Interaction of High-Energy Protons and Alpha Particles with Iodine-127

Abstract

Iodine was bombarded with protons ranging in energy from 0.25 to 6.2 Bev and with 0.25-, 0.50-, and 0.72-Bev alpha particles. Reactions of the type ($p, \mathrm{pxn}$), ($p, 2pxn$), ($p, p{\ensuremath{\pi}}^{+}$), ($p, p2{\ensuremath{\pi}}^{+}$), ($p, n{\ensuremath{\pi}}^{\ensuremath{-}}$), and ($\ensuremath{\alpha}, \ensuremath{\alpha}\mathrm{xn}$) to produce iodine, tellurium, antimony, and cesium isotopes were investigated. Upper limits in the range 0.01 to 0.1 mb were found for cross sections of reactions to produce ${\mathrm{Sb}}^{127}$ and ${\mathrm{Cs}}^{127}$. For ${\mathrm{Te}}^{127}$, upper limits in the range of 1 to 2 mb were found. This and other studies of reactions in which the product has the same mass number as that of the target are discussed in terms of the initial interaction. At all incident energies studied, the cross section for the formation of ${\mathrm{I}}^{126}$ via the ($p, \mathrm{pn}$) or ($\ensuremath{\alpha}, \ensuremath{\alpha}n$) reaction is significantly higher than that of the other ($p, \mathrm{pxn}$) or ($\ensuremath{\alpha}, \ensuremath{\alpha}\mathrm{xn}$) reactions, with the possible exception of ($\ensuremath{\alpha}, \ensuremath{\alpha}4n$). The ($p, \mathrm{pn}$) and probably the ($\ensuremath{\alpha}, \ensuremath{\alpha}n$) reactions appear to be due primarily to knock-on collisions with surface neutrons. The excitation functions for the production of iodine isotopes by proton bombardment decrease between 0.25 and 0.72 Bev but remain relatively constant for higher energies. The ($p, 2pxn$) reactions show a similar effect, but with the excitation functions becoming constant at about 2 Bev. These results are compared with Monte Carlo calculations of the proton-initiated nucleon cascade and of the subsequent evaporation of light particles.