Absorption of 17.6 Mev Gamma-Rays in C, Al, Cu, Sn, and Pb

The gamma-ray absorption cross sections of C, Al, Cu, Sn, and Pb have been measured for the 17.6 Mev gamma-rays produced in the ${\mathrm{Li}}^7(p,\gamma ){\mathrm{Be}}^8$ reaction. A gamma-ray pair spectrometer was used as a detector for the 17.6 Mev gamma-rays in order to avoid counting lower energy quanta produced either in the ${\mathrm{Li}}^7(p,\gamma ){\mathrm{Be}}^8$ reaction, or as secondary radiation in the absorber. The measured cross sections are compared to the sum of the theoretical cross sections for Compton scattering, pair production in the field of the nucleus and pair production in the field of the atomic electrons. For the light elements, the agreement is within the experimental errors of about one percent, but for lead the theoretical value is ten percent above the measured cross section. This discrepancy probably results from the use of the Born approximation in the Bethe-Heitler calculation of pair production. The present results are in good agreement with recent measurements of Adams at 11.04, 13.73, and 19.10 Mev, and of Lawson at 88 Mev.