The technique of fluctuation averaging has been utilized with ($p,\alpha$) reactions to measure subkilovolt average widths $\Gamma$ of excited states for six medium-weight nuclei. The following compound-nucleus widths were determined: 386 ± 80, 465 ± 111, and 390 ± 100 eV at excitations of 15.24, 17.02, and 17.00 MeV, respectively, for the ${}_{}{}^{76}{}_{}{}^{}\mathrm{Se}$ compound nucleus; 114 ± 40, 150 ± 66, 206 ± 34, and 190 ± 65 eV at excitations of 17.36, 18.80, 18.90, and 20.85 MeV, respectively, for ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$; 111 ± 46 and 312 ± 85 eV at excitations of 15.50 and 17.72 MeV, respectively, for ${}_{}{}^{104}{}_{}{}^{}\mathrm{Pd}$; 36 ± 30 and 101 ± 30 eV at excitations of 15.06 and 16.27 MeV, respectively, for ${}_{}{}^{108}{}_{}{}^{}\mathrm{Cd}$; 128 ± 64 eV at an excitation of 15.98 MeV for ${}_{}{}^{110}{}_{}{}^{}\mathrm{Cd}$; and ${58}_{-58\mathrm{}}^{+120\mathrm{}}$ eV at an excitation of 19.29 MeV for ${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$. In all these measurements, excitation functions were measured over a span of several hundred keV in steps of 5- to 15-keV proton energy. The accurately known instrumental resolutions required by the method were primarily determined from the target thicknesses, which were about 10 keV. The required dominance of compound-nucleus reactions was obtained by the use of sufficiently low incident energies and by observations at backward angles. Excitation functions over a span of several MeV and angular distributions were measured in order to select regions of negligible direct reactions.

Comparison of these measurements with widths calculated from the statistical model of the nucleus suggests the need for reduced level densities. The measured widths show no evidence for shell effects.

• Received 17 August 1970

DOI:https://doi.org/10.1103/PhysRevC.3.807