Effects of Collectivity on Target-Fragmentation Reactions

Abstract

A new approach based on the collective tube model is developed for understanding momentum transfer to target fragments in high-energy hadron-nucleus collisions. Collectivity manifests itself in an enhanced dependence of momentum transfer on projectile energy, consistent with experimental results for deep-spallation reactions. The effective target for 149 Tb production from 197 Au by high-energy protons is deduced to consist of 3.1 ± 0.4 nucleons. Extensions of this model to nucleus-nucleus collisions are discussed.Investigations of hadron-nucleus collisions at high energies offer possibilities for probing the space-time character of strong interaction processes at small distances. Experimental data have been interpreted as showing that asymptotic final states of the fastest secondary particles are reached only in distances <:10 fm, 1 and that the immediate product of a hadron-nucleon collision is a state similar to the incident hadron. 2 Such conclusions follow from the observed weak dependence of the number of secondary particles, i.e., the multiplicity, on nuclear size and projectile energy. 3 Cascade development in nuclear matter appears to be suppressed relative to the predictions of a multiple, independent collision model (MICM) which treats the interaction as a series of quasifree particle-particle scattering events. 4 The collective tube model (CTM) 5 represents another approach to understanding such phenomena. A salient point in the CTM is that the incident particle sees a nucleus that is Lorentz contracted to a thin disk. Consequently, nucleons in the path of the incident hadron can be viewed as acting collectively and in a first-order approximation can be considered as a single object, an effective target which may have a mass greater than that of an individual nucleon. Evidence of collectivity has been reported by Vary, Lassila, and Sandel, 6 who found that subthreshold p production data were more consistent with the predictions of the CTM than with those of an MICM which included effects of Fermi motion. While the CTM has been applied primarily to particle production, it has been speculated recently 7 * 8 that it might account for low momentum transfers and sidewardpeaked angular distributions of target-fragmentation products. In this Letter, the CTM is used to analyze quantitatively longitudinal momentum transfers to target fragments from hadron-nucleus collisions. It is shown that experimental data for a proton-induced deep-spallation reaction are consistent with an initial interaction involving several nucleons. Consider the single-particle inclusive reaction in which a projectile of mass m P9 momentum P, and total energy E interacts with a target m r . A mass Am is abraded from the target giving a prefragment (i.e., the precursor of the observed product) which recoils at an angle 0 to the beam direction with a momentum q. The remainder of the system is treated kinematically as a single object having mass w. This "missing mass" includes fragments of the projectile, the Am abraded nucleons, any pa...

Competition of coalescence and “fireball” processes in nonequilibrium emission of light charged particles fromp+Aucollisions

“…For the Au target such an effective target would have a mass of 3.6 nucleons, what fits well with estimated mass of the fireball of the present study and justifies identification of the "fireball" with the "effective target". Furthermore, the deep spallation process of production of 149 Tb, studied by Winsberg et al [14] in proton -Au collisions at energies 1 -300 GeV was also explained by Cumming [15] assuming manifestation of the effective target with the mass of (3.1 ± 0.4) nucleons for proton energies larger than ∼ 2 GeV. This mass again agrees with the "fireball" mass found in our investigations and confirms proposed interpretation of the "fireball".…”

Momentum transfer in the fragmentation of copper by 400-GeV protons

“…It was difficult to understand why such a simple model could successfully fit data for reactions which appeared to involve more conlplex interactions (22). However, recent work (23,24) has shown that eq. [I] with k = 1 can be derived as an approximation from a less restrictive single collision or multiple independent collision model involving small momentum and energy transfers in each step.…”

Mass-yield distribution in proton and heavy ion induced target fragmentation processes

“…(b) In terms of the two-component EP-ET picture [23 l, the collision processes discussed in the present paper are gentle collisions, while those discussed by Cumming [34] are violent collisions. Note in particular that in the latter case, the energy transfer to the RT always increases with increasing bombarding energy and does not tend to a limiting value.…”