Photo-Disintegration of Silver and Aluminum

Abstract

The energy and angular distributions of protons emitted from Ag and Al nuclei when irradiated with the x-rays from a 22-Mev betatron were determined with the use of photographic nuclear emulsions. A maximum x-ray energy of 20.8 Mev was used with Ag and energies of 20.8, 17.1, and 13.9 Mev with Al.With silver the number of neutrons emitted in the same irradiation was determined from the ($\ensuremath{\gamma},n$) induced activity. The ratio of numbers of protons in each Mev interval to the total number of neutrons is compared to the same ratio calculated on the basis of various assumptions as to level density and nuclear reaction cross sections. The comparison suggests that the observed proton spectrum consists of two overlapping components. One is a lower energy group in rough quantitative agreement as to spectrum shape and numbers with calculations from reasonable assumptions as to statistical-model level densities and other nuclear parameters. The second group is a high energy tail (10-14 Mev) of protons definitely outside of the spectrum expected from the statistical model. Moreover, only the high energy group shows angular asymmetry with a preference for emission at 90\ifmmode^\circ\else\textdegree\fi{} to the x-ray beam, which supports the hypothesis that these protons are emitted before the excitation energy is statistically distributed in the nucleus.The ratio of total numbers of protons to neutrons emitted by Ag under 20.8-Mev bremsstrahlung radiation is 0.023\ifmmode\pm\else\textpm\fi{}0.008.With Al protons are emitted with spherical symmetry and with a yield, for quanta above about 14 Mev, as large or larger than the ($\ensuremath{\gamma},n$) yield determined by Hirzel and W\"affler at 17.6 Mev. The shape of the spectrum and the ratio of ($\ensuremath{\gamma},p$) to ($\ensuremath{\gamma},n$) cross sections are compatible with the assumption of a constant or slowly increasing level density in the residual nucleus, as expected for a light nucleus in the energy range involved. From the maximum energy of emitted protons the proton threshold was determined to be 8.6\ifmmode\pm\else\textpm\fi{}0.5 Mev.For use with the theoretical calculation of the proton spectra, the ($\ensuremath{\gamma},n$) cross sections as a function of x-ray quantum energy were determined for ${\mathrm{Ag}}^{107,109}$, ${\mathrm{Al}}^{27}$, and also for ${\mathrm{Cu}}^{63}$. The cross section of ${\mathrm{Ag}}^{109}$ was found to have a maximum of 0.32\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ at 16.5 Mev, ${\mathrm{Cu}}^{63}$ a maximum of 0.10\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$ at 17.5 Mev. For ${\mathrm{Al}}^{27}$ the cross section is still rising at 22 Mev.