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Karl-Heinz Schmidt
A thermometer for peripheral nuclear collisions It is not an easy task to measure the temperature of a nucleus. Several methods have been developed for this purpose: e. g. analysing the spectra of evaporated nucleons, counting the number of emitted nucleons. The excellent isotopic resolution of the fragment separator allows to apply another method, based on the neutron excess of the fragmentation products. If stable nuclei are used as projectiles, they preferentially emit neutrons when they are excited. Due to the Coulomb barrier, the emission of protons is inhibited. Only after evaporating a large number of neutrons, the neutron excess decreases so much that the evaporation of protons sets in and finally competes with the evaporation of neutrons in a kind of equilibrium. Moderate excitation energies can be measured by the amount of neutron excess of the fragmentation residues. This method is best suited for heavy projectiles. It has been applied in the fragmentation of 197Au in an aluminium target. We found that the abrasion of N nucleons induces an excitation energy of N×27 MeV on the average. This value is drastically larger than the amount of surface energy estimated in the original abrasion-ablation model of Bowman and Swiatecki.
The figure shows the measured cross sections of iridium isotopes produced in the reaction 197Au (1 A GeV) + 27Al compared to several calculated curves. The neutron-rich prefragments, produced in the high-energy nuclear collision, predominantly evaporate neutrons. The distribution of final fragments produced after the evaporation stage of the reaction is a measure of the excitation energy acquired in the high-energy collision. The curve calculated with 2 times the energy distribution, estimated from the energies of the single-particle vacancies, is in good agreement with the data. We conclude that the abrasion of one nucleon induces an excitation energy of 27 MeV on the average.
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