Rotation Aligned Negative Parity Side Bands in Light Tungsten and Osmium Nuclei G.D

ROTATION ALIGNED NEGATIVE PARITY SIDE BANDS IN LIGHT TUNGSTEN AND OSMIUM NUCLEI G.D. Dracoulis

To cite this version:

G.D. Dracoulis. ROTATION ALIGNED NEGATIVE PARITY SIDE BANDS IN LIGHT TUNG- STEN AND OSMIUM NUCLEI. Journal de Physique Colloques, 1980, 41 (C10), pp.C10-66-C10-78. 10.1051/jphyscol:19801007. jpa-00220626

HAL Id: jpa-00220626 https://hal.archives-ouvertes.fr/jpa-00220626 Submitted on 1 Jan 1980

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ROTATION ALIGNED NEGATIVE PARITY SIDE BANDS IN LIGHT TUNGSTEN AND OSMIUM NUCLEI

G.D. Dracoulis.

Department of Nuclear Physics, Research SchooZ of PhysicaZ Sciences, Australian National University, P. 0. Box 4, A. C. T. Canberrra, Australia.

Abstract.- Rotation aligned negative parity sidebands have been observed in the light Tungsten and Osmium isotopes. The development from octupole bands to aligned 2-quasiparticle bands is discussed. The hghproton and i13/2 neutron are the likely configurations causing the alignment. Backbending observed in the odd spin negative parity sidebands in lEOOs suggests that both proton and neutron configurations are involved at high spin. New results on backbending in the yrast bands of the light Osmium isotopes are also discussed.

INTRODUCTION would not compete favourably for population in the The study of sidebands in even-even deformed reactions. Even with rotation alignment, the negat- nuclei is an active field which I will not attempt ive parity bands are usually non-yrast and receive to review in the time available here. Rather, I even in the most favourable cases discussed here,

will concentrate on the systematic properties of only a small proportion of the feeding. negative parity sidebands, which show the effects of Although the level schemes I will show are rotation alignment, in Tungsten (Z=74) and Osmium partial schemes, the selection of the aligned neg- (Z=76) isotopes with N=100 to 108. The reasons for ative parity bands is not arbitrary since in the this choice are personal aquaintance with the region, lighter isotopes (N=100 and N=102) they are the but more importantly, the large isotopic range of strongest sidebands observed and with the exception nuclei studied to high spin which reveals the sys- of bands based on 2-quasiparticle isomeric states,

tematic behaviour and occurrence of these bands. the only sidebands identified to high spin. They I will not dwell on the details of experimental are in these lighter isotopes the "yrast negative techniques, however, it is worth remembering that parity" states. the population and decay pattern in (Heavy Ion, xn) Octupole States and Rotation Alignment reactions used to study neutron deficient nuclei Any discussion of negative parity states in preferentially populates yrast states. In deformed this region is at least partly connected with nuclei the yrast states are usually the ground state octupole states, which are well known in the heavy

rotational band members up to about spin 16-20fi (and Tungsten and Osmium nuclei. Their collective

about 3 MeV excitation energy). Sidebands could be nature is established by their preferential populat-

loosely defined as rotational bands with configuration in (d,dt) studies and from their y-decay which ions different Erom the ground state, such as 2 (or involves strong E3 transitions1-'). Neergard and higher). quasiparticle states or other collective voge15) have successfully calculated the properties states (e.g. vibrational states), which necessarily of low-lying octupole states in a wide range of

begin in the region of the pairing gap, about 1 MeV nuclei using a quasiparticle random phase approximat- excitation. If it were not for rotational alignment ion with an octupole-octupole residual interaction.

at high spin in some of these configurations, they In a deformed nucleus the 3- vibration splits into

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801007 states with K' = 0-, 1-, 2- and 3- each with rotation- jl- and i2almost anti-parallel. al bands, the 0- band having (essentially) only odd 21 At high spins (> 10 in this region) a trans- spin members. The bands are successively coupled ition to an aligned quasiparticle configuration is through large Coriolis matrix elements. Because of predicted where the particles have the& spins these interactions the bands (usually only the lowest parallel - the aligned angular momentum being dom- energy band is known) are perturbed. In the Tungsten inated by the contribution of the high spin (unique and Osmium region the lowest states are predominantly parity) quasiparticle. the 2-quasiproton configuration {y2- [514]~r, y2+[402]a} I will keep these predictions in mind in dis- with some 2-quasineutron admixtures. In Tungsten cussing the experimental results, when I refer to the 2- configuration is usually lowest whilst in the possible 2-quasiparticle configurations, proton

Osmium the 3- configuration is seen. and neutron, that contribute to the alignment in the

voge16) has extended the RPA calculations to observed bands. higher spin and also carried out a 2-quasiparticle Negative Parity Bands in Tungsten calculation in a subspace including all states orig- In figure 1 I have collected the partial decay inating in the unique parity subshell (for protons schemes of high spin states in 174~and 176~18), and neutrons) and abput 10 normal parity quasi- 178~19-21), 180W 22-24) and l8ZW 25,26). ~h~ 2- particle states close to the Fermi surface (see octupole band in 1a2w drops in energy in going from footnote 'I. Vogel reached specific conclusions la2w to laow and as we progress to the lighter which are relevant to the results I will show. In isotopes the character of the band changes in sever- summary these were al ways.

l] At low and intermediate spins the negative a) The odd and even spin states separate so parity states are aligned octupole states. The odd that in the lighter isotopes the excitation energies J J-1 and even spin members of the normal rotational bands are approximately Eodd = E even and the AJ=1 cascade

split into separate (AJ=2) 'sequences. The odd spin transitions become weaker, mainly because the trans-

states are favoured in energy and have ition energy is reduced.

I = R+3 b) The decay pattern of the out-of-band El while the even spin states have transitions changes from decays mainly from the band-

I R+2 head (or states close to it), to decays from the where R is the collective rotation (=0, 2, 4 ...). hlgh spin states to the gsb, and predominantly from

That is, the octupole vibration is aligned with the the odd spin states to the gsb. This out-of-band

rotation, and the spacing of the AJ=2 sequences is pattern, which I will comment on below, results in

that of the ground state ("R") configuration, lead- the population being funnelled into the gsb, before

ing to bands with a high apparent moment of inertia. the lowest spin states in the bands can be reached.

The wave furictions are such that the participating That Is, the 4-, 5- states seen in the lightest

quasiparticles have their intrinsic angular moments isotopes are not to be taken as band heads.

Although I will not discuss them,, here, a number of calculations relevant to negative parity hi h spin states have been reported b Flaum and line'), Hjorth et. a18), Neergard, Vogel and RadomskigB, Lin Faessler and Oreizler101, Plosrajczak and ~aesslerll),Krumlinde and ~arshalek~~),Toki, et all3), Zolnowski et a1141, Hamamoto and sagawa15),' Konijn et a116), F.W.N. de Boer et all7) and certainly others. c10-68 JOURNAL DE PHYSIQUE

c) The bands become compressed so that they B(El)/B(E2) is about 10-~b-I for the odd spin have a high apparent moment of inertia. The high states, with a trend toward larger strengths in the

odd spin states J, are at about the same excitation lighter isotopes. The single particle strengths,

energy as the yrast J+l positive parity states in taking the El transition from the 9- state in 174~

174~and 176~where the negative parity states are as an example, would be about 5x10-' Weiskopff Units,

known to high spin. implying low -K components in these states.

Taking these points in a little more detail, El Transitions from the even spin states are

a) The odd-even energy splitting can be seen weaker. This difference has been attributed (see

more dramatically in a plot of AE(J+J-1)/2J vs.2~~ for example refs. 20,27,28)) to the difference be-

given in figure 2. Only small oscillations are ob- tween AK=O and AK=l El transition strengths. At

se-med in le2w, consistent with small second order low spins AK=l transitions are considerably weaker

admixtures of the 0- band. These develop to large than DK=O transitions and since the odd spin states

oscillations for the N=100 and N=102 isotopes. will have both K=O and K=l admixtures (as well as

b) The out-of-band El transitions show several K=2 and K=3 admixtures which will not contribute to

features which I can only outline here. The allowed dipole decays) while the even spin states

strength of the J+J-l El transitions can be estimat- will not have K=O admixtures (remembering that the t ed through the branching ratios in comparison with K"=o- band only had odd spin states ), the transit-

the competing in-band E2 transitions. The ratio ions from the odd spin states will be stronger.

Figure l Partial level schemes showing negative parity sidebands and yrast bands in 174~L182~. (see text for references)

This is n0.t strictly true39), but it is usually assumed that the even spin members will be removed to high energy. al frequency 6w within a band, from the transition

energies. That is

with

The aligned angular momentum with respect to a ref-

erence configuration is given as

i(w) = I~(W) - I~~(W), where g refers to the reference band.

TABLE 1. Branching Ratios and B(E1) ratios.

J-tJ I E B (El) J+J+l gsb Y IkeV: B(E1) J+J-l

Figure 2 (EJ-EJ-1)/2.J VS J~ for the negative

parity bands in Tungsten and Osmium.

A further trend is evident in the branching ratios from the odd spin states. These decay by both J+J+l and J+J-l transitions to the gsb. At low spin these are comparable in intensity, despite the lower energy of the J+J+1 transition. Over a small range of increasing spin the situation changes to a more ratio until at high spins The aligned angular momentum Ix is shown for only J+J-l transitions are observed. The following the 174~and 176~bands in figure 3. The negative table illustrates the trend for two of the tungsten parity bands are seen to have an alignment higher cases. Further examples have been measured in the than that of the gsb, and similar to that observed

Osmium nuclei30). in the yrast band after backbending. By taking the

This effect is probably due to a cancellation gsb as the.reference configuration (extrapolated of the AK=O and AK=l components (see also ref 28)) where necessary through the backbending regiontf) or may indicate a change from the octupole mode the alignment i is obtained. This is shown in fig- which induces the El moment. ure 4 for all the Tungsten isotopes.

c] Moments of Inertia and Rotation Aligned The N=108 isotope 18*w shows little alignment. Angular Momentum ft~nthe even-even nuclei the reference has been In the following discussions I will use the taken as the gsb. In the odd nuclei the'average of the even-even neighbours has been used. There is prescription of Bengtsson and ~rauendorf'l) to considerable &certainty in extrapolations through the backbending region, hdthis should be borne in deduce the aligned angular momentum lx and rotation- mind when comparing relative alignments. c10-70 JOURNAL DE PHYSIQUE

In contrast the alignment in 174~and 176~in - trend through the isotopes was attributed to the ex-

creases from about 1 .S and 2.5 ii in the even and pected sensitivity of the Coriolis mixing to the

odd spin states respectively at Fiw = 0.12 MeV, to position of the neutron Fermi level. The 1 quasi-

about 4 fi in both odd and even spin states at 0.2 neutron states and 1 quasiproton states which might

MeV. The intervening isotopes show the trend of be involved in the negative parity configurations

increased alignment with decreasing neutron number, (since they are the lowest states in the 1 quasi-

correlated roughly with the energy splitting given particle spectrum) are given in table 2.

in figure 2. We note in passing here that the TABLE 2 Low Lying Proton and Neutron Configurations.

alignment observed in lEOw, which approaches 3 ii at protons neutrons

tIie highest spin observed, was interpreted recently j a Nilsson j II Nilsson

as observation of the alignment of the octupole - q/2-[514] 1 fT2 - 7/2-[514] h l l/2 vibration23). i - %-[521] d5/2 + 5/2'14021 j py? Our earlier interpretation of the observed I g7/2 + ')/2+[4041 1 h% - %-L5121 alignment was that the 2 quasiparticle states formed !

by coupling an i13 neutron, and an hq - '/2-[54l]', ila + TZt[633] or ?21[624] h /2 Or h9/2 neutron (not rotation aligned), which are low in

the neighbouring 1 quasiparticle spectra, were the> '~ecause of decoupling the 5/2- or y2- states of this dominant configurations at high spinsl*). The configuration are usually lbwest ?n energy.'

01 I I 1 I I l I I 0.1 0.2 0.3 0.4 ho MeV

Figure 3. Aligned angular mmenta I (see text) vs Kw for the negative X parity bands and yrast bands in and 176~. The alignments for selected bands involving these configurations can be deduced from the known spectra for 175~321, 177~331, 179~34,351, l8lw 26),and

177~~36), 179Re 36) and lEIRe 37). For complete-

Figure 5 Relative aligned angular momenta for sel-

ected odd neutron and odd proton bands in

I I I I l Tungsten and Osmium. Only the a=+% bands 0 .l 0.2 0.3 0.4 ho MeV are shown for configurations with a strong signature dependence. Figure 4 (a) The aligned angular momentum i with

respect to the reference configllrations, the proton or neutron combinations could give rise

for the negative parity bands in the to the alignments observed in the negative parity

Osmium isotopes. The odd spin sequences figure 5. It can be seen from these that either of

are connected by the heavier continuous line. bands in Tungsten since the i13 neutron, and the 12 (b) as for (a) but for the Tungsten proton are both aligned. Comparing these dir-

isotopes. ectly with figure 4 it is apparent that the align-

lpent is only reached above Hw = 0.2 MeV. Therefore, ness one should include the odd proton Ta isotopes if thege configurations are dominant, they are only and the heavier odd Tungsten isotopes but partly for dominant at high frequencies. At low frequencies simplicity, and partly because all the relevant data the alignment is approximately that suggested by are not available, a selection has been made. Vogel, 2 and 3 H in the even and odd spin states

The neutron and proton alignments are shown in respectively. The transition to an aligned quasi-

particle configuration is not seen in 178~and the

6 c10-72 JOURNAL DE PHYSIQUE

heavier isotopes, at least up to the spins so far the gsb through the backbending region have also identified. been observed. The angular momenta I shown in

I will not attempt to make a judgement here on figure 7 for the range of isotopes from A=176 to 182 whether the dominant configuration at high spin is show a clear trend, from strong backbending in lB20s, the neutron or proton one, (the neutron number dep- to a weak but well defined up-bending in 176~s.The endence of the odd protons or neutron does not alignment reached at a frequency of 0.35 MeV is apparently distinguish between them) but will make about the same in each case indicating, as has been further comment on that after showing the Osmium pointed out for the Tungsten isotopesz1), the common results. nature of the crossing band. The aligned (i13 )2 /2 Yrast bands, and Negative Parity Sidebands in light neutron band is accepted as the crossing band in

Osmium Isotopes lE20s 43) although earlier, the h proton configur-

High spin yrast states in the light Osmium ation was suggested to be responsible3'). Further isotopes, down to neutron number 106, lE20s, have evidence that the bandcrossing is consistent with previously been studied, particularly by the the behaviour of the i13 neutron is given in the 12 Michigan and Julich groups38'37'39). we are curr- relative aligned angular momegta of figure 8. As ently studying the isotopes from N=100 to N=106, was seen from figure 7, the bandcrossing occurs at partly with the aim of complementing the Tungsten progressively higher frequencies with decreasing series and will present here new results on these neutron number, but the important point to note is nuclei30s40), some of them preliminary. High spin that the alignments of the i13 1 quasiparticle /2 states in Osmium were populated using (160,xn) reac- bands in lelOs, and from our recent study 40), in tions on enriched Er targets using 160 beams from 1770s, are consistent with the observed alignments the ANU 14UD Pelletron accelerator. In order to in the even nuclei. For simplicity, the i13 band 12 facilitate the study of sidebands, as well as the in 1790s, which we have also identified is not in- usual armory of y-ray spectrosopic techniques we cluded on the plot. More detail on the odd have used a Compton suppressed Ge(Li) spectrometer neutron behaviour in the Osmium nuclei is given in in both singles and coincidence measurements. figure 9. The 4- [521] bands in 1810s and 1770s

Partial level schemes showing the yrast states show anomalies consistent with the magnitudes of the and the rotation aligned negative parity states are alignment gains observed in the yrast bands of the shown in figure 6, together with the results for even neighbours, although at slightly lower frequen-

1840s given in references 41942). Spin assign- cies. The i13 bands, labelled [624]", show /2 ments and parities in the present study are based on anomalies only at higher frequencies. Lieder44 angular distribution measurements and internal con- has suggested that the anomaly in the 9/2+[624] band version electron measurements. in lslOs is due to a crossing with the il three 3/2 Before discussing the negative parity bands I quasineutron band, but involving the 7/2+[633] would like to point out some features of the yrast neutron as well as the a=+% components of the states which have been established here up to spins 9/2+[624] neutron. The situation in the Osmium of about 20 or 22 in 1760s, 178~sand lEOOs. In nuclei differs apparently from other cases where the

1780~and 1800s, candidates for the extensiqns of blocking of the appropriate i13 neutron delays band- /2 Figure 6 Partial level schemes for 1760s - 184~s. (see text

for references)

crossing to considerably higher frequencies. may be a further consequence of the same effect46).

The tempering of the magnitude of the band- TO return to the negative parity bands, we can crossing in the light 0s isotopes may be a demon- follow a development similar to that observed in the stration of the effect of increased interaction Tungsten isotopes. The 3- octupole band identified. matrix elements between the g.s. and the crossing in 18'+0s, unfortunately not to high spin, decays band configurations - small matrix elements leads mainly to the y-band. In 1820s a small energy to strong backbending - which are predicted to have favouring of the odd spin negative parity states is a strong mass dependence31945-47). However, cal- observed with out-of-band decays to both the gsb and culations which correctly treat the rapid change in to the y-band. Lieder et alS1) have also presented hexadecapole deformation which occurs in the region a level scheme for 1820s in a contribution to this are-required to properly examine this point. The conference, in which they have identified a large difference in behaviour between the a=& signatures number of bands including the negative parity band of the 9/2*[624] band observed in 1770s and 17'0s being discussed here. Our results30) agree with

, , c10-74 JOURNAL DE PHYSIQUE

I I I I l I I 0.10 0.20 0.30 0.4 Ao MeV

Figure 7 The aligned angular momentum I vs liw for the yrast (and yrare) sequences in 176~s- l820s.

I I I r 1 0.1 0.2 0.3 0.4 Ro MeV

Figure 8 The relative aligned angular momenta Figure 9 The relative aligned angular momenta

i vs for the yrast (and yrare) i vs iiu for selected odd neutron sequences in lT60s - la20s. bands in Osmium. theirs on all counts except for the placement of the 1 quasiproton spectrum, and in particular its odd spin sequence in the present negative parity neutron number dependence, between the Re and Ta band. In both schemes, an anomaly is seen in the isotopes. An examination of those spectra49-50) even spin sequence at a low frequency. In 1800s shows that the $-[S411 h proton has, as expected, 9/2 only an odd spin sequence has been identified, a similar alignment in the Re and Ta isotopes, but

(although a candidate for the 6- state is observed) is at considerably higher excitation energy in the with the main branches to the gsb, but still with heavy Ta isotopes than in the isotonic Re nuclei. significant branching to the y-band. An anomaly For example, in ~el~l(N=106)the 5/2- state of the in the band spacing is seen at about spin 17. $-[541] configuration is at 357 keV while in 179~a

Similar, well developed, odd spin sequences, and it is at 628 keV. The aligned 2 quasiproton con- separate, more weakly populated even spin sequences figuration in Tungsten, therefore, may be at a high- are observed in both of the lighter isotopes 1760s er excitation energy, in the heavier Tungstens, than and I7*0s. in the isotonic Osmium nuclei - although the light

These bands are, at least superficially, re- nuclei may be similar. Secondly, the negative markably similar to those in the Tung,sten isotopes. parity band in 1800s shows backbending at a low

The El strengths and branching ratios are similar, frequency. If the i13 neutron configuration were 12 the odd-even energy spacing given in figure 2 is responsible for the alignment at 0.2 MeV, then it similar, but there are differences in the alignments. could not be responsible for the gain in alignment

The alignments are shown in figure 4. 1760s at 0.25 MeV because of blocking. The effect of and 17%0sshow higher alignment in the odd spin blocking of the i13 neutron is apparent in the be- 12 states than the even spin states at low frequencies, haviour of the 9/2+[624] 1 quasineutron band in rising to about 45 at Kw = 0.18 MeV, similar to the 1790s (shown in figure 9) which does not show an behaviour in the N=100 and N=102 isotopes of anomaly until a considerably higher frequency than

Tungsten. l800s and lE20s however show even larger the g-15211 band. However, the same argument could alignments, in marked contrast to their Tungsten be applied to the hg proton configuration since in 12 isotones, and in laOOs a large alignment gain is 181~ethe %-I5411 band also does not show backbend- observed at a frequency of 0.25 MeV. ing while the 5/2+[402] band does, and at the same

The same difference is partly seen in comparing frequency, 0.25 MeV, as the negative parity band in the odd-neutron isotopes of Osmium and Tungsten but laoOs. The solution to this dilemma is to postul- jt is not possible here to conclusively identify ate a change in configuration from a 2 quasiproton' the quasiparticle configurations responsible for configuration (involving the h9 proton) to one in- /2 the alignment observed at Kw 0.2 MeV. It may be volving an extra pair of il3 neutrons resulting in > /2 that the h protons still play a significant role the alignment gain at F1w=0.25 MeV. A similar ex- 9/2 since there are two main differences evident in the planation has been given by Ploszajcak and Faessler comparison between Osmium and Tungsten. Firstly, for the backbending observed in the negative the heavy Tungsten isotopes, N ? 106, do not show parity band of 156~r. large alignment. If the protons were responsible, The anomaly in the negative parity even spin this might be attributable to the difference in the sequence in le20s occurs at a frequency of about C10-76 JOURNAL DE PHYSIQUE

0.29 MeV, higher than the anomaly in the gsb, and at I would like to express my appreciation to my about the same frequency as the i13 band in Ia10s. resilient colleagues Dr P.M. Walker, and Dr C. 12 Lieder et alS1) use this as eyidence that the i13 Fahlander. 12 neutrons are responsible for the gain in alignment.

However in laOOs the situation is different since the anomaly in the odd spin sequence occurs at a lower frequency than that in the gsb, or in the ilY2neutron band in 1790s.

Certainly, the situation may be complex, and specific calculations for proton and neutron configurations for this range of Tungsten and Osmium isotopes which properly treat the changing quadrup- ole and hexadecapole deformations are imperative if a clearer understanding is to be reached.

To. return briefly to the introduction of this talk, and the predictions of Vogel; it seems possible to trace the development of the octupole band into separate odd and even spin sequences over a wide range of isotopes. The alignment at low frequencies is consistent with contributions of about 26 and 3H to the even and odd spin states respectively, due presumably to the alignment of the octupole spin. A transition to a band with a higher alignment is seen in most cases. This gain in alignment is consistent with that due to the i13 neutron or the h9 proton which could con- 12 12 tribute to the dominant 2-quasiparticle configurations. , Although it is not possible at present to definitively characterise the negative parity bands at high spin, the systematic trends, and the backbending observed in the negative parity band in

1800s, suggest that the i13 neutrons and h9 12 12 protons both play a role.

I trust that I have convinced you that rotation aligned negative parity bands are a systematically occurring feature of sidebands in this region of the periodic table. REFERENCES

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