Radiation Effects Analysis of a Coherent Laser Detection System

An analysis of the effects of Co60 gamma ray irradiation on the signal-to-noise ratio, noise equivalent power or minimum detectable signal power, and mean squared noise current of a coherent laser detection system has been performed. The detector used in this study was a wide bandwidth, HgCdTe photovoltaic detector operated at 80°K with peak response at 10.6 μm wavelength. The analysis indicates that a threshold for system degradation of approximately 1010 to 1011 γ/cm2-s depending upon the magnitude of standard noise sources such as diode dark leakage current and preamp input resistance noise. Gamma flux thresholds as low as ~107 γ/cm2-s are possible for local oscillator, shot-noise limited performance. A comparison of these calculations with experimental tests produced agreement within approximately a factor of two. Based on the analysis, it was determined that the detector design requirements for reducing gamma noise are similar to those for passive detectors; namely, small detector thickness, low Z materials, low lifetime, low substrate resistivity, and suitable signal processing.