Elastic Scattering of Intermediate-Energy Alpha Particles by Heavy Nuclei

Abstract

The deflected alpha-particle beam of the University of Washington 60-inch cyclotron has been used to study the elastic scattering of 13- to 42-Mev alpha particles by Ag, Ta, Pb, and Th at 60\ifmmode^\circ\else\textdegree\fi{}. The alphaparticle energy was decreased in steps of approximately 1 Mev by placing remotely controlled absorbers in the cyclotron beam. Elastically scattered alpha particles were selected and counted by a differential-range coincidence proportional-counter telescope.Up to a certain critical energy ${E}_{0}$, which increases with $Z$, the cross section for each element decreases with increasing alpha-particle energy in agreement with the Rutherford formula. At higher energies, the dependence is given by the empirical formula $\ensuremath{\sigma}(E)=\ensuremath{\sigma}({E}_{0})\mathrm{exp}{\ensuremath{-}K(E\ensuremath{-}{E}_{0})}$. The slope parameter $K$ is about 0.26 ${\mathrm{Mev}}^{\ensuremath{-}1}$ and decreases slightly as $Z$ increases. Interpretation of results in terms of a semiclassical strong absorption model leads to values of the sum of nuclear radius and alpha-particle radius best fitted by the empirical formula $D=(1.50{A}^{\frac{1}{3}}+1.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ cm.