Photodissociation of Complex Nuclei at Energies between the Mesonic Threshold and 1150 Mev

The photodisintegration of complex nuclei by gamma rays up to 1150 Mev has been studied by exposing nuclear emulsion to bremsstrahlung from the CalTech electron synchrotron and observing the "photostars" produced. Exposures were made at 16 different peak energies between 250 and 1150 Mev. Nearly 10 000 photostars were analyzed for star frequency, prong number, angular distribution, and (at 1143 Mev) the visible energy release per star. The bremsstrahlung yield of multiprong (≥2 prong) stars increases abruptly as photons capable of producing pions are included. The cross section per photon, derived from the bremsstrahlung yield by the photon difference method, is essentially constant at 250 microbarns/nucleon at all energies above 300 Mev. A model for photostar production is given which involves photopion production followed by absorption or scattering of the pion and recoil nucleon. Experimental free-nucleon photopion cross sections are used, together with the Monte Carlo calculations of Metropolis et al., to determine the probability for star formation. Good agreement with both the shape and magnitude of the excitation curve is obtained if nuclear motion is included. Mean and maximum prong numbers for photostars are the same as for stars produced by pions or protons of equal available energy. The mean-free-paths in nuclear matter of pions and protons are short, so that photostar yields are a measure of the integrated total photomeson cross section. More than 95% of the multiprong stars made by 1-Bev bremsstrahlung are made by photons whose energies exceed the pion production threshold of 150 Mev. Most of the 1-prong events are produced by photons below 150 Mev, and the yield is consistent with giant resonance ($\gamma , p$) reactions plus "pseudodeuteron" photodisintegration, a process whose cross section rapidly decreases as the photon energy increases. Variation of mean prong number with energy and comparison with nuclear cascade calculations suggests that the excitation of the residual nucleus is nearly constant at 100 Mev over a wide range of incident photon energies. The visible energy release per photostar shows a linear dependence on prong number, and more than half of the photon energy is carried away by neutral particles.