Conversion coefficients and band assignments in
180
Ta
G. D. Dracoulis,
1
T. Kibe
´
di,
1
A. P. Byrne,
1,2
R. A. Bark,
1
and A. M. Baxter
2
1
Department of Nuclear Physics, RSPhysSE, Australian National University, Canberra ACT 0200, Australia
2
Department of Physics, Faculty of Science, Australian National University, Canberra ACT 0200, Australia
Received 24 April 2000; published 22 August 2000
The conversion coefficients for several bandhead decays in
180
Ta have been measured using pulsed-beam
techniques and the
176
Yb(
7
Li,3n )
180
Ta reaction. The spin and parity of the 520 keV intrinsic state is estab-
lished as 4
+
and several earlier assignments are confirmed. Two-quasiparticle configurations for the 520 and
592 keV states are discussed and following reanalysis of the band properties, a consistent interpretation is
reached. The 520 keV 4
+
state is associated with the favored coupling of the 1/2
-
521 9/2
-
514
configuration while the 592 keV (5
+
) state is identified with the 1/2
-
510 9/2
-
514 configuration.
PACS numbers: 21.10.Re, 23.20.Lv, 27.70.+q
The nucleus
180
Ta has been the subject of considerable
recent interest, driven partly by its status as a rare isotope,
the only one found naturally as an isomer, and considerations
with respect to its formation and survival in nucleosynthesis
which may be affected by its nuclear structure properties.
While not easily accessible by conventional reactions be-
cause of its proximity to stability, we have recently reported
a comprehensive level scheme 1 as has an NBI group 2.
These studies are largely in agreement but there remain a
number of spectroscopic uncertainties and some points of
disagreement. The present study focuses on conversion coef-
ficient measurements aimed at resolving some of these prob-
lems.
Medium-spin states in
180
Ta were populated using the
176
Yb(
7
Li,3n )
180
Ta reaction at 28 MeV, under essentially
the same conditions as the previous study in this laboratory
in which comprehensive - -time coincidence studies and
several reactions were used to establish an extensive level
scheme for
180
Ta 1. At this energy, which is below the
nominal Coulomb barrier, the main population is of states in
179
Ta (4 n channel,
180
Ta (3 n channel, and
177
Lu ( 2 n
channel. The beam from the ANU 14UD Pelletron accelera-
tor was pulsed, with approximately 1 ns pulses, separated by
963 ns, to allow separation of the otherwise complex spectra
by selection of appropriate time periods.
Electron measurements were carried out using the super-
conducting solenoidal spectrometer described by Kibe
´
di
et al. 3. In its lens mode of operation, electrons of a spe-
cific momentum range are transported to a cooled SiLi de-
tector, and the magnetic field is ramped between upper and
lower limits to maximize the efficiency for the energy range
of interest, in this case to encompass electrons corresponding
to conversion of several transitions in
180
Ta, from approxi-
mately 200 to 500 keV. While the spectrometer axis is at 90°
to the beam axis, the electron orbits which are selected cor-
respond to emission from angles near 70°. Gamma rays
were measured simultaneously in a Compton-suppressed Ge
detector placed at about -135° to the beam axis. All data
including the time of electrons and rays with respect to the
pulsed beam and a measure of the instantaneous magnetic
field were recorded in event-by-event mode. The data were
subsequently sorted to produce -time or electron-time ma-
trices for analysis.
Two measurements were made, the first with a 2.2
mg/cm
2
target of
176
Yb as in the previous study 1, the
second with a 0.9 mg/cm
2
target.
The initial focus of the measurements was the 409 keV
decay from a 53 ns isomeric state, which we hoped to sepa-
rate from possible short-lived and long-lived contaminants.
The upper panels of Fig. 1 show electrons and rays ob-
served in a region 8 ns around the beam pulse. The 458
keV ray is predominantly from the M 1 transition depopu-
lating the 5/2
+
402 intrinsic state in
177
Lu 4, which gives
rise to a strong K-electron line at an energy which would
translate because of the difference in binding energy to a
transition in tantalum at 462 keV. This falls close to the
L-conversion line of the 409 keV transition in
180
Ta. The
177
Lu line, however, also has a very long-lived component
because of feeding from the 150 s, 1/2
+
411 intrinsic
state; hence the contaminant can be reduced, as will be dem-
onstrated below. The prompt electron lines of relevance to
180
Ta are K lines of the 316 and 338 keV transitions whose
rays are known to be uncontaminated from the earlier -
experiments. Although relatively weak, they give rise to
clear K-electron lines. The 338 keV transition is from the
514 keV state discussed in 1 while the 316 keV transition
was placed in a nonyrast (1
-
) band as shown on the right of
Fig. 2 which reproduces a modified version of part III of the
180
Ta level scheme from 1, with changes in spin and parity
assignments implied by the new results, and with some er-
rors corrected.
1
The lower two panels of Fig. 1 show the corresponding
electron and -ray spectra obtained by gating in the 8–200
ns region and after subtraction of time-normalized spectra
from later decays. These constraints and manipulations re-
move the contaminations mentioned above and optimize the
intensity of the 409 keV transition from the decay of the 520
keV 53 ns intrinsic state, which is also fed by a 72.2 keV
transition from a 24 ns isomer at 592 keV, as shown in Fig.
2. The K, L, and M lines for the 409 keV transition are
identifiable. The 409 keV ray is clear and no other rays,
1
Several typographical errors and incorrect level and transition
energies pointed out by the compilers have been remedied.
PHYSICAL REVIEW C, VOLUME 62, 037301
0556-2813/2000/623/0373014/$15.00 ©2000 The American Physical Society 62 037301-1