J. Galin

Complete energy damping in 29 MeV/nucleon Pb+Au two-body final-state reactions☆

Abstract Decay products emitted in highly dissipative Pb+Au reactions at 29 MeV/nucleon have been detected using a large area array. Multiplicities of fragments as large as 8 have been detected with a sizeable cross section. The m -fragment exit channels are fully compatible with the formation of a transient excited dinuclear system formed in damped collisions. The excitation energy function shows that fully damped collisions are achieved even for m = 2 indicating that heavy nuclei are able to sustain high excitation energy and end up as evaporation residues.

Fission lifetime measured by the blocking technique as a function of excitation energy in the 24 A.MeV 238U+28Si reaction

Abstract The blocking technique has been used to infer fission lifetimes as a function of excitation energy for uranium-like nuclei formed in the U+Si reactions at 24 MeV/nucleon. The fission lifetimes are found larger than 10 −19 s for excitation energies up to about 250 MeV.

Decay Patterns of Target-like and Projectile-like Nuclei Produced in ^{84}Kr+^{197}Au, ^{nat}U Reactions at E/A=150 MeV

Abstract The reactions 84 Kr+ 197 Au and 84 Kr+ nat U were studied at an incident energy of E A = 150 MeV employing the large-volume neutron multiplicity meter ORION. The observed correlations between the atomic number of projectile-like nuclei (PLN) and neutron multiplicity indicate large deposits of excitation energies in the primary fragments. Angular correlations between fragments from the fission of target-like nuclei (TLN) and secondary PLN show a memory of the reaction plane. No indications for spin effects were found in the TLN-fission fragment distribution.

Emission of complex fragments in the reaction Ar + Au at 44 and 77 A · MeV

Abstract Complex-fragment emission from the 44 and 77 A · MeV 40 Ar + 197 Au reaction has been investigated. Equilibrium and non-equilibrium components have been identified which are discussed in terms of statistical emission from the hot target-like fragment and of a possible persistence of a deep-inelastic process.

Inclusive excitation-energy distributions of hot nuclei from 44 MeV/nucleon Ar- and 32 MeV/nucleon Kr-induced reactions

Inclusive neutron multiplicity distributions were measured by means of 4π liquid-scintillator detectors for Ar and Kr-induced reactions at 44 MeV/nucleon and 32 MeV/nucleon, respectively. For all the systems studied, the observed distributions exhibit a bump structure at large multiplicity, corresponding to highly dissipative collisions. For Ar-induced reactions, the excitation energies necessary to explain the most probable neutron multiplicity associated with these dissipative collisions are estimated, the correspondence between excitation energy and neutron multiplicity being calculated in the framework of the statistical model. The so-obtained values of excitation energies, which are systematically lower than those predicted using the massive-transfer picture, are discussed.

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.

Evolution of fission lifetime with temperature: a straightforward measurement by the blocking technique

The blocking patterns in a single crystal have been used to measure fission lifetimes as a function of the excitation energy in the 238U+28Si reactions at 24 A.MeV. The neutron multiplicity measured on 4 π has been used to infer the excitation energy of the fissioning nuclei. The fission lifetimes measured for uranium-like nuclei with temperatures up to 3 MeV are longer than 10−19 s, a much larger value than those inferred from previous measurements by less direct techniques.