PHYSICAL REVIEW C VOLUME 48, NUMBER 4

Broken reAection symmetry in ""Xe

OCTOBER 1993

S. L. Rugari, * R. H. France III, B.J. Lund, Z. Zhao, and M. GaiA. W. Wright Nuclear Structure Laboratory, Yale University, New Haven, Connecticut 06511

P. A. Butler, V. A. Holliday, A. N. James, G. D. Jones, R. J. Poynter, R. J. Tanner, and K. L. YingDepartment of Physics, Oliuer Lodge Laboratory, University ofLiuerpool, P.O. Box 147, Liverpool L693BX, United Kingdom

J. Simpson

(Received 11 March 1993)

We have applied gamma-ray spectroscopy and selection techniques to study for the first time high spinstates in "Xe. This provides evidence for broken reAection symmetry near the N =Z line in medium-mass nuclei. We discuss simple scaling for enhanced E1 s in nuclei exhibiting broken reflection symme-try.

PACS number(s): 23.20.Js, 11.30.Er, 25.70.Jj, 27.60.+j

In this work we report the observation of high spinstates in "Xe, using in-beam spectroscopy techniquesand deduce for the first time signature(s) of brokenreflection symmetry near the X =Z line. Although "Xeis formed with a cross section (smaller than 1 mb) whichis only approximately 0.2% of the total cross section forthe fusion evaporation reaction, we were able to study itsstructure by using coincidences with selective triggers,such as the Daresbury recoil mass separator [1] (RMS)and the Yale neutron ball [2]. We are, however, only ableto acquire a limited amount of data (due to the smallcross section) that are sufficient to elucidate the structureof "Xe and hence strongly motivate extensions to higherspins in "Xe or to the lighter Xe isotopes such as" Xe.Extensions of this study are now possible with the use ofthe newly constructed large gamma array detectors.

For normal vibrational octupole negative parity states,in slightly deformed transitional nuclei (e.g. ,

"Xe), theE1 transitions matrix elements between J„,„andJ„,„—1 are forbidden in first order, leading to odd-evenstaggering of observed B (E 1 )/B (E2) ratios in these nu-clei. Hence a useful signature for broken reAection sym-metry in transitional nuclei are E1 decays from J„,„ toJ,„,„—1 which are comparable (or even larger) than E 1'sfrom J,dd to J,dd

—1. The observation of brokenreAection symmetry in several nuclei allows one to studyglobal trends of enhanced E1 decays in nuclei, and thepoint of this paper is to investigate such global trends ofenhanced E1 transitions in nuclei.

The nucleus "Xe was studied using the fusion eva-poration reaction Ni( Ni, 2p2n)" Xe at El =243 MeVin two complementary experiments. The first experimentwas performed at the Nuclear Structure Facility at

Current address: Dept. of Physics, George Washington Uni-

versity, Washington, DC 20052.~Current address: Nuclear Physics Lab GL-10, University of

Washington, Seattle, WA 98195.

Daresbury, using the Daresbury recoil mass separatorand the POLYTESSA array of 16 Compton suppressedspectrometers (CSS's) with Ge detectors [3]. The secondexperiment was performed at the Wright Nuclear Struc-ture Laboratory at Yale University, using the Yale neu-tron ball and an array of five CSS-Ge detectors. The re-sults presented here arise from both data sets collected atYale University and at Daresbury Laboratory. The de-tails of these experiments and results will be reportedelsewhere [2], and some performance characteristics ofthe Daresbury RMS [1] and the neutron detectors [4]have been published.

We use these data to test a specific prediction of thecluster sum rule model [5] that suggests that enhancedE1 in di6'erent nuclei should approximately scale like[(N —Z)/A] . This prediction follows isospin selectionrules where E1's in self-conjugate N =Z nuclei are firstorder forbidden. For such X =Z nuclei E1's are possiblein second order, leading to a predicted hindrance [6] ofapproximately (kR) =10 —10 with respect to thesingle particle estimate. In the collective octupole modelenhanced E1's arise from a redistribution efFect caused bythe electric field of the charge distribution of "a pearshaped" nucleus; hence, this mechanism is necessarilyisospin breaking, unlike the case in the cluster modelwhere the predictions follow isospin selection rules.

Gamma-ray transitions in "Xe were identified fromin-beam recoil-gamma coincidence data, using the RMSand the POLYTESSA array. Mass 114 recoils wereidentified in a position sensitive microchannel plate detec-tor placed at the focal plane of the RMS. The heavyrecoils were then stopped in an isobutane filled ionizationdetector from which Z identification was obtained. Aclean "Xe spectrum was obtained by a careful selectionof the recoil ( A =114, Z =54) gamma-ray coincidencedata. The identification of gamma lines in "Xe agreeswith the two gamma lines (449 and 618 keV) previouslyobserved [7] in the beta decay of " Cs. In addition,recoil-gamma-gamma data and recoil —gamma-ray angu-lar distribution data were collected at three angles

0556-2813/93/48(4)/2078(4)/$06. 00 48 2078 1993 The American Physical Society

48 BRIEF REPORTS 2079

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FIG. 1. Level scheme of" Xe. Relative intensities of gammalines (with 10%-20% uncertainties) are given in parentheses.

(0=143, 117', and 101').Further data on "Xe were obtained using the Yale

neutron ball. In this experiment two-neutron plus gam-ma coincidence data were collected. The high segmenta-tion of the neutron ball [2] allowed for identification ofneutrons via time of Aight, in conjunction with a pulseshape discrimination [8]. The multiple scattering of 6MeV neutrons between difterent segments of the neutronball was measured [2] to be of the order of 2%, thus per-mitting the unambiguous identification of the two-neutron channel. The gamma lines observed andidentified in coincidence with two neutrons are mainlyfrom "Xe. Only gamma lines which appeared both inthe neutron ball spectra and in the RMS spectra werefirmly placed in the level scheme of "Xe. Angular distri-butions were also measured in the Yale experiment atfour angles (9=80', 115, 150', and 157'), for gammas incoincidence with two neutrons.

The angular distribution data for transitions within thepositive parity band produce Legendre polynomial ex-pansion 3 z coefficients (average uncertainty +20%) withvalues ranging from 0.2 to 0.7, and values ranging from0.1 to 0.5 for transitions within the negative parity band.These suggest that in-band transitions are stretched E2's.For the 1175 keV transition we obtain from the angulardistribution A2= —0.49(12) with an A~ coefficient thatis consistent with zero, indicating a stretched dipole 3~2transition. The measured Legendre polynomialcoefficients (both Az and A4) suggest that this transitionis more likely to be a mixed E 1 /M2 transition, since theyrequired an anomalously weak M1 transition for themixed M 1/E2 case. The higher spins and parities then

TABLE I. Observed B(E1)'s in" Xe.

E (keV) J;—+Jf B(E1)/B(E2) (fm ) B(E1) (W.u. )

5 4+5 6+

4.6(13)X 108.1(25) X 10

6.0(17)X 101.1(27)X 10

'For B(E2: 5 ~3 )=75 W.u. , see text.

follow from the quadrupole nature of the in-band transi-tions.

The level scheme of" Xe shown in Fig. 1 follows thetrend observed [9] in " ' Xe nuclei. In particular, thelocation of the 5 state (at 1999.7 keV) in "Xe appearsclose to that observed [9] in "Xe (at 1979 keV), whichagain suggests a negative parity assignment for the stateat 1624.7 keV. The trends in these Xe nuclei are similarto that observed in the Ra-Th region where brokenreAection symmetry is well established. Both the heavierXe isotopes and the heavier Ra-Th isotopes exhibit a neg-ative parity band that is high lying with respect to theground state band (g.s.b.), e.g., Ra and "Xe. As oneapproaches shell closure the negative parity band islowered with respect to the g.s.b. and is interleaved withthe ground state band only at higher spins, e.g., at the 9state in both Ra and in "Xe (see Fig. 1). In both casesin the Ra-Th nuclei and apparently in the Xe isotopes wefind broken reAection symmetric configurations that areincreasingly lying lower as one approaches shell closure.

From the 5 state at 2 MeV we observe two E1 de-cays, the 5 —+4+ and 5 —+6+ transitions. Using theangular distribution data we extract the branching ratiosfor these transitions and the B (El )/8 (E2) ratios listedin Table I. For a B( E2: 5 ~3 )=75 W.u. derivedfrom Grodzins systematics with the assumption that theQo for this band is the same as in the ground state band,we extract the B(E1)'s listed in Table I. We note thatthe B(El: 5 ~6+)=10 W.u. is about an order ofmagnitude larger than the average B(E1) observed inmedium-mass nuclei, and approximately two orders ofmagnitude larger than predicted for a two quasi particleconfiguration. As we discuss above, this strong E1 tran-sition is forbidden (in first order) for refiection symmetricvibrational octupole states in nondeformed nuclei (it is aJ,„,„ to J„,„—1 transition); hence, it provides furtherevidence for broken reAection symmetry in "Xe. Wenote that while the forbidden 5 —+6 transition isstrong, the allowed 5 —+4 is very weak. This situationis very reminiscent of the observation in ' Ba [13] wherebroken reAection symmetry is well established and wasrecently analyzed by Ahmad and Butler [14] who con-cluded that it is generally occurring in transitionaloctupole-deformed nuclei. This weak E1 could be due toa cancellation between the contribution of the liquid dropcontribution and the shell correction to the total dipolemoment, as discussed by Butler and Nazarewicz [10].Ahmad and Butler also suggest that such small B(E1)'scould be due to the weaker octupole correlations at lowspins or the smaller (quadrupole) deformation that leadsto a smaller dipole moment. For a smaller liquid drop di-pole moment, the total dipole moment (shell plus liquiddrop) is more sensitive to cancellation effects.

2080 BRIEF REPORTS

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FIG. 2. Average enhanced E1 s in nuclei exhibiting brokenreflection symmetry. The dashed line corresponds to the iso-spinlike dependence predicted [5] in the cluster sum rule modelof the form [(N —Z)!A] . A systematic error of 50% was usedfor B(E1)'s deduced using the Grodzins systematics. In ' Rathe experimental uncertainty of measured [11]absolute B (E 1 )'swas used. See text for a discussion of ' Ba and Ra.

In Fig. 2 we show the measured and extracted 8 (E 1 )'sin heavy even-even nuclei where enhanced E1 decays andbroken reAection symmetry were observed, together withthe B(El: 5 ~6+), observed in "Xe. We refer thereader to the article by Butler and Nazarewicz [10] for acomplete review of measured and extracted 8 (E 1)'s. Inmost cases, except for ' Ra where absolute 8 (E 1 )'s weremeasured [11],the Grodzins systematics were used to ex-tract 8 (E2)'s [12] and deduce the 8 (E 1 )'s shown in Fig.2. We estimate that this procedure for calculating8 (E 1 )'s gives a systematic error of 50%, and theB(E1)'s in Fig. 2 have such an error. We note that in

Ba the enhanced B(El)'s observed for higher spinstates [13] are of the order of 2.5X10 W.u. and aresimilar to those observed in other nuclei. Large Auctua-tions in the B(E1) ratios in ' Ba lead to the smalleraverage value shown in Fig. 2.

Also shown in Fig. 2 is the cluster model prediction [5]for the scaling of enhanced 8 (E 1 )'s of the form[(N —Z)/A] . The B(El ) observed in " Xe is approxi-mately a factor of 25 smaller than the average value of

8(E1) shown in Fig. 2, and the [(N —Z)/A] value for"Xe is about 12 times smaller. The scaling predictionfor 8 (El ) in ""Xe is then only a factor of 2 larger thanobserved, which is in accordance with the expected accu-racy of the data as well as the expected accuracy of thisscaling.

Recent calculations suggest that octupole shapes(hence broken reAection symmetry) should be observed inthe lighter isotopes of Xe [15] near the N =Z line. Thecross section for forming "Xe in (Hl, xnyp) reactions,where HI represents heavy ion, is predicted to be verysmall —in the range of tens of pb, and currently it ap-pears that "Xe is the lightest isotope that is readily ac-cessible for a detailed spectroscopic study of high spinstates. The data presented here should then give a strongimpetus for further studies of the lighter Xe isotopes,where it is predicted [15] that the negative parity statesare lying even lower and are expected to be interleavedwith the positive parity states, even at lower spins (e.g.,5 ) as observed in the lighter Ra-Th nuclei. We notethat in the Ra-Th as well as Ba isotopes, while the nega-tive parity states are dramatically lowered with respect tothe g.s.b. between neighboring isotopes, the 8 (E 1 )'svary by approximately a factor of 2 or so. Hence we mayexpect slightly larger 8 (E 1 )'s in "Xe than reported herefor" Xe.

The collective liquid drop model [16) leads to a scalingof the dipole moments of the form AZ. The dependenceof the neutron-skin term is proportional toNZ/A (I —5), where I =(N —Z)/A and 5 has a compli-cated dependence on 3 and Z. In addition, Auctuations(up to a factor of 2—3) in the values of P+3 and highermultipoles yield to variations in the predicted liquid dropdipole moment of neighboring nuclei that are comparableto variations due to the predicted mass dependence.Hence no simple mass and Z dependent scaling forenhanced E1's can be predicted in the collective octupolemodel. We also emphasize that in this paper we onlyconsider average properties and global trends. Forspecific cases, such as ' Ba and Ra, shell correctionsmay cause cancellations and thus small B(E1)'s . Werefer the reader to Ref. [10] for a detailed discussion ofsuch eQ'ects.

In summary, we report for the first time on high spinstates in "Xe and observed a structure suggestive of bro-ken reAection symmetry. The observed enhanced E1 ap-pears to show simple dependence on isospin and scalingof the form [(N —Z)/A], as predicted in the clustersum rule model. Such a scaling predicts very small8 (El)'s near the N =Z line as expected from isospinsymmetry.

This work was supported in part by U.S. DOE GrantNo. DE-FG02-91ER40609.

[1]A. N. James, T. P. Morrison, K. L. Ying, K. A. Counell,H. G. Price, and J. Simpson, Nucl. Instrum. Methods A267, 144 (1988).

[2] S. L. Rugari, Ph.D. thesis, Yale University, 1992, unpub-lished; S. L. Rugari et al. (unpublished).

[3] P. J. Nolen, D W. Gifford, and P. J. Twin, Nucl. Instrum.

Methods A 236, 95 (1985).[4] M. Gai, S. L. Rugari, R. H. France, B. J. Lund, Z. Xhao,

A. J. Davenport, H. S. Isaacs, and K. G. Lynn, Nature340, 29 (1989).

[5] Y. Alhassid, M. Gai, and G. F. Bertsch, Phys. Rev. Lett.49, 1482 (1982).

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[6] M. Gell-Mann, and V. L. Telegdi, Phys. Rev. 91, 169(1953).

[7] E. Roeckl, G. M. Gowdy, R. Kirchner, O. Klepper, A.Piotrowski, A. Plochocki, W. Reisdorf, P. Tidemand-Petersson, J. Zylicz, D. Schardt, G. Nyman, and W. Lin-denaweig, Z. Phys. A 294, 221 (1980).

[g] S. Pai, W. F. Piel, Jr., D. B. Fossan, and M. R. Maier,Nucl. Instrum. Methods A 278, 749 (1989).

[9] V. P. Janzen (private communication); McMaster annualreport, 1984, p. 54; McMaster annual report, 1985, p. 62(unpublished); and Nucl. Data Sheets 59, 348 (1990).

[10]For a review, see, for example, P. A. Butler and W. Na-zarewicz, Nucl. Phys. A533, 249 (1991).

[11]M. Gai, J. F. Ennis, D. A. Bromley, H. Emling, F. Azgui,

E. Grosse, H. J. Wollersheim, C. Mittag, and F. Riess,Phys. Lett. B 215, 242 (1988).

[12] S. Raman, C. H. Malarkey, W. T. Milner, C. W. Nestor,Jr., and H. P. Stelson, At. Data Nucl. Data Tables 36, 1

(1987).[13]W. R. Phillips, I. Ahmad, H. Emling, R. Holzmann, R. V.

F. Janssens, T.-L. Khoo, and M. R. Drigert, Phys. Rev.Lett. 26, 3257 (1986).

[14] Irshad Ahmad and Peter A. Butler, Annu. Rev. Nucl.Part. Sci. (to be published).

[15]J. Skalski, Phys. Lett. B 238, 6 (1990).[16] C. O. Dorso, W. D. Meyers, and W. J. Swiatecki, Nucl.

Phys. A451, 189 (1986).