The neutron spectra produced by the ${}_{}{}^{89}{}_{}{}^{}Y\mathrm{}(p,n)\mathrm{}{}_{}{}^{89}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}\mathrm{}(p,n)\mathrm{}{}_{}{}^{93}{}_{}{}^{}\mathrm{Mo}$, and ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}\mathrm{}(p,n)\mathrm{}{}_{}{}^{181}{}_{}{}^{}\mathrm{W}$ reactions have been measured at five angles between 15 and 135° 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 $\theta =0.70, 0.68, \mathrm{and} 0.54$ 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 $A=90\mathrm{}$ to about 1 MeV for $A\sim 180$.

• Received 19 April 1971

DOI:https://doi.org/10.1103/PhysRevC.4.607