Abstract

Abstract This paper reports results of digital simulations of low level infrared (IR) processing/detection techniques in presence of zodiacal light background. The study investigates applicability of pulse shape discrimination techniques to detection of IR signals where fast rising target pulses are superimposed on slowly varying background. The quantities that characterize techniques are defined. An improvement of signal-to-background ratio is essential advantage in applying new concept.IntroductionThe scattering of sun light by dust particles that exist In interplanetary space is known as zodiacal light. 1 When these same particles absorb sunlight, they reach temper­ atures of several hundred degrees K and becomes strong infrared radiation sources. For many astronomical observations that utilize Infrared detectors, therefore, zodiacal light constitutes a major source of infrared background. This effect Increases considerably as observation points get closer to sun and near to ecliptic plane. Depending on intensity and spectral content of target radiation source, zodiacal light back­ ground can become fundamental constraint in designing of a satellite IR detection system. The problem becomes extremely formidable if target IR level were a few orders of magnitude lower than background.In many IR detection systems with high background levels, detection outputs are a superposition of fast rising target pulses on a slowly varying background levels. As is true in any measurement, detection efficiency would be enhanced as signal-to- background ratio increases.Recent work2 on pulse shape discrimination in nuclear instrumentation involves iden­ tification of two overlapping pulses of different rise-times and uses one of them with minimum delay to activate a logic circuit. In particular, constant-fraction timing (CFT) is well established over conventional zero-crossing or leading edge timing.3 Nanosecond trigger circuits have been demonstrated.4 Pulse shape discrimination is also utilized for Identifying particle types in conjunction with scintillation counters. 5In particular, rejection of gamma-ray events in fast neutron spectrometers employing organic scintillators has been accomplished by pulse shape discrimination technique. Our emphasis is to generate an target pulse by reducing slowly varying background. The slowly varying background Is reduced by adding to current detector an amplified, Inverted and delayed detector signal. Depending on amplification and delay, background Is reduced and an Improved target can be obtained with time correlation techniques.Shape discrimination conceptThe basic concept of shape discrimination can be illustrated by considering two pulses yg(t) (the background with a rise time tp>g) and yg(t) (the target with a rise time tp>5). The illustration presented here Is based on a principle demonstrated earlier,6 and is similar to time correlation techniques7 for extracting a fast rising component from a complex pulse. The detector output y(t) Is obtained by summing two signals ys(t) and ys(t) as shown In Figure 1. A detector senses only y(t) from which y^(t) must be iden­ tified. Following few steps described below, the improved target signal can be generated.(1) Generate Inverted, amplified (amplification factor g) and delayed (delayed At with respect to t0 as shown in Figure 1)y(±) (t) = -gy(t-At) (1)

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