CROSS SECTIONS OF (p, xn) REACTIONS IN THE ISOTOPE OF LEAD AND BISMUTH

Abstract

Measurements have been made by the activation method of cross sections of (p, xn) reactions in Bi²⁰⁹, Pb²⁰⁶, Pb²⁰⁷, and Pb²⁰⁸. The present results cover x = 3 to 7 in Bi²⁰⁹, 2 to 6 in Pb²⁰⁶, 2 to 4 in Pb²⁰⁷, and 3 and 4 in Pb²⁰⁸, over a total proton energy range from 12 to 85 Mev. The absolute accuracy is about 15%. Each cross section plotted as a function of proton energy rises above its threshold to a peak whose height is of the order of one barn, and then falls again to a low and fairly constant value. The results from x = 3 to 7 are consistent with a compound nucleus plus prompt nucleon–nucleon cascade model using reasonable nuclear parameters, but the experimental (p, 2n) cross section appears to be almost double the value so predicted. Since (p, xn) reactions are dominant in the energy range 10 to 40 Mev., their sum approximates the total reaction cross section; the experimental sum fluctuates around the smooth curve computed for the compound nucleus model with r₀ = 1.3 × 10⁻¹³ cm. The fluctuations are similar to, but more marked than, those in the total neutron cross section of heavy elements in the same energy range. A more detailed theoretical discussion of these results is given by Jackson in the paper immediately following.