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ROTATION ALIGNED NEGATIVE PARITY SIDE

BANDS IN LIGHT TUNGSTEN AND OSMIUM

NUCLEI

G.D. Dracoulis

To cite this version:

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JOURNAL DE PHYSIQUE CoZZoque CIO, suppZe'ment au n012, Tome 41, de'cembre 1980, page C10-66

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. B o x 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 negative parity sidebands, which show the effects of rotation alignment, in Tungsten (Z=74) and Osmium

(Z=76) isotopes with N=100 to 108. The reasons for

this choice are personal aquaintance with the region, but more importantly, the large isotopic range of nuclei studied to high spin which reveals the sys- tematic behaviour and occurrence of these bands.

I

will not dwell on the details of experimental

techniques, however, it is worth remembering that the population and decay pattern in (Heavy Ion, xn) reactions used to study neutron deficient nuclei

preferentially populates yrast states. In deformed

nuclei the yrast states are usually the ground state rotational band members up to about spin 16-20fi (and

about 3 MeV excitation energy). Sidebands could be

loosely defined as rotational bands with configurat- ions different Erom the ground state, such as 2 (or higher). quasiparticle states or other collective states (e.g. vibrational states), which necessarily begin in the region of the pairing gap, about 1 MeV

excitation. If it were not for rotational alignment

at high spin in some of these configurations, they

only

a

small proportion of the feeding.

Although the level schemes I will show are partial schemes, the selection of the aligned neg- ative parity bands is not arbitrary since in the lighter isotopes (N=100 and N=102) they are the strongest sidebands observed and with the exception of bands based on 2-quasiparticle isomeric states,

the only sidebands identified to high spin. They

are in these lighter isotopes the "yrast negative parity" states.

Octupole States and Rotation Alignment

Any discussion of negative parity states in this region is at least partly connected with octupole states, which are well known in the heavy

Tungsten and Osmium nuclei. Their collective

nature is established by their preferential populat- ion in (d,dt) studies and from their y-decay which

involves strong E3 transitions1-'). Neergard and

voge15) have successfully calculated the properties of low-lying octupole states in a wide range of nuclei using a quasiparticle random phase approximat- ion with an octupole-octupole residual interaction.

In a deformed nucleus the 3- vibration splits into

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s t a t e s with K' = 0 - , 1-, 2- and 3- each with r o t a t i o n - a l bands, t h e 0- band having ( e s s e n t i a l l y ) only odd s p i n members. The bands a r e s u c c e s s i v e l y coupled through l a r g e C o r i o l i s matrix elements. Because of t h e s e i n t e r a c t i o n s t h e bands ( u s u a l l y only t h e lowest energy band

i s

known) a r e perturbed. In t h e Tungsten and Osmium region t h e lowest s t a t e s a r e predominantly t h e 2-quasiproton c o n f i g u r a t i o n {y2- [514]~r,

y2+

[402]a}

with some 2-quasineutron admixtures. In Tungsten t h e 2- c o n f i g u r a t i o n i s u s u a l l y lowest w h i l s t i n Osmium t h e 3- c o n f i g u r a t i o n i s s e e n .

voge16) has extended t h e RPA c a l c u l a t i o n s t o h i g h e r s p i n and a l s o c a r r i e d o u t a 2 - q u a s i p a r t i c l e

c a l c u l a t i o n i n a subspace including a l l s t a t e s o r i g - i n a t i n g i n t h e unique p a r i t y s u b s h e l l ( f o r protons and neutrons) and abput 10 normal p a r i t y quasi- p a r t i c l e s t a t e s c l o s e t o t h e Fermi s u r f a c e ( s e e

footnote

'I.

Vogel reached s p e c i f i c conclusions which a r e r e l e v a n t t o t h e r e s u l t s I w i l l show. In summary t h e s e were

l ] A t low and i n t e r m e d i a t e s p i n s t h e n e g a t i v e p a r i t y s t a t e s a r e a l i g n e d octupole s t a t e s . The odd and even s p i n members of t h e normal r o t a t i o n a l bands s p l i t i n t o s e p a r a t e (AJ=2) 'sequences. The odd s p i n s t a t e s a r e favoured i n energy and have

I = R+3

while t h e even s p i n s t a t e s have I R+2

where R i s t h e c o l l e c t i v e r o t a t i o n (=0, 2, 4

...).

That i s , t h e octupole v i b r a t i o n i s a l i g n e d with t h e r o t a t i o n , and t h e spacing o f t h e AJ=2 sequences i s t h a t o f t h e ground s t a t e ("R") c o n f i g u r a t i o n , lead- i n g t o bands w i t h a high apparent moment o f i n e r t i a . The wave furictions a r e such t h a t t h e p a r t i c i p a t i n g q u a s i p a r t i c l e s have t h e i r i n t r i n s i c angular moments

jl

-

and

i2

almost a n t i - p a r a l l e l .

21 A t high s p i n s (> 1 0 i n t h i s region) a t r a n s - i t i o n t o an a l i g n e d q u a s i p a r t i c l e c o n f i g u r a t i o n is p r e d i c t e d where t h e p a r t i c l e s have t h e & s p i n s p a r a l l e l - t h e a l i g n e d angular momentum being dom- i n a t e d by t h e c o n t r i b u t i o n o f t h e high s p i n (unique p a r i t y ) q u a s i p a r t i c l e .

I w i l l keep t h e s e p r e d i c t i o n s i n mind i n d i s - cussing t h e experimental r e s u l t s , when I r e f e r t o t h e p o s s i b l e 2 - q u a s i p a r t i c l e c o n f i g u r a t i o n s , proton and neutron, t h a t c o n t r i b u t e t o t h e alignment i n t h e observed bands.

Negative P a r i t y Bands i n Tungsten

In f i g u r e 1 I have c o l l e c t e d t h e p a r t i a l decay schemes o f high s p i n s t a t e s i n 1 7 4 ~ and 1 7 6 ~ 1 8 ) , 1 7 8 ~ 19-21), 180W 22-24) and l8ZW 25,26). ~h~ 2- octupole band i n 1a2w drops i n energy i n going from la2w t o laow and a s we progress t o t h e l i g h t e r i s o t o p e s t h e c h a r a c t e r o f t h e band changes i n sever- a l ways.

a ) The odd and even s p i n s t a t e s s e p a r a t e s o t h a t i n t h e l i g h t e r i s o t o p e s t h e e x c i t a t i o n e n e r g i e s

J J-1

a r e approximately Eodd

=

E even and t h e A J = 1 cascade t r a n s i t i o n s become weaker, mainly because t h e t r a n s - i t i o n energy i s reduced.

b) The decay p a t t e r n o f t h e out-of-band E l t r a n s i t i o n s changes from decays mainly from t h e band- head ( o r s t a t e s c l o s e t o i t ) , t o decays from t h e hlgh s p i n s t a t e s t o t h e gsb, and predominantly from t h e odd s p i n s t a t e s t o t h e gsb. This out-of-band p a t t e r n , which I w i l l comment on below, r e s u l t s i n t h e population being funnelled i n t o t h e gsb, b e f o r e t h e lowest s p i n s t a t e s i n t h e bands can be reached. That Is, t h e 4-, 5- s t a t e s seen i n t h e l i g h t e s t i s o t o p e s a r e n o t t o be taken a s band heads. Although I w i l l n o t d i s c u s s them,, h e r e , a number o f c a l c u l a t i o n s r e l e v a n t t o n e g a t i v e p a r i t y h i h s p i n

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c10-68 JOURNAL DE PHYSIQUE

c ) The bands become compressed so t h a t they B(El)/B(E2) i s about 1 0 - ~ b-I f o r t h e odd s p i n have a high apparent moment o f i n e r t i a . The high s t a t e s , with a t r e n d toward l a r g e r s t r e n g t h s i n t h e odd s p i n s t a t e s J , a r e a t about t h e same e x c i t a t i o n l i g h t e r i s o t o p e s . The s i n g l e p a r t i c l e s t r e n g t h s , energy a s t h e y r a s t J + l p o s i t i v e p a r i t y s t a t e s i n t a k i n g t h e E l t r a n s i t i o n from t h e 9- s t a t e i n 1 7 4 ~

1 7 4 ~ and 1 7 6 ~ where t h e n e g a t i v e p a r i t y s t a t e s a r e a s an example, would be about 5x10-' Weiskopff Units,

known t o high s p i n . implying low - K components i n t h e s e s t a t e s . Taking t h e s e p o i n t s i n a l i t t l e more d e t a i l , E l T r a n s i t i o n s from t h e even s p i n s t a t e s a r e a ) The odd-even energy s p l i t t i n g can be seen weaker. This d i f f e r e n c e has been a t t r i b u t e d (see more d r a m a t i c a l l y i n a p l o t of AE(J+J-1)/2J v s . 2 ~ ~ f o r example r e f s . 20,27,28)) t o t h e d i f f e r e n c e be- given i n f i g u r e 2. Only small o s c i l l a t i o n s a r e ob- tween AK=O and A K = l E l t r a n s i t i o n s t r e n g t h s . A t se-med i n l e 2 w , c o n s i s t e n t with small second o r d e r low s p i n s A K = l t r a n s i t i o n s a r e considerably weaker

admixtures o f t h e 0- band. These develop t o l a r g e than DK=O t r a n s i t i o n s and s i n c e t h e odd s p i n s t a t e s o s c i l l a t i o n s f o r t h e N=100 and N=102 i s o t o p e s . w i l l have both K=O and K = l admixtures (as well a s

b) The out-of-band El t r a n s i t i o n s show s e v e r a l K=2 and K=3 admixtures which w i l l not c o n t r i b u t e t o f e a t u r e s which I can o n l y o u t l i n e here. The allowed d i p o l e decays) while t h e even s p i n s t a t e s s t r e n g t h o f t h e J+J-l E l t r a n s i t i o n s can be e s t i m a t - w i l l not have K=O admixtures (remembering t h a t t h e

t

ed through t h e branching r a t i o s i n comparison with K"=o- band only had odd s p i n s t a t e s ) , t h e t r a n s i t - t h e competing in-band E2 t r a n s i t i o n s . The r a t i o i o n s from t h e odd s p i n s t a t e s w i l l be s t r o n g e r .

Figure l P a r t i a l l e v e l schemes showing n e g a t i v e p a r i t y sidebands and y r a s t bands i n

1 7 4 ~ L 1 8 2 ~ .

(see t e x t f o r r e f e r e n c e s )

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Figure 2 (EJ-EJ-1)/2.J VS J~ f o r t h e negative p a r i t y bands i n Tungsten and Osmium.

A f u r t h e r t r e n d i s e v i d e n t

i n

t h e branching r a t i o s from t h e odd s p i n s t a t e s . These decay by both J+J+l and J+J-l t r a n s i t i o n s t o t h e gsb. A t low s p i n t h e s e a r e comparable i n i n t e n s i t y , d e s p i t e t h e lower energy o f t h e J+J+1 t r a n s i t i o n . Over a small range o f i n c r e a s i n g s p i n t h e s i t u a t i o n chang- e s t o a more r a t i o u n t i l a t high s p i n s only J+J-l t r a n s i t i o n s a r e observed. The following t a b l e i l l u s t r a t e s t h e t r e n d f o r two o f t h e tungsten c a s e s . F u r t h e r examples have been measured i n t h e Osmium nuclei30).

This e f f e c t i s probably due t o a c a n c e l l a t i o n o f t h e AK=O and A K = l components (see a l s o r e f 28)) o r may i n d i c a t e

a

change from t h e octupole mode which induces t h e El moment.

c] Moments o f I n e r t i a and Rotation Aligned Angular Momentum

I n t h e following d i s c u s s i o n s I w i l l use t h e p r e s c r i p t i o n o f Bengtsson and ~ r a u e n d o r f ' l ) t o deduce t h e a l i g n e d a n g u l a r momentum lx and r o t a t i o n -

a l frequency 6 w w i t h i n a band, from t h e t r a n s i t i o n e n e r g i e s . That i s

with

The a l i g n e d angular momentum with r e s p e c t t o a r e f - erence c o n f i g u r a t i o n i s given a s

i ( w ) = I ~ ( W )

-

I ~ ~ ( W ) , where g r e f e r s t o t h e r e f e r e n c e band.

TABLE 1. Branching Ratios and B(E1) r a t i o s .

J - t J I E B (El) J+J+l

gsb Y IkeV: B(E1) J+J-l

The a l i g n e d angular momentum I x i s shown f o r t h e 1 7 4 ~ and 1 7 6 ~ bands i n f i g u r e 3. The n e g a t i v e p a r i t y bands a r e seen t o have an alignment h i g h e r than t h a t o f t h e gsb, and s i m i l a r t o t h a t observed i n t h e y r a s t band a f t e r backbending. By t a k i n g t h e gsb a s t h e . r e f e r e n c e c o n f i g u r a t i o n ( e x t r a p o l a t e d where necessary through t h e backbending regiontf) t h e alignment i i s obtained. This i s shown i n f i g - ure 4 f o r a l l t h e Tungsten i s o t o p e s .

The N=108 i s o t o p e 18*w shows l i t t l e alignment.

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c10-70 JOURNAL DE PHYSIQUE

In c o n t r a s t t h e alignment i n 1 7 4 ~ and 1 7 6 ~ i n

-

t r e n d through t h e i s o t o p e s was a t t r i b u t e d t o t h e ex- c r e a s e s from a b o u t 1 . S and 2.5 ii i n t h e even and

odd s p i n s t a t e s r e s p e c t i v e l y a t Fiw = 0.12 MeV, t o about 4

fi

i n both odd and even s p i n s t a t e s a t 0 . 2

MeV. The i n t e r v e n i n g i s o t o p e s show t h e t r e n d o f i n c r e a s e d alignment with d e c r e a s i n g neutron number, c o r r e l a t e d roughly with t h e energy s p l i t t i n g given i n f i g u r e 2 . We n o t e i n passing h e r e t h a t t h e alignment observed i n l E O w , which approaches 3 ii a t

tIie h i g h e s t s p i n observed, was i n t e r p r e t e d r e c e n t l y

pected s e n s i t i v i t y of t h e C o r i o l i s mixing t o t h e p o s i t i o n o f t h e neutron Fermi l e v e l . The 1 q u a s i - neutron s t a t e s and 1 quasiproton s t a t e s which might be involved i n t h e n e g a t i v e p a r i t y c o n f i g u r a t i o n s

( s i n c e t h e y a r e t h e lowest s t a t e s i n t h e 1 q u a s i - p a r t i c l e spectrum) a r e given i n t a b l e 2.

TABLE 2 Low Lying Proton and Neutron Configurations.

a s observation o f t h e alignment o f t h e octupole

h l l/2

-

q/2-[514]

1

fT2

-

7/2-[514] v i b r a t i o n 2 3 ) . i d5/2 + 5/2'14021

j

py? - %-[521] protons j a Nilsson Our e a r l i e r i n t e r p r e t a t i o n o f t h e observed

I

g7/2 + ')/2+[4041

1

h%

-

%-L5121 alignment w a s t h a t t h e 2 q u a s i p a r t i c l e s t a t e s formed ! neutrons j II Nilsson

by coupling an i 1 3 neutron, and an hq

/2 O r h9/2

-

'/2-[54l]', i l a + TZt[633] o r ?21[624]

h

neutron (not r o t a t i o n a l i g n e d ) , which a r e low i n t h e neighbouring 1 q u a s i p a r t i c l e s p e c t r a , were t h e >

' ~ e c a u s e o f decoupling t h e

5/2-

o r

y2-

s t a t e s o f t h i s dominant c o n f i g u r a t i o n s a t high s p i n s l * ) . The c o n f i g u r a t i o n a r e u s u a l l y lbwest ?n energy.'

0

1

I

I

1 I I l I

I

0.1

0.2

0.3

0.4

h o

MeV

Figure 3. Aligned a n g u l a r mmenta I (see t e x t ) v s

Kw

f o r t h e n e g a t i v e

X

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The alignments f o r s e l e c t e d bands involving t h e s e c o n f i g u r a t i o n s can be deduced from t h e known s p e c t r a for 1 7 5 ~ 321, 1 7 7 ~ 331, 1 7 9 ~ 34,351, l8lw 26),and 1 7 7 ~ ~ 36), 179Re 36) and lEIRe 37). For complete-

I I I I

l

0 .l 0.2 0.3 0.4

h o

MeV

Figure 5 R e l a t i v e a l i g n e d angular momenta f o r s e l - e c t e d odd neutron and odd proton bands i n Tungsten and Osmium. Only t h e a=+% bands a r e shown f o r c o n f i g u r a t i o n s with a s t r o n g s i g n a t u r e dependence.

Figure 4 (a) The a l i g n e d angular momentum i with

r e s p e c t t o t h e r e f e r e n c e configllrations, t h e proton o r neutron combinations could g i v e r i s e f o r t h e negative p a r i t y bands i n t h e t o t h e alignments observed i n t h e n e g a t i v e p a r i t y Osmium i s o t o p e s . The odd s p i n sequences f i g u r e 5. I t can be seen from t h e s e t h a t e i t h e r of a r e connected by t h e h e a v i e r continuous l i n e . bands i n Tungsten s i n c e t h e i13 neutron, and t h e

12

(b) a s f o r (a) b u t f o r t h e Tungsten proton a r e both aligned. Comparing t h e s e d i r - i s o t o p e s . e c t l y with f i g u r e 4 it is apparent t h a t t h e a l i g n -

lpent i s only reached above

Hw

= 0 . 2 MeV. Therefore,

n e s s one should include t h e odd proton Ta i s o t o p e s i f thege configurations a r e dominant, they a r e only and t h e h e a v i e r odd Tungsten i s o t o p e s but p a r t l y f o r dominant a t high frequencies. A t low frequencies s i m p l i c i t y , and p a r t l y because a l l t h e r e l e v a n t d a t a t h e alignment i s approximately t h a t suggested by a r e not a v a i l a b l e , a s e l e c t i o n has been made. Vogel, 2 and 3 H i n t h e even and odd s p i n s t a t e s

The neutron and proton alignments a r e shown i n r e s p e c t i v e l y . The t r a n s i t i o n t o an a l i g n e d quasi- p a r t i c l e c o n f i g u r a t i o n is n o t seen i n 1 7 8 ~ and t h e

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c10-72 JOURNAL DE PHYSIQUE

h e a v i e r i s o t o p e s , a t l e a s t up t o t h e s p i n s s o f a r i d e n t i f i e d .

I w i l l n o t attempt t o make a judgement h e r e on whether t h e dominant c o n f i g u r a t i o n a t high s p i n is t h e neutron o r proton one, ( t h e neutron number dep- endence of t h e odd protons o r neutron does not apparently d i s t i n g u i s h between them) but w i l l make f u r t h e r comment on t h a t a f t e r showing t h e Osmium r e s u l t s .

Yrast bands, and Negative P a r i t y Sidebands i n l i g h t Osmium Isotopes

High spin y r a s t s t a t e s i n t h e l i g h t Osmium i s o t o p e s , down t o neutron number 106, l E 2 0 s , have previously been s t u d i e d , p a r t i c u l a r l y by t h e Michigan and J u l i c h groups 38'37'39). we a r e c u r r - e n t l y studying t h e i s o t o p e s from N=100 t o N=106, p a r t l y with t h e aim o f complementing t h e Tungsten s e r i e s and w i l l p r e s e n t h e r e new r e s u l t s on t h e s e n u c l e i 30s40), some of them preliminary. High spin s t a t e s i n Osmium were populated using (160,xn) reac- t i o n s on enriched E r t a r g e t s using 160 beams from t h e ANU 14UD P e l l e t r o n a c c e l e r a t o r . In o r d e r t o f a c i l i t a t e t h e study o f sidebands, a s well a s t h e usual armory of y-ray s p e c t r o s o p i c techniques we have used a Compton suppressed Ge(Li) spectrometer i n both s i n g l e s and coincidence measurements.

P a r t i a l l e v e l schemes showing t h e y r a s t s t a t e s and t h e r o t a t i o n a l i g n e d negative p a r i t y s t a t e s a r e shown i n f i g u r e 6, t o g e t h e r w i t h t h e r e s u l t s f o r 1840s given i n r e f e r e n c e s 41942). Spin a s s i g n - ments and p a r i t i e s i n t h e p r e s e n t study a r e based on angular d i s t r i b u t i o n measurements and i n t e r n a l con- version e l e c t r o n measurements.

Before d i s c u s s i n g t h e n e g a t i v e p a r i t y bands I

t h e gsb through t h e backbending region have a l s o been observed. The angular momenta I shown i n f i g u r e 7 f o r t h e range o f i s o t o p e s from A=176 t o 182 show a c l e a r t r e n d , from s t r o n g backbending i n lB20s, t o a weak but well defined up-bending i n 1 7 6 ~ s . The alignment reached a t a frequency o f 0.35 MeV i s about t h e same i n each c a s e i n d i c a t i n g , a s has been pointed o u t f o r t h e Tungsten isotopesz1), t h e common n a t u r e o f t h e c r o s s i n g band. The a l i g n e d ( i 1 3 ) 2

/2

neutron band i s accepted a s t h e c r o s s i n g band i n l E 2 0 s 43) although e a r l i e r , t h e h proton configur- a t i o n was suggested t o be responsible3'). Further evidence t h a t t h e bandcrossing i s c o n s i s t e n t with t h e behaviour o f t h e i13 neutron i s given i n t h e

12

r e l a t i v e a l i g n e d angular momegta o f f i g u r e 8. A s was seen from f i g u r e 7, t h e bandcrossing occurs a t p r o g r e s s i v e l y higher frequencies with decreasing neutron number, but t h e important p o i n t t o n o t e i s t h a t t h e alignments o f t h e i13 1 q u a s i p a r t i c l e

/2

bands i n lelOs, and from o u r r e c e n t study 4 0 ) , i n 1770s, a r e c o n s i s t e n t with t h e observed alignments i n t h e even n u c l e i . For s i m p l i c i t y , t h e i13 band

12 i n 1790s, which we have a l s o i d e n t i f i e d i s not i n - cluded on t h e p l o t . More d e t a i l on t h e odd neutron behaviour i n t h e Osmium n u c l e i i s given i n f i g u r e 9. The

4-

[521] bands i n 1810s and 1770s show anomalies c o n s i s t e n t with t h e magnitudes o f t h e alignment g a i n s observed i n t h e y r a s t bands of t h e even neighbours, although a t s l i g h t l y lower frequen- c i e s . The i13 bands, l a b e l l e d [624]", show

/2

anomalies only a t h i g h e r frequencies. Lieder 44 has suggested t h a t t h e anomaly i n t h e 9/2+[624] band i n l s l O s i s due t o a c r o s s i n g with t h e i l t h r e e

3/2

quasineutron band, but involving t h e 7/2+[633] would l i k e t o p o i n t o u t some f e a t u r e s o f t h e y r a s t neutron a s well a s t h e a=+% components of t h e s t a t e s which have been e s t a b l i s h e d h e r e up t o s p i n s 9/2+[624] neutron. The s i t u a t i o n i n t h e O s m i u m of about 20 o r 22 i n 1760s, 1 7 8 ~ s and l E O O s . I n n u c l e i d i f f e r s a p p a r e n t l y from o t h e r c a s e s where t h e 1 7 8 0 ~ and 1800s, candidates f o r t h e extensiqns of blocking o f t h e a p p r o p r i a t e i13 neutron delays band-

(9)

Figure 6 P a r t i a l l e v e l schemes f o r 1760s

-

1 8 4 ~ s . ( s e e t e x t f o r r e f e r e n c e s )

c r o s s i n g t o considerably higher f r e q u e n c i e s . The tempering of t h e magnitude o f t h e band- c r o s s i n g i n t h e l i g h t 0s i s o t o p e s may be a demon- s t r a t i o n o f t h e e f f e c t of increased i n t e r a c t i o n matrix elements between t h e g . s . and t h e c r o s s i n g band c o n f i g u r a t i o n s

-

small matrix elements l e a d s t o s t r o n g backbending

-

which a r e p r e d i c t e d t o have a s t r o n g mass dependence 31945-47). However, c a l - c u l a t i o n s which c o r r e c t l y t r e a t t h e r a p i d change i n hexadecapole deformation which occurs i n t h e region a r e - r e q u i r e d t o p r o p e r l y examine t h i s p o i n t . The d i f f e r e n c e i n behaviour between t h e a=& s i g n a t u r e s o f t h e 9/2*[624] band observed i n 1770s and 17'0s

may be a f u r t h e r consequence of t h e same e f f e c t 4 6 ) . TO r e t u r n t o t h e negative p a r i t y bands, we can follow a development s i m i l a r t o t h a t observed i n t h e Tungsten i s o t o p e s . The 3- octupole band i d e n t i f i e d . i n 18'+0s, u n f o r t u n a t e l y not t o high s p i n , decays mainly t o t h e y-band. In 1820s a small energy favouring o f t h e odd s p i n n e g a t i v e p a r i t y s t a t e s i s observed with out-of-band decays t o both t h e gsb and t o t h e y-band. Lieder e t alS1) have a l s o p r e s e n t e d a l e v e l scheme f o r 1820s i n a c o n t r i b u t i o n t o t h i s conference, i n which t h e y have i d e n t i f i e d a l a r g e number o f bands i n c l u d i n g t h e n e g a t i v e p a r i t y band being discussed here. Our r e s u l t s 3 0 ) agree with

(10)

c10-74 JOURNAL DE PHYSIQUE

I I I I l I I

0.10 0.20 0.30 0.4

A o

MeV

Figure 7 The aligned angular momentum

I

vs l i w for the yrast (and yrare) sequences in 1 7 6 ~ s

-

l820s.

I I I

r

1

0.1

0.2

0.3

0.4

Ro

MeV

(11)

t h e i r s on a l l counts except f o r t h e placement o f t h e odd s p i n sequence i n t h e p r e s e n t negative p a r i t y band. In both schemes, an anomaly i s seen i n t h e even s p i n sequence a t a low frequency. In 1800s o n l y an odd s p i n sequence has been i d e n t i f i e d ,

(although a candidate f o r t h e 6- s t a t e i s observed) with t h e main branches t o t h e gsb, but s t i l l with s i g n i f i c a n t branching t o t h e y-band. An anomaly i n t h e band spacing i s seen a t about s p i n 17. S i m i l a r , well developed, odd s p i n sequences, and s e p a r a t e , more weakly populated even s p i n sequences a r e observed i n both of t h e l i g h t e r i s o t o p e s 1760s and I7*0s.

These bands a r e , a t l e a s t s u p e r f i c i a l l y , r e - markably s i m i l a r t o t h o s e i n t h e Tung,sten i s o t o p e s . The E l s t r e n g t h s and branching r a t i o s a r e s i m i l a r , t h e odd-even energy spacing given i n f i g u r e 2 i s s i m i l a r , but t h e r e a r e d i f f e r e n c e s i n t h e alignments.

The alignments a r e shown i n f i g u r e 4. 1760s and 17%0s show h i g h e r alignment i n t h e odd s p i n s t a t e s than t h e even s p i n s t a t e s a t low frequencies, r i s i n g t o about 45 a t Kw = 0.18 MeV, s i m i l a r t o t h e behaviour i n t h e N=100 and N=102 i s o t o p e s o f

Tungsten. l800s and lE20s however show even l a r g e r alignments, i n marked c o n t r a s t t o t h e i r Tungsten i s o t o n e s , and i n l a O O s a l a r g e alignment g a i n is observed a t a frequency o f 0.25 MeV.

The same d i f f e r e n c e is p a r t l y seen i n comparing t h e odd-neutron i s o t o p e s o f Osmium and Tungsten but j t i s not p o s s i b l e h e r e t o c o n c l u s i v e l y i d e n t i f y t h e q u a s i p a r t i c l e c o n f i g u r a t i o n s r e s p o n s i b l e f o r t h e alignment observed a t Kw

>

0.2 MeV. I t may be t h a t t h e h protons s t i l l play a s i g n i f i c a n t r o l e

9/2

s i n c e t h e r e a r e two main d i f f e r e n c e s evident i n t h e comparison between Osmium and Tungsten. F i r s t l y , t h e heavy Tungsten i s o t o p e s , N ? 106, do n o t show l a r g e alignment. I f t h e protons were r e s p o n s i b l e , t h i s might be a t t r i b u t a b l e t o t h e d i f f e r e n c e i n t h e

1 quasiproton spectrum, and i n p a r t i c u l a r i t s neutron number dependence, between t h e Re and Ta i s o t o p e s . An examination o f those s p e c t r a 49-50) shows t h a t t h e $-[S411 h proton has, a s expected,

9/2

a s i m i l a r alignment i n t h e Re and Ta i s o t o p e s , b u t i s a t considerably h i g h e r e x c i t a t i o n energy i n t h e heavy Ta isotopes than i n t h e i s o t o n i c Re n u c l e i . For example, i n ~ e l ~ l ( N = 1 0 6 ) t h e

5/2-

s t a t e of t h e $-[541] configuration i s a t 357 keV while i n 1 7 9 ~ a it i s a t 628 keV. The aligned 2 quasiproton con- f i g u r a t i o n i n Tungsten, t h e r e f o r e , may be a t a high- e r e x c i t a t i o n energy, i n t h e h e a v i e r Tungstens, than i n t h e i s o t o n i c Osmium n u c l e i

-

although t h e l i g h t n u c l e i may be s i m i l a r . Secondly, t h e negative p a r i t y band i n 1800s shows backbending a t a low frequency. I f t h e i13 neutron c o n f i g u r a t i o n were

12

r e s p o n s i b l e f o r t h e alignment a t 0.2 MeV, then it could not be r e s p o n s i b l e f o r t h e gain i n alignment a t 0.25 MeV because o f blocking. The e f f e c t of blocking o f t h e i13 neutron i s apparent i n t h e be-

12

haviour o f t h e 9/2+[624] 1 quasineutron band i n 1790s (shown i n f i g u r e 9) which does not show an anomaly u n t i l a considerably h i g h e r frequency than t h e g-15211 band. However, t h e same argument could be a p p l i e d t o t h e hg proton configuration s i n c e i n

12

1 8 1 ~ e t h e %-I5411 band a l s o does n o t show backbend- ing while t h e 5/2+[402] band does, and a t t h e same frequency, 0.25 MeV, a s t h e n e g a t i v e p a r i t y band i n l a o O s . The s o l u t i o n t o t h i s dilemma i s t o p o s t u l - a t e a change i n c o n f i g u r a t i o n from a 2 quasiproton' c o n f i g u r a t i o n (involving t h e

h9

proton) t o one i n -

/2

volving an e x t r a p a i r o f i l 3 n e u t r o n s r e s u l t i n g i n /2

t h e alignment gain a t F1w=0.25 MeV. A s i m i l a r ex- p l a n a t i o n h a s been given by Ploszajcak and F a e s s l e r

f o r t h e backbending observed i n t h e n e g a t i v e p a r i t y band of 1 5 6 ~ r .

(12)

C10-76 JOURNAL DE PHYSIQUE

0.29 MeV, higher than t h e anomaly i n t h e gsb, and a t I would l i k e t o express my a p p r e c i a t i o n t o my about t h e same frequency a s t h e i13 band i n Ia10s. r e s i l i e n t colleagues D r P.M. Walker, and D r C.

12

Lieder e t alS1) use t h i s a s eyidence t h a t t h e i13

12 Fahlander.

neutrons a r e r e s p o n s i b l e f o r t h e gain i n alignment. However i n laOOs t h e s i t u a t i o n i s d i f f e r e n t s i n c e t h e anomaly i n t h e odd s p i n sequence occurs a t a lower frequency than t h a t i n t h e gsb, o r i n t h e ilY2 neutron band i n 1790s.

C e r t a i n l y , t h e s i t u a t i o n may be complex, and s p e c i f i c c a l c u l a t i o n s f o r proton and neutron con- f i g u r a t i o n s f o r t h i s range o f Tungsten and Osmium i s o t o p e s which p r o p e r l y t r e a t t h e changing quadrup- o l e and hexadecapole deformations a r e imperative i f a c l e a r e r understanding i s t o be reached.

To. r e t u r n b r i e f l y to t h e i n t r o d u c t i o n of t h i s t a l k , and t h e p r e d i c t i o n s of Vogel; it seems p o s s i b l e t o t r a c e t h e development of t h e octupole band i n t o s e p a r a t e odd and even s p i n sequences over a wide range o f i s o t o p e s . The alignment a t low frequencies is c o n s i s t e n t with c o n t r i b u t i o n s of about 26 and 3H t o t h e even and odd s p i n s t a t e s r e s p e c t i v e l y , due presumably t o t h e alignment of t h e octupole s p i n . A t r a n s i t i o n t o a band with a higher alignment is seen i n most c a s e s . This gain i n alignment is c o n s i s t e n t with t h a t due t o t h e i13 neutron o r t h e h9 p r o t o n which could con-

12 12

t r i b u t e t o t h e dominant 2 - q u a s i p a r t i c l e c o n f i g u r a t - i o n s . , Although it i s not p o s s i b l e a t p r e s e n t t o

d e f i n i t i v e l y c h a r a c t e r i s e t h e n e g a t i v e p a r i t y bands a t high s p i n , t h e s y s t e m a t i c t r e n d s , and t h e back- bending observed i n t h e n e g a t i v e p a r i t y band i n

1800s, suggest t h a t t h e i13 neutrons and

h9

12 12

protons both p l a y a r o l e .

(13)

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