Antiprotonic heating of nuclei

The bulk properties of nuclear matter such as viscosity, elasticity, heat capacity, and phase transitions from a liquid-like to a gaseous state are not well known. Moreover they can be studied for very small pieces of nuclear matter only. For this work nuclei have to be excited to the very limit of stability and their decay properties carefully registered. The specific excitation process, however, may also stimulate a decay with its own stability limit. The most common excitation method so far, heavy-ion reactions, is indeed very effective in heating nuclei thermally, but at the same time introduces strong rotations, shape distortions and considerable compression in the nuclear complex. It seems difficult to disentangle these effects. These ambiguities can be avoided by a soft or radiation-like heating of nuclei with pions of kinetic energies around 200 MeV. For these pions a nucleus is essentially a black disc. Since a single pion does not transfer enough heat, several pions have to be absorbed simultaneously in the nucleus. This can be accomplished by the annihilation of an antiproton with one nucleon in a nucleus, resulting in about five pions. Antiprotons at low energies annihilate outside the surface - in the nuclear 'stratosphere'. Thus only a small fraction of the produced pions can be absorbed by the nucleus. At high antiproton energies of about 1 GeV the annihilation site is much closer to or even inside the nuclear surface (an 'underground' explosion) with absorption of several pions and production of very hot nuclei with excitation energies of up to 1 GeV. This method of antiprotonic heating is exploited at CERN's LEAR low energy antiproton ring by the PS208 collaboration: HMI-Berlin, GANIL-Caen, TU-Munich, Warsaw, FZ-Rossendorf, IPN-Orsay, IPN-Moscow, and CERN