Nuclear Cross Sections for 1.4-Bev Neutrons

Abstract

Transmission measurements in good and poor geometry have been performed at the Brookhaven Cosmotron to measure the total and absorption cross sections of several nuclei for neutrons in the Bev energy range. The neutrons are produced by bombarding a Be target with 2.2-Bev protons. The neutron detector requires the incident particle to pass an anticoincidence counter and produce in an aluminum radiator a charged particle that will traverse a fourfold scintillation telescope containing 6 in. of lead. Contribution of neutrons below 800 Mev are believed small. The angular distribution of neutrons from the target is sharply peaked forward with a half-width of 6\ifmmode^\circ\else\textdegree\fi{}.The integral angular distributions of diffraction scattered neutrons from C, Cu, and Pb are measured by varying the detector geometry. The angular half-width of these distributions indicates a mean effective neutron energy of 1.4\ifmmode\pm\else\textpm\fi{}0.2 Bev.The total cross sections ${\ensuremath{\sigma}}_{\mathrm{H}}$ and ${\ensuremath{\sigma}}_{\mathrm{D}}\ensuremath{-}{\ensuremath{\sigma}}_{\mathrm{H}}$ are measured by attenuation differences in good geometry of C${\mathrm{H}}_{2}$-C and ${\mathrm{D}}_{2}$O-${\mathrm{H}}_{2}$O, with the result: ${\ensuremath{\sigma}}_{\mathrm{H}}=42.4\ifmmode\pm\else\textpm\fi{}1.8$ mb, ${\ensuremath{\sigma}}_{\mathrm{D}}\ensuremath{-}{\ensuremath{\sigma}}_{\mathrm{H}}=42.2\ifmmode\pm\else\textpm\fi{}1.8$ mb.The cross sections of eight elements from Be to U are measured in good and poor geometry, and the following values of the total and absorption cross sections are deduced (in units of millibrans): Experimental errors are about 3 percent in ${\ensuremath{\sigma}}_{\mathrm{total}}$ and 5 percent in ${\ensuremath{\sigma}}_{\mathrm{absorption}}$.An interpretation of these cross sections is given in terms of optical model parameters for two extreme nuclear density distributions: uniform (radius $R$) and Gaussian [$\ensuremath{\rho}={\ensuremath{\rho}}_{0}\mathrm{exp}\ensuremath{-}{(\frac{r}{a})}^{2}$]. The absorption cross-section data are well fitted with $R=1.28{A}^{\frac{1}{3}}$ or $a=0.32+0.62{A}^{\frac{1}{3}}$ in units of ${10}^{\ensuremath{-}13}$ cm. A nuclear density distribution intermediate between uniform and Gaussian will make the present results consistent with the recent electromagnetic radii.