IR Signal Processing Under Zodiacal Light Background

Abstract This paper reports the results of digital simulations of low level infrared (IR) signal processing/detection techniques in the presence of zodiacal light background. The study investigates the applicability of pulse shape discrimination techniques to the detection of IR signals where the fast rising target signal pulses are superimposed on slowly varying background. The quantities that characterize the techniques are defined. An improvement of signal-to-background ratio is the essential advantage in applying the new concept.IntroductionThe scattering of the sun light by dust particles that exist In interplanetary space is known as zodiacal light. 1 When these same particles absorb the 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, the zodiacal light constitutes a major source of infrared background. This effect Increases considerably as the observation points get closer to the sun and near to the ecliptic plane. Depending on the intensity and spectral content of the target radiation source, the zodiacal light back­ ground can become the fundamental constraint in the designing of a satellite IR detection system. The problem becomes extremely formidable if the target IR signal level were a few orders of magnitude lower than the background.In many IR detection systems with high background levels, the detection outputs are a superposition of fast rising target pulses on a slowly varying background levels. As is true in any measurement, the signal detection efficiency would be enhanced as the signal-to- background ratio increases.Recent work2 on pulse shape discrimination in nuclear instrumentation involves the iden­ tification of two overlapping pulses of different rise-times and uses the one of them with minimum delay to activate a logic circuit. In particular, the "constant-fraction timing" (CFT) is well established over the 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, the rejection of gamma-ray events in fast neutron spectrometers employing organic scintillators has been accomplished by the pulse shape discrimination technique. Our emphasis is to generate an "improved" target signal pulse by reducing the slowly varying background. The slowly varying background Is reduced by adding to the current detector signal an amplified, Inverted and delayed detector signal. Depending on the amplification and the delay, the background Is reduced and an Improved target signal 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 signal with a rise time tp>g) and yg(t) (the target signal 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 signal 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 the few steps described below, "the improved target signal" can be generated.(1) Generate the Inverted, amplified (amplification factor g) and delayed signal (delayed At with respect to t0 as shown in the Figure 1)y(±) (t) = -gy(t-At) (1)