INDC(IND)-35G - IAEA Nuclear Data Services

-30-
Role of Pre-equilibrium Emission on (n.xn) Cross-Sections
R.P. Anand, M.L. Jhingan, S.K. Gupta and
M.K. Mebta
Nuclear Physics Division
Bhabha Atomic Research Centre, Bombay 400 085•
Introduction
The knowledge of (n,2n) and (n,3n) cross sections is quite essential
in the reactor technology. Recently hybrid fusion-fission reactors
have gained considerable importance* The energy of the neutrons from
the fusion reaction D-T is about 14 MeV, above the threshold of (n,2n)
and (n,5n) reactions in most of the reactor materials. These cross
sections are needed in shielding and breeding calculat ions also.
Many of .the nuclides produced in the reactor have short half lives
and it is not possible to measure their cross sections directly. Also
it is interesting to see the role played by the preequilibrium
emission in the case of (n,xn) reactions. Here a simple method to
calculate (n.xn) cross sections has been developed. In our earlier
calculations ' ' the preequilibrium mode of decay was not taken
explicitly but all nonequilibrium effects were taken care by an
empitical factor obtained by Kondaiah 5) t>y -\^& analysis of a large
number of (n,2n) cross sections measured at H MeV. This empirical
factor is valid only around 14 MeV and does not hold good at higher
energy. As a result in our earlier calculations upto about 16 MeV
there was agreement with the measured cross sections but at higher
energy systematically calculated (n,2n) cross sections were lower-,
than measured ones and the reverse was true in case of (n,3n). In the
present work the prsequilibrium mode of decay along with the
equilibrium mode is considered for the first particle emission.
Subsequent emissions are considered to be due to the equilibrium
mode only. In the preequilibrium decay both proton arri. neutron channels
are considered while in the equilibrium decay proton channel is
ignored as in this case proton is likely to have low energy insufficient
to cross the Coulomb barrier. Gamma deexcitation competes with neutron
emission near the threshold as the neutron having low energy is
angular momentum forbidden and further the level density is low at
that energy. This effect is indirectly compensated by using the level
density parameters of Pearlstein*/ which are lower by about a factor
of 2.7 as compared to those of Gilbert and Cameron!?).
Method of Calculation
Cross sections for (n,xn) reactions are calculated on the following
assumptions:
(1) Neutrons are emitted in a statistical manner from the composite
nucleus formed after the capture of incident neutron.