Absorption of -Rays Measured by their Photo-effect in Beryllium

Abstract

IT results from recent determinations of atomic masses, that in the photo-nuclear effect of radium C-rays in beryllium, only the hardest groups, namely, 2.2 × 10⁶ eV., 2.0 × 10⁶ eV. and 1.8 × 10⁶ eV., can be efficient. The absorption of these-rays measured by the number of neutrons produced in beryllium should therefore be very nearly exponential without any preliminary filtering. However, Gentner¹ has found that the absorption curve obtained by this method exhibits a flat maximum for small thicknesses of absorbing material (lead or aluminium) and then decreases, at first with a coefficient corresponding to 0.9 × 10⁶ eV; and afterwards more slowly. Gentner interprets these results by assuming that the observed maximum corresponds to the maximum intensity of the 0.9 × 10⁶ eV. rays developed by the Compton effect from the primary rays, and that this quantum energy represents twice the energy necessary for extracting a neutron from the beryllium nucleus.