H. Rossner

Vaporization and multifragmentation in the reaction 1.2 GeV + Cu and Ag

Abstract Reactions of 1.2 GeV antiprotons with Cu and Ag targets have been studied. It was found that in such reactions, excitation energies are generated in excess of the total binding energy of the nuclei involved. Accordingly, one observes in these reactions the onset of vaporization and multifragmentation processes. In the p + Cu reaction, the vaporization is observed to set in at about 7.5 MeV/nucleon. However, its probability remains relatively small and below 15% up to the highest excitation energies identified in the present studies (approximately 15 MeV/nucleon). The threshold excitation energy for multifragmentation processes, defined as the detection of at least 3 intermediate mass fragments, is observed at about 4 MeV/nucleon for both studied reactions. Multifragmentation is seen to account for no more than approximately 5% and 20% of the corresponding inclusive yield for Cu and Ag targets, respectively.

Fission probabilities of Cu, Nb, Ag, Ho, Au and Bi nuclei excited by stopped antiprotons

The A dependence of nuclear fission induced by stopped antiprotons has been measured. An unambiguous identification of the binary fission decay mode was provided by a coordinate measurement of complementary fission fragments in coincidence using a large-acceptance fission detector based on low pressure multiwire proportional chambers. A deep fissility minimum was observed nearA=100, in agreement with the general behaviour predicted by the liquid-drop model. An unexpectedly low and high fission probability was found for the Ag and Cu nuclei, respectively.

Formation and decay of nuclei heated with high-energy antiprotons

The decay of nuclei excited via the annihilation of 1.2 GeV antiprotons has been investigated. Thanks to the ability to determine the excitation energy, E*, for all events, largely irrespective of their mass partitions, the probabilities of the different channels at play could be estimated as a function of E*. The data show the prevalence of fission and evaporation up to E* = 4 MeV/nucleon, with no hint of a transition towards multifragmentation.

Decay of hot nuclei formed with energetic antiprotons (PS208)

Abstract Energetic antiproton-nucleus interactions allow to build up thermal excitation energies exceeding 800 MeV in nuclei of masses close to 200. We found that for these high excitation energies the fission process still remains a very significant exit channel. At excitation energies close to 800 MeV the fission process approaches 20–50% of the cross section for Ho, Au and U-like nuclei depending on the fission events selection criteria. The average intermediate mass fragment multiplicity at such high excitation energies is equal to about 1 and alpha particle multiplicity is equal to about 5 for all three reactions independently of the exit channel, including fission. A transition towards the multifragmentation is not observed.

Heating nuclei with high-energy antiprotons

The distributions of thermal energy generated in different target nuclei by means of the annihilation of antiprotons onto their surfaces have been investigated. The results confirm that fairly hot nuclei are produced in this approach, as predicted by Intra-Nuclear Cascade calculations. Although the transition towards multifragmentation as the primary decay channel is not observed, these data should allow to pin down the decay patterns of hot nuclei with little excitation of their collective degrees of freedom. In turn this may shed new light on the mechanisms responsible for some events associated with high fragment multiplicities as observed in heavy-ion collisions.