Absorption of 280-Mev Photons

Total cross sections for 280-Mev photons have been measured for a number of elements. The source of photons was the bremsstrahlung spectrum of 310-Mev upper limit, which was obtained from the Cornell synchrotron. P hotons above 250 Mev were detected by a pair spectrometer and the transmission through various samples determined. The measured cross sections in units of ${\mathrm{cm}}^2$/g are 0.01060, 0.0284, 0.0528, 0.0776, 0.1069, and 0.1148 for Be, Al, Cu, Sn, Pb, and U, respectively. The probable statistical errors range from 0.9 to 1.4 percent. These values are lower than the theoretical values by a fraction which is roughly proportional to $Z^2$ and is about 10 percent for Pb, in agreement with other results obtained at lower energies.