New scalings in nuclear fragmentation

Fragment partitions of fragmenting hot nuclei produced in central and semiperipheral collisions have been compared in the excitation energy region 4-10?MeV per nucleon where radial collective expansion takes place. It is shown that, for a given total excitation energy per nucleon, the amount of radial collective energy fixes the mean fragment multiplicity. It is also shown that, at a given total excitation energy per nucleon, the different properties of fragment partitions are completely determined by the reduced fragment multiplicity (i.e., normalized to the source size). Freeze-out volumes seem to play a role in the scalings observed.     

Further experimental evidence for fast fission

Fusion-fission products have been studied for three reactions: Ar + Au, Ar + Bi and Ar + U (5.25–7.5 MeV/u). By measuring symmetric fragmentation components (fission-like events), cross sections for fusion were deduced and compared with the predictions of static and dynamic models. With increasing projectile energy, the width of the mass distributions strongly increases for the two lighter systems. By contrast, for Ar + U it remains essentially constant at a very large value. These results clearly demonstrate that the large increase of the width of the mass distribution cannot be attributed simply to large values of the angular momentum. However, they can be explained by the occurence of a different dissipative process, fast fission, which can be expected if there is no barrier to fission. For the reaction Ar + Au, the total kinetic-energy distributions were also studied in detail. In this case fast fission occurs only at high incident energy. The average total kinetic energy (TKE) was found to be constant with increasing energy whereas the widths of the TKE distribution increase.     

Search for non-strange exotic mesons produced via baryon exchange

Negative results on backward production, via baryon exchange, of exotic non-strange mesons are presented. The reactions π^?p→p_forwardX^?and π^?n→ p_forwardX^?? have been studied with a 12 GeV/cπ^? beam in the Omega spectrometer at CERN. No resonant peak in $\text{X}\text{→}\text{p}\text{p}\text{π}{}^{\mathrm{?}}\text{,}\text{p}\text{p}\text{π}{}^{\mathrm{?}}\text{π}{}^{\mathrm{?}}\text{,}\text{p}\text{p}\text{π}{}^{\mathrm{?}}\text{π}{}^0\text{,π}{}^{\mathrm{+}}\text{π}{}^{\mathrm{?}}\text{π}{}^{\mathrm{?}}\text{π}{}^0$ has been seen. The upper limits obtained on cross sections for exotic meson production $\text{X}\text{→}\text{N}\text{N}\text{π,}\text{N}\text{N}\text{π, 4π}$ are lower than the ?^? backward production cross section in the π^?p→p?^? reaction; this result seems to contradict the predictions of the two-component duality model. Compared to already published experiments in the search for exotics produced via baryon exchange, the sensitivity of this experiment is increased by an order of magnitude.     

Fission and emission of H and He in the reactions of 215 MeV16O with181Ta,208Pb and238U

The reactions of 215 MeV¹⁶O with¹²C,¹⁸¹Ta,²⁰⁸Pb and²³⁸U have been studied. Inclusive measurements for⁴He emission are given from each target, and for fission and^1,2,3H from Ta, Pb and U. For H/He a high-energy, forward-peaked component is observed with characteristics similar to those reported by others. At backward angles a low-energy, nearly-isotropic component is also observed for⁴He that cannot be accounted for by emission from fully accelerated fission products. The spectral shapes for this evaporative component are compared with statistical model calculations, and information is obtained concerning the effective barriers to emission. For the reactions of¹⁶O with¹²C, complete fusion seems to be overwhelmed by incomplete fusion. Fission angular distributions and cross sections are also presented and discussed.     

Experimental observation of enhanced stopping of heavy ions in a hydrogen plasma

Zeitschrift für Physik A Hadrons and nuclei - Enhanced energy loss of 333 MeV 238Uions in a hydrogen discharge plasma with a high degree of ionization has been observed. The ion stopping in a...     

Vanishing of the fusion-like process in heavy-ion collisions

With high resolution collinear fast beam laser spectroscopy in the 451 nm-line of Indium nuclear moments and the change in the mean square radii of the nuclear charge distribution of^105,106 In have been determined.     

Influence of the potential energy surface on the cross sections for deeply inelastic transfer reactions induced by heavy ions

Large differences are observed in the cross sections for deeply inelastic reactions involving the transfer of several charges from ⁴⁰Ca and ⁴⁰Ar projectiles to the same rare-earth targets. The cross sections for four and six charge transfers from projectile to target are enhanced for ⁴⁰Ca incident ions. These differences are qualitatively explained by considering that the drift paths in the (Z, A) plane are governed by a static potential energy surface which describes the composite system. Shell corrections have been included in the calculation of this potential energy surface.     

A Diffusion Equation from the Relativistic Ornstein–Uhlenbeck Process

We derive, in the hydrodynamic limit (large space and time scales), an evolution equation for the particle density in physical space from the (special) relativistic Ornstein–Uhlenbeck process introduced by Debbasch, Mallick, and Rivet. This equation turns out to be identical with the classical diffusion equation, without any relativistic correction. We prove that, in the hydrodynamic limit, this result is indeed compatible with special relativity.     

Nuclear multifragmentation and phase transition for hot nuclei

This review article is focused on the tremendous progress realized during the last fifteen years in the understanding of multifragmentation and its relationship to the liquid-gas phase diagram of nuclei and nuclear matter. The explosion of the whole nucleus, early predicted by Bohr [N. Bohr, Nature 137 (1936) 351], is a very complex and rich subject which continues to fascinate nuclear physicists as well as theoreticians who extend the thermodynamics of phase transitions to finite systems.