States in Dy154 have been located up to I=48+ and their lifetimes measured. Marked structural changes occur along the yrast line with a transition from prolate to oblate shape, followed by an unexpected return to moderate collectivity at the highest spins. Structural changes with increasing energy above the yrast line are also observed

Ahmad, I.

Daly, P.J.

Dichter, B.K.

Drigert, M.

Emling, H.

Garg, U.

Grabowski, Z.W.

Holzmann, R.

Janssens, R.V.F.

Khoo, T.L.

Ma, W.C.

Piiparinen, M.

Quader, M.A.

Trzaska, W.H.

Wang, T.-F.

Carolina Digital Repository

VOLUME 61, NUMBER 1 PHYSICAL REVIEW LETTERSStructural Changes along and above the Yrast Line of ' Dy4 JULY 1988W. C. Ma, ' M. A. Quader, H. Emling, ' ' T. L. Khoo, ' I. Ahmad, ' P. J. Daly, B. K.Dichter, t' t 1 M. Drigert, ~ t'~ U. Garg, Z. W. Grabowski, R. Holzmann, ' ' R. V. F. Janssens, t'~M. Piiparinen, t ~ ~ W. H. Trzaska, t2~ and T.-F. Wanga'~' Argonne National Laboratory, Argonne, Illinois 60439' 'Purdue University, West Lafayette, Indiana 47907University of Notre Dame, Notre Dame, Indiana 46556(Received 29 December 1987)States in ' Dy have been located up to I =48+ and their lifetimes measured. Marked structuralchanges occur along the yrast line with a transition from prolate to oblate shape, followed by an unex-pected return to moderate collectivity at the highest spins. Structural changes with increasing energyabove the yrast line are also observed.PACS numbers: 21.10.Pc, 21.10.Re, 23.20.Ck, 27.70.+qRapid rotation may induce dramatic structuralchanges in some nuclei. The transitional nuclei, whichlie in the regions between spherical and deformed nuclei,should be particularly susceptible to stresses arising fromrapid rotation. Thus, properties of such nuclei provide astringent test of nuclear models. The N=88 isotones,which bridge the sudden change from oblate shapes forN ~ 86 to prolate shapes for N ~ 90, have been a fertiletesting ground. They generally have slightly prolateshapes at low spins, but switch to oblate shapes at higherspins. ' Theory accounts for this switch in terms ofband termination, a process where the shape associatedwith a given configuration having initial prolate defor-mation changes until it reaches the oblate limit. Thisband-termination picture has so far been checked mainlyby comparisons with the energy levels of yrast states ofnuclei in the transitional region. For a more rigoroustest of theoretical models it is important to measure bothyrast and nonyrast levels and, particularly, to determinelifetimes, which provide a direct measure of collectivity.It is also of considerable interest for our understandingof nuclear structure to measure the properties of states athigher spin to ascertain if the oblate coupling schemepersists beyond the maximum spin which can be generat-ed by the valence nucleons.With these motivations, we have identified levels in'66Dyss to very high spin (I =48+) and have also mea-sured the state and feeding lifetimes. The level schemeand B(E2) values confirm the previously observedprolate-to-oblate transition. ' However, at the highestspins a feature not predicted by theory is observed:There is a return to collective behavior at I =40.High-spin states of ' Dy were populated via the reac-tion ' Sn( S,4n) with a 165-MeV beam from ATLAS(Argonne Tandem Linac Accelerator System). Twotypes of targets (' Sn enrichment & 92%) were used: astack of four self-supporting foils which allowed the resi-dues to recoil into vacuum, and a target evaporated on aPb backing, in which the residues stopped in =2 ps. Ineach case, the total Sn thickness was = 1 mg cm . Thedecay y's were detected with use of the Argonne-NotreDame y-ray facility consisting of eight bismuth ger-manate Compton-suppressed Ge spectrometers, of whichthree were positioned at 34', two at 90', and three at146' with respect to the beam direction. A central arrayof fourteen bismuth germanate hexagons, with a detec-tion efficiency of 38% for 662-keV y rays, acted as amultiplicity filter. A total of 135x106 true prompt-coincidence events were obtained for the stacked targetand 55 x 106 for the backed target. The very clean spec-tra from the Compton-suppressed Ge spectrometers al-lowed observation of detailed line shapes for the y raysfrom the backed-target data. Hence lifetime analysis forthe fast transitions (from states with I'=38+-44+ ) waspossible. The stacked-target data are necessary mainlyfor the fast y rays, as some of the lines were significantlysmeared from Doppler broadening in the backed-targetspectra. Finally, lifetimes of longer-lived states (withI'=34+-38+) were measured with the recoil distancemethod with use of the plunger described by Azgui.Details of the experiments will be described in a forth-coming paper.The level scheme for "4Dy deduced from the presentexperiment is shown in Fig. 1. The ordering of the tran-sitions is based on coincidence relationships, cascade-crossover connections, intensity arguments and, for thehighest positive-parity levels, decay-time considerations.Relative y-ray intensities were extracted from the coin-cidence spectra obtained with the stacked target. Spinand parity assignments are based on directional correla-tion ratios. All the transitions reported in Ref. 2 areobserved, although some of them are now placed dif-ferently in the level scheme. The positive-parity se-quence has been extended to spin 48, and the negative-parity sequence to spin 42 (or 43). More than fifty newlines have been assigned, so that many new nonyraststates are located.The y-ray intensity drops quickly at the highest spinsof the positive-parity sequence. Although the intensityof the transition depopulating the 44+ state is still quite46 1988 The American Physical SocietyVOLUME 61, NUMBER 1 PHYSICAL REVIEW LETTERS 4 JULY 19887 (ps)&0.161298.8 '141118— /0.12&0.05 46+„1311.10.23+0.08 44+112'I1476.91.6 x 0.5 (42,43)+ 750.340' ri. I+0.41.2 s0.5877.838+36483.81091289.5751976.5,~34,'32 ~ 323210—1008.83O+,28+,„777.828+,26+i24+22+.20 ~18+/16'~411.0JI4+786.8581.8189.0 W41 715.91211.2„(39) 3g- ~'8 YI63.898 ()1332.0 (37,38) 862.2 770 8~ I843.6 'g7„ 476 (-)848.9 36 „ 35 35- 710.9 3~351 r805.0g ~B94.3 (33)„~747.3 523735.8g34.8 32~ 872.8 ~ 991.6 926Q 974 4.6982.5(31)31-817.1,rrr.9,1034.6 650.5 30 (30 ) L ~'9 93848o.i949.8 „768.7, ~7 983.omr 28„Q7.1 853.3+ 695.9 27 ir887.3 ~— 7626+„ 7618 ~ 508.4~24+ 763.2 866 8 25 ~ 816.2823.0 W, 24'„7652 967684.B I22 s r 769 5 — 718.7 21-, ,623.4760.0685.2 20+„ 696.7 842.9P 72I.3L612.2 18 ir 696.4695.91546 5 493 3 16, -J [6598474 0664 I ~ J )598 pz+ I Ir16 I 49~78596.4 g f588 5 577~1,461 I~ r 10+5569+73.476+ 523 740.6476.92+ 412.20+FIG. 1. ' Dy level scheme. Lifetimes for the higher-spin positive-parity levels are also given.high (9%), for I'&44+ the decay pathway fragments.Thus the 1311- and 1299-keV transitions at higher spinhave intensities of only 4% and ( 2%, respectively.In the negative-parity sequence, the placement of the853-keV (29 27 ) transition is confirmed by allcoincidence considerations while it was not included inthe main decay cascade by previous works. ' In addi-tion to the two parallel y-ray sequences populating the19 and 27 states in the main cascade, an interestingfeature is the presence of a series of dipole transitions be-tween spin 33 and 39By comparing the backed-target data obtained fromthe forward, backward, and 90' detectors, Doppler-shifted line shapes can be seen clearly for the strongtransitions above spin 36+ in the positive-parity cascade.Figure 2 shows an example from the 44+ 42+ transi-tion. The line shapes allow for the extraction of stateand feeding lifetimes by the Doppler-shift attenuationmethod. The slowing-down process of recoils in thetarget and backing material was calculated with theNorthcliffe-Schilling stopping powers after normaliza-tion to the a stopping powers of Ziegler and Chu. Thenuclear stopping was also included following Lindhard etal. s with low-energy ion scattering corrections fromBlaugrund. A computer code was written to fit themeasured Doppler-broadened line shapes. The measuredlifetimes are indicated in Fig. 1 and also in Fig. 3. (Amore sophisticated Monto Carlo treatment ' of theslowing-down process yields the same results within er-rors. ) For the weak 1299-keV transition, only an upperlimit on its deexciting lifetime was obtained, which, how-ever, clearly shows that it is fast. The lifetimes of theI =38+ state obtained from the Doppler-shift attenua-tion and recoil distance methods were in excellent agree-ment.In the y-ray intensities extracted from the backed andstacked targets clear differences were found not only forthe fast y rays above I =36+, but also for the transi-47VOLUME 61, NUMBER 1 PHYSICAL REVIEW LETTERS 4 JULY 19882000 -L 44 ~42l 800—(/)l—& leoo-1400i/00 I I I I I I I I I I I Il090 I I lO l l30ENERGY (keV)2.5-)~ 2.0-l.5—o 1.0-O ~ Ito /n,0.5-0Il0I20I; ~I)36I34234+—32+l30B(F2) {e.u. )I I+ 220m 3I40 50I24+a~' ~I~y"'~W!q.~ 9+416',; ~ ~.'~%29 10l.4+0.3 36'2 A+ I ~ g36+12~ 46+FIG. 2. Line shape of 1125-keV transition observed at 146 .Experiment, thin line; calculation, thick line. Arrows indicateenergies corresponding to full and zero Doppler shift.tions between spin 22+ and 32+ in the positive-parity se-quence. (For the backed target only the intensity of thestopped component was used, so that if a y ray containsa fast Doppler-broadened component its intensity shouldbe smaller than that from the stacked target. ) As theeffective lifetimes of the lower-spin states are very long( ) 10 ps), the fast components in their deexciting y raysmust come from fast "side feeding" (i.e., feeding into thestate under consideration from all other unobserved lev-els) (zf d ( 1 ps), in agreement with the results of Azguief al. 'The observation of both yrast and several nonyraststates makes a stringent comparison with theory2 possi-ble. Figure 3 is a plot of energies versus spin for thepositive-parity levels; transitions connecting the levels arealso indicated. The ground-state band (GSB) is seen tobe crossed at I'=16+ by an aligned vii3i2 S band, whichthen continues as the yrast structure to I'=32+, whilethe GSB continues as an excited band to I'=26+. Bothbands are crossed at these spins by structures whose ir-regular energies and lifetimes (where measured) suggestthe approach toward aligned-particle configurations. ForI'=38+-48+, yet another structure becomes yrast, witha preferential reduced-E2 decay to the 36'+ level. Thetransitions of this last structure, shown as mediumarrows in Fig. 3, exhibit moderate collectivity [Q,=198-243 e fm, 29-43 Weisskopf units (W.u. )l. Acomparison of the results shown in Fig. 3 with a similarplot for the calculated levels —see Fig. 7 in Ref. 2—shows good agreement in the overall features up toI =36+, the maximum valence spin. By locating thenonyrast levels we have thus been able to demonstratethe predicted descent of the oblate aligned-particleconfigurations towards the yrast line and the consequentFIG. 3. Level energy (after subtraction of a rotational term)vs spin for positive-parity levels; connecting transitions areshown, thereby exhibiting the structural connections of states.Transitions within the ground band, the S band, and themoderately collective band at the highest spins are shown asopen, heavy, and medium arrows, respectively; demonstrated orprobable single-particle transitions are shown as thin lines.The numbers by the highest-spin transitions are 8(E2) valuesin Weisskopf units (W.u); 8(E2) values for the ground and Sbands are from Refs. 1 and 4.crossing of the GSB and S band. In the theory aterminating-band picture is predicted at the higher spins,where along a band with given configuration the shapechanges from prolate (y=0') to oblate (y=60')through a triaxial excursion (with intermediate values ofy). The three consecutive yrast transitions, 34i+ 32i+,361+ 341+, and 38i+ 36i+ (E„=761,805, and 1332keV) are of single-particle type (1.4, 7, and 2.4 W.u. , re-spectively). The negative-parity level energies (notdisplayed in Fig. 3) show similarly gaod agreement withtheory; in particular, the unusual series of dipole transi-tions between I'=33 and 39 is in excellent accordwith the theoretical predictions2 of signature degeneratebands.However, above spin 38 the agreement with theory ap-pears to break down for the positive-parity states: Themoderately collective structures observed are quite dif-ferent from the predominantly oblate single-particleyrast states predicted. There is a passibility that thesingle-particle states mix with higher-lying collectivebands to give rise to the observed structures. However,the extended sequence of five collective transitions wouldbe difficult to explain in this manner since the collectivestates are calculated to lie 0.8-2.7 MeV higher.While collective components have been observed in theunresolved y spectra of transitional nuclei at high spin,this is the first time that a change from single-particle toVOLUME 61, NUMBER 1 PHYSICAL REVIEW LETTERS 4 JULY 1988collective behavior has been observed along the yrast linein the discrete-line spectrum. The unexpected return tocollectivity at the highest spin stands out as a generalchallenge for theory to explain. The moderate collectivi-ty may suggest a triaxial shape. The favoring of thecollective structure rather than the expected oblatealigned-particle configurations may suggest that the costof the promotion of particles into higher-lying orbitals(possibly from the next major shell" ) is compensated bydeformation. This structure may represent the initialstage in the excursion towards the larger deformationpresent in the quasicontinuum states in '54Dy (Holz-mann et al. '2) or those prevalent in the heavier prolateDy isotopes. An alternative explanation is that theconfigurations of this structure do, in fact, correspond tothe calculated oblate aligned-particle states with energyminima at y=60'. If this is the case, the moderate col-lectivity may be due to very fiat potential-energy sur-faces extending into the triaxial plane.In summary, an impressive series of nuclear-structurechanges along the yrast line have been observed in ' Dy:from the GSB to an aligned vity2 band, to oblatealigned-particle configurations, and finally an unexpectedreturn to moderately collective states. The observationof several nonyrast levels has also made it possible to see(Fig. 3) a change of structure with an increase in excita-tion energy above the yrast line, and to follow the des-cent of oblate aligned-particle configurations towards theyrast line with increasing spin until they cross both theGSB and vi)3/2 band. The theoretical predictions in theband-terminating picture impressively account for theobserved features for I~ 36. However, the return ofcollectivity at higher spins was not predicted, therebyraising challenging questions about the nature of collec-tivity at the highest spins.We acknowledge helpful conversations with T. Bengts-son, I. Ragnarsson, and M. Riley. This work was sup-ported by the U. S. Department of Energy, NuclearPhysics Division, under Contracts No. W-31-109-Eng-38and No. DE AC02-76ER01672, and the National Sci-ence Foundation under Grant No. PHY84-16025.' Present address: GSI, Darmstadt, West Germany.Present address: Panametrics, Inc. , Waltham, MA 02254.Present address: Idaho National Engineering Laboratory,Idaho Falls, ID 83415.Permanent address: University of Jyvaskyla, Jyvaskyla,Finland.1A. Pakkanen et al. , Phys. Rev. Lett. 48, 1530 (1982).H. W. Cranmer-Gordon et al. , Nucl. Phys. A465, 506(1987).3F. S. Stephens et al. , Phys. Rev. Lett. 54, 2584 (1985);C. Baktash, Phys. Rev. Lett. 54, 978 (1985); S. Patel, Phys.Rev. Lett. 57, 62 (1986).4F. Azgui et al. , Nucl. Phys. A439, 573 (1985).5K. S. Krane et al. , Nucl. Data Tables 11, 351 (1973).6L. C. Northcliffe and R. F. Schilling, Nucl. Data Tables A7, 233 (1970).7J. F. Ziegler and W. K. Chu, At. Data Nucl. Data Tables13, 463 (1974).J. Lindhard et al. , Mat. Fys. Medd. K. Dan. Vidensk. Selsk.33, No. 14 (1963).9A. E. Blaugrund, Nucl. Phys. 88, 501 (1966).H. Emling et al. , to be published.11M. A. Deleplanque, Nucl. Phys. A448, 495 (1986).' R. Holzmann et al. , to be published.49

Structural changes along and above the yrast line of Dy154

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Structural changes along and above the yrast line of Dy154

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