Effect of free-particle collisions in high energy proton and pion-induced nuclear reactions

Abstract

The effect of free-particle collisions in simple "knockout" reactions of the form (a,aN) and in more complex nuclear reactions of the form (a,X) was investigated in a two part study by using protons and pions as the In the second part of this thesis, cross section measurements for (iT.rcN) and other more complex pion-induced spallation reactions were -ixmeasured for the light target nuclei N, 0, and F from 45 to 550 incident pion energy. These measurements indicate rlearly a broad peak in the excitation functions for both (TT.TTN) and fn.X) reactions near 180MeV incident pion energy. This corresponds to the large resonances ob served in the free-particle IT p and ir~p cross sections at the same en ergy. Striking differences in (n,nN) cross section magnitudes are observed among the light nuclei targets. The experimental cross section ratio R _/ + = a(Tr",Tf"n)/a(T7*,TiN) at 180 MeV is 1.7 ± 0.2 for all three targets.The experimental results are compared to previous pion and analogous pro The second stage of this process is the subsequent de-excitation of the excited residual cascade nucleus in a manner similar to that for lew energy nuclear reactions i.e., by emission of nucleons, alpha particles, photons, or even fission fragments. This is termed the "evaporation"stage and has a time scale that is on the order of 10 to 1000 times as long as the cascade stage. Thus, the average de-excitation time for an -20 -18 excited nucleus wouli last about 10 -10 seconds.In light of this mechanism, the wide distribution of product, nu clides formed in a high energy nuclear reaction can be logically explained.Depending upon the complexity of the nucleonic cascade, high as well as low energy deposition events will occur,which in turn, create a wide range of final nuclides after completion of the second or "evaporation" stage of the reaction.-3-This model was further developed by Chew and co-workers (6 Z'j, who proposed the "impulse approximation". This particular approach postulated that the target nucleus would appear to a high energy projectile as a collection of "free" nucleons, rather than an integral cluster, and that the following assumptions should hold (1) The incident particle never in teracts with more than one nucleon at any one time.[2) The amplitude of each incident wave falling on each nucleon is the sane as if the nucleon were a free particle. (3) The binding forces between constituent nucleons in the nucleus are negligible during the strong interaction of the in cident particle with the system.Assumptions (1) and (2) are somewhat oversimplified for systems heavier than the alpha particle. The seemingly paradoxical "impulse"assumption (3) is reconciled by the fact that for very short collision times x, the energy E of the system cannot be determined to better than AE~h/x , a simple application of the uncertainty principle. In spite of these apparent shortcomings, the impulse approximation has been success fully used to correlate a broad spectrum of empirical data and still re mains the basic assumption i...