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

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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 (${\mathrm{C}}_{6}$${\mathrm{H}}_{12}$) and graphite was used. The theoretical attenuation coefficients were calculated using selected Compton and triplet cross sections in addition to the small quasi-deuteron cross sections. A pair cross-section increase of 2.25% was required for carbon, water, and aluminum to bring the total calculated coefficients into agreement with the measured coefficients in the 60-Mev region. The difference between these calculated cross sections and the measured cross sections in the 13-to 50-Mev region has been ascribed to the giant resonance nuclear absorption. A larger high-energy tail to this absorption than predicted by ($\ensuremath{\gamma}, p$) and ($\ensuremath{\gamma}, n$) experiments is indicated.