Sidebands and high-spin states in 182Os

Nuclear Physics A375 (1982) 263-290 © North-Holland Publishing Company

SIDEBANDS AND HIGH-SPIN STATES IN ' g20s Department

of Nucfeur

C . FAHLANDER and G . D . DRACOULIS Physics, Rcawurch School of Physical Sciences, Australian National Umirc" rsity, PO Box 4. ACT 261K1, Australia

Received 23 March 1981 (Revised 17 August 1981) Abstract : High-spin rotational states in 'e Z Os have been studied using singles and coincidence ~-ray techniques and the "°Er(' 60, 4n) reaction . The yrast sequence of states has been extended to spin 24' and 7 sidebands identified . These are the p-vibrational and K = 3 - octupole bands and five two-quasiparticle bands including the K = 8 - (z = 1 .1 ms) isomer band . The octupole vibrational band is rotation aligned and may also mix with two-quasiparticle configurations involving an i  ,Z neutron or an h,,, z proton orbit . The bands with K = 5' - ' and 8 - also involve an i  ,a neutron and have high apparent moments of inertia. As well as the (i  , 2 )' s-band, two other rotation aligned positive-parity bands are observed and they are suggested to be the yrare (i, 3 , 2 )Z neutron bands . The bands are characterized in terms of their most probable Nilsson configurations . The level scheme of '"'Os was reinvestigated and the i  , z neutron band extended to spin ~* .

E

NUCLEAR REAC'T'IONS " °Er(' °O, 4n), E = 81 MeV ; "°Er('°O, Sn)'e'Os, E = 96 MeV ; measured E,, 1,, y!(t), y(ffl. ICC . ' BZ Os deduced levels, J, n, rotational bands, Ge, Ge(Li) detectors, enriched targets .

1 . Introduction

Although rather extensive information exists on the heavier osmium nuclei, experimental data for t820s and lighter isotopes are relatively scarce . The lowlying level structure of teZOs has been investigated in the radioactive decay of tezlr [ref. t )] in which the ground state (g.s.b.) and y-vibrational bands were olr served up to spin 6g , and 4Y respectively . In a subsequent study z) of the (p, 4n) reaction, the level schemes were extended to include the 8Q , 10~ and Sy spin members ofthese bands. The yrast band was observed to spin 20 + following the (a, 4n) [ref. 3)] and (a, 8n) [ref. 4)] reactions and was shown to exhibit a sharp backbend above spin 12. A 1 .1 ms, 8 - isomer has also been identified at 1832 keV [ref. s)]. , The work presented here is a detailed investigation of'820s in which 7 sidebands, including a rotational band based on the 8 - isomer, have been identified . Several of the bands have anomalous spacing, due to Coriolis effects, and their likely 2quasiparticle configurations are discussed. A brief report on the structure of the band based on the 8 - isomer has been published 6). The behaviour of the yrast band, which is extended to spin 24, has 263

264

C. Fahlander, G . D . Dracoulis / Sidebands and high-spin states

also been discussed previously as part of a systematic study of backbending in the light osmium isotopes with A = 176-182 [ref.')] . Since this work was completed, a preliminary partial level scheme including a number of sidebands in ' 820s has been reported 8). 2. Experimental procedure

High-spin states in'820s were populated using the "°Er(160, 4n) reaction with beams from the ANU 14UD Pelletron accelerator. Isotopically enriched metallic Er targets with thicknesses between 1 .9 mg/cmZ and 4.0 mg/cm2 were used . In order to stop the recoiling nuclei, a Pb layer ~ 4-7 mg/cm2 thick was usually evaporated on the back of the targets. For the measurements involving low-energy y-rays, a self-supporting Er target was used to avoid contamination from Pb X-rays. On the basis of an excitation function measurement between 77 and 86 MeV, a beam energy of 81 MeV was selected for the (' 60, 4n) reaction . For the study of ' s 'Os the "°Er( 160, Sn) reaction was used with a beam energy of 96 MeV . 2 .1 . y-y COINCIDENCES (1 ps RANGE)

Several y-y coincidence experiments were performed using different détector combinations . A large-volume Ge(Li) detector was used in conjunction with (a) a second large-volume detector, (b) a thin windowed planar intrinsic Ge detector to enhance the sensitivity to low-energy y-rays, and (c) a Compton-suppressed Ge(Li) spectrometer which had the advantages of improving the sensitivity for weak transitions by reducing the Compton background and reducing the number of storage tapes. Conventional fast-slow electronic circuitry was used and all coincident events were stored on magnetic tapes in a three parameter, y-y-time, event mode . The time range was ~ 1 .0 ~s in all three experiments. The detectors were energy calibrated by simultaneously accumulating a spectrum of 1s20s with radioactive sources of 'sZ Eu and ' 33Ba. The strong lines in 1ezOs were then used for an internal calibration of the coincidence spectra. Relative yray efficiencies were established using radioactive sources placed at the target position . Since the y-transitions in the neighbouring odd osmium isotopes are known from ref. 9) and from the present work, those contaminants could be accounted for. Contaminants from activities produced during the run were measured separately. 2 .2. DELAYED y-y COINCIDENCES (ms RANGE)

A delayed coincidence experiment was carried out to identify the band expected to be based on the K = 8 - 1 .1 ms isomeric state. An "°Er target was bombarded with a pulsed 160 beam with a pulse separation of 1 .6 ms and a pulse width of 0.16

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C. Fahlander, G . D. Dracoulis / Sidehands acrd high-spin states

ms. The y-rays were detected in two large-volume Ge(Li) detectors placed at ±90° to the beam direction and about 2 cm from the target . Due to the long lifetime of the state, particular care was taken to minimize the effects of random coincidences and long-lived activities which make measurements in this time regime extremely difficult . Firstly, the beam intensity was adjusted so that on average about one y-ray was detected per beam pulse (a larger rate would increase both random and activity . y-rays without increasing the real events). Care was taken to avoid activation of the target during this procedure. Secondly, the fast timing signals from the two detectors were gated by slow logic pulses related to the time distribution of the beam . The logic pulse gating the first detector corresponded in time to the actual beam pulse duration and the second detector was gated by the complementary pulse. Thus only in-beam (IB) y-ray pulses from the first detéctor started a time-to-amplitude converter (TAC) and only ôut-of-beam (OB) y-ray pulses from the second detector stopped the TAC. The TAC was set on the 0.8 ms range which was the maximum available. The linear y-ray signal from the IB-detector was then required to wait a maximum time of 1 .0 ms at the analog-to-digital converter (ADC) for the associated OB-detector and TAC signals. Since the IB-signâl could not be held for such a long period without loss of energy resolution it was first digitised. After the arrival of the two delayed pulses the event was recorded on magnetic tape. If these pulses had not arrived within the 1 .0 ms time limit, the IB-signal was still recorded but labelled as not having associated OB and TAC signals. These events were later used for generating the random spectra. 2.3 . ANGULAR DISTRIBUTIONS

Two separate experiments . were perfonmed to determine the y-ray angular distributions . The first consisted of measuring singles y-ray intensities at seven angles between 0° and 90° to the beam direction with the Compton suppressed Ge(Li) spectrometer. In the second experiment, a planar intrinsic Ge detector was used for measurements at five angles between 90° and 150°, and the beam was stopped in a remote, shielded beam dump . In both cases a Ge(Li) detector fixed at 90° to the beam axis served as a monitor. The target was placed at 45° to the beam with a target-detector distance of about 15 cm. An example of a Compton-suppressed singles . spectrum taken at 60° to the beam direction is shown in fig. 1 . Source measurements were made as a function of angle at the target position to check the chamber isotropy and, where â Pb backing was used, to allow for the angle-dependent absorption of low-energy y-rays . 2 .4 . CONVERSION ELECTRON MEASUREMENT

A mini-orange magnetic filter together with a Si(Li) detector was used to measure conversion electrons. The detector was placed at 125° to the beam direction and

C. Fahlander, G. D. Dracoulis 1 Sidebands.and hipp-spin states

26 7

y-rays were simultaneously detected at 55° in a Ge(Li) detector . However, the usefulness. of the data obtained is limited since the spectrum in the low-energy region is very complex and the interesting high-energy transitions de-exciting the observed sidebands were weak. The low intensity is due to the fact that a large number of bands are populated, none of them preferentially, and hence the intensity is fragmented .

Fig. 2. Level sçheme of `e 20s as observed in the "°Er(' 60, 4n) reaction . The width of the transitions represents approximately the intensity of the y-rays . For the 1472 and 2113 IceV bands the odd and even spin sequences have been separated because of an odd/even energy staggering as discussed in sect . 4.

268

C. Fahlander, G. D. Dracoulis / Sidebands and high-spin states

3. Results 3.1 . 's'Os

The level scheme of ' 82 0s shown in fig. 2 is based on the analysis of the y-y coincidence data . A sample of the many coincidence spectra investigated (~ 350) is given 315

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C. Fahlander, G. D . Dracoulis / Sidebands and high-spin states

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C . Fahlander, G . D . Dracoulis / Sidebands and high-spin states TABLE I

Transitions assiEtned to ' BZ Os in the "°Er(' ° O, 4n) reaction Er ')(keV) 94 .0 102 .1 122 .3 127 .0 129 .5 133 .2 152 .4 156 .8 156 .9 158 .0 172 .2 182 .5 182 .7 189 .9 198 .3 206 .2 212 .2 223 .4 224 .7 226 .5 229 .2 231 .8 235 .2 251 .4 253 .1 255 .0 271 .8 273 .5 278 .8 281 .1 282 .E 285 .9 292 .0 308 .E 314 .8 317 .8 326 .2 329 .E 343 .5 343 .5 } 346.4 360 .5 364.1 382.0 384.2 389 .2 393 .9

1, 2 .5(5) 6) 2 .3(5) 1 .6(7) 45 (3) 1 .4(4) 1 .3(6) 2 .6(9) 1 .8(1 .0) 6) 2 .7(1 .5) °) l .l(4) I .1(2) 2 .7(7) ") 11 .7(9) ") 2 .l(3) 2 .8(7) 6) 4.1(3) 1 .6(2) < 0.8 6) 1 .4(3) 1 .3(3) 3 .3(3) < 1 .6 ") 1 .9(4) 2 .2(2) 2.1(3) 1 .4(3) 1 .6(4) °) 100 (5) 3 .9(9) `) 2 .0(1 .2) 6 ) < 1 .1 6) 2 .2(3) 1 .2(2) < 1 .6 3 .9(4) 1 .4(2) 6) 5 .8(6) 0.6(3)

A Z/A o

AalAo

-0.51(14) -0.60(14) 0.15 (2) 0.01 (9) -0.07 (4) - 0.1E (5)

0.20(20) 0.08(20) -0.04 (2) -0 .01(14) -0.05 (5) 0 .0E (6)

-0.83(l7)

-0.02(29)

(-0.07(12)

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-0.93 (2) 0.15 (6)

0 .07 (2) - 0 .08 (7)

0.45 (9) (-0.03 (7) - 0 .79 (2)

-0 .18(10) 0 .03 (8)) 0 .04 (3)

0.10(10) - 0.73 (2) (-0.07 (5) (0.08 (7)

-0 .02(l2) 0 .11 (3) -0 .01 (6)) -0 .01 (8))

0.23 (1) 0 .20 (3)

-0 .08 (2) -0 .09 (3)

0.20 (6) ( - 0.34 (9)

- 0 .08 (7) - 0 .03(10))

0 .34 (4)

- 0 .09 (5)

0 .34 (3)

0 .01 (3)

0 .10 (7)

-0 .03 (8)

0 .27(13)

-0.13(15)

0 .2E (5) 0 .30(11) 0 .25 (2)

-0.12 (6) -0.07(12) -0.07 (2)

3 .9(6) ~°) 2 .0(3) < 0 .5 6) 5 .3(2 .2) °) < 1 .1 3 .7(4) 3 .0(7) `) 85 (6)

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1990 .1 1757 .0 1879 .3 127 .0 2119 .E 2246 .3 2527 .E 2276 .2 2036 .0 2194 .1 2592 .3 1654 .7 2014 .4 2466 .0 2870 .9 2220 .8 2677 .8 2l 19 .E 1879 .3 2420 .4 2449 .7 3305 .3 2913 .4 2701 .2 3166 .4 1654 .7 2972 .E 400 .5 2036 .0 2527.E 2017.8 2276 .2 3265 .2 3573 .E 2194.1 2909 .7 2672.4 3903 .E 2235 .4 2870 .9 2466 .0 1400 .0 2381 .9 2235 .4 2420 .4 2220 .8 794 .4

1896.1 1654.7 1757.0 0.0 1990.1 2113.1 2375 .2 2119.E i879 .3 2036.0 2420.4 1472 .1 1831 .7 2276 .2 2672 .4 2014.4 2466 .0 1896 .1 1654.7 2194.1 2220 .8 3073 .5 2677 .8 2449 .7 2913 .4 1400 .0 2701 .2 127 .0 1757 .0 2246 .3 1735 .1 1990 .1 2972 .E 3265 .2 1879 .3 2592 .3 2346 .2 3573 .E 1891 .7 2527 .E 2119 .E 1039 .E 2017.8 1853 .5 2036.0 1831 .7 400.5

C. Fahlander, G. D. Dracoulis / Sidebands and high-spin states TABLE 1

271

(continued) Transition

398 .0 401 .2 401 .2 416 .9 ~ 417.2 428 .3 432 .4 434.4 435 .1 443 .3 447 .7 453 .8 457 .2 463 .7 479.3 480 .4 480 .5 483 .8 487 .9 488 .E 488 .9 491 .7 494.4 514 .0 523 .1 534.0 534 .0 } 536.8 537 .3 543 .E 543 .8 553.5 559 .0 ~ 561 .1 562 .7 564 .7 567 .2 567 .7 568 .7 580 .8 581 .2 600 .8 617 .E 622.9 638.7 653.8 657 .4 663 .8 694 .5

5 .0(5) 3 .1(1 .4)b)~ ~ 0 .5 °) 6 .7(6) 0 .9(3) 2 .0(6) 3 .8(2 .0) b) 3 .7(5) 5 .5(5) 4 .0(5) < 0 .3 n) l .l(3) 2 .2(1 .0) °) 20 (2) 3 .1(8) 3 .l(8) 72 (5) < 0 .5 4 .1(1 .0) 3 .1(1 .2) < 0 .9 b) 18 (2) 5 .1(2 .4) n) 4 .2(4)

0 .35 (4)

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0 .24 (4)

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0 .13 (3)

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(-0 .22(21) (0 .18 (8)

0.06(24)) 0.01 (9))

0 .09 (6) 0 .35 (6)

- 0 .15 (7) -0 .32 (8)

(0 .71(10)

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0 .34 (3)

- 0 .10 (3)

0 .25 (2)

-0 .07 (2)

0 .33 (3)

-0 .11 (4)

0 .37 (3)

-0 .06 (3)

70 (5)

0 .33 (2)

-0 .10 (2)

8 .6(9) ~ 1 .6(9) 1 .6(8) < 1 .5 17 (2) ~ 1 .0 °)

0 .42 (4)

-0 .10 (5)

-0 .01 (1)

-0.02 (2)

(0.36(11)

-0.04(13))

1 .6(3) 3 .6(6) °) 2 .7(1 .3) b) 2 .1(9) b) < 2 .5 3 .8(1 .2) 2 .2(1 .0) 5 .5(8) °) 2 .3(1 .2) 2 .8(4) 3 .7(4) 1 .0(3) 0.3(3) °) 2 .4(3) 4.0(2 .5)

0 .25 (4)

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(0.25 (8)

-O.11 (5))

0 .2E (7) 0 .23 (7) 0 .73(13)

-0.09 (8) -0.12 (8) -0.13(15)

0.33 (6)

0.04 (6)

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2592 .3 2677 .8 3073 .5 2652 .3 3490 .5 2017 .8 1472 .1 3305 .3 2449 .7 2825 .2 2913 .4 1853 .5 2803 .5 1654.7 3319 .9 2701 .2 3072 .8 1278 .2 3291 .7 3166 .4 2909 .7 1891 .7 2840 .E 3339 .2 2972 .E 1812 .2 2346 .2 3856 .7 3189 .E 3710.0 3617 .3 1831 .7 3850.7 4467 .5 2375 .4 3265 .2 3906.4 3640.5 4059.2 3490.5 1472.1 3573 .E 4467 .9 4479.E 3903.E 4294 .3 4274 .7 4237 .4 4598 .1

2194 .1 2276 .2 2672 .4 2235 .4 3072 .8 1589 .1 1039 .E 2870.9 2014.4 2381 .9 2466.0 1400.0 2346 .2 1190.8 2840.E 2220.8 2592 .3 794 .4 2803 .5 2677 .8 2420 .4 1400 .0 2346 .2 2825 .2 2449 .E 1278 .2 1812.2 3319 .9 2652.3 3166.4 3073 .5 1278 .2 3291 .1 3906 .4 1812.2 2701 .2 3339 .2 3072 .8 3490.5 2909 .7 890 .8 2972 .E 3850 .7 3856 .7 3265 .2 3640 .5 3617 .3 3573 .E 3903 .E

272

C. Fahlander, G. D. Dracoulis / Sidebands and high-spin states TABLE 1

EÏ ') (keV) 702 .9 705 .3 711 .4 727 .3 749 .0 763.4 776.7 790.2 794.7 ~ 795.3 834.9 860.1 860 .2 891 .2 912.E 945 .5 962 .7 991 .3 999 .5 1059 .1 1063 .8 1072 .E 1085 .4 I097 .7 1101 .7 1188 .3 1223 .7 1254.2 1334.E

I, ~ 1 .0 2 .8(4) 1 .0(3) 2 .7(4) 1 .6(5) 1 .9(3) 1 .3(3) 3 .6(4)

(continued)

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0 .32 (5)

- 0 .05 (6)

-O .12 (8) 0 .43 (9) 0 .08 (4)

0.03 (9) -0.14(Il) -0.07 (5)

0 .22 (6)

0 .03 (6)

0 .08 (3)

0 .01 (4)

0 .31 (9) 0 .04 (5) 0 .24(13) 0 .15 (9) 0 .2E (8) -0.06 (5) -0.38 (6) 0.03 (6) ( - 0 .30(23) -0.31(15) 0.34 (8) -0.17(13)

-0 .02(10) 0 .0E (5) -0 .17(15) 0 .13(10) -0 .07 (9) O .14 (6) 0 .2E (7) 0 .01 (7) 0 .12(26)) 0 .03(17) -0 .02 (9) 0 .02(16)

-0.11 (4) -0.26(14)

0 .00 (4) -0 .09(15)

1 .3(5) 2 .9(4) 5 .5(2 .0) ~ l .8(1 .1) 2 .3(3) 3 .7(4) 1 .3(3) 2 .0(3) l .l(2) 3 .4(4) l .5(3) 2 .3(3) 0 .5(2) 0 .6(3) 2 .4(3) 1 .4(3) 0 .9(3) b) < 6 .4 b) 5 .7(6) 0 .8(4)

Transition from

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4940 .3 1896 .1 5191 .0 3073 .5 5023 .7 890 .8 3617 .3 1190 .8 1589 .1 5986 .3 2113 .2 2672 .4 1654 .7 890.8 1039.E 3291 .7 1757 .0 2803 .5 1400 .0 1853 .5 1190 .8 1472 .1 1879 .3 2375 .4 1896 .1 1589 .0 2017 .8 1654 .7 1735 .1

4237 .4 1190.8 4479.E 2346 .2 4274 .7 127 .0 2840 .E 400 .5 794 .4 5191 .0 1278 .2 1812 .2 794 .4 0 .0 127 .0 2346 .2 794 .4 1812 .2 400 .5 794 .4 127 .0 400 .5 794 .4 1278.2 794.4 400.5 794.4 400.5 400.5

The table also lists y-ray energies, intensities and angular distribution coeffcients . ') The errors in the y-ray energies range from f 0 .15 keV for the strong lines to ±0 .40 keV for the weaker lines . b) Intensity estimated from the coincidence data because of contamination in singles . `) Intensity corrected for contamination in singles by the ' 8 'Os line . °) Intensity corrected for contamination in singles by the '"'Os line . °) Angular distribution coefficients given in parentheses are uncertain due to low intensity, contamination or diffiwlt background correction .

in fig: 3 for the large volume Ge(Li) detectors, and in fig. 4 for the intrinsic Ge detector. Careful inspection of the large number of coincidence spectra was required because many ofthe transitions in the level scheme have very nearly the same energy, or are contaminated (see for example the 489 keV gate of fig. 3). Spin assignments were made mainly on the basis of the y-ray angular distributions. These were fitted with functions of the form W(8) = Ao +AZPZ(oos B)+A 4P4 (cos B) and the AZlAo and A4lA o coefficients obtained were compared with theoretical

C. Fahtander, G. D. Dracoulis / Sidebands andhigh-spin states

27 3

values ' °). In some ambiguous cases, a particular spin could be ruled out due to the different decay paths of the state involved, or due to the intensity of the y-ray transition from the state. The timing information obtained from the y-y coincidence data for the observed intrinsic states did not reveal any isomers with lifetimes longer than the experimental limit of ~ 15 ns except for the long-lived K = 8 - isomer . This result is consistent with the work of ref. z) where the existence of isomeric states with half-lives of 10-20 ns was demonstrated for the nuclei ' B80s and ' 860s whereas for '840s and ' BZOs no evidence for delayed transitions was observed . A summary of the information obtained for ' 820s is given in table 1 . It includes energies, assignments, angular distribution coefficients and relative y-ray intensities as obtained both from the singles and the coincidence data. The rejection of peak events in the Compton suppressor was measured ") and the y-ray intensities and angular distributions corrected accordingly. 3.1 .1. The groundstate band. The yrast sequence in ' BZOs has been extended to a probable spin of 24 . The multipole character of the new transitions at 711 keV [(22+) -" 20 + ] and 795 keV [(24+) -" (22+ )] was not established because of their low intensities and because the 795 keV transition is contaminated in singles . The coincidence intensities, however, conclusively establish their order in the yrast band. 3.1 .2. The y-vibrational band. The y-band was previously known up to spin 5. In the present work two new states at 1589 keV and 1854 keV respectively have been found. The higher lying state is assigned spin 7 on the basis of the angular distribution coefficients of the 1059 keV transition . This transition has a large negative Az value and a large positive A4 value which suggests an E2/M1 admixture. Similar angular distribution coefficients are found for the 1000 keV Sy -" 4g transition and for other I -" I-1 transitions between the y-band and the g.s.b. in the rare earth region [see for example ref. ' Z)] . The two transitions de-exciting the 1589 keV level are both contaminated in singles and no multipolarity assignments can be made . The decay of this level to both the 6+ and 4+ members of the g.s.b., however, restricts its possible spins to 6, 5 or 4. The observed states are suggested to be thé 6+ and 7+ band members respectively . 3.1.3. The K = 8 - isomeric band. An isolated rotational band was found with no transitions in prompt or delayed (within the ~ 1 ~s time range) çoincidence with known transitions in' BZOs, although the excitation function measurement suggested its assignment to 'eZOs. An example of a coincidence spectrum within the band is shown by the 523 keV gate of fig. 3. The results of the delayed coincidence experiment are summarized in fig. 5 where both the delayed (OB) and prompt (IB) (with respect to the beam pulse) y-ray spectra are plotted. The only y-rays observed in the OB detector were either activity lines or the five y-rays in the cascade following the decay of the 8 - isomer which accounts for about 20 ~ of the feeding to the 8+ g.s.b. member. In the analysis of the eventby-event data, gates were set on each of these delayed transitions, on their associated

274

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C. Fahlander, G. D. Dracoulis / Sidebands and high-spin states

27 5

Compton background regions, and on the TAC spectrum, and the IB spectra were projected. The real to random ratio, which was about 1 :2, presented a problem since the intensity of the in-band transitions is low (the 206 keV transition for example is only ~ 4 ~ of the 4+ -. 2 + g.s.b. transition intensity) . Random spectra were generated by projecting events in the IB detector which did not have associated OB and TAC signals. These random spectra were subtracted after normalizing to the strongest transitions known not to be involved in the population of the 8 isomer . The individual gates on the five y-rays from the isomeric decay were summed after subtraction of the Compton background and the resulting spectrum is shown in fig. 5 . The only prominent transitions are those previously assigned to the "isolated" rotatiônal band . Thus the band is clearly based on the 8- isomeric state. 3.1.4. The band based on the 1896 keV level. A regularly spaced band with a bandhead at 1896 keV was observed up to spin 14. It feeds into the g.s.b. at spin 6+ via an 1102 keV transition, and into the y-band at spin 4+ via a 705 keV transition . Since the 1102 keV transition has a characteristic stretched dipole angular distribution, and the 705 keV transition is also a dipole, a firm assignment of spin 5 is made for the 1896 keV state. The spin assignments for the higher lying states of the band are based on the observed crossover and cascade transitions and on their angular distributions. The band is observed to spin 14 but some evidence for a 567 keV 16 -" 14 transition exists . The 567 keV gate shows all transitions in the 1896 keV band including the 544 keV transition but has not been included in the level scheme of fig. 2 since the coincidence relationship can not be confirmed in any of the othér gates. An estimate of the K-electron conversion coefficient aK for the 70S keV transition indicates that it is of E1 character thus suggesting negative parity for the band. The 1896 keV band given in ref. e) does not include the94 keV transition connecting the first two states . This transition was observed in our coincidence measurement with the small volume intrinsic Ge detector, and is important in the discussion of rotation alignment (subsect . 4.5). 3.1 .5. The band based on the 1471 keV level. The 1472 keV level is proposed to be the bandhead of a perturbed rotational band. It decays to the 4* state of the g.s.b . by a 1073 keV transition whose angular distribution coefficients are not well determined, but which are, within the uncertainties, consistent with thoseof a stretched dipole transition . The 1472 keV level also decays to the 3Y and 2y states and its spin is therefore assigned as 3 . The 1655 keV level decays by a 1254 keV stretched dipole to the 4* member of the g.s.b. suggesting a spin of 3 or 5. Since it also decays to the 6 + g.s.b. state, and the 5+ and 4+ states of the y-band, the spin 3 alternative can be eliminated . The 1879 keV level feeds the 6+ g.s.b. state by a 1085 keV transition, whose angular distribution coefficients are also consistent with a stretched dipole and it decays to the 5 - state by a 225 keV stretched quadrupole transition . A spin of 7 is therefore assigned to the 1879 keV level. The upper limit on the Kconversion coefficient of aK < 3.8 x 10 -3, obtained for the 1254 keV dipole transition, eliminates the M1 possibility and determines the parity to be negative .

27 6

C. Fahlander, G. D. Dracoulis / Sidebands and high-spin states

The cascading 102 and 122 keV transitions are contaminated in singles, but were partially resolved in the intrinsic Ge detector allowing their respective angular distributions to be obtained by line shape fitting. Their AZ coefficients have large negative values (table 1) suggesting mixed multipole transitions and therefore MI/E2 character. The 1757 keV level is therefore assigned spin 6 and negative parity . The crossover transitions are, where measured, consistent with stretched quadrupoles. Our placement of the band does not agree with that of ref. s) since they do not include the 7 - -" 5 - transition of energy 225 keV and the two cascading transitions of 102 and 122 keV. These transitions are evident in the 315 keV gate of fig. 3 and the 1254 keV gate of fig. 4 projecting the small-volume detector . The placement of the 1879 keV 7 - level is also determined by the 1085 keV 7- -~ 6+ transition . Further, the present scheme includes a 183 keV transition which connects the S and 3 - states of the band . Gates set on the in-band transitions show two y-rays with the same energy, viz. the 14 - -" 12 - and 3 - -. 2Y 581 keV transitions. The latter transition was not observed in the work of ref. 8). Instead they proposed two 581 keV transitions in the band, corresponding to the 14 - -. 12- and 16 - ~ 14 - transitions, with the 569 keV y-ray as a possible 18 - -" 16 - transition . The coincidence data of the present work cannot conclusively exclude a second in-band 581 keV y-ray but unambiguously establish the 3 - -~ 2Y 581 keV transition . At low spins, the out-of-band transitions become more energy favoured and compete strongly with the in-band transitions. This is a possible reason why we have not observed the expected 4- member of the band. There is some evidence for weak 219 keV (possible 6 - -~ 4- ) and 117 keV (possible 5 - -" 4-) transitions but, in the absence of supporting data, they have not been included in the level scheme . 3.1 .6. The band above the 2113 keV level. A band with the lowest observed state at 2113 keV is observed up to a possible spin 20 . The angular distribution coefficients for the 1098, 727 and 777 keV transitions which de-excite the 2375, 3074 and 3617 keV states are consistent with those for stretched quadrupole transitions and spins 10, 14 and 16 respectively are assigned to these levels . The intervening band member at 2672 keV feeds the g.s.b. at spin 10+ via an 860 keV transition which is contaminated in singles by a dipole transition, the 5 - -+ 6â transition . Since the measured AZ coefficients for the doublet are positive, the 860 keV y-ray associated with the band discussed in this section must have a positive AZ coefficient and therefore is probably a quadrupole. The 2672 keV state also decays to the 12+ g.s.b. member via a 326 keV y-ray whose angular distribution coefficients are consistent with those of an I -" Itransition and it is fed by a 401 keV in-band transition from the spin 14 band member, suggesting a spin 12 assignment . Thestate at 2113 keVdecays to the 8+ g .s.b. member via an 835 keV transition, whose angular distribution is consistent with an 1-. I transition suggesting a possible spin of 8 for the 2113 keV state.

C'. Fahlander, G. D. Dracoutis / Sidebands and high-spin states

277

No in-band transitions are observed below spin 12 because the intensity is transferred from the sideband to the g.s.b. as the out~ôf--band transitions gain in energy over the in-band transitions at low spin. The out-of-band transition rates can be estimated by assuming the in-band transitions to be enhanced quadrupoles and using the known B(E2) ~ alue of the.2 + -" 0+ g.s.b. transition ' 3). If the out-of-band quadrupole transitions were of M2 character they would have a strength of about 103 Weisskopf units. Thus an E2 character, rather than M2, is favoured, leading to a positive-parity assignment for the band . The angular distribution coefficients for the 133 keV transition, connecting the 2246 keV and 2113 keV states, suggest an M 1/E2 multipole character, and those for the .152 keV transition, connecting the 2528 keV and 2375 keV states, are consistent with a stretched dipole . The 2246 keV and 2528 keV levels aie therefore assigned spins of 9 + and 11 + respectively . 3 .1.7. The band above the 2804 keV level . Members of a weakly populated band feed into the g.s.b. at spin 10+ and 12 + through two stretched quadrupole transitions of energy 991 keV and 745 keV. The strength of these transitions can be estimated in the same way as in the previous section yielding a large enhancement over the M2 single-particle rate . An E2 character is therefore . favoured and the band is assigned positive parity . 3 .1.8. The band above the 1735 keV level. The spins of the levels above the 1735 keV state have not been determined uniquely. The 1335 keV transition depopulating the 1735 keV level was weak in singles and the measured angular distribution coefficients have large uncertainties. This transition is however, within the uncertainty, consistent with a stretched dipole suggesting spin 3 or 5 to the 1735 keV state. The 1224 keV y-ray de-exciting the 2018 keV level was contaminated , by an activity line and no angular distributions wére obtained. This state also decays via a 428 keV transition of stretched dipolé character to the 6y state whose spin, however, is also not unambiguously assigned . The 2018 keV , level therefore has possibly spin 5 or 7. Because of the relative intensities of the transitions compared. with that of the other bands, spins 5 and 7 are favoured for the 1735 and 2018 keV states respectively . The in-band transitions are weak or contamined and no multipolarity assignments can be made to support the proposed band structure. A sequence of states feeding into the y-band at spins Sy and 7Y via the 492 and 382 keV y-rays is also observed. Spins have not been assigned to these lévels but they could be thé even spin members ôf the present band. 3 .2 . ~si(~

The level scheme of's1Os, which has previously been studied 9 ), was reinvestigated . Two new y-rays with energies of 629 and 709 keV were observed corresponding to the ~+ -" ~+ and ~+ -" ~+ transitions respectively of the ~+ [624] i,3is neutron band .

C. Fahlander, G. D. Draroulis / Sidehands and high-spin states

27 8

4. Discussion We will first consider the g.s. rotational band and then discuss the structure of the sidebands in terms of their probable 2-quasiparticle configurations. Some of the expected lower-lying 2-quasiproton and 2-quasineutron configurations obtained from the known 1-quasiparticle states in the neighbouring odd proton and neutron nuclei, and from the Nilsson model, are shown for reference in table 2. TABLE 2

Some low-lying 2-quasiproton and 2-quasineutron configurations obtained from known I~uasiparticle states in the neighbouring odd proton and odd neutron nuclei and from the Nilsson model Proton configuration

K*')

Neutron configuration

K")

~ - [514] ® } [541] ~ * [402] ® } C541] [51a]

®~ ui`[~] -[~ 5] ® 3'C~l

] * [~]

® i [514]

') The unfavoured K-value is given in parentheses.

The 2-quasiparticle states are, according to the strong-coupling model, doubly degenerate with two different values of the projection K of the angular momentum, K = ~Sl t f SlZ ~, where Sg t. 2 are the projections of the angular momentum of the two individual particles. The degeneracy is removed by residual interactions and the favoured K-value, for which the intrinsic spins of the unpaired nucleons are anti-parallel ' 4), is given in table 2. The unfavoured K-value is given in parentheses. In fig. 6, the moment of inertia parameter (2.~/ltz) is plotted as a function of the rotational frequency squared (ft2mZ) for the observed bands. A striking feature is that all of the bands, except the y-vibrational band, have a high apparent moment of inertia relative to the g.s.b. indicating that rotation alignment is important for these bands. This alignment will be discussed for the 2-quasiparticle bands in subsect . 4 .5, and in subsect. 4.6 the alignment of the octupole vibration and competing 2-quasiparticle configurations are discussed. 4.1 . THE GROUND STATE BAND

The properties of the yrast states for the even tez-taBOs isotopes have been studied by Warner et al. 3). The yrast bands of the lighter "e-tBOOs nuclei have recently been investigated in this laboratory') . All these cases show backbendïng or the effects of bandcrossing above spin 12. It is believed that the phenomenon of backbending is due to the intersection of

C. Fahlander, C. D . Dracoulis / Sidebands and high-spin states

279

Fig. 6. The moment of inertia parameter 2 .1/h 2 versus the rotational frequency squared (h Z CO Z ) for the rotational bands of ' 82 0s. The parameters were calculated using the formulae 2.f/hZ = (4l-1)/E~,(1-+ 1-2), with h~~z

Ey(I~1-2) =/(

l(!+1)-

(I - 2x1-1)) 2

fortheg.s .b.

a 2-quasiparticle band (the Stockholm or s-band) with the g.s.b. The higher moment of inertia of the crossing band is attributed to rotation alignment of the intrinsic spin by the Coriolis force, the effect being largest for high-! orbitals having lowLl projections ' °) . The specific high Î orbitals involved in the Os region have however been somewhat controversial . The anomaly was first suggested to be due to the rotation alignment of the (h#)2 proton configuration 9) although it is now fairly well aooepted that the (i,~)Z neutron configuration' 6) is responsible for backbending in this region . This is further supported by the work of ref. '). The backbending behaviour for the yrast band of te20s is demonstrated in the moment of inertia plot of fig. 6. Except for the two new transitions observed in the present work, no new information has been obtained . 4 .2 . THE y-VIBRATIONAL BAND

The assignment of levels of the y-vibrational band up to spin 5 + was suggested by Yamazaki et al. Z). These states are confirmed in the present work and two more members of the band, the 6Y and 7Y states, are suggested.

280

C. Fahlarrder, G. D. Dracoulis / Sidebands and high-spin states

The moment of inertia parameter (fig . 6) is lower than .the g.s.b. moment of inertia and splits into two different curves for the odd- and even-spin states respectively . Further, the level energies deviate from those of a strongly coupled band and the even-spin members are energy favoured over the odd ones. This is seen as an oddeven energy staggering in a plot of E(I~E(I-1)l2I versus 2IZ (fig. 7). A strongly 15

4

b W

w

5 o__a --~

10

Octupole band ~band K 8

band

~~ Kr 5~ 1 band o-0 r 200

Fig. 7. The parameter

2113 keV band

400 600 2 12

(E,-E,_,)/21

as a function of 21 2 for sidebands in

'e20s .

coupled band would give a smooth curve in this kind of plot [cf. the curves for the K = 8 - and K = St -~ bands] and the appreciable deviation observed here might be attributed to band mixing arising from the rotation-vibration interaction. The observed energy staggering and consequent splitting into two separate sequences can also be accounted for in the asymmetric rotator model, as shown by Toki and Faessler ") . They give predictions of energies of the g.s.b. and y-vibrational bands for a wide range of even W, Os and l't nuclei . The excitation energies and the ordering of the levels of the y-band depends sensitively on the asymmetric ydeformation which is fitted in ref. ") to the earlier experimental data and for ' eZOs a best fit is obtained for y = 14.0°. The calculations remain in good , agreement with our extension of the y-band up to spin 7. 4 .3 .

CHARACTERIZATION OF THE K =

8-

ISOMER BAND

The rotational band built on the 1 .1, ms 8 - isomer is observed up to spin 19 - . The 8 - state was identified previously as the 2-quasineutron configuration v{~+ [624] x ~- [514]} e _ on the basis of the systematic appearance of 8- states in the

C. Fahlander, G. D. Dracoulis / Sidebands and high-spin states

28 1

TABLE 3

Cascade mixing ratios in the K = 8 - band and the parameter (g  - gx)/Qo Initial spin

E,~,_, (keV)

~b~')

b b)

10

206

0.81(l8)

-0 .87(40)

1 I

12

229 25l

0.63 (8) 0.53(10)

- 0.56±ô :ii -0 .42_`ô :is

(gc - 9e)lQo -0. 024 ±ô :ôôi -0 .031 _'û:ôôâ -0 .038'_ô :ôôs

') From the rotational model with K = 8 and the measured branching ratios . n) From the angular distributions.

N = 106 isotopes s). A sensitive test of the proposed configuration can be obtained from an analysis of the decay properties of the statés of the rotational band established here. Table 3 lists the experimental E2/M 1 mixing ratios, S, for the cascading transitions, as obtained in two different ways . Firstly, assuming pure K and using the rotation model, the mixing ratio can be deduced from the cascade/crossover branching ratios . The sign of S is not determined and the absolute values are listed in column 3 of table 3. Secondly, the mixing ratio is obtained from the angular distribution coefficients . These were corrected for partial alignment as deduced from the alignment of the stretched E2 transitions in the g.s.b. The large negative values of the Az coefficients (table 1) are only consistent with a negative sign for 8 and the deduced values are listed in column 4. These determine the sign and magnitude' of the parameter (grc - gR)/Qo, where Qo is the intrinsic quadrupole moment and gt: and grt are the intrinsic and rotational g-factors respectively . Taking the average of the (gK - grt)lQo values as -0.031(2), Qo = 6.0(6) and gx = + 0.24(2) from systematics in this region, a value of gK = 0.05(3) is obtained, supporting the proposed 2-quasineutron configuration which would have gK x 0 if pure. For a 2-quasiproton configuration, grc : .. + 1 .0 and a limit of 5 8 ~ is therefore placed on the proton admixture. Similar long-lived isomers have been found in the N = 106 isotopes s " to-zt) "8 Hf(T = 0.6 s) and to°W(T = 5.0 ms). In the case of te°W [ref. zz)], a rotational band was identified and assumed to be based on the 8 - isomer . The y-rays associated with this band show the same large negative angular distribution coefficients as our results for tBZOs and an analysis of the corresponding data allows a similar limit to be placed on the proton configuration for te°W as for tezOs, i.e. <_ 8 %. In contrast the K = 8 - isomer of t'BHf is known to have a 36 ~ admixture of the 2-quasiproton configuration n{~ + [404] ® ~- [514]} e_ [refs. zt " z3)] . The absence of mixing in ' BZOs and 'e°W is attributed to the change in proton Ferrai level which moves the 8 - 2-quasiproton level to a higher excitation energy .

28 2

C. Fahlander, G. D. Dracoulis / Sidebands and high-spin states

4.4 . CHARACTERIZATION OF THE 1896 keV BAND

The band based on the 1896 keV state has a regular energy spacing with a high apparent moment of inertia (fig . 6). The cascade mixing ratios S were deduced from the cascade/crossover branching ratios as well as from the cascading angular distribution coefficients as in subsect. 4.3 . Two values of a were extracted from the angular distributions but only the values with positive sign were consistent with the magnitude of the S-value obtained from the cascade/crossover branching ratios (table 4). The parameter (gtc - ge)/Qo is also given in the table and is, within TABLE 4

Same as table 3 but for the K = 5' -' band Initial spin

E,~,_, (keV)

7

130

9 10

190 212

12

253

8

11

157

235

~b~ < 0.75

0.52_±ô :ie 0.36(5) 0.24(5) 0.33(5)

0.28±°0 :°,ô

b

(y,;-yx)/Qo

+0 .17(8)

+0 .103_'ô ;ô3~ +0 .035_* ô :ôô9

+0 .27(5) +0 .23(8)

+0 .055±ô :ôôi +0 .076±$ :ôiô +0 .067 ±ô :ôôé +0 .070_`â ;ôiâ

the experimental errors, constant with spin . This lends support to the proposed rotational structure and a positive mean value for (grc-ge)/Qo of +0.061(6) is obtained . With gR and Qo as given in subsect. 4.3 we find grc = + 0.61(5) suggesting a mixture of 2-quasiproton ( ~ 60 ~) and 2-quasineutron ( ~ 40 ~) configurations . The lowest observed state of this band is assigned spin 5 and negative parity and it decays both to the 4+ member of the y-band and to the 6 + g.s.b. member with 705 keV and 1102 keV E1 transitions respectively . Assuming K = 5 for the band, the degree of K-forbiddenness, n, for the 705 keV transition is 2, and the 1896 keV level might be expected to be isomeric . The lifetime can be estimated by comparison with an isomeric E1 decay in tB°Os za) which has a degree of Kforbiddenness of n 4 and a hindrance factor of about 5 x IOB . From this a conservative estimate of ~ 10' for the hindrance of the 705 keV transition is obtained which would give rise to a lifetime of ~ 10 ns for the 1896 keV state. This lifetime is below our experimental limit. The observed lack of isomerism does therefore not rule out an assignment of K = 5 for the configuration of the band. Further, the high moment of inertia of the band indicates that admixtures of low-K components are likely to be present. Possible low-lying proton and neutron configurations are a{} + [400] ® ~ - [514]} 4 - and v{~ + [624] ® ~}-[521]}s-, which would lead to a mixed, predominantly K = 4 and 5, assignment . The ~ + [624]i,~ neutron orbital could account for the high moment of inertia as will be further discussed in the next section.

C. Fahlander, G. D. Dracouüs J Sidebands and high-spin states

283

4.5 . ROTATION ALIGNMENT OF THE 2-QUASIPARTICLE BANDS

As was pointed out earlier, all of the observed sidebands (except the y-band), show a high apparent moment of inertia, consistent with Coriolis effects due to the presence of a highy orbital . A quantitative estimate of the Coriolis effects can be made from the rotation aligned angular momentum as discussed by Bengtsson and Frauendorf 2s) who use the cranking model to treat the quasiparticle bands in 0.3

0.1 -~ ~/~ ~

,.

0.2

0.3

9/2* [624] band 1J2- [521] band

~. 7/2 [514] band K=8 band K=5~~ band Gsb Octupole band 1735 keV band 2113

keN band K-8

a__ . Q 2113

keV band K-1

n-o 2804 keV band

Fig. 8. Aligned angular momentum ih as function of the rotational frequency tuu for the l~uasiparticle bands ~*[624], }- [521] and } -[514] of 'B'Os and for the observed rotational bands of ' 820s . The parameters were deduced using the prescription of Bengtsson and Frauendorf ~°).

284

C' . Fuhlmrder, G . D. 1)rucoulic j Sidchunds und high-spin .stutes

a rotating deformed potential . In their prescription, the aligned angular momentum (its) of the quasiparticles is obtained with respect to a reference configuration, which is generally the g.s. configuration containing only the angular momentum of the collective motion, as a function of the rotational frequency (~a~) . The reference is extended to high spins through the parameterization I(u~) _ (.~°+ .9,cvz)~ where .4 ° and .~, are parameters fitted to the g.s.b. rotational energies before the backbend . The plot of ifi versus flcu is shown in fig . 8 for the bands of ' sZ Os. The fitted parameters have the values .Qo = 23 .5 MeV- ' ~ / Z and .~, = 125 .0 McV__ 3 ha which is very similar to the values given by Frauendorf c~t ut. Z`') . Also shown in this figure are the alignment of the I-quasiparticle bands of. ' e 'Os, for which the reference configuration is taken as the average of the g.s.b. values in the even-even neighbours . For this case the parameters have the values .9 = 23.0 MeV - ' ~ ~z and .~ 1 = 147 .6 MeV - ~ ti4 respectively . The ~ + [624] neutron band splits into two separate curves due to the difference in signature (a), a quantum number introduced by Bohr and Mottelson Z'), which reflects the symmetry with respect to a rotation of 180° around an axis perpendicular to the symmetry axis. According to Bengtsson and Frauendorf the aligned angular momentum is an additive quantity. That is, the sum of the alignment for the individual 1quasiparticle components should equal the 2-quasiparticle alignment . Both the K = 8 - and the 1896 keV (K = 4, 5) bands have a ~ + [624] neutron component and they both show very similar alignment . For example, at ficu z 0.10 MeV, the K = 8 - band has an alignment of i x 2 .8tZ and the 1896 keV band has i x 2.9ß while at ~a~ ~ 0.24 MeV the alignments are 4.2 and 4.3 respectively . The ~+[624] 1-quasineutron component has a similar aligned angular momentum at these frequencies . The aligned angular momentum in the K = 8 - and 1896 keV bands, taking into account an uncertainty of about 1 unit of angular momentum, thus approximately equals the value in the ~+ [624] band alone. The addition ofthe second quasiparticle, the ~ - [514] neutron in the case of the 8 - band, would lead to an aligned, angular momentum significantly higher than that observed experimentally. The possible source of this discrepancy in additivity might be in the definition of the reference configuration . As pointed out by Peker et al . zs) the g,s .b. will contain Coriolis antipairing effects (CAP) which will be reflected in the parameter .3~ and which are unlikely to be present to the same extent in the 2-quasiparticle bands. The CAP effects increase with rotational frequency, giving rise to an apparent decrease of rotation alignment at high frequency, as is clearly seen for the s-band of fig . 8. Further, in the light osmium region the . CAP effects are pronounced and the deformation, particularly the hexadeeapole moment, is changing rapidly . Both the CAP efFects and the deformation are sensitive to the neutron configurations and in particular to blocking of specific neutron orbitals . In contrast to the situation in the osmium region, similar comparisons of additivity for 1-, 2- and 3-quasiparticle states in thé light ~Hf isotopes s°) show that the additivity is reasonably well followed . However, the Hf cases have a predominantly t

~

3

Zs)

z9,

C. Fahlander, G. D. Dracoulis J Sidehands and high-spin stales

2R5

2-quasiproton core configuration (whereas the present states are mainly neutron configurations) and may not be as sensitive to the changes in thé reference configuration as the number of quasiparticles is altered . In discussions of rotation alignment one must therefore bear in mind this uncertainty introduced by the reference configuration, especially at higher rotational frequencies. Notwithstanding this uncertainty, since CAP effects in the sidebands ' aré reduced dué to the blocking of the occupied orbitals, a relative comparison of the alignment between the sidebands can be made. It may also be possible to use the 2-quasiparticle bands somewhat in the choice of the reference parameters as discussed by Bengtsson s `) . The positive-parity band with its lowest observed state at 2113 keV (1 = 8+) has a large alignment as shown in fig. 8. The alignment deduced depends to some extent on the K-value chosen for the band . This is demonstrated for the even-spin sequence which is plotted in fig. 8 for both K = 8 and K = 1 . For K = 8 the alignment reaches i ~ 7~ at fi~ x O .16 MeV whereas for K = 1 the maximum alignment is i ~ 8~ at ~m 0 .20 MeV. Due to the large admixture of low K-values, expected for a rotation aligned band, an average value of K ~ 1 is more probable, with somewhat higher K-values for the low-spin states . The Coriolis effects assumed to be responsible for the alignment are evident in the energy staggering of the oddand even-spin states . The 2804 keV positive-parity band also has a high alignment, as shown in fig. 8 for K = 1, but is slightly lower than for the 2113 keV band. The high degree of alignment for the two bands, which is larger than the alignment of the 8 - and 1896 keV bands, and for the 2113 keV band, nearly as large as that of the s-band, suggests that more than one aligned quasiparticle component is involved . One possibility for the 2804 keV band is the (h t )Z proton configuration. The proton ~} - [541] h t band known in `e 'Re [ref. 9)], however, shows an aligned angular momentum of the same order as the ~±[624]i,~ neutron band. One would therefore expect the 2804 keV band to have the same alignment as the s-band and the (h~)z assignment is therefore unlikely för this band. The (h~)~ configuration, will only give rise to a low-lying even-spin sequence, and therefore is not a likely configuration for the 2113 keV band, which has both an even- and an odd-spin sequence . The two positive-parity bands are candidates for the next lowest (yrare) rotation-aligned bands produced by two i, 3iz neutrons, expected from the rotational alignment model (RAL) of Stephens and Simon 1 s). The lowest (yrast) even-spin rotational band, produced by the (i, 3~z)Z neutron configuration, is the s-band which intersects the g.s.b. and produces the backbending discussed in subsect. 4.1 . The position of the yrare states, which will originate mainly from the v{~+[624] ® ~+[633]} and v{~+ [624] ® u+ j615]} configurations (which for pure K would correspond to K = 8 and K = 10) can be estimated from the Nilsson single-particle energies as well as from the 1-quasiparticle energies known from the neighbouring odd isotopes . Both estimates yield an excitation energy for the 8 + state in reasonable

286

C . Fahiander, G. D . Draroulis / Sidehands and high-spin .states

agreement with the observed 2113 keV state. The estimated energy for the 10+ state lies close to the energy of the expected 10+ state of the 2804 keV band, obtained by an extrapolation from the observed 12 + , 14+ , . . sequence . These two positive-parity bands decay to the g.s.b. with relatively high-intensity transitions from states above the bandhead. If the in-band transitions are enhanced quadrupoles of the same strength as the 2 + -. 0+ g.s.b. transition, the out-ofband E2 transition strengths would be enhanced by a factor of ~ 15 over the Weisskopf estimate . This implies large admixtures of low-K components since the K-selection rules otherwise would inhibit the out-of-band transitions. A consequence of this mixing is that a bandhead as such may not be identified since the out-of-band transitions reduce the intensity remaining in the band at low spin. The short lifetime of the 2113 keV 8+ level is also consistent with admixtures of low-K components . The reduction of intensity within the band would account for the non-observation of the 10+ state of the proposed v{~+[624] ® ~+[615]} band. A band with the same configuration is assigned in 'azW [ref. az)], with a similar energy for the 10+ state as that expected in the 'BZOs case, but with a mean life of 1 .4 ACS . The long lifetime and the fact that the band only decays to the g.s.b. from the bandhead, in contrast to its proposed ' 820s equivalent, implies less admixture of low-K components. The K-components are mixed through Coriolis coupling which is large in ' BZOs but will be smaller in 'azW because of the higher neutron Fermi level. The smaller Coriolis coupling in 'szW is also consistent with the non-observation of backbending which implies the absence of a low-lying s-band az). 4 .6. THE OCTUPOLE VIBRATIONAL BAND AND ROTATION ALIGNMENT

The negative-parity states above the 1472 keV level are suggested to be the members of the K = 3 - octupole vibrational band. The excitation energies of these states are close to the energies of the K = 3 - octupole band assigned in teaOs [ref. 'a)], and the decay pattern of the proposed K = 3 - band of ' SZOs is similar to the isaOs case, with decays to both the g.s.b. and to the y-vibrational band. Neergârd and Vogel aa) have been able to reproduce the properties of lowlying octupole states in a wide range of rare-earth nuclei using a random phase approximation (RPA) with an octupole-octupole residual interaction. The octupole vibration splits with the quadrupole deformation into four components with K = 0 - , 1 - , 2- and 3 - which mix through Coriolis coupling between the components . The decay properties of the octupole band in ' 820s show several features, including the strength of the out-of-band E1 transitions, which provide evidence for such K-admixtures . The strength of the 1-" 1-1 E1 transitions can be compared with the competing in-band E2 transitions through the ratio {B(E1)/B(E2)}, estimated from the y-ray branching intensities. Taking the transition from the 5 - state as an example, this

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ratio is found to be about 2 x 10 -a b - ' which is similar in strength to the out-ofband E1 transitions for the K = 2- octupole band of the isotone' e°W. The absolute B(E1) values for 'e ZOs can be obtained by assuming the in-band E2 transitions to have the same strength as the known 2+ -. 0+ g.s.b. transition, and are found to be of the order of 10_e W.u. . In contrast, dK = 3 (n = 2) transitions would have inhibition factors of the order of 10' [see subsect. 4.4 and ref. 3s)] . Further evidence for K-admixtures can be obtained from a comparison between the E1 decays from the odd- and even-spin states, and also from the ratio of 1 ~ I+ 1 and I -+ I-1 E1 decays from the odd-spin states . The strength of the out-of-band transitions depopulating the odd-spin states are higher than those from their even counterparts . For example, the estimated E1 strength of the 7 - --" 6g transition is a factor of two or more higher than that of the unobserved 8 - -" 88 transition . This difference occurs because there is no admixture of the K = 0- component in the even-spin members of the octupole band since a K = 0- band only consists of odd-spin states . The E1 transitions between the octupole band and the g.s.b. are restricted by the K-selection rule to dK = 0 or 1 and further, the dK = 1 transitions are usually more inhibited than the dK = 0 transitions. Thus the out-of-band decays from the even-spin states, which only have a dK = 1 component, are weaker than those from the odd-spin states which have both components . A related effect is that the ratio of the I -" 1+ 1 and I -" I-1 transition strengths from the odd-spin states varies with spin. The 3 - state, for example, ofily decays by an I -" I+ 1 transition, despite the higher energy of the I ~ I-1 path . In contrast, the 5 - state decays by both possible paths, but the 7~ state only via an I -" I-1 transition . The absence of the I --~ I-1 transition in the decay of the 3 - state may be due to a cancellation between the dK = 0 and dK = 1 components as discussed in ref. 36). The change in decay pattern for the states with higher spins would then indicate a change in the K = 0 and K = 1 components of the wave functions of the states . Similar decay patterns have been observed for the octupole bands in nearby W nuclei a') and in, for example, "ZYb [ref. 3s)] but have not been fully explained. The I = 3- and S - spin members also decay with both 1-~ I and I -" 1-1 transitions to the y-band and the branching ratio B(E1 ; I -~ I-1)/B(E1 ; 1-. I) can be deduced. The Alaga intensity rules applied to these branching ratios are not in agreement with any pure Kvalue again suggesting Kadmixtures. Thus the above decay properties of the K = 3 - band lend support to the suggestecj importance of the Coriolis interaction. Another consequence of the Coriolis mixing is the compression and distortion of the energy spacing in the band. This results in a high apparent moment of inertia (fig. 6), and a staggering in energy for oddand even-spin members (fig. 7). The band effectively splits into an odd- and an even-spin sequence with a slight favouring in energy for the odd-spin states . In tenors of an alignment of the angular momentum of the octupole vibration, the odd energy favouring can be understood since the odd-spin states can be aligned

2R8

C' . Fahlandrr, G. D. Drucoulis / Sidehands and high-spin stales

completely (I - R+3 where R = 0, 2, 4, . . . is the collective rotation), while the even spins can only be aligned partially (I - R+2). In an extended RPA calculation, Vogel 3v) shows that a transformation from an aligned octupole to an aligned twogizasiparticle configuration takes place at some critical angular momentum I~ 9 in the light rare-earth nuclei, with an additional loss in collectivity of the octupole vibration. The aligned angular momentum, i, for the 3 - band (fig . 8) demonstrates that an alignment of 3tß is reached at a rotational frequency of iur~ - 0.09 MeV for the odd-spin sequence and increases to Sti at itcu - 0.22 MeV. Although care should be taken because of the uncertainty in i, as discussed earlier, the increase of 2tß is significant, and may be indicative of the proposed transition from an aligned octupole vibration to an aligned band where 2-quasiparticle states are more dominant. The specific 2-quasiparticle configurations involved in the octupole band of ' gZOs are not known . For the heavier osmium nuclei (' ae - ' e°Os) the lowest octupole states are predicted s4) to be the K = 3 - states and to consist predominantly of the {~ + [402] ® -'z'--[505]}3- proton configuration with some admixture of neutron configurations . The neutron admixture is likely to decrease when going to the lighter ' SZOs nucleus. In the related W nuclei, where the lowest octupole state has K~ = 2- [see refs. zz . a°-az)], the dominant 2-quasiproton configuration is predicted aa) to be the n{~ + [402] ® ~ - [514]} Z - configuration with admixtures of the v{~ - [503] ® ~+ [615]} 2 and v{~ - [512] ® ~ + [624]} Z - configurations which are calculated to decrease with neutron number. This is in reasonable agreement with the experimental result from radioactive decay studies of'e2W [ref. aa)], and in similar studies of' 8°W [ref. aa)], which is the isotope of'SZOs, the 2-quasiproton configuration is suggested to be dominant . The rz{ ~ + [402] ® ~- [505] } 3 - configuration may possibly account for the alignment at lower frequencies of the K = 3 - band in ' BZOs. The larger alignment at higher frequencies, however, suggests that configurations involving aligned particle orbits are more strongly admixed . Possible configurations are n{~+[402] ® ~[541]} 3 -, n{~+[404] ® ~ - [541]} s - and v{~ - [512] ® ~+[624]}Z- . The alignment of the } - [541]h t proton band and the ~+[624]i,~ neutron band are approximately the same, and either of them could account for the observed alignment. In principle it is possible to distinguish between neutron and proton configurations by extracting the value of (gk -gR)/Qo from the mixing ratios of the cascade transitions within the K = 3 - band. The resulting intrinsic g-factor of about 0.1 would imply a neutron configwation but, because of the uncertainties due to the low, and admixed K-values, such a conclusion must be tentative. 4.7 . THE 1735 keV BAND

The parity of the 1735 keV band is not known but the lowest observed state is most probably a spin 5 state. A possible configwation for the intrinsic state is therefore the {~ - [514] ® ~ - [514]} s+ 2-quasiproton configwation . The proton

C. Fahlander, G. D. Draeoulis l Sidehands and high-spin slates

289

] - [541]h t orbital would account for the observed alignment of the band (see fig. 8) thus supporting the proposed configuration. Further experimental information is necessary to make more definite conclusions about this band . 5. Summary An extensive level scheme for teZOs has been obtained. On the basis of their decay properties, K-assignments and the observed rotation alignment, most of the bands have been identified with specific 2-quâsiparticle configurations . Of the two vibrational bands observed, the y-band and the octupole band, the former does not show rotation alignment. In contrast, the latter exhibits alignment of the octupole phonon at low spins, and the effects of alignment of individual 2-quasiparticle configurations at higher spins. The rotation alignment, which dominates the properties of the' $ ZOs level scheme, in the sense that only these favoured configurations compete succesfully for population, occurs through the strong Coriolis coupling in the i,a,a neutron or h~ proton orbitals . Only the y-band does not fit into this category, and it is populated essentially indirectly . The effect of rotation alignment is evident even in the band based on the 1 .1 ms 8 - isomer, the strongest populated sideband observed, whose other properties would qualify it as a strongly coupled band. We thank Dr I. G. Graham for çomputer program development, A. H . F. Muggleton for target preparation and acknowledge the support of the technical and academic staff of the 14UD pelletron accelerator facility in this work. References 1) A. 1. Akhmadzhanov, B. Bayar, R . Broda, V . Valyus, N. G. Zaitseva, H. W. Seibert, K. M. Kamalkhodzhaev, G. Muziol, A. F. Novgorodov, W. Neubert, M . Finger, U. Hagemann and H. Shtrusnyi, Bull . Acad . Sci. USSR 36 (1973) 1820 2) T. Yama7aki, K. Nishiyama and D. L. Hendrie, Nucl . Phys . A209 (1973) 153 3) R. A.~ Warner, F. M. Bernthal, J. S. Boyno, T. L. Khoo and G. Sletten, Phys. Rev. Lett. 31 (1973) 835 4) W. F. Davidson, R. M. Lieder, H. Beuscher, A. Neskakis, Z. Seres and C. Mayer-BBricke, Z. Phys. 264 (1973) 235 5) J. Burde, R. M. Diamond and F. S. Stephens, Nucl. Phys . 85 (1966) 481 6) G. D. Dracoulis and C. Fahlander, Phys . Lett . 97B (1980) 355 7) G. D. Dracoulis, C. Fahlander and M. P. Fewell, Phys . Rev. Lett 45 (1980) 1831 8) R. M. Lieder ; Proc . Int. Conf. on band structure and nuclear physics, New Orleans 1980, Nucl. Phys . A347 (1980) 69c 9) A. Neskakis, R. M. Lieder, M. Müller-Veggian, H. Heuscher and W. F. Davidson, Nucl. Phys . A261 (1976) 189

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