Observation of a full set of coexisting intruder excitations in 98Zr: Evidence for correlated excitations?

Volume 177, number 3,4

PHYSICS LETTERS B

18 September 1986

O B S E R V A T I O N O F A F U L L S E T O F C O E X I S T I N G I N T R U D E R E X C I T A T I O N S IN 9SZr: EVIDENCE FOR CORRELATED EXCITATIONS? ~ R.A. M E Y E R , E.A. H E N R Y , L.G. M A N N Nuclear Chemistry Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA

and K. H E Y D E Institute of Nuclear Physics, Proeftuinstraat 86, B-9000 Ghent, Belgium

Received 17 March 1986; revised manuscript received 3 July 1986 Results from multiparameter in-beam spectroscopy studies of 98Zr58 reveal the existence of two distinct sets of levels with strong intra-set transitions. The nine levels identified in the excited set are found to correlate well with the first nine levels of I°2Ru. Similar sets in Zr isotopes and isotones of 98Zr58 are shown for this mass region.

The available p r o t o n shell-model states b e y o n d Z = 38 are the 2pa/2 and 199/2 orbitals.For the 4oZr nuclei with N >~ 50, the g r o u n d state and first excited 0 + state can be described as linear combinations of the p r o t o n (2pw2) 2 a n d (199/2) 2 configurations. As the neutron n u m b e r in the Zr nuclei increases the amplitude of the proton(1g9/2) 2 configuration in the 0 + excited level increases [1-5] and there is at the same time a steady decrease in excitation energy for this 0 + level. Excitation of p r o t o n pairs from below the Z = 38 subshell can contribute to this particular excited state giving rise to an even larger occupation of the p r o t o n lg9/2 orbital, especially when the neutron n u m b e r if N > 56 (where the 3sl/2 and lg7/2 neutron orbitals are starting to fill). This is caused by the large p r o t o n - n e u t r o n interaction energy between the lg9/E-proton and lg7/2-neutron orbitals c o m p a r e d to either the 2pl/2-proton and lgv/E-neutron or the lg9/2-proton and 2d S/E-neutron orbitals. Such a mutual p r o t o n - n e u t r o n polarization effect has been used This work supported in part by US DOE through contract Nr. W-7405-ENG-48 and in part by NATO research grant RGO 565/082/D1.

by F e d e r m a n and Pittel [6] to explain the onset of deformation in the A - 100 mass region. Also, in the S r - Z r - M o region, the (d, 6Li) cross section exhibits large strength to the first 0 + excited state [1-3,5] and can be taken as evidence for strong proton-pair neutron-pair correlations in this level for the A = 100 mass region. Such correlations form a necessary basis for invoking real alpha clustering. The latter, however, would imply the observation of dipole states near the first excited 0 + states. However, our in-beam conversion electron studies, c o m b i n e d with our gamma-ray studies, show no evidence for the presence of associated negative parity states. We have reviewed recently the occurrence of intruder states in odd mass nuclei near single closed shells and have given evidence for their occurrence in e v e n - e v e n nuclei [7]. However, todate, explicit evidence has not been found for the occurrence of an entire set of collective excitations built on an intruder configuration in e v e n - e v e n nuclei [8], although there are numerous cases of observed yrast levels associated with intruder configurations [9-11]. We have performed in-beam g a m m a - r a y singles and coincidence spectroscopy studies with the

0 3 7 0 - 2 6 9 3 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

271

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96Zr(t, p)98Zr r e a c t i o n using m u l t i p a r a m e t e r (proton-gamma-gamma-time) c o i n c i d e n c e techniques at the L a w r e n c e L i v e r m o r e N a t i o n a l L a b o r a t o r y i n - b e a m s p e c t r o s c o p y system at the Los A l a m o s I o n B e a m Facility. A n a l y s e s of these d a t a have revealed a n u m b e r of previously u n o b served levels a n d have d e t e r m i n e d j~r values for k n o w n levels [12]. In table 1 we give the present status of the s p i n - p a r i t y values that result from the analysis of o u r d a t a a n d f r o m previous studies. In fig. l a we show the excited states of 98Zr b e l o w 3.3 M e V where we have p l a c e d the levels into two sets. The first set has its m a i n deexciting transitions leading to the 98Zr g r o u n d state while the second set has its m a i n deexciting transitions leading to the 854 keV 0 + level. W h e r e possible we give the ratio of the B(E2) values for the J to J - 2 transition that stays within the set c o m p a r e d

Table 1 Levels observed in 98Zr. Energy

jr

Spin-parity comment

0 854 1223 1437 1590 1744 1806 1843 1859 2048 2104 2487 2491 2568 2613 2800 3217

0+ 0+ 2+ 0+ 2+ 2+ 34+ 04 4+ 24 3+ 64 44 24 584

A) A) A) A) A) A) A) B,C,D) A) A) C,E) C,E) (tentative value) C,D,F) C,E) (tentative value) C,E) A) C,D,E) (tentative value)

A) Assignment from Nuclear Data Sheets [12]. B) Angular distribution in (t, p) cross section work consistent with I = 4 and new transitions observed in our work are consistent with a 4 + assignment. c) Deexcitation branching ratios and population from higher energy levels support given J= value. D) Sole population to J - 2 level, transition intensity in our (t, p) experimental data, and (t, p) excitation functions agree with other (t, p) in-beam experiments for the case of J to J - 2 cascades. E) Previously unobserved. F) Populated by gamma-de-excitation of the known 5 level. 272

18 September 1986

with the J to J - 2 transition that crosses to the o p p o s i t e set. Using unified shell-model calculations. F e d e r m a n and Pittel [6] have shown that, for the excited 0 ÷ state, a larger t h a n expected o c c u p a n c y of the p r o t o n lg9/2 o r b i t a l will occur due to the p r o m o tion of extra p r o t o n s from b e l o w the Z = 38 subshell closure. D u e to the larger 1g9/2 o r b i t a l occ u p a t i o n , the n e u t r o n lg7/2 o r b i t a l has been shown to be lowered with respect to the 2d5/2 o r b i t a l t h r o u g h the residual p - n i n t e r a c t i o n [6]. This lowering will facilitate the p r o m o t i o n of n e u t r o n pairs across the 2d5/2 subshell closure, l e a d i n g to a larger o c c u p a t i o n of the l g v / 2 - n e u t r o n o r b i t a l a n d thus giving rise to p r o t o n - p a i r , n e u t r o n - p a i r correlations in the 0 + excited state. Thus, from the view of the n u m b e r of n u c l e o n pairs available (particle plus hole pairs), we might expect that 98Zr* should l o o k like a°ZRu. I n fig. l b we show the set of levels associated with the 854 keV 0 + state of 98Zr, which we term 98Zr*, j u x t a p o s e d to the k n o w n levels in l°2Ru [13]. T h e picture of the 0 + excited state, in which the o c c u p a t i o n s of the l g 9 / 2 - p r o t o n a n d the lg7/2n e u t r o n orbitals are e n h a n c e d at the expense of the orbitals b e l o w the respective gaps, is typical of the g r o u n d - s t a t e structure in the heavier M o isotopes [6]. Thus, when the M o isotopes are used as targets for the (d, 6Li) reaction, large cross sections [3] into the excited 0 + states of the e v e n - e v e n Z r nuclei, where the d e p l e t e d p o p u l a t i o n of the orbitals b e l o w the gaps is preserved show up as well as small (d, 6Li) cross sections to the g r o u n d states. The a b o v e a r g u m e n t s are further s u p p o r t e d b y the very low energy n e e d e d to excite alpha-like p a i r i n g v i b r a t i o n s ,t ( E x ( a ) ) in this m a s s region. A n estimate of E x ( a ) a m o u n t s to 2 B E ( A , Z ) B E ( A - 4, Z - 2) - B E ( A + 4, Z + 2) (see fig. 2a for a c o n t o u r plot of E x ( a ) ) a n d gives values on the o r d e r of 1 to 0.5 M e V for the 96'98Zr nuclei. Even lower energies seem to occur in the nucleus l ° ° M o which is j u s t two p r o t o n s outside a Z = 40,

,a Since we concentrate on nuclei with a neutron excess, the energy E,,(a) should be related more to two-proton, two-neutron pairing vibrations than to real alpha-pairing vibrations. Therefore, we call the excitations alpha-like pairing vibrations.

Volume 177, number 3,4

PHYSICS LETTERS B

18 September 1986

b (8 + )

5-

3217

IC

2+

2613

(4 +) ' ~ . ~ ~+

2568 2491

4+

(3+)? . / ~ .

2487

EO

C

8+

d5/2

2573

5+--T~/~2441

~'~'2048

1806

4+

2+ ~2

d5/2

5

2104

0+ 3-

g7/2

IC IC IC

17

g7/2

2800

2+

IC

1

2+

1590

0+

1437

1859 ~.~

EO

+)

0+ 5

1892

2

+

~

4

+

~

6+ 2+

1759

(4 +)

1714 ~

6+

1637 / "7"~'~

~_-Z--~3~)_L~1833-

0+

2363

1843 1744

. . . .

2+

1223

854

/

/

1873

~

//

4~../.~1798

".. 2~

158o

4+

1106

11

EO

0÷

1169

2*

1008 932

4+

2÷

0+

~ ( 8

2~/"~2265

0

0+

98 40Zr58

371

01

CALC.

-- _ -

0÷

1004

/

/ / / ~ /'

- - 4-~2-÷" L J ~ 9- -8- 9 j - . v - "

--'~ <"~

890

2+

0+

/

368

0 ~

98Zr*

0+

944

2+

475

O*

0

/

~

ti'

102Ru

Fig. 1. (a) Levels of 98Zr below 3.3 MeV observed in the present experiment. Numbers at the side of the levels are the ratio R which we define as the J to J - 2 intraset B(E2) value divided by the J to J --2 B(E2) value for the interset transition (i.e. a transition to a level in the opposite column). We use EO to denote a 0 + level that has its largest EO transition to the opposite set of levels and IC to denote levels where we only observe transitions that stay within the band. (b) Illustration of the increased number of nucleon pairs in 98Zr* over those in 97Zr (n.b. in each case, the right-hand column illustrates the expected distribution of neutrons). (c) Comparison of the intruder state set of levels (98Zr*) with known levels in l°2Ru (right side) and IBM-2 calculations (left side).

N---56

core

[3]. T h e

alpha

separation

energy

S,,(A, Z)---- -Q~,(A, Z ) for t h e s e m e d i u m h e a v y nuclei, w h e r e the p r o t o n a n d n e u t r o n F e r m i levels a r e q u i t e d i f f e r e n t , c a n serve as a n i n d i c a t i o n o f the e a s e o f f o r m a t i o n o f s t r o n g p r o t o n - p a i r , n e u t r o n - p a i r c o r r e l a t e d states. T h e a l p h a s e p a r a t i o n energy does have a rather small value of - 4 MeV in this A - 100 m a s s region, a l t h o u g h t h e s m a l l e s t

v a l u e s of t h e a l p h a s e p a r a t i o n e n e r g y o f - 2 M e V r e s u l t in t h e R u n u c l e i w i t h N = 52 (see fig. 2b f o r a c o n t o u r p l o t o f the a l p h a - s e p a r a t i o n e n e r g y in t h e A - 100 region). I n p e r f o r m i n g I B M - 2 c a l c u l a t i o n s to d e s c r i b e 98Zr*, a g o o d e s t i m a t e o f t h e p r o t o n a n d n e u t r o n b o s o n n u m b e r s is d i f f i c u l t to m a k e d u e to the p r e s e n c e o f s u b s h e l l o c c u r r e n c e for b o t h p r o t o n s 273

PHYSICS LETTERS B

Volume 177, number 3,4 3

2 1

0

0

0.5

can be seen in fig. lb, the calculations agree well with both the known l°2Ru levels and the set of levels built on the first excited 0 ÷ state in 98Zr (i.e.

44

l

42

98Zr*).

Z 40

38

36

°51 50 mN 4 3

52

2

54

56

I

I

58

60

3

2

4

a 62

5

42 Z 40

36

50

52

54

1

I

t

56

58

60

b 62

mN

Fig. 2. Contour plot for the 40 ~
[14].

and neutrons. For 98Zr neutron pair excitation across the N = 56 subshell gap actually does not change the neutron boson number N, counting from N = 50, since this merely results in a redistribution of the neutron 0 ÷ pair distribution (N, = 4). The effect of increased lg7/2 neutron orbital occupation through the p r o t o n - n e u t r o n interaction, on the other hand, can change the proton pair distribution by lifting extra proton pairs from the Z = 38 shell into the lgg/2 proton orbital. This can be taken to lead to a larger effective value for N,~ (i.e. we use N,,(eff.)- 3). Thus, the increased collectivity associated with the 98Zr* excitation is simulated in a phenomenological way (see fig. lc). The parameters used in our IBM-2 calculation were taken from Van Isacker and Puddu [15]. As 274

18 September 1986

Strong mixing between intruder configurations and the ground state vibrational configurations has been observed [8,10,16-20]. Calculations, treating this mixing in a detailed way within the IBM-2, were first carried out by Duval and Barrett [21] for the Hg nuclei. In nuclei such as 11°'112'114Cd, where the two different types of configurations have close lying levels of the same spin and parity, the level gamma-ray deexcitation patterns can be significantly altered with respect to the unperturbed situations [8,10,17]. Indication of strong mixing in 98Zr may be manifested in the deexcitation of the 98Zr 1843 keV (4 +) level where detailed experimental analysis can give only a limit to the out-of-band transition (due to the doublet nature of the gamma-ray photopeak in the in-beam data as well as previous beta decay experiments). Further indication of large mixing in 98Zr can be illustrated by studying the ratio R = [ B ( E 2 ; 2 i ~ 0i)/B(E2; 2 i ~ 0gs) ]. The known intruder band head in 96Zr is at 1581 keV and lowers to 854 keV in 98Zr. Thus the intruder states can be expected to mix more strongly with the ground state vibrational states in 98Zr compared with 96Zr. This is reflected in the ratio R for 96Zr compared with the one we observe in 98Zr (see fig. 3a): R = 115 in 96Zr while R = 11 for 98Zr, a decrease by a factor of ten. This strong mixing also gives rise to the large monopole matrix elements observed in 98Zr for transitions between the first four 0 ÷ levels [181. Comparison of the isotopes and isotones of Zr suggests that the strong proton-pair, neutron-pair correlated structures extend over a wide range of nuclei. In fig. 3a we show the systematics of the levels of Zr that exhibit the largest occupation of protons in the lg9/2 orbital and neutrons in the lgT/2 orbital. We juxtapose the 0 + excited state and associated 2 + states for the A ~< 98 Zr nuclei to the ground state bands for the A > 98 Zr nuclei. The levels of this series display a steady progression beginning with the closed neutron shell nucleus 9°Zr that has its first (excited) 0 ÷ level at 1761 keV and associated 2 ÷ level at 2081 keV

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Volume 177, number 3,4

18 September 1986

a

(6 + )

2180

(8 + )

2363

6+

1637

2+ 20~2 (15.6)

(,2 +

(8 + ) (1551)

4+

1276

2+ 1067 ( _> 100)

2+

644 (115)

4+

989

2+

368

(11) 0+

0+

117611 90

(8 + ) (1685)

1531}?

92

0+

0+

0+

116o91

115811

113001 94

96

6+

1063

4+

564

2+

212

0+

865

4+

428

2+

152

0+

18531 98

6+

I oJ 100

I

ol

102

40 z r b

2+ " ~ >

200

8+ (8 +) >14 35 2

9-2+

6+

~

2+

o÷

-"~

.! .z_

0+

1-

,tl

(4 + )

(2+ )

2+

O-

41

-'N

~2O0

4+

2*/--

58

2+ 485

2+

0+

0+

0+

Imgl

I~1

I o I

Fig. 3. (a) Comparison of known intruder band members in N = 50-58 Zr nuclei with ground state yrast bands in N = 60 and 62 Zr nuclei. The box below the J'" = 0 + state gives the excitation energy of the intruder state while the value given in parenthesis under the 2 + level is the B(E2) ratio R. (b) Comparison of proton-pair, neutron-pair correlated states in the n = 58 even-even isotones with 38 ~< Z ~<42. The position of the set of states associated with the 695 keV 0 + level in 100 Mo is shown in boxes. The values to the right of the levels are the values of the B(E2) ratio R (see text).

275

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higher in excitation energy, to l°2Zr which has a fully developed rotational ground state b a n d that possess a m o m e n t of inertia parameter of h2/2j = 25.3 keV [20]. The characterization of 98Zr* and its association with l°2Ru fits well into the progression from vibrational via gamma-soft deformed to fully rotational as the lg7/2-neutron occupancy grows. This comparison is a further manifestation of the increasing collectivity as the lgT/2 neutron orbital is populated first by polarization and eventually at N > 58 where it becomes the ground state configuration (see discussion by F e d e r m a n and Pittel [6]). Thus, the " s u d d e n " onset of deformation previously thought to occur in this mass region m a y be an artifact of correlating different structural types. Inspection of fig. 2a suggests that we should be able to identify similar structures in the isotones of 98Zr58. The energy contour (fig. 2a) suggests that while these alpha-like correlated structures should be expected at higher excitation energies in 96Sr58; they should become the ground-state set in 1°°M058. Jung has studied the levels of 965r p o p u lated in the beta decay of mass separated 96Rb [22]. In fig. 3b we show that there is a set of levels in 965r built on the 1229 keV 0 ÷ level which behave in a way similar to those we observe in 98Zr in the case of l°°Mo, the levels have been investigated by (n, n'y) reaction spectroscopy [23] and more recently by 96Zr(TLi, p2n)l°°Mo in-beam spectroscopy [24] as well as via beta-decay spectroscopy of l°°Nb using the recoil mass separator J O S E F [25]. In fig. 3b we show the combined results of these studies where, albeit less complete than what we find for 98Zr, we can again identify two sets of levels for l°°Mo. In this case, consistent with the prediction based on the alpha-like correlation energies, the ground state set corresponds well with the 98Zr* set of levels. A consequence of our characterization of 98Zr is that the first 2 + levels associated with the ground state configuration occur at 1590 keV which is close to the 1750 keV energy of the first 2 + level in 96Zr. This indicates that the ground state configuration of 98Zr retains fairly well the double-closed subshell nature k n o w n in 96Zr. This retention of double-closed subshell nature receives further support from the 2 7 / 2 - shell-model isomer recently 276

18 September 1986

f o u n d in 97y [25], which is one p r o t o n hole in 98Zr. Thus, in this special case the double-closed subshell nature m a y be taken to extend beyond more than a single nucleus. In summary, by performing in-beam g a m m a - r a y spectroscopy studies using the 96Zr(t, p)98Zr reaction, we have been able to identify a set of eight levels that are related to the 854 keV 0 ÷ intruder state in 98Zr. This excited set, 98Zr*, correlates well with the k n o w n excited levels built on the ground state of e v e n - e v e n l°2Ru and is shown to c o m p a r e well to IBM-2 calculations using N= (effective) = 3, N~ = 4. Further, we are able to show that similar structures occur in the neighboring e v e n - e v e n isotopes and isotones of 98Zr58 and that their excitation energies follow alpha-like correlation energies for this mass region. Juxtaposition of the excited structures in A ~< 98 Zr isotopes to ground state bands in A > 98 Zr isotopes shows a smooth transition from vibrational via gamma-soft to symmetric-rotor structure. One of the authors (K.H.) is grateful to the N F W O and I I K W for financial support while another of us (R.A.M.) wishes to thank N A T O and the Institute for Nuclear Physics at the Rijksuniversiteit Gent for their support and hospitality during part of this work.

References [1] A. Saha, G.D. Jones, L.W. Put and R.H. Siemssen, Phys. Lett. B 82 (1979) 208. [2] A.M. Van den Berg and R.H. Siemssen, Nuovo Cimento 81 (1984) 318. [3] F. Catara, L. Ferreira, A. Insolia, A. Vitturi and R. Broglia, Nucl. Phys. A 372 (1981) 237. [4] W. Mayer, D. Pereira, K.E. Rehm, H.J. Scheerer, H.J. Korner, G. Korschinek, W. Mayer, P. Sperr, S. Pieper and R. Lawson, Phys. Rev. C 26 (1982) 500. [5] J. Janecke and F.D. Bechetti, in: Nuclear physics, eds. C.H. Dasso, R.A. Broglia and A. Winther (North-Holland, Amsterdam, 1982) p. 253. [6] P. Federman and S. Pittel, Phys. Rev. C 20 (1979) 820. [7] K. Heyde, P. Van Isacker, M. Waroquier, J.L. Wood and R.A. Meyer, Phys. Rep. 102 (1983) 291. [8] A. Mheemed, K. Schreckenbach, G. Barreau, H.R. Faust, H.G. Borner, R. Bissot, P. Hungerford, H.H. Schmidt, H.J. Scheerer, T. von Egidy, K. Heyde, J.L. Wood, P. Van Isacker, M. Waroquier, G. Wenes and M.L. Stelts, Nucl. Phys. A 412 (1984) 113.

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[9] A.F. Barfield, B.R. Barrett, K.A. Sage and P.D. Duval, Z. Phys. A 311 (1983) 205. [10] D. Kusnezov, A. Bruder, V. Ionescu, J. Kern, M. Rast, K. Heyde and R.A. Meyer, Phys. Rev. C (1985), to be published. [11] J. Bron, W.H.A. Hesselink, A. Van Poelgeest, J.J.A. Zalmstra, M.J. Uitzinger, H. Verheul, K. Heyde, M. Waroquier, P. Van Isacker and H. Vincx, Nucl. Phys. A 318 (1979) 335. [12] H.-W. Muller, Nucl. Data Sheets 39 (1983) 467. [13] P. de Gelder, D. de Frenne and E. Jacobs, Nucl. Data Sheets 35 (1982) 443. [14] A.H. Wapstra and G. Audi, Nucl. Phys. A 432 (1985) 55. [15] P. Van Isacker and G. Puddu, Nucl. Phys. A 348 (1980) 125. [16] R. Julin, J. Kantele, M. Luontama, A. Passoja, T. Poikolainen, A. Backlin and N.A. Jonson, Z. Phys. A 296 (1980) 315. [17] J. Kantele, in: Heavy ions and nuclear structure, eds. B. Sikora and Z. Wilhelmi (Harwood Academic, New York, 1984) p. 391.

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[18] K. Kawade, G. Battistuzzi, H. Lawin, H.A. Selic, K. Sistemich, F. Schussler, E. Monnand, J.A. Pinston, B. Pfeiffer and G. Jung, Z. Phys. A 304 (1982) 293. [19] F. Schussler, J.A. Pinston, E. Monnand, A. Moussa, G. Jung, E. Koglin, B. Pfeiffer, R.V.F. Janssens and J. van Klinken, Nucl. Phys. A 339 (1980) 415. [20] R.A. Meyer, Hyp. Int. 22 (1985) 385. [21] P.D. Duval and B.R. Barrett, Phys. Lett. B 100 (1981) 223; Nucl. Phys. A 376 (1982) 213. [22] G. Jung, Ph.D. Thesis, Justus Liebig Universit~it (Giessen, Fed. Rep. Germany, 1980); H. Behrens, Nucl. Data Sheets 35 (1982) 281. [23] G. Molnar, I. Dioszegi, A. Veres and M. Sambataro, Nucl. Phys. A 403 (1983) 342. [24] D.E. Hook, J.L. Durell, J. Lukasiak and W.R. Phillips, Schuster Laboratory Annual Report (1984) p. 55. [25] G. Menzen, Ph.D. Thesis, Universitat Koln (Cologne, Fed. Rep. Germany, 1985).

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