Determination of Nuclear Level Densities at an Excitation Energy of 20 MeV

The level-density ratios in ${\mathrm{Fe}}^{56}$ and ${\mathrm{Ni}}^{60}$ at excitation energies of approximately 10 and 20 MeV have been evaluated by combining calculations based on the statistical theory of nuclear reactions and experimental measurements of level widths $\Gamma$ determined from studies of cross-section fluctuations. The importance of each of the various parameters in the statistical-theory calculation was studied as a function of the spin $J$ of the compound nucleus. Assuming a Fermi-gas model with shell and pairing-energy corrections, the evaluated level-density ratios give values of the level density parameter $a$ of $6._{-0.9\mathrm{}}^{}{}_{}{}^{+0.6\mathrm{}}$ and $6.7_{-1.3\mathrm{}}^{}{}_{}{}^{+0.8\mathrm{}}$ Me${\mathrm{V}}^{-1\mathrm{}}$ for ${\mathrm{Fe}}^{56}$ and ${\mathrm{Ni}}^{60}$, respectively. These values of $a$ are in excellent agreement with those made at excitation energies of less than 10 MeV. Hence, within the framework of the Fermi-gas model these results give independent evidence, in addition to the evidence already discussed in the literature, that the $\Gamma$ values determined from cross-section-fluctuation measurements are those characteristic of the compound nucleus.