Equilibrium and Nonequilibrium Contributions to the (p,n) Cross Sections of Y, Nb, and Ta

Abstract

The neutron spectra produced by the $^{89}Y(p,n)^{89}\mathrm{Zr}$, $^{93}\mathrm{Nb}(p,n)^{93}\mathrm{Mo}$, and $^{181}\mathrm{Ta}(p,n)^{181}\mathrm{W}$ reactions have been measured at five angles between 15 and 135\ifmmode^\circ\else\textdegree\fi{} for proton energies between 7.8 and 14.8 MeV. It is concluded that the data at low bombarding energies are consistent with a compound-nuclear-reaction mechanism. A constant-temperature level density for energies below the neutron binding energy is found to be appropriate for the residual nuclei $^{89}\mathrm{Zr}$, $^{93}\mathrm{Mo}$, and $^{181}\mathrm{W}$, with nuclear temperatures $\ensuremath{\theta}=0.70, 0.68, \mathrm{and} 0.54$ MeV, respectively.For bombarding energies above 10 MeV contributions from noncompound processes appear and at 14.8 MeV these are comparable in magnitude to those produced by compound-nuclear ($p,n$) reactions. On the basis of the angular distributions and the bombarding-energy dependence the relative fraction of the noncompound cross section due to direct and to pre-equilibrium reaction mechanisms is estimated.The extracted average doorway-state width varies from 350 keV near $A=90$ to about 1 MeV for $A\ensuremath{\sim}180$.