A Nuclear Thermostat

Many people will remember from high school that chemical substances and reactions behave differently at different temperatures (only a few degrees separate life from death!). In this respect, the thermostat, a thermally controlled environment, has been crucial for developing scientific understanding of matter.  Nuclear matter and reactions are no different; they, too depend on temperature.  However, measuring nuclear temperatures is not straightforward, and the fact that this has now become possible (for heavy-ion reactions at Fermi energies) is a significant achievement. A collaboration of physicists  from Rochester University and Washington University at St. Louis derives thermal information from light particles boiled off nuclear reaction products, using a calorimeter assembled from very efficient neutron detectors (SuperBall, top image) and charged-particle detectors (Dwarf array, bottom image).

New investigations have been performed by colliding very heavy ions (e.g., ¹³⁶Xe on ²⁰⁹Bi) using the National Superconducting Cyclotron Laboratory at Michigan State University.  The bombarding energies (62 MeV per nucleon) should have been sufficient to vaporize the system several times over.  Instead, the experiments have demonstrated a surprising behavior of colliding nuclei: They break apart into many small pieces called clusters, at temperatures much below the hundred billion degrees or so predicted by theories of thermal disintegration.  Such properties of nuclear matter could have important consequences for the fundamental science of small quantal systems, our understanding of clustering in micro-cosmos, and cosmology in its effort to understand objects and processes occurring on the scale of our universe.

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This page was last updated on July 24, 2002.

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