Photosensor readout detector having dynamic reset rate

Abstract

An integrating circuit (12) includes a capacitor (22) which charges at a rate proportional to the instantaneous amplitude of an input signal V.sub.IN generated by a photosensor (14) to produce an integrated output signal V.sub.OUT. The input signal V.sub.IN is proportional to the photon rate upon the photosensor (14), which is the quantity of interest. The output signal V.sub.OUT is non-destructively oversampled at a rate which is much higher than the Nyquist rate of the input signal V.sub.IN to produce sample signals V.sub.S1 which are differenced and later recombined to produce a replica of the output signal V.sub.OUT. The capacitor (22) is discharged and the output signal V.sub.OUT thereby reset to zero whenever the amplitude of the output signal V.sub.OUT reaches a predetermined maximum value V.sub.RESET. The output signal V.sub.OUT is thereby 'folded' into a sawtooth waveform, which has a dynamic range of V.sub.RESET divided by a minimum value. The photon rates of large amplitude signals, which can cause several resets during the observation period, are later reconstructed by averaging the slopes of the sawtooth waveform. The photon rates of small amplitude signals are later reconstructed by summing the differenced outputs during a Nyquist sample interval. Anomalous sample signals V.sub.S1 with very large amplitudes corresponding to short duration 'gamma' events such as nuclear radiation are detected, deleted and replaced with sample signals having amplitudes interpolated from the amplitudes of adjacent sample signals.