Antiproton-nucleus annihilation at rest

Abstract

Cooled antiprotons from the LEAR storage ring facility have been stopped in targets from A = 6 to 238 to investigate the processes following annihilation at the nuclear surface. Among the recent results are the following: The measured spectra can be fitted to N(p2) = N0 exp (–p2/p02), where p is the momentum of the ejectiles and p0 ≈ 360MeV/c for all ejected particles from protons to alpha particles and for all 16 targets except 6Li. The proton spectra show an additional low-momentum component attributed to evaporation. The measured residual nucleus distribution from stopped p annihilation in Cu, when compared with INC calculations and with residual mass distributions from energetic π+, π- and proton interactions with Cu, indicate that the energy set free in the annihilation process is used only in part as excitation energy. The residual nucleus distribution from a 238U target shows that both symmetric and asymmetric fission take place. Detection and analysis of coincident fission products from 209Vi, 232Th and 238U enabled us to determine the individual and total mass and energy distributions of the fragments, their opening angle, the average excitation energy and momentum of the nucleus at scission, and the number of nucleons emitted before and after scission. We also found that the process is asymmetric for low excitation energies and symmetric for higher ones. INC calculations have reproduced the experimental results.