A consistent study of precompound and compound-nucleus emission mechanisms in neutron-induced reactions

Abstract

The exciton model in its closed form and the Weisskopf-Ewing model have been consistently applied to neutron-induced reactions. The analysis has been performed for 12 nuclei in the range A=45-209. The primary particle-emission is considered to be governed by an admixture of precompound and compound-nucleus mechanisms, while all consecutive particles are considered to undergo pure compound-nucleus emission. The calculated primary-neutron spectra and excitation functions for (n,p), (n, 2n) and (n, 3n) reactions at E,-4-24MeV have been compared with experimental data. The average best-fit values of the exciton-model free parameter K have been found to be K = 700 MeV 3. Departures form this value appear to cancel out structure effects introduced into transition rates through the structure-sensitive single-particle level density.

Isotopic effects in the (n, p) reaction cross section on heavy targets and the statistical preequilibrium model

“…The value of K used in the preequilibrium calculations reflects itself in the value of the quantity REv(E) (see (8)), which describes the balance of preequilibrium and evaporation emission in the strong reaction channels. The dominant channels in fastneutron-induced reactions on heavier targets are the (n,n') and (n,2n) channels.…”

“…The value of the parameter K = 700 MeV 3 was suggested in our previous work [8]; it was obtained primarily from the analysis of (n, nl) spectra and (n,p) excitation functions (and used to calculate (n,2n) and (n, 3n) excitation functions) on a number of odd-Z-even-N targets. The calculations performed in this way reproduce the slope of the observed isotopic trend.…”

“…REv(E) is the fraction of the incoming flux which forms the compound nucleus and O-~v(e, fi; E) would be the cross section for the (e, fi) reaction if only the compound-nucleus formation and decays (assuming no precompound emissions) were present [8]. The quantity REv(E ) can be calculated (neglecting direct processes) from…”

“…The formalism was described in detail in our previous papers [, [8][9][10] where acs(C~, El) is the cross section for the formation of the composite system by an incoming particle e of energy E i, W~(n,E, efl is the emission rate, and T(n, E) is the time the system spends in an n-exciton state. The emission rate W~(n,E, sp) of particles fi with energy ~a and reduced mass #b from an n-exciton state (p, particles; h, holes) is given by…”

“…The transition rates Jr_+, calculated by using the golden rule and the density of accessible final states corrected for the Pauli principle [8,11], are proportional to the averaged squared matrix element [M] 2. This quantity is expected to behave approximately as ]MI z =K.A-3E -1 (6) where K is a free parameter.…”