The Neutron Absorption Limit in Cadmium

Abstract

In order to find the high energy absorption limit in Cd the absorption in boron of the transmitted neutrons was measured. The slow neutron absorption coefficient in boron is found to be 38 ${\mathrm{cm}}^2$/g in agreement with Amaldi and Fermi. The Cd penetrating neutrons are inhomogeneous in energy and therefore are not absorbed exponentially but according to the law $\frac{(1-e^{-x})}{x}$, where $x=\kappa \delta$, $\kappa$ is the absorption coefficient for neutrons just penetrating Cd and $\delta$ is the thickness of boron traversed. $\kappa$ is found to be 9.5 ${\mathrm{cm}}^2$/g B. A sheet of Cd 0.5 mm thick reduces the neutron counts to 8.4 percent of the total; 5.7 percent are absorbable in heavy layers of boron. Assuming the $\frac{1}{v}$ law for the absorption of neutrons in boron the absorption limit in Cd is found to be 0.41 ev. With the aid of data from the wheel experiment of Rasetti and collaborators the resonance energy and the half-width are calculated and are $E_r=0.18\mathrm{}$ ev and $\Gamma =0.15\mathrm{}$ ev, respectively. An estimate is made of the neutron width which turns out to be ${\Gamma }_n=1.3\mathrm{}\times {10}^{-3\mathrm{}}$ ev.