Temperatures of Multifragmentation Processes

One of general expectations for a system undergoing a phase transition is an enhanced heat capacity at temperatures where the phase transition occurs, reflecting the latent heat required to transform from one phase to the other. For nuclear matter, enhanced heat capacities are expected at temperatures of the order of 5-10 MeV, due to the transformation from the Fermi liquid which can be found in the interior of large nuclei to a gas consisting of free nucleons. Stastistical model calculations, shown to the right, predict a "plateau" in the relationship between temperature and density at termperatures of the order of 5 MeV.

Experiments at the NSCL are focused upon determining whether such a plateau may exist for the hot but finite nuclear systems that are formed by colliding two heavy nuclei. Results of first generation experiments, which measured temperature by comparing the yields of nuclei in quantum states at low excitation excitation energy to those at higher excitation energy, are shown on the left, The data reveal only a gradual increase in temperature with incident energy of the beam. In these early experiments, however, it was not possible to ensure that the two nuclei were undergoing a head-on collision, where the temperature would be larger instead of a grazing collision, where the temperature would be smaller.

Presently, researchers at the NSCL are reexamining this trend by performing measurments for central (head-on) collisions. This requires the use of 4p detectors, such as the NSCL 4p, neutron Superball, or Miniball/Miniwall arrays to determining the centrality of each collision.

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William G. Lynch, 1997