In a series of measurements with Gammasphere, striking differences were found between the yrast and negative parity bands in 238-240Pu and those in 241-244Pu. These differences can be linked to variations with mass of the strength of octupole correlations. At the highest spins, 238-240Pu are found to exhibit properties associated with stable octupole deformation, suggesting that a transition with spin from a vibration to stable deformation may have occurred

Ahmad, I.

Amro, H.

Bhattacharyya, P.

Carpenter, M.P.

Chowdhury, P.

Cizewski, J.

Cline, D.

Greene, J.P.

Hackman, G.

Janssens, R.V.F.

Khoo, T.L.

Lauritsen, T.

Lister, C.J.

Macchiavelli, A.O.

Nisius, D.T.

Reiter, P.

Seabury, E.H.

Seweryniak, D.

Siem, S.

Sonzogni, A.

Uusitalo, J.

Wiedenhöver, I.

Wu, C.Y.

Carolina Digital Repository

VOLUME 83, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 13 SEPTEMBER1999Octupole Correlations in the Pu Isotopes: From Vibration to Static Deformation?I. Wiedenhöver,1 R. V. F. Janssens,1 G. Hackman,2 I. Ahmad,1 J. P. Greene,1 H. Amro,1,3 P. K. Bhattacharyya,4M. P. Carpenter,1 P. Chowdhury,5 J. Cizewski,1,6 D. Cline,7 T. L. Khoo,1 T. Lauritsen,1 C. J. Lister,1A. O. Macchiavelli,8 D. T. Nisius,1 P. Reiter,1 E. H. Seabury,5 D. Seweryniak,1 S. Siem,1,9 A. Sonzogni,1J. Uusitalo,1 and C. Y. Wu71Argonne National Laboratory, Argonne, Illinois 604392University of Kansas, Lawrence, Kansas 660453North Carolina State University, Raleigh, North Carolina 27695,and Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708-03084Purdue University, West Lafayette, Indiana 479075University of Massachusetts, Lowell, Massachusetts 018546Rutgers University, New Brunswick, New Jersey 089037University of Rochester, Rochester, New York 146278Lawrence Berkeley National Laboratory, Berkeley, California 947209University of Oslo, Oslo, Norway(Received 29 April 1999)In a series of measurements with Gammasphere, striking differences were found between the yrastand negative parity bands in238 240Pu and those in241 244Pu. These differences can be linked tovariations with mass of the strength of octupole correlations. At the highest spins,238 240Pu are foundto exhibit properties associated with stable octupole deformation, suggesting that a transition with spinfrom a vibration to stable deformation may have occurred.PACS numbers: 21.10.Re, 23.20.Lv, 25.70.De, 27.90.+beastsngceto.se--heisndu--tatan-a-Geeeci-er-n-Octupole correlations have attracted much interest othe years, especially in deformed nuclei, because theyoften associated with some of the lowest collective modbeing observed [1,2]. Specifically, in most nuclei of thrare earth and actinide regions, rotational bands of stawith odd spins and negative parity appear at excitatienergies of#1 MeV which are interpreted as structurebased on one-phonon octupole vibrations. In nuclei whthese correlations are much stronger, these negativeity states lie even lower in excitation energy, and staboctupole deformation may occur. In nuclei around146Baand 224Th, bands with levels of alternating spin and paity, connected by strong electric-dipole transitions, reresent the best experimental evidence for the rotationoctupole-deformed nuclei. It has been shown empiricathat rotation can stabilize the octupole shape [1–3].The energy displacement between the positive anegative parity states can be used as an empirical msure of the strength of the octupole correlations. Thdisplacement has been investigated by, among others,los and von Brentano [4,5] who showed that it depenstrongly on angular momentum. Furthermore, thesethors also suggested that nuclei exhibiting dynamical otupole effects at low spins might develop static octupodeformation at sufficiently high angular momentum.The present Letter examines the properties of the yrand lowest bands of negative parity in a numberPu isotopes. It is shown that in the238 240Pu isotopes,where the octupole correlations are the strongest, levelalternate spin and parity become interleaved at the highspins I  30h̄, as in the octupole bands of146Ba and0031-90079983(11)2143(4)$15.00verareesetesonsrepar-ler-p-ofllyndea-isJo-dsu-c-leastofofest224Th discussed above. Furthermore, particle alignmenand transition rates are found to be affected by the strocorrelations. Thus, the available experimental evidensuggests that the evolution from an octupole vibrationan octupole-deformed rotation may have been observedThe data discussed below have been obtained in aries of so-called “unsafe” Coulomb excitation measurements, with thick targets at beam energies above tbarrier, where the feeding of the highest spin statesenhanced. This technique was pioneered in Ref. [6] awas successfully used in earlier work on Cm and Pnuclei [7]. In the present case, beams of208Pb ionsat an energy of 1300 MeV from the ATLAS accelerator bombarded targets consisting of0.3 mgcm2 lay-ers of 240,242,244Pu (98% enriched) electroplated onto50 mgcm2 Au backing foils. These three targets provided information not only on the even Pu nuclei, bulso on the odd isotopes241,243Pu produced via single-neutron transfer. Such a reaction was used to gather dn 238Pu as well. In this case, a239Pu target with char-acteristics similar to those just described was used in cojunction with an odd-neutron207Pb beam at 1300 MeV.In all cases gamma rays were detected with the Gammsphere array, comprised of 101 Compton-suppressedspectrometers. Events were written to tape when thror more suppressed Ge detectors fired in prompt coindence. In excess of108 events were collected with eachtarget. Most of the subsequent data analysis was pformed on the quadrupleg coincidence events, wherethe data were sorted into cubes gated on known trasitions from the nuclei of interest. Additional gating© 1999 The American Physical Society 2143VOLUME 83, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 13 SEPTEMBER 1999conditions were placed on the g-ray sum energy and mul-tiplicity detected in Gammasphere. Proper subtraction ofrandom coincidence events proved essential in order toremove the contamination of the spectra by the intense grays from 197Au Coulomb excitation. The data analysiswas performed with the programs of the RADWARE pack-age [8]. A detailed discussion of all the level schemesand other experimental results obtained in the course ofthese studies is beyond the scope of the present Letter andcan be found in Ref. [9].Figure 1 compares the aligned spins ix as a functionof rotational frequency h̄v for the bands of interest here,i.e., for the yrast sequences and for the lowest negativeparity cascades in 238 244Pu. A number of interestingfeatures clearly stand out: (i) all the Pu isotopes with massA $ 241 exhibit a strong alignment in their respectiveyrast bands at h̄v  0.25 MeV, (ii) this alignment isnot present at all up to the highest frequencies observedin 239Pu and 240Pu and is delayed at least up to h̄v $0.28 MeV in 238Pu, (iii) the behavior of the alignmentcurve of 240Pu is distinctly different from that of allthe other even-even Pu isotopes, (iv) all negative parityexcitations show the 2 3h̄ alignment characteristicof the octupole phonon, and (v) an additional gain inalignment occurs only in the negative parity bands of theisotopes where a drastic upbend occurs along the yrastline, while, in contrast, (vi) the relative alignment betweenthe two bands decreases in 240Pu.The observation of sudden gains in ix of 9 10h̄ in242Pu and 244Pu complements the original findings ofRef. [10] by delineating completely, for the first time,a backbending in an actinide nucleus. This ix value isconsistent with the alignment of a pair of i132 protons, aspredicted in Refs. [10–12]. To the best of our knowledge,all the available calculations [10–12] indicate that thesame strong proton alignment should occur around h̄v FIG. 1. Aligned spins ix of the yrast and octupole rotational bands in the Pu isotopes. In all cases the same reference is subtracted,with the Harris parameters J0  65h̄2 MeV21 and J1  369h̄4 MeV23.21440.25 MeV in the lighter Pu isotopes, yet this expectedalignment gain is not seen in 239,240Pu and is delayed in238Pu. The effect is particularly striking in 240Pu: onlya small, smooth increase in alignment is observed over arange of four to six transitions beyond the point wherethe backbending occurs in the heavier Pu nuclei (Fig. 1).There is no sign of a band interaction either in the yrastor in the octupole band [13]. The remainder of this paperdiscusses a possible reason for this striking observation.In all the even-even Pu nuclei, the first excited bandis of negative parity and is associated with an octupolevibration [7]. It is noteworthy that the first 12 levelis the lowest in excitation energy (597 keV) in 240Pu.It is located at 605 keV in 238Pu, at 781 keV in 242Puand around 950 keV in 244Pu [14]. This observation,together with the fact that octupole correlations are knownto significantly alter alignment patterns seen in reflectionsymmetric nuclei [2,15], makes it worthwhile to searchthe data for additional indications of stronger octupolecorrelations near A  240. Figure 2 presents the energystaggering, SI, between the odd-spin, negative parityand even-spin, positive parity bands. This staggering isdefined asSI  EI 2EI 2 1 I 1 1 1 EI 1 1I2I 1 1(1)and is a measure of the extent to which the two bandsof opposite parity can actually be regarded as a single,rotational octupole excitation, i.e., the degree to which theodd-spin, negative parity level of spin I has an excitationenergy located in between those of the two neighboringeven spin, positive parity states with respective spinsI 2 1 and I 1 1. In the Pu isotopes with A $ 242, thevalues of SI decrease with spin before leveling off, abehavior characteristic of many well deformed actinidenuclei, e.g., 236,238U (not shown), which reflects theVOLUME 83, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 13 SEPTEMBER 1999FIG. 2. Comparison of the energy staggering SI as afunction of spin I in the Pu isotopes and in 220Ra and 222Th(Ref. [16]), two of the best examples of octupole deformednuclei.presence of band crossings associated with the alignmentgains visible in Fig. 1. Remarkably, the SI values for238,240Pu continue to decrease up to the highest spins(Fig. 2), where they become small and comparable tothose seen in 222Th and 220Ra [16], two of the bestexamples of nuclei with static octupole deformation.Thus, at spin I $ 25h̄ the level sequences that are seenin the two lightest even Pu isotopes display the energyspacings characteristic of an octupole rotational band.Moreover, the interleaving at high spin of states withopposite parity is also realized in the odd 239Pu nucleus.The octupole vibrational bands built on the two signaturesof the 63112 orbital, originally proposed in Ref. [17],have been firmly established and extended in the presentwork. For both sequences (Fig. 2) the SI values mirrorthe trend seen in the two even neighbors. Also, levelswith the same spin but opposite parity are located close inenergy: the 4921 and the 4922 levels are 17 keV apart,the two 532 states are separated by only 8 keV, andthe 572 levels are within 26 keV. Hence, these statesappear to form so-called parity doublets, as would beexpected for odd nuclei with static octupole deformation[1,2]. Thus, at least from the point of view of thelevel energies, the three lightest Pu isotopes behave likeoctupole deformed rotors at the highest spins. It isworth pointing out that the decrease in relative alignmentbetween the two bands in 240Pu noted above is consistentwith this interpretation.Further indications about the importance of the oc-tupole correlations come from the ratio of the transi-tion dipole D0 and quadrupole Q0 moments shown inFig. 3 for the even Pu isotopes. This ratio is extractedfrom the experimental E1E2 branchings with the fol-lowing expression:D0Q0s512I020 j I 2 2 0I010 j I 2 1 0sBE1BE2. (2)FIG. 3. Ratio of transition dipole and quadrupole momentsextracted from the E1 and E2 branchings E1:I2i ! Ii 211E2:I2i ! Ii 2 22 as a function of the spin Ii with theexpression given in the text. The values of the transitiondipole moment D0 given on the right hand side are for 240Puonly. They have been calculated assuming rotational E2-matrixelements with the Q0 moment of the ground state band.From the figure, it is clear that the strength of the E1transitions grows with smaller mass number, as expectedfor the increase in the strength of octupole correlationsnoted above. Furthermore, the out-of-band E1 decaysbecome increasingly competitive with the in-band E2decays as the spin in the negative parity band increases.The effect is most pronounced for 240Pu, and perhapsalso in 238Pu, but the data are less precise in this case.Under the assumption that the BE2 values are constantwithin a rotational band, this result suggests an increase inE1 (i.e., octupole) collectivity with angular momentum.It is also possible to derive values at each spin for theinduced intrinsic dipole moment D0 in 240Pu, assuming aconstant quadrupole moment Q0  25.9 e b adopted fromthe measured BE2 value of the 21 ! 01 ground statetransition [18]. It can be seen from Fig. 3 (right handvertical axis) that the D0 values at high spin become quitelarge; D0  0.2 e fm for I $ 21h̄ in 240Pu. Such valuesare of the same order as the dipole moments observed inthe light Th nuclei [19] D0  0.2 0.3 e fm, which areamong the best examples of octupole-deformed nuclei [2].While the experimental evidence discussed abovepoints towards the importance of octupole correlationsand, possibly, of octupole deformation in the understand-ing of the level sequences of 238 240Pu, some cautionremains in order. First, at the highest spins whereoctupole deformation may have set in (Fig. 2), the levelenergies display the expected pattern of interleavedstates with odd and even spins, but the E1 transitionsconnecting levels of opposite parity have so far not beenseen. For example, the observation of the E1 strengthsof the 272 ! 261 and the 261 ! 252 transitions wouldconstitute further support for the interpretation presentedhere. Unfortunately, as the positive and negative paritylevels become interleaved, the E1 transitions are strongly2145VOLUME 83, NUMBER 11 P H Y S I C A L R E V I E W L E T T E R S 13 SEPTEMBER 1999suppressed by the energy factor. The expected ratio ofprobabilities BE1; 261 ! 252BE2; 261 ! 241 is0.016 e fme b while the measured upper limit for thisratio at spin 26 is 0.05 e fme b; i.e., the detection ofthese transitions is beyond the statistical accuracy of thepresent data. Also, it should be recognized that while theratios of the reduced E1 and E2 transition rates are verysuggestive of octupole deformation, they are perhaps notas conclusive as one would like. Figure 3 shows that theratios in 244Pu are substantially smaller than in 238,240Pu,but that those of 242Pu come closer to the 240Pu values.Ratios similar to those seen in 242Pu have also beenreported in other cases of octupole vibrations such as 238U[6]. Presumably, the increase in BE1 strength with spinseen in most octupole bands reflects the importance of thedynamical effects discussed by Jolos and von Brentano[4,5] and, hence, should be expected.To summarize, the role of octupole correlations hasbeen investigated in a number of unsafe Coulomb ex-citation experiments on Pu nuclei. In the lightest iso-topes, where the correlations are the strongest, theirimpact translates in the absence 239,240Pu or delay infrequency 238Pu of the strong proton alignment seenin the heavier Pu nuclei. Furthermore, at the highestspins, the correlations are such that the yrast and theoctupole bands merge into a single sequence of levelswith alternating spin and parity, large intrinsic dipole mo-ments can be inferred from the BE1BE2 ratios, andso-called parity doublets occur in 239Pu. Thus, the ex-perimental evidence suggests that a transition from anoctupole vibration to stable octupole deformation mayhave occurred. Detailed microscopic calculations areneeded to fully account for the enhanced importance ofoctupole correlations near 240Pu. In particular, the roleof the octupole-driving orbitals needs to be fully ex-plored. In this context, it is striking that in the imme-diate odd-even neighbors of 240Pu the Dl  3, DV 0 particle-hole configurations p	32252132165121and n	72162472274321 [20] come close in energyto the Fermi surface (within 0.5 MeV) and are expectedto play a role in the ground state configuration.Stimulating discussions with L. Egido, F. Iachello,P.-H. Heenen, and W. Nazarewicz are gratefully acknowl-edged. This work is supported by the U.S. Departmentof Energy, Nuclear Physics Division, under ContractsNo. W-31-109-ENG-38, No. DE-FG02-94ER40848,No. DE-AC03-76SF00098, and by the National ScienceFoundation. The authors are indebted to the office of2146Basic Energy Sciences, U.S. Department of Energy,through the transplutonium element production facilitiesat the Oak Ridge National Laboratory, for the use of Puisotopes. One of us (S. S.) acknowledges support from aNATO grant through the Research Council of Norway.[1] I. Ahmad and P. A. Butler, Annu. Rev. Nucl. Part. Sci. 43,71 (1993), and references therein.[2] P. A. Butler and W. Nazarewicz, Rev. Mod. Phys. 68, 349(1996), and references therein.[3] J. F. C. Cocks et al., Phys. Rev. Lett. 78, 2920 (1997).[4] R. V. Jolos and P. von Brentano, Phys. Rev. C 49, R2301(1994).[5] R. V. Jolos and P. von Brentano, Nucl. Phys. A587, 377(1995).[6] D. Ward et al., Nucl. Phys. A600, 88 (1996).[7] G. Hackman et al., Phys. Rev. C 57, R1056 (1998).[8] D. C. Radford, Nucl. Instrum. Methods Phys. Res., Sect. A361, 306 (1995).[9] I. Wiedenhöver et al., in Proceedings of the InternationalConference on Nuclear Structure ’98, Gatlinburg, TN,AIP Conf. Proc. No. 481 (AIP, New York, 1999), p. 527;I. Wiedenhöver et al. (to be published).[10] W. Spreng et al., Phys. Rev. Lett. 51, 1522 (1983).[11] J. Dudek, W. Nazarewicz, and Z. Szymanski, Phys. Scr.T5, 171 (1983), and references therein.[12] J. L. Egido and P. Ring, Nucl. Phys. A423, 93 (1984).[13] It could be argued that the aligned band is not observeddue to an accidentally small mixing at the band crossingresulting in the population of the ground band only inCoulomb excitation. This would, however, not explainthe absence of alignment in the negative parity band of240Pu, the reduction in relative alignment between the twobands, nor the absence of alignment in all the bands of239Pu.[14] R. Firestone and V. S. Shirley, Table of Isotopes (JohnWiley and Sons, Inc., New York, 1996), 8th ed., Vol. II.[15] S. Frauendorf and V. V. Pashkevich, Phys. Lett. 141B, 23(1984).[16] J. F. Smith et al., Phys. Rev. Lett. 75, 1050 (1995).[17] M. Devlin et al., Phys. Rev. C 47, 2178 (1993).[18] C. E. Bemis, Jr., F. K. McGowan, J. L. C. Ford, Jr., W. T.Milner, P. H. Stelson, and R. L. Robinson, Phys. Rev. C 8,1466 (1973).[19] P. Schuler et al., Phys. Lett. 174, 241 (1986).[20] It should be realized that the configurations suggested hereare mixed. For example, the n	721624 Nilsson stateis mixed with the n	721613 orbital at the deformationsof interest.

Octupole correlations in the pu isotopes: From vibration to static deformation?

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Octupole correlations in the pu isotopes: From vibration to static deformation?

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