${\pi }^0$ production cross sections are calculated with use of a Boltzmann master equation which assumes a nucleon-nucleon collision relaxation mechanism for the reaction of 35-MeV/nucleon ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ with ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, and ${}_{}{}^{184}{}_{}{}^{}\mathrm{W}$ targets, and for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ at energies of 60, 74, and 84 MeV/nucleon. Results support a nucleon-nucleon collision mechanism as one possible interpretation of experimental yields.

• Received 2 January 1985

DOI:https://doi.org/10.1103/PhysRevLett.54.2215