Calculations of the response of shielded detectors to gamma rays at MeV-range energies

Nuclear instruments designed to detect gamma rays at energies from 0.1 to 10 MeV respond primarily to the electrons produced by gamma-ray scattering and absorption in either the instrument itself or in the surrounding materials. Although tabulated attenuation coefficients are very useful for estimating macroscopic quantities such as bulk energy depositions, such quantities are averages over several different phenomena at the microscopic level. For detectors with active elements that are thin compared with an electron range, the competing effects of inscattering and outscattering result in complicated responses, as evidenced by the strong energy dependence of the resulting pulse-height spectra. Thus, for some applications the macroscopic averages are entirely sufficient, but for others a full microscopic analysis is needed. The author first reviews the literature on the responses of several types of detectors to gamma rays at energies below 10 MeV, and then they use a series of simple Monte Carlo calculations to illustrate the important physics issues. These simple calculations are followed by thorough studies of the energy and angle responses of two proposed instruments, including their responses to instantaneous pulses of large numbers of simultaneous incident photons.