SUBTHRESHOLD PION DYNAMICS AS A SOURCE FOR HARD PHOTONS BEYOND PROTON-NEUTRON BREMSSTRAHLUNG IN HEAVY-ION COLLISIONS

We report on the first measurement in heavy-ion collisions of hard pho tons of extremely high energies that extend the photon spectrum to 5 t imes the beam energy per nucleon. The photon spectrum was measured for the systems Kr-86 + Ni-nat at 60A MeV and Ta-181 + Au-197 at 40A MeV. The data are interpreted using the Dubna cascade model. The radiative channel of pion-nucleon interaction pi + N --> N + gamma, involving s ubthreshold pions produced in the nuclear medium, dominates all other processes considered for the production of the very energetic photons. This channel was never considered so far at beam energies below the p ion threshold.