Properties of (γ,n) Cross Sections

The mean energies of photons producing the reactions ${\mathrm{Cu}}^{63}(\gamma ,n){\mathrm{Cu}}^{62}$, ${\mathrm{Zn}}^{64}(\gamma ,n){\mathrm{Zn}}^{63}$, and ${\mathrm{C}}^{12}(\gamma ,n){\mathrm{C}}^{11}$ have been found by measuring their absorption coefficients for many values of $Z$. Monitor and detector were made of the element investigated. The absorbers were Be, C, Al, Ti, Fe, Ni, Cu, Zn, Se, Mo, and Sn. Less than 1 percent of the radiation striking the detector originated in the absorber. In this geometry, ($\frac{1}{Z}$) ×absorption cross section was a linear function of $Z$. The Compton cross section was separated from the pair cross section by considering its different $Z$-dependence. The mean energy was evaluated in two ways: (1) from the observed pair cross section and the Bethe-Heitler formula, and (2) from the observed Compton cross section and the Klein-Nishina formula. A correction was applied for the deviation of pair cross section from the value given by the Born approximation. With this correction, the values of the mean energy were found to be as follows: