Interaction of High-Energy Protons and Helium Ions with Niobium

Production cross sections were measured radiochemically for isotopes of niobium, zirconium, copper, nickel, and sodium produced in niobium targets bombarded with 240, 320, 500, and 720 MeV protons and with 320, 500, 720, and 880 MeV helium ions. For the proton bombardments these cross sections were also calculated by the Monte Carlo method with an electronic computer by use of the conventional two-step model of high-energy reactions. Interpolated results of a previous calculation by Metropolis and co-workers were used to simulate the effect of the initial high-energy cascade. These results were used in turn as input data for an evaporation calculation. A comparison of the theoretical yields of final products with the observed yields indicates that the theory accounts fairly well for low-deposition-energy products (niobium and zirconium isotopes), and quite well for high-deposition-energy products (copper and nickel isotopes). The theory fails completely to account for the yields of sodium isotopes, whose production must be ascribed to fragmentation, as noted previously by others. No Monte Carlo calculations were made for helium-ion-induced reactions. However, a comparison of yields of products of helium-ion- and proton-induced reactions shows a remarkable similarity at all energies and for all products. The main difference is a greater yield by a factor of two in the case of helium-ion bombardments. The implications of this for the mechanism of fragmentation are discussed. During the course of this work a 15-min positron emitter was discovered and identified as Nb⁸⁸.