Mar 16, 1995 - mcm (267.0, 314.7 and 359.6 keV) cut from another matrix of y4-data have been ... of structure 2 (see below) playing a dominant role in the ..... We wish to thank the crew and technical staff at the, now defunct, NSF at Daresbuy.
I. Phys. G Nucl. Part. Phys. 21 (1995) 911-936. Printed in the UK
Highly excited A I = 1 structures in 193Hg N Fotiades’, S Harissopulos’ , C A Kalfasl , S Kossionides’ , C T Papadopoulosz, R Vlastou*, M Serrisz, M Meyes, N Redon3, R Duffait3, Y Le Coz’, L Ducroux’, F Hannachi4, I Deloncle4, B Gall4, M G Porquet4, C Schuck4, F Azaiez5, J Duprats, A Korichi5, J F Sharpey-Schafer6, M J Joyce6, C W Beausang6, P J Dagnal16, P D Forsyth6, S J Gale6, P M Jones6, E S Paul6, J Simpson’, R M Clarks, K HauschildS and R Wadsworths Institute of Nuclear Physics, NCSR Demolaitos, 15310 Athens, Greece National Technical University of Athens, 15773 Athens. Greece Institut de Physique Nuclhaire de Lyon, WZP3-CNRS, Universiti Claude Bemard. P-69622 Vllleurbanne Cedex, France CSNSM,INZF3-CNR.5, F-91405 Orsay Campus. France F”. INZP3-CNRS. F-91406 Orsay Campus. Fmce Oliver Ladge Laboratow, University of Liverpool. Liverpool U 9 3BX, UK Daresbuy Laboratory. Warrington WA4 4AD, UK Department of Physics, University of York, York YO1 SDD, UK I
’
Received 2 December 1994, in final form 16 March 1995
Abstract Highly excited states in the nucleus %g have been investigated by in-beam p r a y spectroscopic techniques using the EUROGAM may. The reaction lr0Nd (48Ca5n) at a beam energy of213 MeV was used M populate states of lg3Hg.me level scheme has been considerably extended (up to 10.7 MeV) and enriched from earlier studies. Two new su’uctures of competing dipole and quadrupole transitions were observed. Experimental E(MlYB(E2) ratios were determined for the two smctures and compared with theoretical estimates. They were also compared with similar structures in the neighbouring Hg and Pb nuclei.
NUCLEAR REACTIONS lS0Nd(“Ca,Sn), E(‘sCa) = 213 MeV; measured y-y coincidences, angular correlations ‘=Hg; deduced levels, decays, Ex,Ey, I”. J . Enriched targets, Ge detectors. Cranked shell model interpretation. 1. Introduction
The I9’Hg isotope has been the subject of investigation in several speclroscopic experiments [I-71. The results show the existence of several band sanctures in the low-lying level scheme of this isotope, interpreted as rotational bands built on various rotation-aligned quasi-neulron configurations [Z]. The observation of superdeformation in this isotope [3] has generated a great deal of attention and the data from subsequent experiments were used not only to investigate this phenomenon, but also to gain information on the lower-lying states. In the neighbouring Pb isotopes [&I81 as well as in I9’Hg [I91 and in I9‘Hg [20], intense dipole bands have been reported. These bands consist of dipole aansitions, suggested as M1 due to intensity-balance arguments, with regular and irregular sequences. Weak crossover 0954-3899/95/070911+7.6$19.50 @ 1995 IOP Publishing Ltd
91 1
912
N Fotiades et a1
quadrupole transitions are also observed in some dipole bands in Pb isotopes and in all bands in Hg isotopes. The excitation energy of most of these structures is still unknown, because the transitions connecting them to lower-lying states remain unobserved. However, the bandhead excitation energies are expected at moderate excitations (4-6 MeV). Such a band has also been reported in 193Hg141. A common feature in the interpretation of all dipole bands is the need for high-K configurations involving deformation-aligned hg/z and i l y z proton orbitals to account for the large B(M1) values characterizing them. These configurations are coupled to the neutron configurations present at lower angular momenta to reproduce the high angular momenta of the dipole bands. These near-oblate bands exhibit larger B(Ml)/B(E2) ratios in the Pb nuclei than in the Hg nuclei. This difference has been studied in terms of different intrinsic structures for these bands and/or different deformations. Total Routhian surface (TRS) calculations, reported in [20], give similar deformations for these bands in Hg and Pb nuclei, thus favouring an interpretation based on different intrinsic structures. Prolate deformation in this region has also been invoked to account for the existence of irregular sequences observed in I9’Hg [211 and in 193Hg[5]. The alignment of nucleons occupying the rhlllz. u i l y z and uhg12 high4 orbitals isconsidered responsible for the shape change from oblate collective towards the non-collective prolate shape. Such excitations are energetically cheaper than the configurations involved in the case of dipole bands, because they do not include the ah9p and ~ i 1 3 / 2excitations which lie across the 2 = 80 proton gap. We must point out here that the prolate sequence of lg3Hg [5] is the same as that reponed previously as an oblate dipole band [4]. The fact that the same sequence has been interpreted in two entirely different ways shows that the phenomenon we are dealing with is not yet clearly understood. In this paper we present two new A I = 1 structures in ImHg locating them precisely at moderate excitation energies and spins. Furthermore, we give evidence for different configurations in these structures from the configurations in the Pb isotopes. In addition, we discuss the questions raised in the different interpretation of the previously known A I = 1 structure in this isotope [4, 51. Finally, we discuss briefly some new results in the general decay scheme of this isotope. 2. Experiment The reaction I5ONd 5n) Ig3Hg was used to populate high-spin states of 19”g at beam energy E(48Ca) = 213 MeV. The 20 MV tandem Van de Graaff accelerator at the Nuclear Structure Facility at Daresbury (UK)was used to provide the beam. The target consisted of two stacked self-supporting foils of isotopically enriched IsoNd each of thickness 500 pg cm-*. Gamma-rays were detected using the EUROGAM detector array which consisted of 44 large-volume n-type hyperpure Ge detectors, each surrounded by a BOO escape suppression shield [22]. A total of approximately lo9 coincident events, of unsuppressed fold five or higher, were collected and stored on magnetic tapes for subsequent off-line analysis. The events were unpacked into doubles and stored into two two-dimensional 4096 x 4096 channel matrices. The first matrix was symmetrized and used to investigate the coincidence relationships and to discover the new structures. The second matix was especially built to establish the directional correlations of the y-rays Z(15Sn-900)/I(900-15~),where Z(81-82) is the intensity of y-rays recorded by a detector at angle 81 while gated on a detector at angle 6 (DCO ratio). A DCO (directional correlation from oriented states) value near 1.0 indicates an E2 and near 0.5 a dipole transition, when calculated by gating on an E2 y-ray, while, by gating on a dipole transition, 1.0 indicates
Highly excited AI = 1 structures in "'Hg
913 L
914
N Fotiades et a1
a dipole and 2.0 indicates an E2 transition. In addition, several matrices from triples or quadruples have been built in order to produce clean spectra, mainly for the investigation of the structures at high excitations. v1
' r)
24
Gate : 943.5 keV
figure 2. Specmm gated at the 943.5 keV uansition showing band 4 (uppcr part) and spectrum produced by addition of the gates on the 983.4 and 1081.5 keV transitions showing band 7 (lower pait). All transitions are quoted in keV.
3. Results 3.1. General decay scheme
From the analysis of the former matrix the previously known level scheme [Z, 4, 51 has been enriched and extended to higher energy and spin. Some of this work has previously been presented in 161. The level scheme of this work is in general agreement with works previously reported on this isotope and it is shown in figure 1 with nine bands present (1-9). Several new sidefeeding and linking transitions between these bands have been observed. Bands 3,5 and 6 have been extended up to spin 49/2+, (53/2+)and (55/2-),respectively. A new, very weak path (150.5, 1206.6 keV transitions) connecting band 5 to band 1 has been established. Below band 8 the existence of a 989.0 keV transition which was uncertain in [2], has been verified. Moreover, since this transition is in coincidence with band 8, the 19.9 keV transition should exist, but remains unobserved, because it is below the energy threshold of the detection system. Band 4 is entirely new. It extends up to spin (51/Zt) and deexcites through 549.5 and 943.5 keV transitions to levels 37f.2" of bands 3 and 5, respectively. In addition, the
~
Highly excited AI = 1 structures in Ig3Hg
915
continuation of band 7 above level 45/2- has been found. Two spectra (y3-data) showing bands 4 and 7 are seen in figure 2. The 298.6, 561.7 and 719.6 keV transitions reported in [5] to feed level 45/2- above band 8 are probably misplaced. Our data corroborate the fact that these transitions do belong in Ig3Hg,but that they are involved in structure 1 and the deexcitation out of this structure (see figure 3). Information on the energy, intensity, DCO values and assignment (if known) for the new transitions of the general decay scheme, as well as for all transitions of the three structures and their connections to the lower levels. can be found in table 1. 3.2. Structures a t higher excitations In addition to the enlargement of the general decay scheme three structures have been observed at higher excitations. ' h o of them are entirely new, while the third has already been investigated in previous works [4, 51. They are all shown together in the partial level scheme in figure 3. In the same figure the levels of the bands of the general decay scheme where these smctures deexcite, as well as all the transitions connecting the structures to these levels, are included. The first thing we notice when looking at the partial level scheme in figure 3 is that the three structures exhibit similar features: (i) they consist of direct transitions competing favourably with crossover transitions; (ii) the DCO values suggest that the crossover transitions are quadrupoles (L = 2), while the direct transitions are dipoles ( L = 1); (iii) we obtain a better intensity balance by assuming that the dipole transitions are pure MI; (iv) the deexcitation out of these structures proceeds through a region where the level pattern is rather complicated before feeding the lower bands. The first three features are similar to those reported for the dipole bands in the neighbouring Pb [8-18] and Hg [19, 201 isotopes. The fourth feature is similar to the properties reported for the irregular sequences in Hg isotopes 15, 211 suggesting that the region of deexcitation out of the structures is a similar phenomenon to these irregular sequences. We discuss each structure separately below. 3.2.1. Structure I In figure 3 the first new AI = 1 structure is shown together with all the transitions connecting it with the lower bands 2 and 8. This sbllcture extends up to 10.7 MeV excitation energy which is the highest excitation energy experimentally observed in the Ig3Hg isotope so far. The ordering of the transitions is based on the coincidence relationship between them and on intensity arguments (corrected for internal conversion, if necessary). Intensity calculations in conjunction with theoretical estimates of internal conversion coefficients [23] favour an M1 multipolarity for the 138.8 keV and 257.8 keV dipole transitions and result in a better intensity balance assuming that all dipole transitions are M1. From the DCO values we measured it is not possible to define the spin and parity of the levels of structure 1 uniquely. We tentatively suggest the spin and parity given in figure 3. A scenario of spins reduced by Iii for each level is not to be excluded. In figure 4 (upper part) a specmm gated on several lines of this structure shows all its members (y4-data). The determination of the excitation energy of the structure is based on several paths (e.g. 512.8, 881.5, 626.8.719.8,and546.0 keV transitions path) observed to connect the structure
917
Highly excited A I = 1 structures in 1 9 3 ~ g
Table 1. Excitations. energies, intensities. Dco ratios and assignment for all transitions discussed
in this work.
General decay scheme
y-
$2'
1614.9
1595.0
19.9
7.361.4
2210.9
1505
9(1)
1.12(30)
?+
Y
Z3" 2
1955.3
1743.6
211.9
14(1)
0.47(6)
p-
H
a+ 2
5407.0
5179.1
227.4
lZ(1)
382.0
0.0
382.0
E
2443.0
2048.6
394.7
120)
3055.5
2554.9
5w.3
c5
1000
471(+)
T 0.98(1)
~
(qc)
y'
H
fit 2
i 9L-
H
9-
H
y+
39+
$?+
3979.0
3429.4
549.5
8(1)
4271.9
3710.0
561.9
17(3)
l.lO(4)
q-
H
4543.1
3979.0
564.1
3%1)
0.82(20)
?+
H
5221.0
4543.1
677.9
28(1)
0.84(30)
q'
H
5259.6
4543.1
716.5
15(1)
0.68(10)
5270.7
4533.3
737.4
92(4)
0.99(1)
4749.1
3979.1
169.4
36(2)
1.11(8)
$?-
?+ ?+
4' 9$+
c)
Y
ys+ 2 ' Q
(++)
6022.9
5221.0
801.9
1N1)
5557.3
4749.1
808.2
23(1)
1.11(10)
4823.2
3979.7
843.5
21(1)
055(6)
q+ q"'
6287.8
5418.8
869.0
lO(3)
6356.2
5418.8
937.4
l2U)
H
3979.0
3035.5
9435
370)
0.40(4)
(?+) (?+) 9'
Y
9' ?+
6254.1
5270.7
983.4
24(1)
0.91(8)
9-
c
9-
1595.0
606.0
989.0
23(1)
1.01(10)
y+
c
U ' 1
6772.7
5758.4
1014.3
17(1)
0.73(9)
(y-)
H
T 'I-
6781.1
5758.4
1022.7
5(2)
++
a2
3613.5
2554.9
1058.6
7(1)
H
7335.6
6254.1
1081.5
lO(1)
0.82(20)
(T-)
c
3710.0
2621.6
1088.5
l7(1)
1.17(20)
$?-
H
H
y+ ?+
ct
49+
H
T
?+ y?-
918
N Fotiades et al Table 1. continued. Excitationd
Energyb
htensitf
Ei (keV
E, Rev)
(keV
('/d
6897.4
5758.4
1139.0
2210.9
1004.7
1206.6
4321.5
3035.5
1286.0
5302.1
3739.8
1562.0
4724.3
4651.4
72.9
6278.7
6164.8
113.9
4724.3
4580.1
144.5
7697.6
7559.3
138.8
0.52(6)
5198.4
4976.8
221.7
0.50(3)
5660 1
5420.2
240. I
4976.8
4724.3
252.5
0.51(2)
8254.1
7996.8
257.8
0.6W
6278.7
6004.8
274.2
0.43(6)
7996.8
7697.6
298.7
0.54(3)
6278.7
5963.3
315.6
0.51(2)
4976.8
4651.4
325.5
5538.0
5198.4
339.4
7135.8
6781.5
354.7
0.47(20)t
8610.8
8254.1
356.1
0.47(20)'
5562.0
5198.4
363.6
0.35(7)
5963.3
5562.0
401.1
059(8)
7559.3
7135.8
4229
0.46(3)
5963.4
5538.0
425.5
7996.8
7559.3
437.5
m(7ay
6004.8
5562.3
4426
0.49(4)1
5420.2
4977.0
443.2
0.49(4$
9535.7
9080.7
454.4
1.02(IOj'
5660.1
5198.4
461.5
Assignment
KO
(1:
H
1: )
H
y-
25+
Zl+
T
T H
AI(+)
T
q+ s+ 1
Highly excited A I = 1 structures in IS3Hg Table 1. continued.
9080.7
8610.8
470.6
50(1)
5198.4
4724.3
4742
33(1)
6781.5
6278.7
502.4
410)
7559.3
7046.3
512.8
9(1)
5963.4
5420.2
5435
5(1)
8254.1
7697.6
5565
436)
6164.8
5606.9
557.7
166)
7697.6
7135.8
561.8
61(4)
8610.8
7996.8
614.0
4~5)
10149.6
9535.7
614.5
27(4)
6278.7
5660.1
618.7
72(6)
6164.8
5538.0
626.8
S7(1)
6278.7
5562.0
716.7
19(3)
5538.0
4817.9
719.8
556)
5562.3
4817.9
7444
29(5)
5963.3
5198.4
765.0
17(2)
7559.3
6781.5
777.6
49(1)
5606.9
4817.9
789.0
lO(1)
6004.8
5198.4
806.0
Il(1)
6781.5
5963.3
818.2
36(1)
9080.7
8254.1
826.6
3q1)
7135.8
6278.7
857.1
786)
7046.3
6164.8
881.5
37(1)
9535.7
8610.8
9x.9
20(1)
1021.6
17(5)
1068.9
16(3)
1145.0
15(5)
1149.0
24(5)
1177.7
20(1)
10149.6
10713.4
9080.7
9535.7
919
920
N Fotiades et a1 Bble 1. continued. Excitation"
Ei Rev)
E/ &e\')
Energyb
IntensifyC
KO
(keV
( 1:
Assignment c 1
'7
SWctUIc 2
6833.6
6585.3
252.3
11(4)
7104.6
6833.6
267.0
47(2)
5876.0
5574.0
302.2
6Q)
7415.5
7104.6
314.7
32(1)
6585.3
6256.0
325.4
9(2)
7778.7
7415.5
359.6
28(3)
5574.0
5198.4
375.8
c5
8186.2
7778.7
411.0
20(1)
8616.5
81862
426.9
12(1)
472.3
4(1)
6833.6
6256.0
577.6
IO@)
7415.5
6833.6
581.9
IO(2)
606.1
6(1)
0.61(4)
(q-)
H
(q-1
(?-)
H
Cy-)
(43 1.06(7)'
Cy-)
H
(y-) (?-) (9-1
H
(q-)
(y-)
H
(9-1
(q-)
H
(q-)
c
($-)
0.39(8)
0.56(4)
2.24(50)'
($-)
(T-) (q-)
U H
(F-) (y-)
7778.7
7104.6
674.1
21(1)
2.50(30)'
(y-)
c
5876.0
5198.4
678.0
7(1)
2.25(60)'
(%-)
c
(T-) (T-)
6585.3
5876.0
709.3
9(2)
2.06(70)'
Cy-)
H
(y-)
8186.2
7415.5
770.7
20(1)
2.55(60)'
8616.5
7778.7
837.8
22(1)
1.55(20)'
881.7
19(5)
938.0
13(2)
1026
e5
SWcNre below level
4651.4
4580.1
71.3
4057.4
3743.2
314.2
lO(3)
2476.6
2149.0
327.7
44(1)
2149.0
1745.7
403.2
37(1)
2476.6
2048.6
428.1
39(5)
3586.5
3079.5
507.0
U)(])
+-
Y
[q-) H (4-) (q-) (y-) H
4651 keV
0.59(20)
9(p-) q-
H
1.22(10).
y-
H
9cp-,
H
t+ r*
-
?-
p729(y-)
High@ excited AI = 1 structures in Ig3Hg
92 1
Table 1. continued
Excitation' Ei (keV) E, (Rev)
Energyb (keV)
2476.6
1955.3
521.3
3586.5
3061.9
524.5
5562.0
4976.8
585.2
4651.4
4057.4
594.1
3079.5
2476.6
602.9
4271.9
3586.5
685.7
2476.6
1745.7
731.1
4651.4
3743.2
908.2
3586.5
2621.6
965.0
4057.4
3083.0
974.4
4580.1
3586.5
993.6
3061.9
2048.6
1013.4
4651.4
3586.5
1064.8
4651.4
3356.8
1294.4
5407.0
5302.1
105.2
5302.1
5179.1
123.0
7057.2
6897.1
160.4
6897.1
6699.3
197.6
5927.1
5691.5
235.6
5691.5
54m.0
284.5
6992.7
6699.3
293.4
7299.5
6992.7
306.7
7414.7
7057.2
357.3
6692.3
6324.1
367.8
7784.1
7414.7
369.7
6699.3
6324.1
375.4
5691.2
5302.1
389.6
6324.1
5927.1
397.0
Intensity'
PIm) low
DCO ( I:
292
Assignment H I; )
n2
922
N Fotiades et a1
7141.6
6692.3
449.3
30(1)
8248.3
7784.1
464.0
43(4)
8746.1
8248.3
497.9
21(1)
7414.7
6897.1
517.6
17(1)
5927.1
5407.0
520.1
131(3)
7299.5
6699.3
600.2
8(2)
6324.1
5691.5
632.6
SS(5)
7784.1
7057.2
726.9
57(1)
6692.3
5927.1
764.6
54(1)
6699.3
5927.I
772.2
154(3)
8248.3
7414.7
833.6
38U)
7541.2
6692.3
848.9
33(9)
5407.0
4533.3
873.4
630)
8746.I
7784.1
962.0
36(1)
9268.6
8248.3
1020.3
30(2)
9782.4
8746.1
1036.3
8(1)
8837.4
7784.1
1053.3
T h e uncertainty on Ihe excitation varies from 0.2keV to 1.0 k V . bThe uncertainty on the y-ny energies varies from 0.2 keV to 0.4 k V for the stroog vansitions and from 0.8 keV to I .O keV for the weakest ones. CIntensitiesderived from coincidence gates, %CO values obtained by gating on a AI = 1 transition (see text). 1x0values for energetically close y-rays which were impossible to separate.
to the upper part of band 8 (above spin 41/2-). The gates in the 546.0 and 719.8 keV transitions shown in figure 5 include some of the connections out of the structure, giving evidence for the correctness of the proposed level scheme. The existence of the 563 keV transition on the top of the structure is uncertain due to low statistics. The 72.9 keV transition involved in the deexcitation of the structure and proposed to connect levels (43/2-) and 4112- at 4651 keV is also uncertain. Since all transitions above level (43/2-) are seen in coincidence with the transitions below level 41/2-, the 72.9 keV transition should exist. Unfortunately, this transition is obscured from the x-rays of the Hg isotopes and cannot be firmly established by our data. This is also the case for a 71.3 keV transition below the 41/2- level. Finally, the observation of the crossover 144.5 and 325.5 keV transitions is also uncertain due to low statistics. This structure has a maximum intensity of -20% of the intensity of the lowest 382.0 keV
Highly excited AI = 1 structures in Iq3Hg
923
5 t-7
94 X
3 2 1
0 Gate : Structure 2
5 N
0 7
4
X 3 2
1 0
200
400
EY(keV)
800 ‘O0
Figure 4. Spectra in coincidence with s t l u c m s 1 and 2. The specwm for structure I is produced by addition of the gates on 17 transitions (138.8, 221.7, 252.5, 257.8, 298.7, 315.6, 356.1, 454.4.470.6.502.4.556.5.777.6. 818.2, 826.6, 924.9, 1068.9 and 1177.7 keV) cut from a matrix built from y4.data. For the spectrum of shllcm 2 lhe gates on three transitions of this mcm (267.0, 314.7 and 359.6 keV) cut from another matrix of y4-data have been added. All new uansitians seeo in coincidence with these stluctures and not assigned in this work are markd with an asterisk. All transitions a e quoted in keV.
transition in this nucleus. This value represents the intensity of the 298.7 keV line, corrected for conversion, plus the intensity of the 556.5 keV line. Below levels (61/2-) and (59/2-) at 7559 keV the intensity of the main sequence of M1 transitions gradually diminishes as a deexcitation out of the structure sets in. This fragmentation is more intense below level (53/2-) at 6279 keV where the pattern of transitions is rather complicated. A path out of structure 1 (-25%) passing through level (45/2-) at 4818 keV (above 546.0 keV transition) feeds band 8. Another -45% of the intensity passing through several paths feeds level (43/2-) (below 252.5 keV transition). The way out of the remaining -30% of the intensity of structure 1 still remains unobserved. However, whatever the way out, -95% of the intensity feeds bands 2 and 8 (-30% at level 31/2- of band 2 and -65% at
924
N Fotiades et a1 5 0
Gate: 719.8 keV
4 3
2 N
I
9 xo -L(D 1 .
'
, . , .
,
,
,
,
'
$20 0 16
..
.
,
,,
Gate: 546.0 keV
12
a 4
0 200
4w
500
600
700
E,(keV) Figure 5. Specva in coincidence with 546.0 and 719.8 keV transitioos (y'-data) showing some of the connections out of structure 1 (425.5 and 744.4 keV transitions). The absence of the 252.5, 221.7 and 363.6 keV transitions in bolh gates and of 401.1 keV transition in the gate of 719.8 keV transition is hdicative of the relative position of these Innsitions in the level scheme. For simplicity energies are quoted only for the present MI transitions in key. The absent transitions are marked with an asterisk.
level 33/2- of band 8). Finally, the level (43/2-) (below 252.5 keV transition) which gathers -45% of the intensity out of structure 1 is worth discussing here. The same level is also fed by a path out of structure 2 (see below) playing a dominant role in the deexcitation of the two structures. The total intensity gathered in thii level is -25% of the intensity of the 382.0 keV line (intensity of 252.5 and 474.2 keV transitions corrected for internal conversion). Since the 72.9 keV transition connecting this level to the lower 41/2- level at 4651 keV is obscured it is impossible to determine how much of this intensity is transferred to the 41/2- level. However, -35% of this intensity is seen to fragment in several paths below the 41D- level. All fragmentation paths feed bands 2 and 8. Since almost all the intensity of structures 1 and 2 (see below) ends up in bands 2 and 8 we conclude that there should exist many more, as yet unobserved, fragments out of this level (or out of the (43/2-) level) feeding these bands.
3.22. Structure 2 Structure 2 is shown in figure 3 together with structures 1 and 3. This structure extends up to 8.6 MeV excitation energy. The determination of the excitation energy of the structure is based on the transitions observed to connect it with the level (47/2-) at 5198 keV (the observation of the 302.2 and 375.8 keV transitions is uncertain
Highly excited A I = 1 structures in L93Hg
925
due to low statistics, and because these y-rays are obscured by, the strong 302.9 and 375.2 keV transitions of the general decay scheme, respectively). Thus, the uncertainty (16) characterizing the spin of this level also characterizes the levels of structure 2. In figure 4 (lower part) a spectrum gated on several lines of this structure shows all its members (y4-data). This structure has a maximum intensity of -6% of the intensity of the lowest 382.0 keV transition (intensity of 267.0 keV line plus intensity of 581.9 keV line). The decay out of structure 2 below level (47/2-) at 5198 keV follows the same paths as the deexcitation out of structure 1, i.e. it proceeds mainly via the 221.7 and 252.5 keV transitions and then fragments towards bands 2 and 8. An accurate intensity percentage for this way out is difficult to give because of the second 252.3 keV transition involved in this linking. However, 30% is a rough estimate for this way out. Moreover, there is a direct connection of this structure to bands 2 and 8, although the linking transitions have not been firmly established, largely because of the complexity of the level pattern and the low statistics. Some of the transitions involved in this connection are the 606.1,881.7,938.0 and 1026 keV transitions. As for structure 1,95% of the intensity of structure 2 ends up in bands 2 and 8 (-45% at level 31/2- of band 2 and -50% at level 33/2- of band 8). Finally, a weak connection towards the lower levels of band 5 is possible but the linking transitions are unknown.
3.2.3. Structure 3 The last A I = 1 structure experimentally investigated in this work is structure 3 shown in figure 3. It is strongly populated in this reaction with maximum intensity, as calculated from the 284.5 and 520.1 keV transitions, -38% of the 382.0 keV line intensity and extends up to 9.8 MeV excitation energy. This structnre has been investigated in previous works [4,5] and, here, we have enriched it and tentatively extended it up to spin 71/2. The existence of 522.2 keV and 514.1 keV lines is not firmly established because they are obscured from the 520.1 keV line of this sequence and the 523.2 keV and 512.8 keV lines of the general decay scheme. Our data agree on the structure being built directly above the 5407 keV level [5] and fragmenting towards several bands of the general decay scheme 14, 51. We also agree on a positive parity being more likely for this structure, as proposed in [5]. A spectrum gated on several transitions of this structure (y3data) clearly shows all its members in figure 6. The structure was reported as collective in origin by Roy et a1 [4] and then as a single-particle structure by Deng eral [5]. One of the arguments in 151 on the non-collectivity of this structure was the absence in the coincidence spectra of the 357.3 keV line assigned as I 6 w Z 4 in 141 and as 59/2 H 55/2 in [5] and in the present work. Another argument in [5] was based on the comparison of the highly irregular S(*) % I/AE,(Ml) moment of inertia of this structure, as opposed to the regular moments in the Pb dipole bands. Our coincidence spectra justify the non-existence of the 357.3 keV line, thus confirming the experimental validity of the first argument. Such spectra gated on 357.3 and 726.9 keV are shown in figure 7. Clearly, the former transition is not present in the gate of the latter and vice versa. Hence, the two transitions are not in coincidence. The existence of a second 357 keV transition in gate 357.3 keV can be attributed to contamination fiom another isotope. In [4] a 572 keV transition (I+5 H 1+3 and 57/2 H 53/2) is considered possible but difficult to establish because it is obscured by the 573.0 keV line of band 8. Since this A I = 1 structure is in coincidence with band 8, the fact that no trace of the 572 keV line can be found in the gate of 573.0 keV in our data suggests that this line does not exist. One can also estimate the expected intensity of the 357.3 and 572 keV transitions from the measured intensities of the MI transitions
+
+
926
N Fotiades et a1
*
(namely 7.0 & 0.5 % and 9.7 0.5 % of the 382.0 keV pray intensity for the 160.4 keV and 197.6 keV y-rays) and the expected B(Ml)lB(EZ) ratios of the I 6 (59/2) and I + 5 ( 5 7 ~ levels, ) assuming that B(MI)/B(E2) values are in the interval 2 4 /2/(eb)2 for all states in this structure (see below for a justification of the selection of this interval). This method gives intensities of 2.5 & 0.8 % and 20 -+ 6 % for the 357.3 and 572 keV y-rays, respectively. The fact that such intensities are not observed experimentally provides further justification of the non-existence of these transitions.
+
Figure 6. Specwm io coincidence with SWC~UE 3, The specbum is produced by addition of the gates on eight msitions (160.4. 197.6, 235.6, 2845, 369.7.397.0, 464.0 and 491.9 keV) cut from a rmtrix built from y’-data AU transitions x e quoted in keV.
4. Discussion
4.1. Neutron conjgurations
The configurations in the lower level scheme of this isotope have been extensively investigated [l, 21 and include only neutron excitations lu, 3v and 5v out of a collectivelyrotating oblate core. The extension of the general level scheme reported here is in agreement with this interpretation. The configurations and the cranked shell model (CSM) notation for all bands of the general decay scheme are seen in table 2. Briefly discussing the configurations in table 2, we first note that the proposed configurations include i13/2, p3/2 and hglz neutrons. In CSM calculations the iI3l2 orbitals generate very steep Routhians producing large alignments when decoupling. Furthermore. the corresponding backbending effects are extremely strong. The decoupling of these nucleons is expected to take place at low and moderate frequencies because, due to the small deformation, the Coriolis force is large. AU these features were corroborated by the experimentally deduced large alignments of the bands and the rotational frequencies where the alignments occur [2]. Our data were found to be in general agreement with the previously reported results for the bands of the IwHg isotope.
Highly excited AI = 1 structures in lP3Hg
927
Figure 7. PM of spectra gated on 357.3 and 726.9 keV transitions demonsating the lack of coincidence between these transitions Ihe fonner transition is not present in the gate of the latter and vice versa. The energy of some &er msitions characteristic of Ig3Hg isotope is also quored in keV.
Table 2. Configurations and CSM notation for all bands of the general decay scheme in '"Hg. Band
CSM
notation
Neutron configuration
A ABP ABCEF A B m ABC ABCDF ABCDE
ABE B
Here, it is worth discussing band 4, which is a new band. Band 4 exhibits many similarities to the ABCEP band in the neighbouring IP1Hgisotope [21] such as:
928
N Fotiades et al both bands are characterized by a positive parity and their bandhead lies at about the same excitation energy; both bands deexcite towards the same neighbouring bands, i.e. ABE and ABCEF both bands have an alignment of -201i.
Based on these similarities we propose the same assignment (ABCEF')for band 4. This assignment corresponds to a v i ~ 3 / Z v p ~ p v hconfiguration gp where the F' Routbian originates from the vhslz orbital. This is an N = 5 low-$2 orbital with a large down slope as a function of Tw. It crosses the F ( v p s p ) orbital at low frequencies (e0.1 MeV) and becomes yrast as the rotational frequency increases [21]. In the TRS calculations of the same reference the aligned uhslz orbital drives the nucleus towards a y = -80' deformation, while the bands ABC and ABCEF are characterized by a I/ = -60" deformation. Thus, a shape change seems to emerge in the interpretation of this band.
4.2. Configurations at higher energy involving proton excitations In this section we shall discuss the nature of the three structures in figure 3 and of some of the levels between them and the general decay scheme. Looking at the partial level scheme in figure 3 we can clearly identify three different regions: (i) a lower region governed by bands of E2 transitions and extending up to approximately 4.5 MeV, (ii) an intermediate region of complicated level pattern extending up to approximately 7.0 MeV; (iii) an upper region, more regular than the intermediate one, governed by structures of dipole transitions competing favourably with quadrupole crossover transitions and extending up to approximately 10.0 MeV. The limits of these regions cannot be strictly defined, since there is a certain overlapping between them, e.g. in the case of the deexcitation out of structures 1 and 2 the complicated level pattern holds down to 2.0 MeV, creating the irregular structure between bands 2 and 8 in figure 3 (the transitions of this structure are grouped under the heading 'Structure below level 41/2- at 4651 kv'in table 1). This indicates that the various phenomena generating these regions can coexist. In figure 8 the excitation of the levels of the three regions (relative to a rigid rotor reference) are plotted as a function of J ( J 1) ( J being the spin of the levels). It can be seen that the three regions can coexist and become yrast in different parts of the plot. Structure 3 is yrast in the lower part of the upper region, while in the upper part of this region structure 1 becomes yrast, a fact that explains the large intensity they gather in these regions. The lower region has been discussed in the previous section. The discussion of the intermediate and upper regions is the aim of this section. In order to accomplish this we shall first try to exploit the similarities of these regions to corresponding ones in neighbouring isotopes, and then compare the experimental data with theoretical calculations performed for Ig3Hg. As mentioned in a previous section, the three structures of the upper region have similar features to the dipole bands observed in the neighbouring Pb [S-181 isotopes as well as in 19*Hg [19] and in IgaHg [20] isotopes. These features are summarized below.
+
(i) Direct dipole transitions, suggested as M1, compete favourably with crossover quadrupole (E2) transitions. In most cases in the Pb isotopes the sequence of M1 transitions is regular and the band is then characterized as regular. If the sequence of
~
Highly excited AI = 1 structures in 193Hg
A
>, 2
6-
v
.
P
+
3 v
Ig3Ha
A
J
o A
Experimental Yrast and Yrare States
4-
$e I!.” oA A
;;3
.
0
8 ‘x 2 -
w
If,
laF0
0
200
,
XA
General States
A
Intermediate States Structure 1
,
Structure2 ,
0
600
8
0
V ,
929
1000
J(J+I) (ti2)
,
Structure3
,
1400
Figure 8. Experimental yrast and yrare states in lmHg observed in lhis work plotted in an E, - A x J ( J + 1) versus J ( J + 1) plane.
M1 transitions is irregular the band is characterized as irregular. This is the case for the two dipole bands reported in 19’Hg, the one dipole band reported in ‘96Hgand the minority of the dipole bands in Pb isotopes (mainly in the lighter isotopes). Based on this classification, we note that of the three structures in the upper region in ’93Hgonly structure 2 is regular. Structures 1 and 3 are irregular, as is the case for all dipole bands reported in the neighbouring Hg isotopes so far. However, in all three structures the energies of the quadrupole crossover transitions exhibit a gradual increase, as in the case of the bands of the lower region, and hence the two quadrupole sequences of each structure can be considered as two signature partners with small signature splitting. This is also the case in one of the two dipole bands in 19’Hg and in the dipole band in 196Hg.In the Pb isotopes the quadrupole transitions are scarcely observed because they are weaker and hence in the majority of cases no signature partners can be identified. (ii) The dynamic moments of inertia 3(’) are small. Typical values lie in the interval 1&30h’MeV-’. This is also the case for all three structures of the upper region in IBHg. This can be seen in figure 9 where the dynamic moment of inertia 30) = dZ/du % 4/A E,(E2) of the three structures is plotted against the rotational frequency of the nucleus. Indeed, it can be seen that the 3(’) momen& of inertia are generally small, varying between 10 and 30@MeVP over a wide range of frequencies. (iii) The B(Ml)/B(EZ) ratios are generally large. In the Pb isotopes the values reported vary in the interval 10-40 p2/(eb)2. In the Hg isotopes the B(MI)/B(E2) values are quenched. They vary between 2 and 6 fiZ/(eb)’, but can still be considered large compared with typical values of the lower decay scheme which are much smaller (e.g. less than 0.1 p’/(eb)* for the 27/2-, 31/2- and 35/2- levels of band ABF). This quenching is also the case in the values deduced from ow data for the three structures. These values are shown in figure 10 in the next section where they are compared with theoretical calculations. Indeed, it can be seen from this figure that the experimental B(Ml)IB(EZ) values lie in the interval 2-4 $/(eb)Z for all three structures. Based on these similarities we can suggest that the three structures in the upper part of figure 3 are a manifestation of a phenomenon similar to that producing the dipole bands in the neighbouring Pb and Hg isotopes.
930
N Fotiades et a1 60 .'
Structure 3
N v N
v
30
Structure 2
20 Figure 9. Experimental dynamic moments of inertia [3@)w 4/A$@2)1
structure 1
10 .
0.2
0.4
the signature pariners of structure I (open and filled circles). structure 2 (open and filled squares) and structure 3 (open and filled diamonds) of '=Hg, versus frequency for
0.6
fiw (MeV) Structure2
I
v
24
28
32
36
24
28
32
3t
Spin I (h) Flyre 10. Experimental B(MIVBCE2) values far some of the levels in strucNres 1 (squares). 2 (diamonds) and 3 (circles). Solid lines represent theoretical calculations for various configurations ( a b , e and d represent the Configurations nhspiup nhgpi13ph;& c3 vi:3l2, nh& @ vi:3l2, and rh;ph& @ vi&. respectively).
Trying to exploit these similarities we note that, in order to account for the large B(Ml)/B(EZ) ratios in all neighbouring dipole bands, high-K proton configurations have been proposed which can produce large B(M1) values and hence large B(Ml)/B(EZ) ratios. The high-K values in these configurations are produced by two quasiprotons from the kglz and/or i13/2 high-S? orbitals. The high spin of the levels is then reproduced by coupling these configurations to high-J neutron configurations. In the Hg nuclei the high-S? proton orbitals which lie above the Fermi level are the [505]9/2- and [514]7/2- arising from k9lz orbital at zero deformation and [606]13n+ arising from i13p orbital at zero deformation (see figure 11). Indeed, these orbitals can produce high-K combinations such as: z ( ~ ~ / z ~ I ~ / z ) K = a(kgl,)K=s. II
93 1
Highly excited A I = 1 structures in lP3Hg
Pmton ingle parhclelevels WoodsSaxon wienlial
- 0 1
-0.4
'
'
-0.3
I
-0.2
'
-0.1
0.0
Figure 11. Proton orbitals involved in be configurations of Hg isotopes. A Woods-Saxon potential has been used.
Of these combinations, the second is favoured by the theoretical calculations of B(Ml)/B(E2) ratios performed for various configurations for the '93Hg isotope. These calculations are described in the next section. Indeed, a configuration involving two hgp proton excitations is found to lie closer to the experimental data. However, it must be pointed out that the quenched values experimentally observed in the Hg isotopes, as compared with those in the Pb isotopes, can be theoretically reproduced by inclusion of two aligned proton holes from the hlllz orbital in the configurations (see next section). Theoretical calculations have also been performed for the dipole bands in '=Hg [19] and lPaHg [ZO] and resulted in similar conclusions. In the majority of instances where dipole bands have been observed in this atomicmass region the absolute excitation energy of these structures has been impossible to determine from the experimental data. However, in the present work, the data have been rich enough to allow the determination of the absolute excitation energy of all the structures by observation of an intermediate region of transitions. As mentioned in a previous section, this intermediate region exhibits similar features to the irregular sequences reported in the Hg isotopes [5, 211 which were interpreted as being of single-particle character. These features are summarized below. (i) The energy spacings are irregular, producing a complex level pattern. The decay between these states proceeds via competing dipole and quadrupole transitions, favouring sometimes one multipole and sometimes the other. This is also the case in the intermediate region in IP3Hg.
932
N Fotiades et a1
(ii) There is a level which gathers the decay out of the sequences, below which a fragmentation towards several rotational structures sets in. In 19'Hg a 41R- level gathers the bulk of the intensity of the irregular sequence in this isotope [21] and fragments into a large number of branches. A similar level (4651 keV) exists in the lower part of the intermediate region in '93Hg below structures 1 and 2 and fragments towards bands 2 and 8 via several paths. For structure 3 the results of this work and of [4, 51 show that the same role is played by the 47/2(+) level (5407 keV). (iii) In experiments with backed targets the lifetimes of the transitions of these sequences are longer than the stopping times in the backing. The upper limits in the B@2) values deduced from these lifetimes are much smaller than the typical values in the levels of the bands in the lower region as would be expected for transitions of single-particle character. In our experiment no backing has been used. However, the results of the experiment with backed mget in [5] report, that for y-rays above the 47/2(+) level at 5407 keV no Doppler broadened line shapes were observed, suggesting lifetimes longer than a few picoseconds. Based on the above features we propose that the intermediate region is one of singleparticle character. Furthermore, we note here that the configurations proposed for the 41/2level, above which the irregular sequence in I9'Hg has been built [21], are based on the coupling of two hlllz proton holes to the neutron excitations suggested for the lowest-lying negative parity 21D- state, which is the bandhead of band ABE in I9'Hg. We must point out here that such configurations drive a nucleus towards prolate deformation [21]. The situation is almost identical in 193Hg,urging us to propose the following possible configuration for the 41/2- level at 4651 keV (("(h'1/2)-2),=Lo
" (i:3/2P3/2),,,,,z),=j1/2.
The i:312 p 3 p three-neutron configuration which is present in the proposed configuration characterizes bands 2 and 8 (ABF and ABE) of '93Hg [2] and reproduces the angular momentum of the bandhead of these bands. An addition of lOIi angular momentum, gained by the alignment of the proton holes, reproduces the spin of the 41/2- level. Note that the h l l p proton holes used in this configuration differ from those used in the configurations of the upper region (see next section) because the intermediate and upper regions are characterized by different nuclear shapes (see next paragraph). For the 47/2(+) level at 5407 keV it is still uncertain to suggest a possible configuration since this deexcites simultaneously towards many rotational bands of different parity in the lower region. In TRS calculations performed for Hg isotopes the presence of h i g h 4 proton orbitals in the configurations proposed for the dipole bands drives the nucleus to a triaxial nearoblate, weakly collective shape (y = -75") [4, 201. On the other hand, the single-particle structures in the Hg isotopes have so far been connected to a shape change towards a prolate non-collective shape ( y = -120") [5, 211. Based on the similarities of the intermediate and upper regions of the 193Hgisotope to the single-particle structures and the dipole bands in the neighbouring isotopes, respectively (see discussion above), we can suggest similar shape changes for the newly observed regions in lg3Hg.Hence, we propose a shape change from collective oblate in the lower region towards non-collective prolate in the intermediate region and triaxial near-oblate in the upper region. These shape changes render the 193Hg isotope a 'y-soft' one. Such a softness to y-deformation in the Hg isotopes has been predicted in the theoretical calculations of 1241. Moreover, this scenario of successive shape changes is in accordance with the theoretical calculations reported in [25] for 194Hg,but valid also for all neighbouring Hg
Highly excited AI = 1 structures in 193Hg
933
isotopes. Indeed, these calculations predict a successive shape change from oblate collective ( y = -65”) towards prolate non-collective ( y = -120”) and hiaxial weakly collective ( y = -SO“), before the prolate ( y = 0’) superdeformed minimum becomes yrast.
4.3. B(MI)/B(E2) ratios
Assuming that all dipole and quadrupole transitions are MI and E2 and adopting zero mixing ratios for all A I = 1 transitions, B(MI)/B(EZ) ratios can be measured for the AI = 1 structures of ‘93Hg. For these levels for which such a measurement was possible the values were found to vary between 2 and 4 p’/(eb)’. Here, we mention the one order of magnitude difference of these values from those reported in the neighbouring Pb isotopes which vary from 20 to 40 p*/(eb)’. In order to answer the question whether this difference can be attributed to different intrinsic structures for these nuclei, theoretical calculations of the B(MI)/B(E2) ratios have been performed for various possible configurations in ‘%Hg. The model used here was that introduced by Donau and Frauendorf 12.61 in the form used in [27]. Both experimental points and theoretical curves are plotted in figure 1O(i) for structures 1 and 2 and in figure IO($ for structure 3. The theoretical calculations of the B(MI)/B(EZ) ratios have been canied out for four configurations. We chose these configurations based on the general conclusions of the previous discussion, i.e. we need high-K proton configurations coupled to high-J neutron orbitals. More specifically, we chose two deformation-aligned h9/2 and i13/2 proton orbitals coupled to aligned i l 3 p neutrons. Such configurations have also been used in the neighbouring I9’Hg [I91 and 196Hg[ZO] isotopes. In I9’Hg, four-neutron configurations have been successfully used in the interpretation of the dipole bands in this isotope [19]. It is therefore reasonable to use one more neutron in the corresponding configurations in 193Hg,Moreover, we need five neutrons in order to reproduce high angular momenta which characterize the states of the structures. Thus, two of the four configurations (those labelled as a and c in figure IO) used in the calculations are z h9/2
@v
z h&‘ @ v if3lz.
These configurations are characterized by high angular momenta (-2.56). In the calculations fully aligned nucleons have been assumed, i.e. we used K = 11 and K = 8 for the mixed a(hglzi13/2) and the pure ~ ( h &proton ~ ) configurations, respectively, and ix = 22.56 for the five neutrons. A quadrupole moment Q = -4.06 eb (value corresponding to an oblate p = 0.136 deformation) has been used. For the mixed proton configuration the gyromagnetic factor (g = 0.96) has been taken from experimental results [28],while for the pure proton configuration we used the value g = 0.78 [29]. For the collective gyromagnetic factor we assumed that gR = Z/A = 0.415. It can be seen from figure IO that the B(Ml)/B(EZ)values for the a and c configurations are rather large compared to the experimentally deduced values. In order to quench these values hllp proton holes can be used in the configurations. Such excitations have already been applied to the configurations of the dipole bands in I9’Hg 1191 and ‘%Hg I2.01. Thus, in two of the four configurations (those labelled as b and d in figure IO) two fully aligned h l l p proton holes (i,= 106) have been used instead of two i13p neutrons. The corresponding configurations are x h9/Z i13/z €3 x h& €3 U i&
h$’ @ z h;‘
@ v i:312.
934
N Fotiades et a1
The presence of proton holes in the configurations reduce the theoretical B(Ml)/B@2) values due to their positive g-factors compared with neutrons, thereby reproducing quenched values in the Hg isotopes with respect to that of the Pb isotopes. Moreover, these configurations produce a better approximation to the spin of the levels of the structures (-28h). The quenching in the case of configuration b is not enough to bring the values for this configuration close to the experimental points (see figure IO) while, in the case of configuration d the quenching reproduces the experimental values successfully. Hence, we propose the r h& @ r h;& @ v i:3/2 configuration as a possible interpretation of the three structures. This configuration is similar to the configurations used in the dipole bands of 19*Hg [I91 and 196Hg[ZO], but different from those used in the Pb nuclei [S-IS] which do not involve rotation-aligned proton holes. This configuration indicates that the AI = 1 structures have a different intrinsic structure in Hg isotopes from those in the Pb nuclei. Finally, we note that the contribution of p 3 / 2 neutrons in the case of structures 1 and 2 is possible (because these structures have a negative parity), although since this orbital adds little alignment it has been considered as negligible in the calculations. The use of h1ll2 proton holes could qualitatively explain the larger excitation energies of the dipole bands in the Hg isotopes than in the Pb isotopes. Indeed, the few cases where the excitation energies of such bands have been experimentally determined show larger excitation energies in the Hg isotopes [19, present work] than in the Pb isotopes [S. 15, 171. In Pb isotopes the high-K h9/2 and i13n proton orbitals are occupied from proton excitations originating from the SI/Z orbital (see figure 11). Since more energy is needed to excite protons from the hll,? orbital in the high-K orbitals than from the s1,z orbital, we expect the configurations involving h l l p proton holes in Hg to appear at high excitation energies. The difference of the B(Ml)/B(E2) ratios for the AI = 1 structures in Ig3Hg and Pb isotopes could be also at’uibuted to different deformation effects. However, TRS calculations performed for 19?Hg in [4] predict weakly-deformed oblate shapes similar to the ones reported in Pb nuclei. The similarity between the nuclear shapes of these isotopes can only account for a small fraction of the difference in the experimental B(Ml)/B(EZ) ratios. Hence, we conclude that the one order of magnitude difference in the B(Ml)/B@Z) ratios can only be accounted for by different intrinsic configurations. 5. Conclusions
In summary, we have extended the level scheme in L93Hgup to 10.7 MeV and enriched it with many new transitions. Two new high-spin AI = I structures were observed. They consist of competing dipole and quadrupole transitions and their total maximum contribution to the reaction channel is -26%. A third AI = 1 structure in the same isotope, reported previously [4, 51, has been investigated further. All these structures are characterized by high-K proton configurations coupled to high-J neutron and proton-hole configurations. Below all three structures an intermediate region of irregular energy spacings has been observed to connect these Structures to the lower region of the rotational bands. The intermediate region exhibits similar features to the single-particle structures observed in the neighbouring isotopes, suggesting that it is a region of non-collectivity. The configurations proposed for the AI = 1 structures are different from those suggested for the dipole bands in the neighbouring Pb isotopes, since they involve aligned h l l l Z proton holes. This indicates different inhinsic smctures in Hg and Pb nuclei, while the possibility of different deformations in these isotopes is minimized by TRS calculations. Comparison with similar situations in neighbouring isotopes and the theoretical
Highly excited AI = 1 structures in '93Hg
935
predictions for the shape change in the Hg isotopes lead us to propose a shape change from collective oblate in the lower region to non-collective prolate in the intermediate region and triaxal near-oblate in the region of the three structures. The successive shape changes proposed for this isotope render it one of the most 'y-soft' nuclei in this atomicmass region. In order to be entirely sure about this scenario, measurements of the lifetimes of the states proposed as collective or non-collective are needed. We hope that this will be the aim of an experiment in the near future. Acknowledgments We wish to thank the crew and technical staff at the, now defunct, NSF at Daresbuy for excellent collaboration. The EUROGAM project is supported jointly by SERC (UK) and IN2P3 (France). One of the authors (NF) acknowledges the receipt of an NCRS Demokritos postgraduate studentship and another five (MU,PJD. S E , PMJ and RMC) an SERC postgraduate studentship during the course of this work. The authors acknowledge support from the EU (contract number SC1-CT91-687). Finally, JS acknowledges support from the NATO collaborative research programme. References 111 Lieder R M, Beuscher H,Davidson W F, Neskakis A and Mayer-BWcke C 1975 Nucl. Pkys, A 248 317 [21 Hiibel H, Byme A P. Ogaza S,Stuchbery A E, Dracoulis G D and Gultormsen M 1986 Nucl. Phys. A 453 316 131 Cullen D M e! a1 1990 Phyi Rev.Lett. 65 1547 [41 Roy N, Becker J A, Henry E A, Brinkmm M 1,Stoyer M A. Cizewski I A. Diamond R M. Deleplanque M A, Stephens F S, Beausang C W, and Draper J E 1993 Phys. Rev. C 47 R930 [SI Deng J K, Ma W C, Hamilton J H. Garrett J D. Baldash C. Johnson N R, Lee I Y, McCowan F K, Pilotte S. Yu C H and Nazxewicz W 1993 Phys. Len. 319B 63 I61 Fotiades N er a1 1993 Pmc. In?. Conf on the Future of Nuclear Spectroscopy (Crete) (Athens: INP NCSR Demokritos) p 91 Joyce M 1 et nl 1993 Phys. Rev. Lett. 71 2176 Fant B, Tanner R I,Butler P A. James A N, Jones G D. Poynter R 1, White C A, Ying K L, Love D J G. Simpson J and Connell K A 1991 J. Phys. G: Nucl. Port. Phys. 17 319 Kuhnert A, Stoyer M A, Becker I A. Henry E 4 Brinkman M 1. Yates S W, W a g T F, Cizewski J A, Stephens F S, Deleplanque M A . Diamond R M, Macchiavelli A 0. Draper I E, Aviez F, Kelly W H and Korten W 1992 Phys. Rev. C 46 133 Wang T F, Henry E A, Becker J A, Kuhnert A, Stoyer M A, Yates S W, Brinkman M I,Cizewski I A, Macchiavelli A 0. Stephens F S,Deleplanque M A. Diamond R M, Draper J E, Azaiez F, Kelly W H, Korten W, Rubel E and Akovali Y A 1992 Phys. Rev. Len. 69 1737 Bddsiefen G, Hiibel H, Mehta D. 7hirumala Rao B V, Birkental U,Fr6hlinsdorf G, Neffgen M, Nenoff N, Pancholi S C. Sin& N, Schmitz W,Theine K, Wilkau P, Grawe H, Heese I. Kluge H, Maier K H, Schnmm M. Schubart R and Maier H I 1992 Phys. Left. 275B 252 Clark R M. WadswOnh R, Paul E S, Beausang C W, Ali I, Astier A, Cullen D M.Dagnall P I, Fallon P. Joyce M J, Meyer M, Redon N, Regan P H, Namrewiu Wand Wyss R 1992 Phys. Len 275B 247 Clark R M. Wadsworth R, Paul E S, Beausang C W. Ab I, Astier A, Cullen D M, Dagnafl P 1, Fallon P. Joyce M J, Meyer M. Redon N, Regan P H, SharpeySchafer I F, Nararewicr W md Wyss R I992 Z Phys. A 342 371 Baldsiefen G, Hiibel H, Azaiez F, Bourgeois C. Hojman D, Karichi A. Penin N and Sergolle H 1992 2 Phys. A 343 245 Baldsiefen G,Birkental U, Hiibel H. NenotT N, Thhmala R m B V,Willsau P, Heese 1, Kluge H, Maier K H, Schubm R and Frauendorf S 1993 Phys. Len. 298B 54 Dagnall P 1, Beausang C W, Fallon P, Fonyth P, Paul E S, Shqey-Schafer J F, Twin P J, Ali I, Cnllen D M, Joyce M I, Smith G, Wadsworth R, Clark R M, Regan P H, Astier A. Meyer M and Redon N 1993 3. Pkys. G:Nucl. Part. Phys. 19 465
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