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 photons of extremely high energies that extend the photon spectrum to 5 times the beam energy per nucleon. The photon spectrum was measured for the systems {sup 86}Kr+{sup nat}Ni at 60 {ital A} MeV and {sup 181}Ta+{sup 197}Au at 40 {ital A} MeV. The data are interpreted using the Dubna cascade model. The radiative channel of pion-nucleon interaction {pi}+{ital N}{r_arrow}{ital N}+{gamma}, involving subthreshold 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 pion threshold. {copyright} {ital 1996 The American Physical Society.}