decay of
97
Ag: Evidence for the Gamow-Teller resonance near
100
Sn
Z. Hu,
1
L. Batist,
2
J. Agramunt,
3
A. Algora,
3
B. A. Brown,
4
D. Cano-Ott,
3
R. Collatz,
1
A. Gadea,
3
M. Gierlik,
5
M. Go
´
rska,
1
H. Grawe,
1
M. Hellstro
¨
m,
1
Z. Janas,
5
M. Karny,
5
R. Kirchner,
1
F. Moroz,
2
A. Plochocki,
5
M. Rejmund,
1
E. Roeckl,
1
B. Rubio,
3
M. Shibata,
1
J. Szerypo,
5
J. L. Tain,
3
and V. Wittmann
2
1
Gesellschaft fu ¨r Schwerionenforschung, D-64291 Darmstadt, Germany
2
St. Petersburg Nuclear Physics Institute, RU-188-350 Gatchina, Russia
3
Instituto de Fı ´sica Corpuscular, Dr. Moliner 50, E-46100 Burjassot-Valencia, Spain
4
Michigan State University, East Lansing, Michigan 48824
5
Institute of Experimental Physics, University of Warsaw, PL-00681 Warsaw, Poland
Received 11 February 1999; published 23 July 1999
In two complementary measurements, a cubelike array of 6 Euroball-Cluster germanium detectors and a
total-absorption -spectrometer were used to investigate the decay of
97
Ag, a three proton-hole nucleus with
respect to the
100
Sn core. The half-life and Q
EC
value of the decay of the 9/2
+
ground state of
97
Ag were
determined to be 25.94 s and 6.9811 MeV, respectively. A total of 603 rays 578 new was observed, and
151 levels 132 new in
97
Pd have been identified. An interesting -delayed cascade was observed, which
comprises 6 -transitions with a deexcitation pattern involving an initial increase of the level spin. The
Gamow-Teller GT -decay strength distributions from the two measurements reveal a large GT resonance
around 4 MeV with a width of about 1.8 MeV. The hindrance factor for the total GT strength, summed from
the ground state up to 6 MeV excitation energy in
97
Pd, amounts to 4.36 with reference to a shell-model
prediction. This factor is discussed in comparison with a core polarization and a Monte Carlo shell-model
calculation. S0556-28139905208-5
PACS numbers: 23.40.-s, 27.60.+j
I. INTRODUCTION
Since the the identification of the doubly closed-shell
nucleus
100
Sn and some of its neighboring isotopes 1,2, the
study of these very neutron-deficient isotopes has attracted
considerable interest. The experimental progress in this field
includes, e.g., the measurement of the mass of
100
Sn and
100
In 3, the in-beam spectroscopy of
99
Cd 4 and
98
Cd 5,
the observation of proton radioactivity for
105
Sb 6 and
112
Cs 7, and the -decay studies of
94
Ag 8,
100-104
In
9–11, and
101
Sn 12.
In particular, the unique nuclear structure features of nu-
clei in the region below
100
Sn make decay interesting, as it
is characterized by a fast g
9/2
˜ g
7/2
Gamow-Teller GT
transition here only the nuclei situated in the ‘‘south-east’’
of
100
Sn are considered. Within the extreme single-particle
shell model, such transitions involve protons in the mostly
filled g
9/2
orbit, with the corresponding GT partner shell
g
7/2
being mostly empty. This model predicts the total GT
strength, summed over all the final states, to be
B GT =
N
9/2
10
1 -
N
7/2
8
B
0
GT , 1
where N
9/2
denotes the number of protons filling the g
9/2
orbit, N
7/2
the corresponding value for the g
7/2
orbit, and
B
0
(GT) =17.78 the B (GT) value of
100
Sn. However, the
B (GT) values obtained from experiments are significantly
smaller than those from theoretical predictions. This ‘‘hin-
drance’’ or ‘‘quenching’’ of GT transitions can be expressed
as the ratio between the theoretically and experimentally de-
termined GT strengths. For example, a GT hindrance factor
of the order of 4 has been found 13 for the N =50 even-
even nuclei
96
Pd and
98
Cd by comparing shell-model predic-
tions with the experimental B (GT) values for -decay.
In an attempt to explain the observed GT hindrance,
Towner 14 has considered the effects of pairing correla-
tions, core polarization and higher-order configuration mix-
ing. As we will discuss below, the core-polarization and
higher-order effects are both large and together can account
for most of the observed hindrance. However, one probably
needs correlations between the 0 g
9/2
and 0 g
7/2
orbitals which
go beyond the core-polarization model, such as those incor-
porated in the recent Monte Carlo shell-model calculations
15, in order to fully account for the observed hindrance.
The GT strength can be experimentally determined by
measuring -delayed particles and rays. The electromag-
netic radiation is normally measured with high-resolution
germanium detectors. However, in the cases of odd-even and
odd-odd nuclei with high Q
EC
values, it is expected that a
significant part of the total -decay strength is distributed
over many daughter states at large excitation energy, where
the level density is very high. Since the feeding to indi-
vidual levels is often very weak, and moreover the deex-
citation might proceed through several partly parallel cas-
cades, standard high resolution - spectroscopy is generally
insufficient to determine the complete GT-strength distribu-
tion due to its limited detection sensitivity.
Alternatively, the strength can be obtained from total-
absorption spectrometry by using 4 detectors. A highly ad-
vanced version of a total-absorption spectrometer TAS has
been installed at the mass separator on-line to the heavy-ion
accelerator UNILAC of GSI 16 for studying the decay of
nuclei around the doubly magic nucleus
100
Sn and around
the semimagic nucleus
146
Gd. This instrument consists of a
PHYSICAL REVIEW C, VOLUME 60, 024315
0556-2813/99/602/02431517/$15.00 ©1999 The American Physical Society 60 024315-1