Neutron Photoproduction Cross Sections of Silicon, Phosphorus, and Sulfur

The absolute ($\gamma , n$) cross sections of natural silicon, phosphorus, and sulfur have been measured using the bremsstrahlung from the 70-MeV synchrotron at the University of Virginia. Yield points were taken at 0.5-MeV intervals from 12 to 30 MeV in each case. The absolute measurements were made utilizing a Halpern-type detector, a modified version of an NBS ionization chamber, and a calibrated Ra-Be neutron source. A minimum of 20 runs was made for each element, with less than 1% standard deviation in the yield points contributed by the major isotope. The data were analyzed from 12 to 30 MeV by the Leiss Penfold technique using 2 interlacing 1-MeV bin widths and also using ½-MeV bin widths. Our results show that each cross section is composed of several discrete resonances, with at least 4 resonances resolved in each cross section.