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SPIN ASSIGNMENT OF NEUTRON RESONANCES
VIA (n, g) AND (n, a)
REACTIONS
M. Przytula
Several methods exist, and have been used in the past,
for determining the spins of neutron resonances. However the present
review is limited to those techniques which exploit the spin dependence
of (n, g) and (n, a)
spectra. These methods, which turned out to be very productive, are
essentially based on the systematics of the g
and a decay for the various multipolarities.
In the case of radiative capture, two methods, and the results achieved,
are described in detail: i) the multiplicity method; ii)
the low-lying level population method. Both techniques apply to those
compound nuclei for which radiative capture can be described by the
statistical model, namely most nuclei with atomic weight A>90-100,
with the possible exception of those having magic or near-magic proton
or neutron numbers. The first method has the merit of having provided
large amounts of spin assignments at times which were technologically
less advanced than the present ones: its main drawback however is the
fact of not being applicable to odd--even target nuclei. The second
method is not subject to this limitation while presenting at the same
time spin effects much larger than those of the multiplicity method.
The obtained results have been used to provide estimates of the spin
cut-off parameter s and to study the possible
spin dependence of the s-wave neutron strength function. Spin
assignments of p-wave resonances have helped in the analysis
of the parity nonconservation measurements performed by the TRIPLE thus
improving the estimates of the weak spreading width Gw.
In this context it has been found that the population of the low-lying
states depends significantly not only on the parity of the initial state.
Finally, spin assignments based on (n, a)
reaction are described for the target nuclei 143Nd and 147Sm.
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