Thermal excitation and decay of nuclei from antiproton-nucleus interactions at 1.22 GeV - art. no. 034616

The formation and subsequent decay of nuclei excited via the annihilation o f 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 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 Me V were recorded, a condition fulfilled for 100% of the annihilation events. The distributions of excitation energy (E*) of the transient hot nuclei ha ve been investigated for a large range of target nuclei, E* being determine d event by event from the total multiplicity of light particles. The averag e 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 t he U target, in good agreement with the predictions of an intranuclear-casc ade model. The distributions extend to E*>8 MeV/nucleon for Cu and E*>5 MeV /nucleon for Au, with cross sections exceeding 18 of sigma (reac). Thanks t o 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 o f fission and evaporation up to E*=4-5 MeV/nucleon for Au and U. The fissio n probability Pa, was measured for the first time over the full range of E* . The reproduction of the data by statistical models is reasonable, provide d that the ratio a(f)/a(n), is adjusted for the different targets and a tra nsient time shorter than 1x10(-21) s is considered. The experiment has allo wed the fission probability to be investigated as functions of the associat ed neutron and light-charged particle multiplicities. The intermediate mass fragment multiplicities rise smoothly with E* up to about 1 unit at E*=1 G eV for Au and U, with no indication of significant contribution from anothe r process than evaporation, Heavy residues have been measured quite abundan tly at the highest E*. with most of their kinetic energy arising from the r ecoil effects in the evaporation stage. Overall, the data allow a coherent picture to be established. consistent with the hot nucleus retaining conven tional decay properties.