Photographic Plate Evidence for the γp-Reaction in Be⁹

Abstract

THE reaction Be⁹ γp Li⁸ was first observed by Ogle, Brown and Conklin¹ in irradiations made with a 22-MeV. betatron employing counter techniques to observe the delayed β-particle emission from the lithium-8 and the α-particles from the beryllium-8. More recently, Baldwin² has reported that the relative cross-section shows a maximum at 32-MeV. γ-ray energy. Because lithium-8 is unstable, the reaction proceeds by three stages: and leads to the production of a three-particle star in photographic emulsions which are not electron-sensitive. Such stars have been observed in beryllium-loaded Ilford plates type C₂ and E₁ irradiated with 24-MeV. γ-rays from the synchrotron at this Establishment. Of the twenty-one events observed to date, that of highest energy is shown in the photomicrograph of Fig. 1, where the proton, recoiling lithium-8 nucleus and the two α-particles from the break-up of the beryllium-8^* nucleus are visible. The distribution in range of the protons emitted is shown in Fig. 2, from which it will be seen that in about half the cases the proton escapes from the emulsion. As there are insufficient events to enable escape corrections to be applied, results on the variation of cross-section with energy cannot be given at this stage. However, the mean value of the cross-section over the energy-range 18–24 MeV. can be calculated from the results and is (2.5 ± 1.0) × 10⁻²⁸ cm.². In deriving this value, the density of beryllium in the emulsion was taken as 0.0079 gm./c.c. (a figure supplied by Messrs. Ilford, Ltd.), the theoretical spectrum calculated by Heitler³ was assumed and γ-ray intensity measurements with a thick-walled ionization chamber were employed.