Dependence on Atomic Number of the Nuclear Photoeffect at High Energies

Abstract

A measurement was made of the number of neutron-proton coincidences observed when 320-Mev bremsstrahlung bombarded D, Li, Be, C, O, Al, Ti, Cu, Sn, and Pb. If one normalizes the data for the number of neutron-proton pairs in a nucleus (i.e., by dividing by $\frac{\mathrm{NZ}}{A}$) it is found that the observed coincidences decrease as $A$ increases.It is possible to quantitatively account for this $A$ dependence by correcting for the probability that two nucleons will escape from inside a nucleus without either having a collision. The probability of escape is a function of the nuclear radius, $R$, and the mean free path, $\ensuremath{\lambda}$, in nuclear matter. For medium weight elements the observed neutron-proton pairs are produced with a cross section given by ${\ensuremath{\sigma}}_{Z,A}(\mathrm{coincidences})\ensuremath{\cong}3.0(\frac{\mathrm{NZ}}{A}){\ensuremath{\sigma}}_{\mathrm{D}}P(\frac{2R}{\ensuremath{\lambda}}),$ where ${\ensuremath{\sigma}}_{D}$ is the cross section for the photodisintegration of the deuteron and where $P(\frac{2R}{\ensuremath{\lambda}})$ is the probability-of-escape factor. For two nucleons emitted at 180\ifmmode^\circ\else\textdegree\fi{}, the form of $P(x)$ is $P(x)=(\frac{3}{{x}^{3}})[2\ensuremath{-}{e}^{\ensuremath{-}x}({x}^{2}+2x+2)]$The formula for the cross sections is shown to be what one would expect if the fundamental mechanism in complex nuclei is the same as that suggested by Wilson for the photodisintegration of the deuteron. The constant, 3.0, depends on the cube of a neutron-proton pair interaction distance. A less naive treatment also involves a nucleon pair correlation function.