Crossings in alternating-parity bands of neutron-rich Ba nuclei Page: 4 of 9
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were analysed since y-y coincidences provide higher statistics, allowing the study of weak
transitions. Two, two-dimensional spectra (matrices) were created. Along one axis of each
matrix were sorted energies of y rays emitted in any direction. Along the other axis were
sorted energies of y rays emitted at t900 (first matrix) or at N180 (second matrix) with
respect to the direction of the first y ray. The DCOR for a y1 transition in a Y1-y2 cascade
was defined as the ratio of coincidence rates N(-yl,ry2) obtained from the first and the second
matrix, respectively, by gating on the same y2 line. DCOR values for transitions in 144Ba
and 146Ba nuclei are listed in Table 1 (DCOR listed in Table 1 are associated with y1 of
unknown multipolarity, y2 being a stretched quadrupole transition). These values should
be compared against theoretical predictions for the EUROGAM II array: for a stretched
quadrupole transition DCOR=0.89 independent of the spin value of the initial level; for
a stretched, unmixed dipole transition DCOR=1.09 independent of the spin value of the
initial level; for a non-stretched, unmixed dipole transition DCOR=0.81 for a 6 -+ 6 -+ 4
cascade and DCOR=0.82 for a 8 -+ 8 -+ 6 cascade and for a stretched, unmixed dipole
transition in a 3 -+ 4 -+ 2 cascade DCOR=1.19.
To find parities of levels, directional-polarization correlations were measured with the
help of the clover detectors, acting as Compton polarimeters [10]. In the EUROGAM II
array, the clover detectors were arranged in two parallel rings and the radiation source was
placed between the centre of the two rings. For analysis, -y-y coincidences were selected
with one y ray (y1) scattered inside a clover detector and the other (72) observed in a
conventional Ge detector at approximately 90* to the rings of clover detectors. Two y---y
matrices (PER and PAR) were created out of these data. Along one axis of both matrices
were sorted 72 energies. Along the other axis were sorted y1 energies resulting from the
add-back of signals detected in a clover detector. These energies were accepted only when
two elements of a clover detector fired, which indicated Compton scattering either in a
direction perpendicular (event sorted into the PER matrix) or parallel (event sorted into
the PAR matrix) to the direction of 72. The linear polarization Pexp(y), of a y transition
with energy E, was measured using the formula:
Pe~pey) - 1 a(Ey)N(y) - NI (E7) (1)
Q(Ey) a(Ey)Ns(y) + N(y)
where N (-) and N11 (y) are numbers of counts in the y line of an energy E., found in the
gated spectra obtained from the PER and PAR matrices, respectively, by gating on the
same, stretched quadrupole y2 transition. Q(Ey) is the sensitivity of the clover polarimeter
taken from ref.[10] and a(Ey), called the geometrical calibration, is a correction factor for
instrumental asymmetry. The geometrical calibration was determined by measuring a
ratio a(Ey) = N (Ey)/N1(Ey) using unpolarized radiation. Two -y--y matrices were sorted,
where y1 was an add-back signal and y2 was observed by any of EUROGAM detectors.
The isotropic distribution of y2 ensured that y1 detected by the clovers was unpolarized.
The geometrical calibration found in this work was a(Ey)=0.980(5), independent of Ey.
Linear polarization values for y transitions in 144Ba and 146Ba nuclei obtained in this
work are listed in Table 1. They can be tested against theoretical values for directional-
polarization correlations Pth calculated for the EUROGAM II array (upper[lower] sign
applies for electric[magnetic] transition): for a stretched quadrupole transition Pth= 0.14
independent of spin value of the initial level; for a stretched unmixed dipole transition4
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Urban, W.; Jones, M.A. & Durell, J.L. Crossings in alternating-parity bands of neutron-rich Ba nuclei, report, July 22, 1995; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc691559/m1/4/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.