ni isotopes. The resulting coincidence spectra contained usually severaI strong new y-lines belonging to the same nucleus as well as the lines which could be identified as the known gamma transitions emitted from the heavy reaction partners - Pb isotopes. In coincidence gates set on gamma transitions from a specific Pb isotope the corresponding Ni isotope lines could also be easily recognized in the spectra. Fig. 1 shows examples of coincidence spectra obtained for gates set on the (a) 1425 keV 2+ --+ O+ transition in 66Ni and (b) 803 keV 2* + O+ transition in *06Pb. The prominent lines belonging to the same nucleus as the gated transition are marked with black dots; other symbols are used to mark main transitions from the reaction partners. As expected for such integrated reaction data the coincidence spectra reflect the complexity of processes taking place. @Ni most naturally arising from the pick up of two neutrons appears in the exit channel not only together with 206Pb, but lighter load isotopes also show up as heavy reaction partners. In those cases clearly neutrons have been evaporated, but it remains unknown whether the excited light or heavy primary reaction product was the source of this secondary neutron emission. Fig. 2 displays another example of the coincidence spectrum with the gate set on the 1017 keV 9/2+ + 5/2- transition in 65Ni In this case the delayed coincidence condition was used since the 9/2+ state in 65Ni’was found to be isomeric. The spectrum shows gamma transitions of 65Ni located above the isomer and gammas from the lead partner nuclei, which also precede in time the (j5Ni isomeric decay. The quantitative analysis of coincidence spectra provided intensities of gamma transitions located in the level schemes of the studied Ni isotopes. At the same time the intensities of the corresponding cross coincidence Pb isotope lines were analysed and gave the dis~bution of heavy partner nuclei accomp~ying a specific Ni isotope in various processes. The distributions of Pb isotopes observed in coincidence with prominent lines from 63Ni to 67Ni are displayed in Fig. 3. It should be emphasized that each point in the figure represents the intensity of one selected transition (indicated in the figure caption) in a given Pb isotope and the value does not correspond to the total isotope yield. The reno~alis~ion of these results to extract the more complete yields is attempted in the other part of the analysis concerned with the reaction aspects [ 61. For the purpose of the present work we emphasize only the features of the results of Fig. 3 which help to reinforce the identification of various Ni nuclei. For the relatively used
626
gate 1425 keV X
600
1000
energy
1400
l
68N;
x
eoepb
1600
[keVJ
Fig. 1. Gamma spectra in coincidence with the 2’ -+ O+ transitions in (a) &Ni and (b) 206W. Lines in the same nucleus as the coincident transition and in the reaction partners are marked. The insert shows the low energy part with the isomeric 58 keV transition.
better known 63Ni and 64Ni nuclei the heaviest observed Fb isotope partners are correspondingly 2WPb and *e’Pb as expected for simple one neutron transfer and inelastic scattering_ The spectrum of lighter Pb isotopes corresponds to processes involving neutron evaporation e.g. up to 6 neutrons were evaporated in some reactions where 64Ni is present in the exit channel. The earlier identification of the 1017 keV transition with the @Ni nucleus [4] is now strongly reinforced by the co~~spo~ding Pb isotope spectrum of Fig. 3 which shows 207Pb as the main partner. Similarly for the 1425 keV line the identification with the known %G transition is evident by the strong yield of 206Pb and complete absence of heavier Pb isotopes. Somewhat less trivial is the identification of 67Ni since the observed cross coincidences were very weak, apparently due to the long lived isomer present in the nucleus. Nevertheless the 694 keV transition known in 67Ni from the radioactive decay f2] shows the expected partnership with the 205Pb and *04Pb nuclei and not with any heavier. The right hand side of Fig. 3 displays similar results for gates set on transitions from various Fb isotopes where the Ni isotope yields are again represented by the intensity of
I: Puwiat et d/Nuclear
Physics A 574 (1994) 623-641
627
I
X
gate1017keV
0 65Ni + zO4Pb Cl zO5Pb A zO6Pb
X z’J7Pb
200
600
1000 1400 1800 2200 2600 energy [heV]
Fig. 2. prompt y-spectrum in coincidence with the delayed 1017 keV transition in 65Ni. The gamma lines preceding the isomeric decay in 65Ni and lines in tbe reaction partners are marked. Note that, along with the known strong 570 keV and Doppler broadened 898 keV transition, the recently identified [41 new line at 2485 keV is also marked as belonging to *07Pb.
one selected transition. These results confirm the previous identifications and show the much narrower dis~butions of Ni isotopes which practically do not extend beyond 63Ni. Within this systematic presentation we also show the results for the 2485 keV line that has been previously [4] identified with the 207Pb nucleus in a somewhat tentative way. The clear observation of the 65Ni partner and the absence of @jNithat is easily detected makes this assignment now firm. From the results discussed above it is apparent that each Ni isotope, including the 64Ni projectile, is produced in a variety of processes ranging from quasielastic to deep inelastic reactions which involve large excitation energies, that are frequently released by neutron evaporation. Consequently, the integrated production for each of them leads to the population of states in a broad range of excitation energy and spin. The results of the spectroscopic analysis of these excitations are presented in the next section.
3. Results and discussion In this section we review and briefly discuss the spectroscopic results obtained in the present experiment for Ni isotopes with A > 64. The 63Ni nucleus was populated with fairly low yield and no new information has been extracted beyond that existing already in the literature. We note that the 8.6( 8) ns half-life determined by us for the 63Ni ygg/2 isomer agrees well with the earlier measured [ ‘71 more accurate value of 9.4( 3) ns.
628
T Paw& et al./Nuckar
Physics A 574 (1994) 623-641
20QPb 779keV
,
64Ni
208Pb 2615 keV
l
l
1346keV 104.
104: *
1017keV
*
l
104
207P b 570keV
.
102’
Pb partner
63
64
65
66
67
Ni partner
Fig. 3. Distributions of Pb isotopes observed in cross coincidence with the indicated gamma lines from 63Ni to
67Ni (left) and corresponding Ni isotopes distributions for gates set on transitions from the F’bpartner nuclei (right). The intensity of the strongest Iine in the partner nucleus coincident with the indicated transition is presented, the scale is arbitrary but identicat for all cases. The detection Iimit for the heaviest isotopes marked with an atrow is crucial for the identification procedure (see text). The results indicated for the 2485 keV transition of XJJFh give a clear evidence supporting the tentative identification presented in Ref. [4].
3.1. Even Ni isotopes
64Ni used in our experiment as a projectile is the heaviest stable isotope and naturally the best known in the presently discussed series. It has been studied using many reactions predominantly populating the low spin states. Recent (a,2p) experiments f I] provided essential information on the high spin states confirmed later in the ( t2C, *‘C) transfer reaction [ 81. All of these results compiled in Ref. [ 91 include only scarce information on the y-radiation in the @Ni. Thus the level scheme displayed in Fig. 4 and established in the present experiment fills this gap by showing the observed gamma de-excitation of many states which were mostly known from other studies. Except for five gamma
629
7: Pnwlat et al./I?uclear Physics A 574 (1994) 623~64i 5812.5
r+
1280.4
4962.4 I 430.3 4532.1
7-
.
*+t
102u.4 2*
2276.8
1264.3
1346.0
*
*
t
I
nn
64 28
+
2+
Of
.
Nl 36
Fig. 4. The deduced level scheme for blNi. The width of the arrows indicates the gamma intensity.
transitions
for the first time. Their relative intensities account for the integrated population of a specific state in 64Ni produced in several processes as described in the previous section. The numerical values of relative intensities are given in Table 1. The spin-parity values come from the previous studies and the observed gamma
decay
all others are observed
nicely
supports
these assignments.
For two states at 3396 and 3750 keV the
direct population from the well established 5 state at 3849 keV strongly favors the 4+ ch~acte~~tjon thereby removing the existing (2+, 3,4+) ~biguity. The observed main yrast decay clearly indicates that the states at 5812 and 4532 keV are the highest spin 8+ and 7- states established in Ref. [ 1] with much less accurate excitation energies of 5810( 50) and 4600(50) keV. We leave the level at 4962 keV unassigned which apparently corresponds to the 4965(5) keV level previously seen in the (p,p’) and (t,p) reaction [ 91. At the same time we suggest a tentative 6- assignment for the 4173 keV level which is strongly populated in the yrast decay, and was also observed in the (p,p’) reaction. The strong feeding from the 7- state and depopulation through the .5- state
Z Pawtat et al./Nuclear
630
Physics A 574 (1994) 623-641
Table 1 Energies, intensities and placement of gamma transitions observed in 62-67Ni nuclei Ei (keV)
.& (keV)
Ii *
If
64Ni
99.6(6) 189.0(4) 236.5(5) 323.4(2) 359.4(2) 430.3(6) 452.9(6) 583.4(6) 785.7(5) 930.8( 1) 1239.0(3) 1264.3( 1) 1284.0(6) 1346.0( 1) 1474.8(5) 1521.5(4) 1820.4(5) 2049.8(4) 2214.4(5)
0.3( 1) 1.2(2) 0.5(2) 6.6( 5) 3.1(3) 0.5( 1) 0.8( 1) 0.4( 1) 1.3(2) 29.5( 15) 7.2(7) 65.0( 20) 0.3( 1) 100.0 1.0(2) 1.6(3) 1.0(3) 1.6(4) 1.1(3)
3849.3 3749.7 4085.5 4172.7 4532.1 4962.4 3849.3 3749.7 3396.0 2276.8 3849.3 2610.3 3560.6 1346.0 4085.5 2867.5 3166.4 3396.0 3560.6
5- h 4+ 4 (6-) 4 7- ---t 5- + 4+ 4+ ---t 2+ 5- 4 4f ---t 3- + 2+ d --t o+ (2+, 4+) 4 4f + 3- h
4f 355(6-) 74+ (2+, 4f) 4f 2+ 4+ 2+ 2+ o+ 4f 2+ 2+ 2+ 2+
65Ni
310.4(2) 382.4(4) 488.3(4) 616.5(3) 629.4( 3) 692.7(5) 720.6(2) 1003.5(4) 1017.0( 1) 1168.7(2) 1491.6(6) 1502.3(2) 1610.3(2) 1889.5(5)
71.0(30) 2.2(4) 3.0(4) 4.6(5) 21.0(20) 2.7(3) 7.1(5) 1.4(3) 100.0 14.6( 12) 1.1(3) 18.6( 15) 18.4( 15) 1.9(5)
310.4 692.8 4011.1 3522.8 692.8 692.8 2906.3 3522.8 1017.0 2 185.7 4011.1 2519.3 1920.7 2906.3
3/23/2(17/2+) (15/2+) 3/23/2(13/2+) (13/2+) 9/2+ (11/2+) (17/2+) (13/2+) 5/2+ (13/2+)
---f --, --+ 4 ---t -+ --t --t + h --P -+
5/23/2(15/2+) (13/2+) l/25/2(11/2+) (13/2+) 5/29/2+ (13/2f) 9/2+ 3/29/2+
66Ni
58.0( 5) 354.3(5) 355.9( 1) 471.1(4) 490.1(2) 1085.5(3) 1245.7(3) 1404.8(6) 1425.1( 1) 1760.3( 1) 1945.8(3)
1.3(2) 2.9(3) 69.0( 30) 3.4(5) 22.0( 20) 10.7( 12) 8.9(7) 2.3(4) 100.0 82.0(30) 9.0(8)
3599.3 3725.2 3541.3 4070.4 4089.4 5174.9 2670.8 6579.7 1425.1 3185.4 3370.9
-
(5-)
(5_)
--+ ---) 7- h 8+ + (lo+) 2f + (4+) --f -
(4+) (6-) (6-) 72+ 8+ o+ 2+ 2+
100.0 100.0
1007.2 694.1
9/2+ 5/2- d
5/21/2-
67Ni
313.1( 1) 694.1( 1)
(6-)
Z twit
et al. /Nuclear Physics A 574 (1994) 623641
631
66 28 Nl 38 l
Fig. 5. Partial level scheme of @Ni from the present investigation.
favors strongly the spin value of I = 6. Whereas a 6+ state at this low energy is very unlikely a negative parity 6- state is expected; this assignment is further supported by the observation of a similar state in &Ni as discussed below. In the 66Ni nucleus only three gamma ~ansitions were known from the %Yo (3+) radioactive decay studies [ 21. On the other hand the (cr,2p) experiments [ 1 ] established four high spin states at 3.39 (5-), 4.05 (7->, 4.76 (5-,8”) and 5.17 MeV (8+). In the present experiment the 66Ni was populated in the 2n quasielastic transfer reaction as well as in deep inelastic processes. Apparently these complex reactions contribute significantly to the overall a6Ni production since the observed population of high spin states is much more pronounced than in the 64Ni case (see Table 1). Cons~uently, the established level scheme shown in Fig. 5 involves mostly yrast states. Based on
632
L Paw&
et al./Nuclear
Physics A 574 (1994) 623-641
104
rA % 3
103;
E
102
j
I
,,lL. 0
10
20
time
30
40
[ns]
Fig. 6. The decay curves of the 1017 keV line in 65Ni (upper part) and the 1425 keV line in 66Ni (lower part). The dashed line represents the prompt curve obtained from the fast 1346 keV 2+ - Of transition in @Ni.
the ((u,2p) results we assign the states at 5175 and 4089 keV as the 8+ and 7- states correspondingly. In analogy to the 64Ni levels the states at 3541 and 3 185 keV which are strongly populated in the yrast decay are the 5- and 4+ states respectively; the mismatch of the 5- energy with the (cq2p) result can be attributed to the large energy uncertainty of the latter one. We note that the nearly 600 keV higher energy of the 4+ excitation in 66Ni compared to the 64Ni case is a simple consequence of the filling of the f5/2 neutron orbital at the neutron number N = 38 which closes an easy way to form the 4+ excitation. The gamma transitions below the 5- state did show a delayed component and the analysis of the time spectra indicated the presence of a Tt/2 = 4.3(4) ns isomer. The high energy of the 1760 and 1425 keV transitions involved in the isomeric decay gave the time resolution which enabled not only the half-life determination shown in Fig. 6, but also allowed to establish that the 471, 490, 1085 and 1405 keV transitions precede the decay of the isomer. With the 356 keV transition showing clearly the prompt component the low energy 58 keV transition (see Fig. 5) is the only candidate to be considered as the isomeric transition. Thus in the scheme of Fig. 5 the 4.3 ns isomer was located at the 3599 keV level and assigned as the 6- state in analogy to the 64Ni nucleus scheme. We leave unassigned the weakly populated new states at 3371, 3725 and 4070 keV. A possible correspondence with the states from the shell model calculation will be considered in the last section of this work. Few words should be devoted to the 2671 keV level which we characterize as the (yf;Ikpt,2) 3+ basing on the interpretation of the known %Zo radioactive decay [ 21. The $‘Cos9 ground state assigned tentatively as the 3+ state of the 7rf;)2vpt~2 doublet decays almost exclusively
T ?bviat et aL/Nucleur PizysicsA 574 (1994) 623-641
633
to the 2671 keV level in @Ni. Considering possible ways of this decay it is clear that
the Gamow-Teller transition changing one of the fs/z neutrons into the fT,q proton must be strongly favoured over other possibilities. Thus the final state in @Ni should be the simple neutron particle-hole f$pr,z excitation which is expected around the observed 2.7 MeV energy. Moreover, the complete absence of any /3 strength to the lowest ‘j6Ni 2+, which almost certainly involves a similar f;;ikpl/z component would then indicate a 4+ rather than a 3+ ~signment for the @Co ground state. Summ~~ng the results for the even Ni isotope we note that along with the fairly complicate nature of states with spin values I 6 4 the most transp~ent inte~retation of the @Ni and %Ni level schemes can be given for the high spin states. The negative parity states involve one g9/2 neutron and, as expected, their excitation energies are significantly lower in 66Ni compared to the 64Ni isotope. Whereas the 5- is predominantly of vgg/zpt/z nature, the 7- and 6- states structure must involve the yg9,2f;;!2 excitations. The presence of the 6- isomer in @jNi confirms this simple inte~retation and yields the ret~dation factor of 50 for the forbidden f5/2 +p1/2 Ml position. The even more pronounced lowering of the 8+ state excitation energy with increasing neutron number is naturally related to its est~lish~d two~neu~on g$,z structure. It would be highly desirable to extend the present info~ation including the closed N = 40 @Ni isotope, for which the 5- isomer and the further lowering of the 8+ state is expected. Indirectly our data indicate that the 68Ni isotope is produced in the present experiment, but as yet we were not able to extract any specific info~ation on its gamma transitions.
3.2. Odd
Ni
isotopes
The excited states of @Ni populated in the present experiment are shown in the level scheme of Fig. 7 and nume~c~ data on gamma intensities are given in Table 1. Since 65Ni is predominantly produced in the qu~iel~tic In transfer one observes a strong ~pulation of non-yrast states known from the earlier (d,p), (n,r) and j3 decay studies [ 103 ; these states are distinguish~ in Fig. 7 by firm spin-parity ~signmen~. Among them the ygg/2 excitation ten~tively assigned at 1013f 10) keV from the (dq) data [ 111 is strongly supported by the present result. The 1017 keV t~sition, un~biguously assigned to the 65Ni isotope f see Fig. 2) shows a decay displayed in Fig. 6. The resuhing 25(5) ns isomeric half-life perfectly matches the B(M2) (8912 --+fs/z) derived from the identical known transition in the a3Ni isotope. In the analysis this 9/2+ isomer half-life was exploited to obtain clean spectra in coincidence with the delayed 1017 keV transition thereby signi~cantly enhancing the detection sensitivity for the generally much weaker populated high-spin states. Five new levels were establish~ as shown in the scheme of Fig. 7 with tentative spin-p~ity ~signments which are based on simple expectations and observed gamma decay. Whereas the shell model c~culations presented in the next section support in general these ~signments it is clear that the ~sumption of 17/2+ for the 4011 keV level was crucial in suggesting the spin-pity ch~cte~~tion of the four lower lying new states. At 4 MeV excitation energy 1712’ is naturally expected as the
634
T. Pawlat et al./Nuclear Physics A 574 (1994) 623-641
5l2'
65 28
.
NI37
1007.2
9/2+
>0.3ps
6w.qr
694.1 0.0
67 28
I
a-
.
NI39
Hg. 7. The level schemes of the 65Ni and 67Ni isotopes. The 9/2+ prior to this study are marked with firm spin-parity assignments.
highest possible
spin from the coupling
isomer and the low-spin
states known
of the g9/2 neutron with the 4+ core excitations
of 5$j2 or vf5/2p3/2 type. Much less complete results were obtained for the 67Ni isotope and the experimental difficulty arises from the presence of the long lived isomer which virtually excludes the possibility to establish coincidence relationships with any transition feeding the isomer. The scheme of Fig. 7 shows the simple decay of this isomer naturally interpreted as the vggl2 state in 67Ni. The 694 keV gamma transition de-exciting the fs/2 67Ni state known to be populated strongly in the Gamow-Teller /3 decay of 67Co [2] was seen in the coincidence projection. The 694 keV gate gave only one strong coincident gamma line of 3 13 keV, except for the weak cross coincidence 205,204Pb lines discussed earlier
I: Pawlat et al./Nuclear
Physics A 574 (1994)
623-641
635
and confirming the 67Ni nucleus identification. The resulting 1007 keV excitation energy coincides with the 1020( 22) keV level suggested from the (cr,%e) reaction study I:121. Apparently the 313 keV gamma line corresponds to the 9/2+ --+ 5,f2- M2 isomeric transition from the neutron gs/z state, which is expected in the N = 39 67Ni at much lower energy than in the 63Ni and 65Ni isotopes. The similar B(M2) values of about 2 ( e~/2~c)2 fm2 for the 63Ni and 65Ni M2 positions allow to predict the value of M 8 ,US for the 67Ni isomeric half-life. Restricted by the present expe~ment~ conditions we can give only the lower limit of 0.3 ,us.
4. Shell model caIculations for Ni isotopes Shell model calculations for low-lying, low-spin states in Ni and Cu isotopes have been performed by Koops and Glaudemans [ 131. Assuming an inert 56Ni core and a p3/2, f5/2, pi/z model space for protons and neutrons they used the modified surface delta interaction (MSDI) and the isospin formalism. In the spirit of this approach selected Ni high spin states were calculated in a highly truncated model space including the g9/2 neutron orbital [ I]. More recently a complete set of realistic two-body matrix elements (tbme) was derived for the full p3/2, f5/2, PI/Z, gg/2 model space [ 141, which enables a comparison to the schematic MSDI. The realistic interaction has been used successfully for the corresponding proton space in N = 50 nuclei [ 14,15]. Its application to the Z = 28 neutron space allows an investigation of the concept of valence mirror nuclei. Therefore in the present work the two different residual interactions were used and adapted to the full N = 28-50 neutron shell UP~,Q, f5/2, pi/z, gg/2. The core nucleus was 56Ni and free parameters of the various model interactions were fitted to 25 selected single-, two- and three-particle states in 64Ni to 68Ni. Occupation of the p, f orbitals was left free up to their m~imum values of 2j + 1, whereas occupation of the 8912 orbit was limited to < 4. This turned out to be important to reproduce the splitting of the (f,p)“-2 x (gs,2)2 and (f,p)“-’ xg9,~2states of opposite parity, which cannot be achieved with any further truncation of the gs/2 occupation. In a first approach following Ref. [ 131 the MSDI was used in a model space including the g9,q orbital, which requires a renormalization of the interaction strength as compared to Ref. 1131. As a first step the single particle energies for P~,Q, fs/z and pt,2 were taken from previous work [ 131 as they are essentially fixed by the respective single particle states in 57Ni. The T = 1 interaction strength A1 and the gs/2 single particle energy were treated as free parameters. To allow for modification of the single particle energies (spe) in the mid shell nuclei 64-68Ni due to proton ph-excitation, in a second step the spe were optimized with respect to the level schemes of these nuclei. Finally, the monopole term Bt was fitted to the ground state binding energies of the Ni isotopes. In the second approach we used the realistic set of tbme, further on referred to as S3V, which was derived by Sinatkas et al. [ 141 from the Sussex nucleon-nucleon interaction [ 161 employing the Kuo-Brown method [ 171. As the interaction was renormalized with respect to a ‘@%n core and the single particle energies were adjusted to fit the spectra
‘I Paw& et al./Nuclear
636
Physics A 574 (1994) 623-641
Table 2 Shell model parameters Single particle (hole) energies [ MeV]
Interaction P3f2
MSDI
A1 = 0.38 MeV
-10.20
core
$2
PI/2
g9/2
-9.14
-8.97
-7.41
“Ni
Bt = 0.438 MeV s3v
see text
8.22
7.84
7.30
-3.41
68Ni
-9.42 8.78
-10.20 7.59
-8.30 7.07
-6.43 -3.21
56Ni @Ni
of N = 48-50 nuclei in the upper half of the proton and neutron shell, a new set of spe had to be found by a fit to the Ni isotopes. As in the MSDI approach this was performed with special emphasis on the 64-68Ni level schemes. To fit the ground state binding energies a general shift of 0.456 MeV was applied to all diagonal tbme and all single particle energies were reduced by 0.827 MeV relative to the 56Ni core. The final interaction parameters and single particle energies used in the two approaches are listed in Table 2. It should be noted that with the full model space the MSDI strength parameters A1 and B1 as compared to Ref. [ 131 are reduced to nearly their “canonic” values 25/A MeV [ 181. The single particle energies found for the two model approaches are quite different, note the p3/2-f5/2 inversion. For comparison energies relative to a 68Ni core are also listed in Table 2.
the single particle (hole)
5. Discussion of shell model results The results of the shell model calculations are compared to the experimental level schemes of 64Ni 66Ni 65Ni and 67Ni in Figs. 8-10. The experimental levels used in the fitting proiedure’ are marked by asterisks. In view of the large distance of 8-l 1 neutrons from the core nucleus 56Ni and the neglect of proton core excitations the overall agreement with experimental levels up to 6 MeV excitation energy is satisfactory. The mean level deviation (MLD) as defined in Ref. [ 191 is 0.317 MeV for both the MSDI and S3V sets of tbme. The main observations in comparison of experiment and shell model results can be summarized as follows.
5.1. Even isotopes
(9 In both calculations
(ii)
the first 2+ and 3- states, which presumably contain some collectivity, are predicted too high in excitation energy as a consequence of the limited configuration space (Figs. 8 and 9). The level order and the splitting of the two-particle states In = 5-, 6-, 7- with their main configurations g9/2p1/2, g9/2p3/2 and g9/2f5/2 is poorly reproduced in contrast to the well predicted (g9/2);+ state. In the SD1 approach this is mainly due to the vanishing interaction for states of unnatural parity as I” = 6-. The S3V
1: Pawlat et af./Nuclear Physics A 574 (1994) 623-641
637
+
-8
6248
-
8+
-
7-
8+
l 581.?
+
-6 -
10+ +
5615 --9 ---__6+
7-
-6+ -2 -
4962 4756 4693
5; _
7-
7-
l 4532
4431
6-
:.----$
-
-4
8' s-
+
-2
-4
D
4+
+
-2
3-
0+
-
4+
-2
4+ 2+ +
3166
(2+,4+)
-
4+
o+
s3v
-
4064 3935 -
6+ - s-
z+
*%A 3561
$
-
+
(6-l
iE
'2867
0+
'2610
4+
2277
2+
'1346
2+
3100
3-
2724
0+
%9%
2+ 4+
1565
2+
2+
-0
+
MSDI
0
0+
ECCP
0
0+
EXP
64Ni,, Fig. 8. Experimental and shell model level schemes for @Niqe _ and the experimental scheme of the valence-mirrornucleus $Kr, interaction seems to overcompensate this effect. Inspection of the model wave functions for the I” = 5-, 6-, 7- multiplet shows that these states are mixed with predominant gg/zf;i: (> 70%) and g9/2p1/2 (> SO%) character for the I” = 6-, 7- and .5- states, respectively. Therefore their relative positions can hardly be influenced by reasonable variations of the spe, but are an inherent consequence of the residual interaction. This raises the problem of explaining the retarded 6- ---+ 5- Ml transition, giving rise to an isomer in 66Ni. In the case of pure g9/2f5/2 and g9/2p1/2 configurations, respectively, I-forbiddeness would be a straightforward explanation. We have calculated the Ml transition strength using effective single particle Ml matrix elements for p3i2, f5/2 and pl,~ 1203. The effective g9/2 Ml matrix element was taken from the g-factor of the (g/2+) level in 63Ni [ 71. The
638
I: Pawtat et al./Nuclear Physics A 574 (1994) 623-641
-
6560
lo+ -
-
10
$
(lo+)
+
'517%
8+
*go-
?-
+
-4
-
7-
-
g:
m
34: 6_
-
74' s-
-
x+ 3+ +
-0
-$ -
+
*
-0
SW
g3
*3185
(4*)
3'
-o+
-2
3725 a': 3371
-
2671
3+
'2445
o+
*142S
2+
2+
-0
MSDI
+
0
0+
EXP
~p~~~e~~ and shell model level schemes for @Ni. Experimental levels included in the fitting procedureare markedby asterisks. Fig. 9.
B(M1 ) values are 8.9 and 50 mWu for the MSDI and S3V approach, respectively, which agree well with the experimental value of 21(2) mWu. Also in 66Ni the I” = 3+ state with predominant configuration f;/\pt,z and the
f$g;,* * I” = lO+ state are well accounted for in both theoretical approaches. (iii) In Fig. 8 for imprison the valence mirror nucleus to 64Nis6 the N = 50 nucleus $Cr is shown. The data are from Ref. [ 15 f . A nice one to one correspondence is established, which is only distorted for states carrying collective strength that are subject to core excitation. This does, however, not imply, that the corresponding states are pure valence nucleon states and not affected by core excitations. It has been shown that the 8*, 9+ and lO+ states in %r are strongly influenced by
T: Pawht et aL/Nuclear Physics A 574 (1994) 623-641 -
17/2+
'4011
(17/2+)
3523
(15/2+)
2903
(13/2+)
'2519
(13/2+)
'2186
w/2+)
-s/2+ -
13/2+
-
11/2+
-
13/2+ 5t2+
-
17/2:
----cafe+
1921
-Q/2+
639
-3f2
5/2+
*+ l 1017
9/Z*
693
3/2-
* 310
3/2-
*
$$I
--3$2
-
1l2-
-s&c
-3l2--
s3v
5l2-
MSDI
l/2-
y
Es@
@Ni,,
0a -3/2-
-
-
s3v
w
-
Ql2+
-
3/2-
9/2+ 5/2-
-
5/2-
1/2-
-
l 1007
g/2+
694
512-
l
l/2-
0
MSDI
1/2-
EXP
67Ni3,
Fig. 10. Same as Fig. 9 for 6sNi (a) and 67Ni (b).
the yg$ds,2
core excitation
[ 151, and the co~sponding
7if;‘ifg,t2 con~guration
should be at least equally effective in @‘Ni. Neve~el~s the similarity of the valence mirror level schemes is su~rising, as the proton single particle level sequence for a 78Ni core differs considerably [ 141 from 56Ni. 5.2. Odd isotopes (i)
With the readjusted single particle energies (see Table 2) the Iz = 3,12-, 5/Z-, l/2- and Q/2+ single particle (hole) states in 65*67Niare well described. The fact
640
T: Pawiat et al./Nuclear
Physics A 574 (1994) 623-641
that the 56Ni spe have to be changed towards midshell can be taken as evidence for proton core excitations, which according to the Sn and Pb isotopes should be strongest in midshell. The treatment of lplh and 2p2h proton core excitation due to computer capacity limitations were beyond the scope of the present model approach. (ii) In 65Ni for the three-quasiparticle states of main configuration (f,p)$gg,z and spin/parity 5/2+-17/2+ similar deficiencies of the model calculations are observed as for the I* = 5--7- two-quasiparticle states in the even isotopes. While the centroids of the two multiplets are well accounted for, the splitting is too small in both model approaches. Therefore the level sequence of yrast and yrare high spin states is not well reproduced for similar reasons as in the even isotopes. In suck this first attempt to give a consistent shell model desc~ption of neutron rich Ni isotopes accounts reasonably well for the gross structural features of one-, twoand three-particle states. There are still open problems concerning the trend of single particle levels from 56Ni to the N = 40 subshell closure, which for the moment do not allow a meaningful extrapolation towards ‘*Ni. More experimental data, especially on 69,70Ni,would help to clear the situation. The role of proton core excitations has to be investigated, which is only feasible with severe truncation in the neutron configuration space. The concept of valence mirror nuclei works nicely, and core excitations seem to have only little influence on it. This is all the more surprising as the proton and neutron single particle level sequences are different. The realistic interaction (S3V), first used successfully for proton excitations in N = 50 nuclei [ 14,151, in the N = 28 neutron space gives only marginal improvement over the schematic MSDI interaction.
Acknowledgement
This work has been supported in part through the agreement on scientific cooperation between Poland and Germany and by the Polish Scientific Committee under grant no. 2243 19203.
References [ 11 U. Fister, R. Jahn, Pvon Neu~n-Cosel, i? Schenk, T.K. Trelle, D. Wenzel and U. Wienands, Phys. Rev. C 42 (1990) 2375. f21 U. Bosch, W.-D. Schmidt-Ott, E. Runte, P Tidemand-Pettersson, R. Koschel, E Meissner, R. Kirchner, 0. Klepper, E. Roeckl, K. Rykaczewski and D. Schardt, Nucl. Phys. A 477 (1988) 89. [3] R. Broda, M.A. Quader, P.J. Daly, R.V.F. Janssens, T.L. Khoo, WC. Ma and W.M. Drigert, Phys. L&t. B 251 (1990) 245. 141 M. Schramm, H. Gmwe, J. Heese, H. Kluge, K.H. Maier, R. Schubart, R. Broda, J. Grebosz, W. Kr6las, A. Maj and J. Blomquist, Z. Phys. A 344 (1992) 121. t51 M. Schramm, H. Grawe, J. Heese, H. Kluge, K.H. Maier, R. Schubart, R. Broda, J. Gqbosz, W. K.r6las, A. Maj and J. Blomquist, Z. Phys. A 344 (1993) 363. [61 W. Kr6las, R. Broda, J. Grebosz, A. Maj, T. Pawtat, M. Schramm, H. Grawe, J. Heese, H. Kluge, K.H. Maier and R. Schubart, IFJ Krak6w Ann. Rep. (1992) p. 57, HMI Berlin Ann. Rep. (1992) p. 243.
T. Pawlal et al. /~~c~~ar Physics A 574 (I 994) 623441
[7] t8l [9] [IO]
641
W. Mliller, H.H. Bert&at, H. Haas, B. Spellmeyer and W.-D. Z&z, whys. Rev. B 40 (1989) 7633. A. Boucenna, L. Kraus, I. Linck and T.U. Ghan, Phys. Rev. C 42 (1990) 1297. B. Singh, Nucf. Data Sheets 62 (1991) 614. M.R. Bhat, Nucl. Data Sheets 69 (19933 215. [Ill T.R. Anfinsen, K. Bjemdal, A. Graue, J.R. Lien, G.B. Sandvik, LG. Tveita, K. Ytterstad and E.R. Cosman, Nucl. Phys. A 157 (1970) 561. 1121 R.T. Kouzes, D. Mueller and C. Yu, Phys. Rev. C 18 (1978) 1587. [ 131 J.E. Koops and l?W.M. Glaudemans, Z. Phys. A 280 (1977) 181. [ 141 J. Sinatkas, L.D. Skouras, D. Strottman and J.D. Vergados, J. Phys. G 18 (1992) 1377, 1401. [ 151 G. Winter, R. Schwengner, J. Reif, H. Prade, L. Funke, R. Wirowski, N. Nicolay, A. Dewald, R von Brentano, H. Grawe and R. Schubart, Pttys. Rev. C 48 ( 1993) 1010. [ 161 JR Elliott, A.D. Jackson, H.A. Mavromatis, E.A. Sanderson and B. Singh, Nucl. Phys. A 121 (1968) 241. [ 171 T.T.S. Kuo and GE. Brown, Nuci. Phys. 85 (1966) 40; A 114 ( 1968) 241. I181 P.J. Brussaard and RW.M. Glaudemans, Shell model applications in nuclear spectroscopy (NortbHolland, Amsterdam, 1977) p. 116. [ 191 R. Gross and A. Frenkel, Nucl. whys. A 267 ( 1976) 85. [20] P.W.M. GIaudem~s, M.J.A. de Voigt and B.F.M. Steffens, Nucl. Phys. A 198 (1972) 609