Production of Photostars by Bremsstrahlung of 250 to 500 Mev

The yield of photostars in nuclear emulsion as a function of bremsstrahlung energy has been found to increase rapidly from 6.8 millibarns per $Q$ (total beam energy/maximum beam energy) at 250 Mev to 22.8 millibarns per $Q$ at 500 Mev, for stars of 2 or more prongs from silver nuclei. The cross section for silver was derived by averaging the emulsion nuclei (excluding H) in proportion to their atomic weights. The total photodissociation cross section is estimated to be at most 20% larger due to the addition of one-prong and zero-prong stars. The cross section per photon is derived by the photon difference method, and is found to be about 30 millibarns per silver nucleus above 300 Mev. This very large cross section is approximately 100 times the free nucleon total photomeson cross section. Therefore, photostar production from reabsorption of real pions produced in nuclear matter requires a very short mean free path ($\lambda \approx 1\times {10}^{13}$ cm).