X-Ray Attenuation Coefficients from 13 to 80 Mev for Hydrogen, Carbon, Water, and Aluminum

The x-ray attenuation coefficients for hydrogen, carbon, water, and aluminum have been measured in the energy range from 13 to 80 Mev by placing varying lengths of attenuators in a 90-Mev bremsstrahlung beam in a good geometry experiment using a large sodium-iodide total-absorption spectrometer as the detector. In the hydrogen case, a difference method employing cyclohexane (C₆{H}₁₂$) and graphite was used. The theoretical attenuation coefficients were calculated using selected Compton and triplet cross sections in addition to the small quasi-deuteron cross sections. A pair cross-section increase of 2.25% was required for carbon, water, and aluminum to bring the total calculated coefficients into agreement with the measured coefficients in the 60-Mev region. The difference between these calculated cross sections and the measured cross sections in the 13-to 50-Mev region has been ascribed to the giant resonance nuclear absorption. A larger high-energy tail to this absorption than predicted by (γ, p) and (γ, n) experiments is indicated.