The levels of 185Ir populated in the decay of 185m + gPt

Nuclear Physics A325 (1979) 421-444 ; © North-HollarulPublLrhlng Co., Amsterdam Not to be roproduoed by photoprint or microfilm without written permission from the publisher

THE LEVELS OF'" sIr POPULATED IN THE DECAY OF'asm+~ C. SCH>JCK

Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, 1N1P3, 91406 Orsay, France

J. GENEVEY-RIVIER

Institut des Sciences Nucléaires, IN2P3, USMG, 53, Auenuedes Martyrs, 38026 Grenoble Cédex, France

V. BERG

Institut de Physique Nucléaire, IN2P3, 9/406 Orsay, France

A. KNIPPER and G. WALTER

Centre de Recherches Nucléaires, IN1P3, 67037 Strasbourg Céder, France

C. RICHARD-SERRE IN2P3, CERN, 121! Geneva 23, Switzerland

and Institute

A. HÛGLUND of Physics, University of Stockholm,

Sweden

(The ISOLDE Collaboration) Roceived 8 March 1979

Abstract : The decay scheme of ' esm + aPt has been investigated using isotopically separated samples produced by the 1SOLDE facility. A level scheme of `eslr has been established . The } + [402] and } + [400] positive parity bands have been observed . The negative parity levels belonging to the h9 ~Z and h  i~ orbitals are discussed in the framework of the asymmetric rotor-plus-particle model.

E

RADIOACTIVITY `esm .'esPt [from Pb(p, 3pxn) Hg -+ Au -. Pt ; mass separated ; mea sured Er, lr , E~~, yy-coin, y~ delay, ce~e delay ; deduced ICC. ' ss lr deduced levels, !, n, T, ~~, M1/E2 admixtures, B(M1), B(E2). Ge(Li), Si(Li). Magnetic spectrometers.

1. Introduction

Recent studies in the odd-proton nuclei belonging to the transitional region of neutron-deficient isotopes around A = 190 have shown a number of specific features . Systematic works on isotopes of Tl (Z = 81) [refs. t, z)], Au (Z = 79) [refs . 3 - a)] and Ir (Z = 77) [ref$. 9 _ t e)], in the region between sphericity and stable axial deformation, suggested a continuous transition from well-deformed prolate to weakly deformed oblate shapes through soft fluctuating triaxial forms 421

422

C. SCHÜCK et al,

including various kinds of shape coexistence. In these nuclei, most information has been obtained by investigating the negative parity levels built on the h~ and the h,~ quasiparticle states. The structure of these levels has been found very sensitive to the deformation characteristics. In odd-A iridium isotopes systematic studies have been undertaken by complementary on-line and in-beam experiments 1 z -'s) . The h,~ system in ' 8' -1 a9l r was interpreted assuming a h~ hole coupled to an asymmetric core' s " "), while the positive parity bands were consistent with prolate axial deformation. The h~ system could be intérpreted either by coupling a h # proton to an asymmetric core ") or by classical Coriolis calculations assuming a prolate axial deformation i3) . Recently, new calculations have been performed on ie'Ir [refs. le-ZO)] . The introduction by Toki and Faessler 1 g) of the VM1 prescription has improved the quantitative description of thé triaxial core model. In the model of Leander ' 9 ), the coupling ofan odd particle to an enharmonic even core suggested that the triaxiality of 1 s' lr is not stable but has a dynamical origin . In order to test the evolution of the h~ and the h~ systems further from stability, we have here extended the systematics down to the' es Ir isotope. Preliminary results on this isotope had been obtained at ISOLDE 1 [ref. 11 )]. Two isomers were observed in iaspt with half-lives T t = 70.9±2.4 min and T} = 33.0±0.8 min. The stronger y-lines were determined and two isomers in the nanosecond rangé observed ~ isslr . Considering that the ground state of'BSPt decayed to the high-spin levels of' aslr with 70.9 min half-life, and that the low-spin levels were fed by the 33 min half-life a ' zi), and by systematic comparison of N = 107 isotopes, spin ~+ was assigned to iaspt ground state and spin ~- to the 128.7 keV isomeric state a ' s) . They will here be referred to as tasapt and iasmpt respectively . We present here the results on the levels of'BSIr fed by the decay of 1as Pt. The results of an in-beam study of iaslr will be presented in the following paper zZ) . 2. Experimental procedare 2.1 . SOURCE PREPARATION AND DATA ACQUISITION

A complete set of spectroscopic measurements was performed with the new ISOLDE 2 facilities Zs) on line with the reconstructed 600 MeV synchrocyclotron at CERN. iasHB was produced by bombardment of a metallic lead target with the proton beam, and the excited states of 1 es Ir were populated in the decay chain iasHg ~ iasAu ~ ias~+mPt ~ iaslr . Some of the measurements were made off-line, the A = 185 ion beam being collected on 1 mg/cm 2 aluminium foils . The singles y-measurements and the y-y coincidences were performed on-line, the mass separated activity being collected on a mylar tape and moved in front of the detectors. The singles and coincident spectra were recorded with an on-line intertechnique Plurimat 20 computer

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allowing a 1200 events/s coincidence rate. The coincidences were treated using the ARIEL computing facilities with the IBM 370-135 at Orsay. 2.2 . GAMMA-RAY SPECTROSCOPY

The low-energy spectrum from 20-400 keV was measured with a Ge(Li) X-ray detector with 0.9 keV resolution at 100 keV. A spectrum obtained with one source is shown in fig. 1 . Ge(Li) detectors of 8 ~ and 10 ~ efficiency were used for singles y and y-y coincidences. A typical y-ray spectrum and some gates from a y-y series are presented in figs . 2, 3 and 4 respectively . 2.3 . ELECTRON SPECTROSCOPY

Si(Li) detectors . Due to the existence of the two Pt isomers, electrons and y-ray spectra had to be recorded simultaneously in order to get the multipolarities of the transitions independently of their feeding. Two sets of multipolarity measurements were performed with two different ydetectors. The Ge(Li) X-ray detector was used for the measurement of the low energy part of the y-spectrum and the 10 ~ Ge(Li) detector for the high energy part . The electrons were, in both cases, measured with the same Si(Li) detector, 3 mm thick, and with a resolution of 3 keV. An electron spectrum recorded with one source is shown in fig. 5 . The low energy spectrum (0-250 keV) . This has been recorded with a 1 °/oo resolution magnetic semi-circular spectrograph with photographic detection and pre-acceleration . Shelters allowed to discriminate the different half-lives. Magnetic

iooo Fig. 2. Partial y-ray spectnun obtained with the 10 ~ Ge(Li) detector (33 min isomer favoured).

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inductions of 4.5 x 10-3T and 1 .6 x 10-ZT were used ; in the former case a 15 kV pre-acceleration allowed to observe electron lines down to 1 keV (Auger M). The photographic plates were analyzed with digital microdensitometers Optronics at the ESO Laboratory (CERI~ and PDS at the CDSI Institut d'Optique (Orsay za))_ The data were recorded on magnetic tapes and written as a bidimensional matrix

426

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Fig. 4. Coincident gate sequence. The 251.2, 253.1 and 255.1 keV gates between two background gates .

at ARIEL (Orsay) were they could be treated classically by plotting and curves sitting programs. Typical spectra are shown in figs . 6, 7 and 8.

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42 8 2.4 . HALF-LIFE MEASUREMENTS

A double-lens Gerholm-Lindskog type Z s) spectrometer with 3 ~ transmission was used for e --e- coincidences and lifetime measurements . The half-lives of three levels in the nanosecond range (5.8 keV, 135.3 keV and 229.6 keV) have been

Fig. 6. Low energy electron microdensitogram from a film obtained with the ß-spectrograph (B=4.5x10_aT).

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measured using delayed coincidence techniques ; the results will be presented in detail in sect . 4. 3. Experimental results

Table 1 summarizes the results obtained from the various singles and coincident electron and y-ray spectra. Due to the existence of the two Pt isomers, at least two measurements with different sets of values of collecting, waiting and counting times were made for each experiment in order to favour in turn either isomer . t s smPt ~ The relative intensities between the transitions fed by different ways ('esaPt, or a complex feeding) were strongly dependent on the timing set. Column 2 gives the relative intensities for a spectrum favouring the 33 min isomer . Column 3 indicates the main feeding : ' e saPt or t s smPt . The normalisation for absolute conversion coefficients have been made with the theoretical conversion coefficient aK ze) of the 135.3 keV transition which appeared to be a pure E2 from L-subshell ratios . The multipolarities were evaluated by using the most characteristic data available (Irsubshell ratios or absolute conversion ze) coefficients) and compared with the theoretical values . The two last columns indicate the main y-rays observed in coincidence with one particular transition and the deduced position of that transition in the level scheme . Some transitions appeared to be double or even triple, which could be deduced from the coincidence data or from the low-energy electron measurements with the ß-spectrograph . The level scheme of 1 a slr is presented in fig. 9. A wide range of spins (-~ to ~) is

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observed due to the different spin values of the two isomers in tespt . Spins higher than ~ are fed by the ~+ ground state ( 185gPt) while lower spins are mainly fed by the 128.7 keV ~- isomeric state ('esmPt), a complex feeding occuring generally for low energy levels . 4, The level scheme of '"s1r As in te'lr, we observe two groups of negative parity levels belonging to the h,t and h,~ orbitals and one group of positive parity levels . 4.1 . LEVELS RELATED TO THE } - [541] STATE

The ground state spin had been measured by ABMR method z') and interpreted as the ~~ - [541] state from the h~ orbital . It was expected that for deformations higher than ß = 0.22 the ~ - [530] state would be lowered below the ~+[402] state and would appear as the ground state za) . Experiment shows that a ground state spin change oaurs between' 8'Ir and tes Ir (fig. 10). In in-beam experiments, a decoupled band built on ~~ - [541] is excited up to spin z (see following paper zz)). The ~ -" ~ 152.8 keV transition is observed in the radioactive decay. The ~- level at 5 .8 keV. As shortly mentioned before at), a 5.8 keV E2 transition has been observed with the semicircular spectrograph and the double lens spectrometer and assigned as the transition between the ~} - [541 ] level and the ~ ~- [541 ] ground state. In the low-energy spectrum obtained with the ß-spectrograph using a h 9~2 18~ rr

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LEVELS OF ' 8'lr (1)

43 7

B = 4.5 x 10 - ' T induction and 15 kV pre-acceleration (fig. 8) two lines of 2.7 keV and 3 .2 keV corresponding to M and M, conversion lines of the 5.8 keV transition could be observed between M and L Auger spectra. The N-lines are mixed with L Auger. The very low energy lines appear broadened, due to the implantation of the separated ions into the source backing (accelerating potential 50 kV). The half-life of the 5.8 keV level was obtained by measuring delayed coincidences between the 152.8 K and the 5.8 M electrons in the double lens ß-spectrometer. The detection of the 5.8 M electrons was enabled by their pre-acceleration in a potential of 15 kV. A prompt comparison curve was obtained by recording 152.8 K-L Auger coincidences using the same source . The slope of the delayed curve showed that the 5.8 keV level has a half-life of 5 + 1 ns. This measurement locates definitely the ~- band head of the decoupled band, which is mainly ~~ - [541]. The measured transition probability, B(E2) = 1 .4 e2 ~ b2 = 200 B(E2)e.P ., isconsistent with an intraband E2 transition and gives strong support to the ground state assignment . The ~- level at 135.3 keV. The 135.3 keV level, which decays to the ~- ground state by a pure E2 transition, is interpreted as the intrinsic state ~~ - [541] . The half-life of this level was measured with the double lens spectrometer . Coincidences between 119.8 K and 135.3 L electrons gave rise to an asymmetric time distribution curve due to the half-life of the 135.3 keV level. A comparison curve was obtained by measuring 80 K-ß coincidences from a 144Ce source . The slope of the delayed curve, evaluated by the convolution method 29), gave Tß (135 .3 keV level) = 0.29±0.3 ns, which leads to a transition probability B(E2) = 1 .8 e2 . bz. The other levels form two distinct groups with spins respectively higher and lower than the .~ level belonging to the ht orbital, with a predominant component of [541] coupled with ~- [532] and ~ - [530] . The i - 465 .9 keV level is observed in the in-beam experiments as the basis of the dI = 1 band sa). 4.2 . LEVELS RELATED TO THE ~- [505] STATE

The half-life of the 646.6 keV level had been measured by the delayed coincidence method' 1 ) as 19±3 ns and confirmed by the in-beam experiments z2) (T} = 21 .5 f 2 ns). The 640.8 keV transition is assumed to connect the 646 .6 keV ~-- [505] level (which appears as an isomeric state from 193Ir to issIr) to the 5.8 keV ~~- [541] level. The 646.6 keV level is observed in (a, xn) experiments as the band head of a dl = 1 ~ band excited up to spin ~ (and tentatively z) 22). A group of levels feeding the 646.6 keV isomer is built according to coincidence results. As the time resolution was 30 ns, coincidences are clearly seen. Multipolarity assignments are difficult to determine because some transitions are mixed with or energetically very close to transitions placed elsewhere in the level scheme .

43 8

C. SCHLICK et at .

In the y-y spectrum favouring ' SS~Pt decay, coincidences with the 191 .4, 243.0, 298 .1, 370.1 and 706.2 keV appear in the 640.8 keV gate. Relative intensities of the 253.1 and 298 .1 keV transitions in the 640.8 keV gate lead to an estimation of the intensity of the 253.1 keV transition : lY = 3 f 1 (table 1 units) . The small 253.1 keV K-line in the ß-spectrograph spectrum settles the energy of the transition, and its intensity is consistent with M1 multipolarity. Fig. 4 shows the 253 keV gate clearly separated from the 251 keV and the 255 keV gates. The multipolarity of the 370 keV transition was difficult to determine. Two 370 keV transitions are placed in the level scheme, but from coincidence intensities it can be deduced that more than 80 ~ of the y-intensity feeds the 646.6 keV level . The 370 keV K-line is mixed with the 307 keV L-line . The intensity of 370 K obtained after subtraction of the estimated 307 L intensity leads to a conversion coefficient aK(370) = 0.06f 0.03 which is consistent with E2 or M 1 + E2 multipolarity . From considerations relative to heavier odd-A iridium systematics (figs. 11 and 12), we propose E2 multipolarity for the 370 keV transition and spin ~ for the 1016.7 keV level. 4 .3 . THE POSITIVE PARITY LEVELS

As in heavier iridium nuclei, the mixed i + [402] and i + [400] bands are fed by the i e spt decay up to spin ~. The ~+ level at 229.6 keV. The ~+ [402] state which is the ground state in heavier isotopes is located at 229.6 keV in ' a slr and decays by a strong E1 transition to the [541] ground state. Delayed coincidences between 229.6 keV electrons and X-rays showed that the half-life of the 229.6 keV level is T~ = 2 .1 ±0.3 ns . In this experiment the ß-y coincidences from a e°Co source gave the prompt comparison curve. We propose that this level is the same as the 235±5 keV level the half-life of which had been measured by delayed y-coincidences as 2.1 f 0.2 ns [ref. ")]. This measured half-life leads to hindrance factors relative to the single particle transition probabilities : F~,, (229 .6 keV) = 1 .8 x lOs and F,,(94 .3 keV) = 4 x lOs . The other low energy positive parity levels . These are placed on the grounds of coincidence results. They are mainly connected by low energy transitions, some of which have very close energies . The multipolarity of these transitions are deduced from the semicircular spectrograph measurements (figs . 6 and 7) where the corresponding lines could be easily resolved . It should be noticed that the 103.1 keV transition connecting the intrinsic states ~ + [400] and ~+ [402] has E2 multipolarity . The M 1 hindrance between these states appears as a characteristic of odd-A iridium isotopes. The reduced M1 transition probability is less than 6.2 x 10 -e (eh/2M)Z for the very strong 106.4 keV E2 transition in' a'Ir [ref. iz)].

LEVELS OF 'eslr (1)

439

5. Discussion The characteristic features of the level structure of odd-A iridium isotopes from ' e'Ir to ' 9 'Ir are observed in ' 85 1r with a regular increase of the quadrupole deformation. It should be noticed that due to the important range of spins fed in the tasPt decay, the' 851r level scheme obtained by radioactivity is more complex than that of'$'Ir [ref. ' z )], which is only fed by the ~- [ref. Z')] ground state of'8'Pt. The h r system . From the experimental B(E2) value for a transition from 1; to If between members of the same rotational band, the intrinsic quadrupole moment Qa can be calculated and the deformation parameter e z deduced. The values B(E2) = 1 .4 e z ~ b2 for the 5 .8 keV transition between the ~ and ~ rotational members of the ~ - [541] band corresponds to a quadrupole moment Qo = 5 .5 b, which in turn leads to a deformation e z = 0.18 . The value B(E2) = 1 .8 e2 bZ for the 135.3 keV transition between the ~ and ~ rotational members of the same band leads to Qo = 7.1 b and e2 = 0.22 . The h~ system, including in-beam excited levels and levels fed by radioactivity, forms a very complete structure from spin } to spin i' and will be discussed in detail in the following paper zz), since the ~-parameter is essentially sensitive to the side ~

h m2 experiment

theory 121/21

0.5

Fig. 11 . Comparison of experimental and theoretical energies of the h~~ levels . The parameters used in the asymmetric rotor-plus-particle calculations were .1F = 0.5, d = 0.7 MeV, ß = 0.21, y = 22°.

440

C . SCHÜCK et al.

band dl = 1 which is.reached by in-beam experiments. We shall only note here that there is no need for y-asymmetry up to spin ~ and that the levels reached in this work are very similar to the corresponding levels in 's'lr (fig . 10). The ~ and ~ states cross between ' s'lr and isslr. The same feature is observed between 's'Au and 'asAu and cannot be obtained with classical parameters in the asymmetric rotor description. Levels originating from the h~ orbital are observed in Tl, Au and lr nuclei with energies rapidly decreasing with increasing deformation (actually increasing N) and are identified as the ground state band in ieslr and issAu [ref. 4 )] . The values of the lowest ~- level excitation energy were calculated by Blomgvist a °) for Tl and extended by Dionisio et al. s') for Au and Ir in the framework of the Nilsson model . The calculated differences between the minima of the potential energy surfaces for the ~ - [541 ] proton and the ~ + [402] proton appear in satisfactory agreement with the experimental evolution of the ~- levels . The h,~ system . The levels belonging to the h,~ system built on the isomeric i - [505] state (including the levels observed by in-beam studies) are tentatively interpreted in the framework of the Meyer-ter-Vehn asymmetric rotor-plusparticle model ") by coupling a h,~ hole state to a ' s6 Pt rigid triaxial core . The configuration space is restricted to the single h,~ shell. In our calculations the free parameters are the asymmetry parameter y and the Fermi level ~ F as defined in ref. "). The quadrupole deformation parameter ß is related to y and to the energy of the first 2+ state of the even-even core by the formula sz) E2i -- ßzA~ls

4 sinz(3y)

The occurrence of seven levels with spins values ranging from ~ to i in an energy interval of 706 keV can be explained by a y-asymmetry higher than 20°. The comparison with experimental values (fig . 12) seems satisfactory with deformation values ß = 0.21 and y = 22°. It is to be noticed that the y-parameter is close to the y = 21° value ofthe asymmetry deduced from the relative energies ofthe two first 2 + levels of the inept even core sz). Experimental and theoretical results are compared in fig. 12. The energy is very sensitive to the Fermi level location : the ~, F parameter was adjusted to ~ F = 0.5 MeV which is halfway between the u- [505] and the ~ - [514] levels l') . In the experimental systematics of fig. 12 the general trend for increasing axial deformation and decreasing triaxiality with decreasing N from ' 9' Ir (ß = 0.18, y = 27°) i s) to isslr (ß = 0.21, y = 22°) appears with the decrease of the i energy and the smooth increase of the -~ energy . A specific feature of odd-A gold and iridium nuclei is the strong hindrance factor of the M1 transition connecting the i - isomeric state from h,~ to the ~- band head of the decoupled band, which is Fv,,(zl -" ~) = 5 x lOs in ls'Ir and Fv(i -" ~) _

LEVELS OF ' es lr (1)

44l

23n

~ 1.5 N

(21n )- -----_

_

w

w

_

19/2

1 ______ 5/2~ 11n3 15/îz -__ (9nß 15/2 ___ (11nî1- ' 17n

0.5

(7/21

-__

~

_

23/2 21/2

_-

nnz

13/23 ----__~ 19/2 11/2a ______ 17/2 7/2 : __ _ 15/22 ______ 11/zß ____-5/2 __ ---9n 15n _____13/2

______ _

-____`

-

-~~__ -___

7 /2

0 L

11n

165I r 77

.37214mL____~ ~171t9s 11/2 ~} -__ fl 3~s____ 0 197I r 169I r 191I r 77 77 77

Fig. 12 . Systematics of the low energy levels of the h t i ~= family in odd-A iridium nuclei .

4 x 10 5 in 'aSIr. In gold nuclei, the corresponding hindrance factor is an order of magnitude weaker : Fv,,(i -" ~) = 1 .5 x 104 in ' 9 Au [ref. ')] and 3.3 x 10 4 in tsSAu [ref. 8 )] and is interpreted as due to the change from prolate to oblate shape. This interpretation does not hold for odd-A iridium nuclei where there seems to be very small overlap between the h ~ particle configuration and the h,~ hole configuration. It should be noticed that in both gold and iridium nuclei the ~- state coming from the h~ orbital is assumed to result from the coupling of a h~ particle with a prolate even core. It can be interpreted in the strong coupling limit as the ~ rotational level built on the ~ - [541] Nilsson state (~}-[541]) . In the odd-A gold nuclei, the u_ isomeric state is assumed as resulting from the coupling of a h,~ hole with an oblate even core and can be interpreted as the } - [550] Nilsson level. The u_ -~ ~- M1 isomeric transition corresponds therefore to the ~ i [550] -" ~~ - [541] transition which implies a change from oblate to prolate shape, but is K-allowed. In the odd-A iridium nuclei, the ~- isomeric state is assumed to result from the coupling of a h~ hole with a prolate even core. In the strong coupling description, it corresponds to the ~- [505] intrinsic state. The - -. ~- M1 isomeric transition g

C . SCHÜCK et at.

44 2

7/2 1 /2+ C4007 -___

~/2+

v H

w

5/2±

22ps____


~Ps ~3/2 5/t 9.t ns _1 /2 ~.Q229 _Z.OL____ fl-____0-____0-____!~ 3/2 ~eSlr ~~Ir ~89 Ir ~9~Ir t93Ir +..~ t ==_

0

-

n

__

GO

1/2+00007 3R+ [4027 1/2+ C400] 3/2+C4027

Fig . 13 . Systematics of the } * [402] and } * [400] low energy levels in odd-A iridium nuclei .

is then ~u - [505] -+ ~~- [541] which is strongly K-forbidden (dK = 5). This K-forbiddenness could explain the important hindrance of the i -" z M1 transition in odd-A iridium nuclei . The positive parity levels . The systematics of low-energy positive parity levels from ' 93Ir to' es Ir show a smooth variation with an increase of the quadrupole deformation (fig. 13). As was noticed in subsect. 4.3, the M1 component rate between the two band heads ~} * [400] and ~ + [402] drops very quickly from ' 9' Ir to 1 a' lr B(M 1) = 1 .8 x 10- s (elt/2Mc)2 in ' 891r where the 94 .3 keV transition is E2 + 5 % M2 and B(M1) < 6x 10 -e (elt/2Mc)Z in le'Ir where the 106.4 keV transition seems tô be a pure E2. The corresponding 109.8 keV transition in lasl r appears also to be an E2. It is to be noticed that for these isotopes the e z deformation lies in the region of the d N = 2 pseudo-crossings between the ~} * [400] and -~*[660] states and the ~+ [402] and ~* [651 ] states respectively . One possible explanation of the M1 hindrance could be that for a given Ir isotope, the main component of the ~* band iaslr the is N = 4 (6) while the main component of the ~* band is N = 6 (4). In evidence for supplementary ~+ and ~+ levels indicates the presence of another intrinsic state. The ~ + [642] state from the i,~ orbital could be a good candidate. 6. Conclusion iaslr The extension to of systematic studies of the low-energy levels of odd-A revealed the same specific trends as earlier observed in these iridium isotopes has isotopes, regularly varying with increasing prolate deformation. A very complete structure built on the ~- [541] state belonging to the h~ orbital is strongly fed but does not appear to be very sensitive to triaxiality. The quadrupole deformation could be measured through B(E2) experimental values. The system of levels built on the u- isomeric state originating from the h,~ orbital has a density of levels which could be explained with the triaxial assumption.

LEVELS OF ~sslr (1)

443

The positive parity levels show the same regular variation with increasing deformation . It would be interesting to compare the light odd-A iridium systematics with an asymetric rotor model including several j-orbitals 33) . We would like to acknowledge fruitful discussions with Drs. J . Meyer- ter-Vehn and R. Piepenbring. We are indebted to Dr . J. Meyer- ter-Vehn for the communication of his triaxial rotor-plus-particle program . The assistance of the ISOLDE staff is sincerely appreciated. We thank S . Equilbey and A. Marion for their help for the microdensitometer treatment and B. Merlant for the data handling programs. References 1) 2) 3)

4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20)

21)

J . O. Newton, S. D. Cerilov, F. S. Stephens and R. M. Diamond, Nucl . Phys . A148 (1970) 593 J. O. Newton, F. S. Stephens and R. M. Diamond, Nucl . Phys . A236 (1974) 225 M . A. Deleplanque, C. Gerschel, M. Ishihara, N. Perrin, V. Berg, C. Bourgeois, M. G. Desthuilliers, J. P. Husson, P. Kilcher and J. Letessier, J. de Phys. Lett . 36 (1975) L205 ; M. A. Deleplanque, C. Gerschel, M. lshihara, N. Perrin, B. Ader, C. Bourgeois, J. P. Husson, P. Kilcher and J . Letessier, J. de Phys. Lett . 36 (1975) CS-97 C. Bourgeois et al., Proc. lnt. Conf. on nuclei far from stability, Cargèse, 1976, p. 456 C. Bowgeois, P. Kilcher, J. Letessier, V. Berg and M . G. Desthuilliers, Nucl . Phys. A295 (1978) 424 ; C. Bourgeois, thesis, Orsay, France (1977) E. F. Zganjar e~ al., Phys . Lett . 58B (1975) 159 A. C. Kahler, L. L. Riedinger, N. R. lohnson, R. L. Robinson, E. F. Zganjar, A. Visvanathan, D. R. Zolnowski, M . H. Hughes and T. T. Sugihara, Phys . Lett . 72B (1978) 443 V. Berg, R. Foucher and A. Hoglund, Nucl. Phys . A244 (1975) 462; V. Berg, C. Bourgeois et R. Foucher, J. de Phys. 36 (1975) 613 M. G. Desthuilliers, C. Bourgeois, P. Kilcher, J. Letessier, F. Beck, T. Byrski and A. Knipper, Nucl Phys . A313 (1979) 221 and private communication A. Bâcklin, V. Berg and S. G. Malmskog, Nucl . Phys . A156 (1970) 647 V. Berg, S. G. Malmskog and A. Bâcklin, Nucl . Phys . A143 (1970) 117 M. Finger et al., CERN Report 70-29 (1970) C. Sébille-Schück, M . Finger, R. Foucher, J. P. Husson, J. Jastrzebski, V. Berg, S. G. Malmskog, G. Astner, B. R. Erdal, P. Patzelt and P. Siffert, Nucl . Phys . A212 (1973) 45 ; C. Sébille-Schück, thesis, Orsay, France (1972) S. André, 1. Boutet, J. Rivier, 1. Tréherne, J. Jastnebsk~, J. Litkasîak, Z. Sujkowski and C. SébilleSchück, Nucl. Phys. A243 (1975) 229; Proc . Int. Symp . on highly excited states in nuclei, Jûlich, 1975, p. 67 S. André, J. Genevey-Rivier, J. Tréherne, J. Jastnebski, R. Kaczarowski and J. Lukasiak, Phys. Rev. Lett . 38 (1977) 327 J. Lukasiak, R. Kaczarowski, J. Jastnebski, S. André and J. Treherne, Nucl . Phys. A313 (1979) 191 P. Kemnitz, L. Funke, H. Sodan, E. Will and G. Winter, Nucl . Phys . A245 (1975) 221 J. Meyer-ter-Vehn, Nucl . Phys. A249 (1975) 111, 141 H. Toki and A. Faessler, Nucl . Phys. A253 (1975) 231 G. Leander, Nucl . Phys. A273 (1976) 286 Ch . Vieu, S. E. Larsson, G. Leander, I. Ragnarsson, W. de Wieclawik and J. S. Dionisio, Proc . Int. Conf. on nuclei far from stability, Cargèse, 1976, p. 431 ; Ch . Vieu, S. E. Larsson, G. Leander, I. Ragnarsson, W. de Wieclawik and J. S. Dionisio, J. of Phys . G4 (1978) 1159, 531 C. Sébille-Schück, V. Berg, J. Genevey-Rivier, Â. H3glund, A. Huck, A. Knipper, G. Walter and C. Richard-Serre, Proc . Int. Conf. on nuclei far from stability, Cargèse, 1976, p. 444

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22) S. André, J. Genevey-Rivier, J. Treherne, R. Kaczarowski, J. Lukasiak, J. Jastrzebski and C. Schück, Nucl. Phys . A325 (1979) 445 23) H. L. Ravn, L. C. Carraz, J. Denimal, E. Kegler, M. Skarestad, S. Sundell and L. Westgaard, Nucl . Insu . 139 (1976) 267 24) A. Marion and S. Equilbey, CDSI, lntemal Report (1978) 25) T. P. Genholm and J. Lindskog, Ark. Fys. 24 (1963) 171 26) R. S. Hager and E. C. Seltzer, Nucl . Data 4A (1968) 15 27) H. Rubinsztein and H. M. Gustafsson, Phys. Lett . 58B (1975) 283 28) S. G. Nilsson, C. F. Tsang, A. Sobiczewski, Z. Szymanski, S. Wycech, C. Gustafson, I. Lamm, P. Môller and B. Nilsson, Nucl . Phys . A131 (1969) 1 29) B. Olsen and L. Bostr3m, Nucl . Insu. 44 (1966) 65 30) J. Blomgvist, Nuclear theory progress report, State Univ. New York, Stony Brook (1969), p. 29 31) J. S. Dionisio, Ch. Vieu, W. de Wieclawik, R. Foucher, M. Beiner, S. E. Larsson, G. Leander and I. Ragnarsson, J. of Phys. G2 (1976) L183 32) A. S. Davydov and G. F. Filippov, Nucl . Phys. 8 (1958) 237 33) S. E. Latsson, G. Leander and I. Ragnarsson, Nucl . Phys . A307 (1978) 189