High-spin states in doubly odd
176
Re and signature inversion in h
9/2
i
13/2
structures
M. A. Cardona,
1,2
A. J. Kreiner,
1,2,4
D. Hojman,
1,2,4
G. Levinton,
1
M. E. Debray,
1,2
M. Davidson,
3,4
J. Davidson,
3,4
R. Pirchio,
3
H. Somacal,
1,2
D. R. Napoli,
5
D. Bazzacco,
6
N. Blasi,
7
R. Burch,
5
D. De Acun
˜
a,
5
S. M. Lenzi,
6
G. Lo Bianco,
7
J. Rico,
5
and C. Rossi Alvarez
6
1
Departamento de Fı ´sica, Comisio ´n Nacional de Energı ´a Ato ´mica, 1429 Buenos Aires, Argentina
2
Escuela de Ciencia y Tecnologı ´a, Universidad de San Martı ´n, 1650 San Martı ´n, Argentina
3
Departamento de Fı ´sica, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina
4
CONICET, 1033 Buenos Aires, Argentina
5
INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy
6
Dipartimento di Fisica, Sezione di Padova, Padova, Italy
7
Dipartimento di Fisica and INFN, Sezione di Milano, Milano, Italy
Received 17 August 1998
High-spin states in doubly odd
176
Re were investigated by means of in-beam -ray spectroscopy techniques
using the multidetector array GASP. Excited states of
176
Re were populated using the
165
Ho(
16
O,5n ) reaction
at a beam energy of 101 MeV. Seven rotational bands have been observed and their configurations have been
discussed. Alignments, band crossing frequencies, and electromagnetic properties have been analyzed in the
framework of the cranking model. Signature inversion phenomena in the h
11/2
i
13/2
and h
9/2
i
13/2
structures are discussed. In the latter case signature inversion is traced to a large repulsive matrix element of
the p -n force acting in the maximally aligned J =11 state. S0556-28139904702-0
PACS numbers: 21.10.Re, 21.60.Ev, 23.20.Lv, 27.70.+q
I. INTRODUCTION
The study of doubly odd nuclei has provided, for about
two decades now 1, a fruitful ground for the discovery and
discussion of a number of interesting nuclear structure phe-
nomena. One recurrent theme 1–3 has been the attempt to
establish a general classification scheme for the coupling
modes of two nonidentical valence nucleons, leading to
semidecoupled 1,4, doubly decoupled 4,5, and com-
pressed structures 2. A particularly interesting phenomenon
discovered along the way was that of signature inversion
6,7. The present study of the doubly odd nucleus
176
Re is
framed into the above context. It has been reexamined using
the GASP detector array at the Legnaro Tandem Facility
yielding a wealth of rotational structures where the above-
mentioned ideas can be tested. In particular a signature in-
version is found at high spins in the h
9/2
i
13/2
semide-
coupled structure, a feature recently discovered in
162,164
Tm
and
174
Ta 8,9 and its origin discussed in terms of the ex-
perimental proton-neutron force present in
208
Bi 10.
A partial study concerning the doubly decoupled band in
176
Re has already been published 11 and preliminary re-
sults of the level scheme have been reported earlier 12.
II. EXPERIMENTS AND RESULTS
A. Measurements
High-spin states of
176
Re were populated through the
165
Ho(
16
O,5n ) reaction at 101 MeV. The target consisted of
a2 mg/cm
2
Ho rolled foil, backed with a 1.5 mg/cm
2
evaporated Bi layer. The beam was provided by the Tandem
XTU accelerator of Legnaro and -rays emitted by the
evaporation residues were detected using the GASP array
13, which consisted for this coincidence experiment of 39
Compton suppressed large volume Ge detectors, a planar de-
tector and a multiplicity filter of 80 bismuth germanate
BGO elements, providing the sum-energy and -ray mul-
tiplicity used to select the different reaction channels. Events
were collected when at least three suppressed Ge and three
inner multiplicity filter detectors were fired. With this condi-
tion a total of 10
9
events were recorded. The data corre-
sponding to Ge energies ( E
) were sorted into fully symme-
trized matrices and cubes. The large number of triple
coincidences offered the possibility to generate matrices
gated by transitions of the different bands of
176
Re and to
obtain very clean double-gated spectra. A nonsymmetrized
matrix of the planar against all the large volume Ge detectors
was very useful to examine the low-energy region of the
spectra with the high-energy resolution of the planar detec-
tor. Known bands in
176
Re 14 were extended up to
28–29 , and the assignment of new bands to
176
Re was
based on coincidence data, multiplicity distributions, coinci-
dences with Re K x rays and the knowledge of the neighbor-
ing Re isotopes. The most intensely populated nuclei were
175,176,177
Re 15–17,
175,176
W 15,16, and
173
Ta 18 corre-
sponding to the 6 n , 5 n , 4 n , 5 np , 4 np , and 3 n chan-
nels, respectively. In the off-line analysis of the data the 5 n
channel leading to
176
Re was enhanced by setting a proper
gate on the multiplicity spectrum of the BGO ball. A matrix
of -ray energy in the Ge detectors vs BGO multiplicity was
used to find assignments of transitions to different reaction
products. Figure 1 shows the BGO multiplicity spectra ob-
tained by setting gates on pure transitions belonging to
177
Re,
176
Re, and
175
Re. In the spectra the mean value of the
multiplicity distribution is indicated, the variation of the av-
erage multiplicity with the number of evaporated neutrons is
apparent. According to this analysis and to the coincidence
with Re K x rays, the 320.3 and the 504.1 keV transitions
were assigned to
176
Re Figs. 1c and 1d. The Ge times
( t ) were measured with respect to the fast signal provided
by the multiplicity filter. For each Ge detector a matrix E
PHYSICAL REVIEW C MARCH 1999 VOLUME 59, NUMBER 3
PRC 59 0556-2813/99/593/129818/$15.00 1298 ©1999 The American Physical Society