Identification of the K
8
rotational band in
138
Gd
D. M. Cullen, N. Amzal, A. J. Boston, P. A. Butler, A. Keenan, E. S. Paul, and H. C. Scraggs
Oliver Lodge Laboratory, Department of Physics, University of Liverpool, Liverpool L69 7ZE, United Kingdom
A. M. Bruce
Department of Mechanical Engineering, University of Brighton, Brighton, BN2 4GJ, United Kingdom
C. M. Parry
Department of Physics, University of York, Heslington, York, YO1 5DD, United Kingdom
J. F. C. Cocks, K. Helariutta, P. M. Jones, R. Julin, S. Juutinen, H. Kankaanpa
¨
a
¨
, H. Kettunen, P. Kuusiniemi, M. Leino,
M. Muikku, and A. Savelius
Department of Physics, University of Jyva ¨skyla ¨a ¨, P.O. Box 35, FIN-40351, Jyva ¨skyla ¨ , Finland
Received 27 April 1998
A K
=8
-
collective rotational band has been established upon the 6 s isomeric state in the very neutron-
deficient nucleus
138
Gd. The band was observed using a technique involving the correlation of -ray transi-
tions across the isomeric state. The single-particle configuration of the isomer has been deduced from the
I =2 to I =1 intensity branching ratios. In addition, a series of other -ray transitions were observed which
are reasoned to be part of a higher-lying four quasiparticle structure which decays through the K
=8
-
isomeric state. The properties of the K
=8
-
band are discussed in relation to the systematic properties of
K
=8
-
bands in the neighboring N =74 isotones. S0556-28139806608-4
PACS numbers: 21.10.Re, 23.20.Lv, 27.60.+j
I. INTRODUCTION
High-K multiquasiparticle isomers are a general feature of
the midshell nuclei in the mass 170–180 region of the
nuclear chart 1. These K isomers exist because of the large
number of high- orbitals around the Z =72– 74 proton
Fermi surface ( K is the projection of the total intrinsic an-
gular momentum onto the nuclear symmetry axis of a de-
formed nucleus. As a consequence of the charge indepen-
dence of the nuclear force, similar high-K states are known
to exist in the mass 130–140 region which are based upon
the N =72– 74 neutron orbits. For example, K
=8
-
iso-
meric states are known in all of the even-even N =74 iso-
tones,
54
128
Xe 2,
56
130
Ba 3,
58
132
Ce 4,
60
134
Nd 5,
62
136
Sm 6,
and
64
138
Gd 7 with half-lives ranging from nanoseconds Xe
to milliseconds Ba,Ce. The half-lives of these states are a
result of the K -selection rule which implies that -ray tran-
sitions involving large changes in K are hindered 8. Al-
though the decay mode of these mass 130–140 isomeric
K
=8
-
states generally differs, in
56
130
Ba,
60
134
Nd,
62
136
Sm,
and
64
138
Gd it proceeds via an E 1 transition to the I
=8
+
member of the yrast band. However, despite this similar de-
cay mode the B ( E 1) transition strengths are observed to
vary by a factor of 40. Reasons for this variation have been
suggested to be due to either a change in the underlying
structure of the isomeric state itself or that of the yrast 8
+
state to which the isomer decays 7.
The observation of isomeric states, without the observa-
tion of the associated rotational band, is generally related to
the conditions under which an experiment is performed. For
example, in a previous study of
138
Gd 7 a K
=(8
-
) iso-
mer with half-life 6 1 s was observed. However, because
that experiment was only concerned with detecting delayed
-ray transitions, which were emitted by isomeric states, no
prompt -ray transitions were collected, i.e., the experiment
was insensitive to the prompt members of the K
=(8
-
)
rotational band built upon the isomeric state. The importance
of establishing the K
=8
-
rotational band is that it allows
the underlying single-particle configuration of the isomer to
be confirmed through the I =2 to I =1 -ray intensity
branching ratios. As a consequence, the single-particle con-
figuration of the isomeric state in Ref. 7 was not unambigu-
ously deduced. In another study of
138
Gd 9, only prompt
data were recorded and even though it might have been pos-
sible to observe the transitions in the rotational band it would
not have been possible to link the band into the known level
scheme.
This paper describes the result of an experiment which
populated high-spin states in
138
Gd in order to establish the
K
=8
-
rotational band.
138
Gd is the most neutron-deficient
N =74 nucleus which can realistically be studied using a
stable beam and stable target combination with a fusion-
evaporation reaction 9. In this experiment, a K
=8
-
rota-
tional band was established in
138
Gd by using a technique of
correlating prompt and delayed -ray coincidences across
the 6 s isomeric state. From the intensity branching ratios
of the prompt transitions in the K
=8
-
rotational band,
| ( g
K
-g
R
)/ Q
0
| values have been extracted which confirm the
underlying single-particle configuration of the isomer. In ad-
dition, a series of higher-lying transitions was observed
which is reasoned, on the basis of the systematics of similar
bands in this region, to be associated with a four-
quasiparticle structure. These additional transitions also de-
cay through the K
=8
-
two-quasiparticle isomeric state in
accordance with the K -selection rule 8. This measurement
PHYSICAL REVIEW C AUGUST 1998 VOLUME 58, NUMBER 2
PRC 58 0556-2813/98/582/8465/$15.00 846 © 1998 The American Physical Society