Persistence of Odd-Even Effect on Nuclear Level Densities at High Excitation Energy

Abstract

Excitation functions and angular distributions for the reaction $^{54}\mathrm{Cr}(p, \ensuremath{\alpha})^{51}\mathrm{V}$ have been measured between 6.5 and 20.0 MeV for the ground state and the first two excited states in $^{51}\mathrm{V}$ at 0.32 and 0.92 MeV, with spins ${\frac{7}{2}}^{\ensuremath{-}}$, ${\frac{5}{2}}^{\ensuremath{-}}$, and ${\frac{3}{2}}^{\ensuremath{-}}$, respectively. From the angular distributions and the excitation functions, a compound nuclear mechanism is supported for the population of the ${\frac{5}{2}}^{\ensuremath{-}}$ and ${\frac{3}{2}}^{\ensuremath{-}}$ levels. At higher bombarding energies there is evidence that a simple pickup mechanism dominates for producing the ${\frac{7}{2}}^{\ensuremath{-}}$ level. From the fluctuation analysis of the excitation functions for the ${\frac{5}{2}}^{\ensuremath{-}}$ and ${\frac{3}{2}}^{\ensuremath{-}}$ levels, the decay width of the compound nucleus $^{55}\mathrm{Mn}$ was obtained as a function of energy. From the differential cross sections and decay widths, the level density of $^{55}\mathrm{Mn}$ was deduced at excitation energies between 15 and 23 MeV. The results were combined with low-energy level-density data previously obtained with a magnetic spectrograph. These results, when compared with similar data for even-even $^{56}\mathrm{Fe}$, show that the energy shift required to match level densities for odd and even nuclei is the same at low and high excitation energies. The energy dependence of the level density cannot be reproduced over the whole energy range with level-density expressions employing conventional odd-even corrections. The same conclusion is supported by statistical-model calculations of the excitation functions.