Hard photon and neutral pion production in cold nuclear matter

The production of hard photons and neutral pions in 190 MeV proton induced reactions on C, Ca. Ni, and W targets has been for the first time concurren tly studied. Angular distributions and energy spectra up to the kinematical limit are discussed and the production cross-sections are presented. From the target mass dependence of the cross-sections the propagation of pions t hrough nuclear matter is analyzed and the production mechanisms of hard pho tons and primordial pions are derived. It is found that the production of s ubthreshold particles proceeds mainly through first chance nucleon-nucleon collisions. For the most energetic particles the mass scaling evidences the effect of multiple collisions. (C) 2001 Elsevier Science B.V. All rights r eserved.