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Register a new userLimiting Temperatures in Multifragmentation
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Authors
W. Trautmann
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The systematic data set on isotopic effects in spectator fragmentation collected recently at the GSI laboratory permits the investigation of the N/Z dependence of the nuclear caloric curve which is of interest in several respects. In particular, new light is shed on the proposed interpretation of chemical breakup temperatures as a manifestation of the limiting temperatures predicted by the Hartree-Fock model. The obtained results are discussed within the general context of temperature measurements in multifragmentation reactions.
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In this contribution we show that the biggest fragment charge distribution in central collisions of Xe+Sn leading to multifragmentation is an admixture of two asymptotic distributions observed for the lowest and highest bombarding energies. The evolution of the relative weights of the two components with bombarding energy is shown to be analogous to that observed as a function of time for the largest cluster produced in irreversible aggregation for a finite system. We infer that the size distribution of the largest fragment in nuclear multifragmentation is also characteristic of the time scale of the process, which is largely determined by the onset of radial expansion in this energy range.
The neutron emission in the fragmentation of stable and radioactive Sn and La projectiles of 600 MeV per nucleon has been studied with the Large Neutron Detector LAND coupled to the ALADIN forward spectrometer at SIS. A cluster-recognition algorithm is used to identify individual particles within the hit distributions registered with LAND. The obtained momentum distributions are extrapolated over the full phase space occupied by the neutrons from the projectile-spectator source. The mean multiplicities of spectator neutrons reach values of up to 12 and depend strongly on the isotopic composition of the projectile. An effective source temperature of T approx. 3 - 4 MeV is deduced from the transverse momentum distributions. For the interpretation of the data, calculations with the Statistical Multifragmentation Model for a properly chosen ensemble of excited sources were performed. The possible modification of the liquid-drop parameters of the fragment description in the hot environment is studied, and a significant reduction of the symmetry-term coefficient is found necessary to simultaneously reproduce the neutron multiplicities and the mean neutron-to-proton ratios <N>/Z of Z <= 10 fragments. Because of the similarity of the freeze-out conditions with those encountered in supernova scenarios, this is of astrophysical interest.
The fragmentation of the quasi-projectile is studied with the INDRA multidetector for different colliding systems and incident energies in the Fermi energy range. Different experimental observations show that a large part of the fragmentation is not compatible with the statistical fragmentation of a fully equilibrated nucleus. The study of internal correlations is a powerful tool, especially to evidence entrance channel effects. These effects have to be included in the theoretical descriptions of nuclear multifragmentation.
Multifragmentation of a ``fused system was observed for central collisions between 32 MeV/nucleon 129Xe and natSn. Most of the resulting charged products were well identified thanks to the high performances of the INDRA 4pi array. Experimental higher-order charge correlations for fragments show a weak but non ambiguous enhancement of events with nearly equal-sized fragments. Supported by dynamical calculations in which spinodal decomposition is simulated, this observed enhancement is interpreted as a ``fossil signal of spinodal instabilities in finite nuclear systems.
The isotope yields of fragments, produced in the decay of the quasiprojectile in Au+Au peripheral collisions at 35 MeV/nucleon and those coming from the disassembly of the unique source formed in Xe+Cu central reactions at 30 MeV/nucleon, were measured. We show that the relative yields of neutron-rich isotopes increase with the excitation energy in multifragmentation reaction. In the framework of the statistical multifragmentation model which fairly well reproduces the experimental observables, this behaviour can be explained by increasing N/Z ratio of hot primary fragments, that corresponds to the statistical evolution of the decay mechanism with the excitation energy: from a compound-like decay to complete multifragmentation.
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