Thermal excitation and decay of nuclei from antiproton-nucleus interactions at 1.22 GeV

Abstract

The formation and subsequent decay of nuclei excited via the annihilation of 1.22-GeV antiprotons have been investigated at the low energy antiproton ring (LEAR). Both neutrons and charged products, from protons up to fission fragments and heavy residues, were detected over a solid angle of $4\ensuremath{\pi}$ by means of the Berlin neutron ball (BNB) and the Berlin silicon ball (BSiB), respectively. All events associated with an inelasticity greater than 10 MeV were recorded, a condition fulfilled for 100% of the annihilation events. The distributions of excitation energy ${(E}^{*})$ of the transient hot nuclei have been investigated for a large range of target nuclei, ${E}^{*}$ being determined event by event from the total multiplicity of light particles. The average excitation energies are about twice as large as for annihilations at rest, and range from 2.5 MeV/nucleon for the Cu target to 1.5 MeV/nucleon for the U target, in good agreement with the predictions of an intranuclear-cascade model. The distributions extend to ${E}^{*}>8$ MeV/nucleon for Cu and ${E}^{*}>5$ MeV/nucleon for Au, with cross sections exceeding 1% of ${\ensuremath{\sigma}}_{\mathrm{reac}}.$ Thanks to the capability of determining ${E}^{*}$ for all events, largely irrespective of their mass partitions, the probabilities of the different decay channels at play could be estimated as a function of ${E}^{*}.$ The data show the prevalence of fission and evaporation up to ${E}^{*}=4--5$ MeV/nucleon for Au and U. The fission probability ${P}_{\mathrm{fis}}$ was measured for the first time over the full range of ${E}^{*}.$ The reproduction of the data by statistical models is reasonable, provided that the ratio ${a}_{f}{/a}_{n}$ is adjusted for the different targets and a transient time shorter than $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21}$ s is considered. The experiment has allowed the fission probability to be investigated as functions of the associated neutron and light-charged particle multiplicities. The intermediate-mass fragment multiplicities rise smoothly with ${E}^{*}$ up to about 1 unit at ${E}^{*}=1$ GeV for Au and U, with no indication of significant contribution from another process than evaporation. Heavy residues have been measured quite abundantly at the highest ${E}^{*},$ with most of their kinetic energy arising from the recoil effects in the evaporation stage. Overall, the data allow a coherent picture to be established, consistent with the hot nucleus retaining conventional decay properties.