Neutron Reaction Cross Sections of Selenium and Iron at 14.4 MeV

Fast-neutron activation cross sections of selenium and iron have been measured with an improved technique that takes advantage of the high resolution of Ge(Li) detectors. Powdered samples, mixed with iron and/or aluminum powders to act as neutron-flux monitors, were irradiated with 14.4±0.3-MeV neutrons, and the decay of gamma spectra was followed with a thin Ge(Li) detector having a resolution of 4 keV (full width at half-maximum). The following cross sections (in millibarns) have been determined: ${\mathrm{Se}}^{74}(n, 2n){\mathrm{Se}}^{73g}$, 293±29; ${\mathrm{Se}}^{74}(n, 2n){\mathrm{Se}}^{73m}$, 65±15; ${\mathrm{Se}}^{74}(n, p){\mathrm{As}}^{74}$, 107±11; ${\mathrm{Se}}^{76}(n, 2n){\mathrm{Se}}^{75}$, 808±81; ${\mathrm{Se}}^{76}(n, p){\mathrm{As}}^{76}$, 56±6; ${\mathrm{Se}}^{78}(n, p){\mathrm{As}}^{78}$, 24±2.4; ${\mathrm{Se}}^{78}(n, \alpha ){\mathrm{Ge}}^{75(m+g)\mathrm{}}$, 6±1.0; ${\mathrm{Se}}^{80}(n, 2n){\mathrm{Se}}^{79m}$, 680±100; ${\mathrm{Se}}^{80}(n, \alpha ){\mathrm{Ge}}^{77(m+g)\mathrm{}}$, 2-9; ${\mathrm{Se}}^{82}(n, 2n){\mathrm{Se}}^{81m}$, 894±89; ${\mathrm{Se}}^{82}(n, 2n){\mathrm{Se}}^{81g}$, 225±45; ${\mathrm{Fe}}^{54}(n, p){\mathrm{Mn}}^{54}$, 259±26; and ${\mathrm{Fe}}^{54}(n, \alpha ){\mathrm{Cr}}^{51}$, 90±10. Systematic trends in these cross sections are discussed and compared with the available theoretical computations. The $2p_{\frac{3}{2},\frac{1}{2}}$ proton subshell-closure effect in the Ga-Ge-As region for ($n, \alpha$) cross sections suggested by Chatterjee is confirmed. For ${\mathrm{Se}}^{81}$ the spin cutoff parameter is calculated to be 2.9±0.4 from the isomer ratio $\frac{{\sigma }_m}{{\sigma }_g}=4.0\mathrm{}\pm 0.9$, and the ratio of the moment of inertia $I$ associated with the spin distribution to the rigid-body moment of inertia $I_R$ is obtained as $\frac{I}{I_R}\approx 1.0$.