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Published: 1984

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Pion production in heavy ion collisions at energies below the free nucleon-nucleon threshold, i.e., at energies, where the pion mass exceeds the kinetic energy of a projectile nucleon in the zero momentum frame, has been discussed for a long time in terms of probing the intrinsic Fermi motion in the projectile and target nuclei. However, as the beam energy is lowered, this process gets more and more unlikely and below a certain energy it should not be observed anymore. In a sharp cut-off Fermi gas model this occurs at E/sub lab/=50 MeV/u. However, as long as the total center of mass energy exceeds the pion mass, pions still can be produced by a cooperative sharing of the beam energy of several (or all) projectile nucleons. The experiments presented here are meant to extend the experimental information into that kinematic domain and represent the up to now lowest beam energy, where pion production has been identified unambiguously. The production of a pion of 100 MeV kinematic energy with a 35 MeV/u 14N beam requires Fermi momenta as high as approx. 350 MeV/c or alternatively 60% of the total beam energy. The information from the present experiments together with the results of previous experiments at higher beam energies of 44 MeV/u and 60 to 84 MeV/u allows one to distinguish between the alternative production mechanisms.