Calculations and measurements of the energy-dependent response of a shielded gamma-ray detector

Instruments designed to record high-intensity gamma-ray flashes must have fast time response, wide dynamic range, and good rejection of photon backgrounds at lower energies. In principle, plastic scintillators can easily provide the necessary time response and dynamic range; like other photon detectors, however, they must be carefully shielded to reduce their low-energy sensitivity. This shielding is often complicated by the need to use different optical sensors to cover the full dynamic range, which each sensor requiring a separate opening through the shielding. In this detector, a high-sensitivity photomultiplier tube handles low-intensity signals, and a silicon photodiode covers high intensities. These electronic components, particularly the diode, may also respond directly to incident radiation, so localized shielding must be provided. To reduce the detector`s total mass, the scintillator and photodiode are enclosed in a relatively thick, tight-fitting inner shield, which is surrounded by a thin outer shield to reduce the leakage through any gaps. Although efficient, this arrangement demands careful design and testing. This report describes such an analysis, which uses Monte Carlo simulations to develop a comprehensive model of the detector at photon energies from threshold to above 10 MeV. Included are discussions of the fundamental responses of the unshielded silicon diode and plastic scintillator, explanations of the effectiveness of different shielding materials, studies of calibration sources, and comparisons with laboratory tests