Neutron-Capture Cross Sections by Capture-Gamma Counting

A method is described for determining neutron capture cross sections and locating capture resonances by capture gamma counting. Breit-Wigner parameters for the strong resonance in cadmium are determined to be $E_0=0.177\mathrm{}$ ev, ${\sigma }_{c0\mathrm{}}=7600\mathrm{}$ barns, $\Gamma =0.110\mathrm{}$ ev. Resonances have been found in cadmium at 19 and 28 ev with evidence of unresolved resonances at about 100 ev and above; ${\sigma }_{c0\mathrm{}}\Gamma \cong 3$ barn-ev for the 19-ev level, and ${\sigma }_{c0\mathrm{}}{\Gamma }^2\cong 9$ barn ${\mathrm{}\left(\mathrm{ev}\right)\mathrm{}}^2$ for the 28-ev level. Comparison transmission measurements on cadmium show the capture-gamma method to be quicker and more sensitive under some circumstances than transmission measurements for locating weak capture levels. Factors affecting the relative sensitivity of the two methods are discussed. A resonance was found in strontium at 3.58 ev; its strength (${\sigma }_{c0\mathrm{}}{\Gamma }^2$) is approximately 8 barn-${\mathrm{}\left(\mathrm{ev}\right)\mathrm{}}^2$. Resonances were found in barium at energies of 25, 93 and 380 ev, with strengths (${\sigma }_{c0\mathrm{}}{\Gamma }^2$) estimated to be 8, 90, and 550 barn ${\mathrm{}\left(\mathrm{ev}\right)\mathrm{}}^2$, respectively. Measurements on a thick silver sample show that the method can be used to measure $\frac{{\sigma }_c}{{\sigma }_t}$ through a resonance over a fairly large range of values of transmission and cross sections. The strength of the 5.13-ev resonance in silver is determined to be ${\sigma }_{c0\mathrm{}}{\Gamma }^2=345\mathrm{}\pm 30$ barn ${\mathrm{}\left(\mathrm{ev}\right)\mathrm{}}^2$.