Experimental observation of a two-phase state of expanding nuclear matter at E p=1 GeV

Abstract

A double-arm time-of-flight spectrometer was used to study the thermal multifragmentation of 238U, 232Th, and 197Au nuclei that is induced by 1-GeV protons. It is established that the pseudoevaporation channel, where a single heavy fragment of mean mass in the range 80–90 amu is produced in association with approximately ten smaller clusters (〈M〉=7–10 amu) emitted anisotropically at low velocities, is dominant at this energy. The pseudoevaporation cross sections change from 10±5 mb for 197Au to 120±40 µb for 238U. For 238U and 232Th, it is found that, in the region M<8 amu, the power law is violated, which is explained by the decay of part of small primary clusters in the course nucleon exchange through the “gas” phase within the expanding nucleus.