Production ofTb149from Gold by 2.2-GeV Protons

Abstract

Angular distributions and differential range spectra at angles of 15, 60, 90, 120, and 165° to the beam direction have been measured for ${\mathrm{Tb}}^{149}$ recoiling from thin gold targets irradiated with 2.2-GeV protons. The observed mean velocities decrease monotonically from a value of 0.264 ${\mathrm{}(\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{m}\mathrm{u})\mathrm{}}^{1/2}$ at 15° to 0.149 ${\mathrm{}(\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{a}\mathrm{m}\mathrm{u})\mathrm{}}^{1/2}$ at 165°. The ratio of integrated intensity in the forward hemisphere to that backward is 3.10. The experimental results are consistent with the symmetry requirements of a two-step model for the reaction, provided that there is a positive correlation between $v_{\parallel }$, the forward component of velocity due to the first step, and $V$, the velocity due to the second step. However, there are substantial quantitative discrepancies between the experimental results and calculations based on the cascade-evaporation model which is commonly used to describe the two steps of a high-energy nuclear reaction. The calculations predict a value of $〈v_{\parallel }〉$ twice as large as that observed and a much weaker correlation between $V$ and $v_{\parallel }$. These effects are discussed in terms of possible contributions from heavy-fragment emission in both steps of the reaction. Systematics of the average energies of various recoil products suggest an empirical correlation that may be useful in distinguishing fission from "deep spallation."