High-resolution position readout for proportional counters with fluorescence gating capability

摘要

Abstract: We describe a new position readout scheme, applicable to proportional counters, which provides the attributes required for large format, high energy x-ray detectors, such as that proposed for the Eixon x-ray monitor instrument on the ESA INTEGRAL mission. Large format detectors for coded mask imaging require a position resolution of typically less than 1 mm, in order to over-sample the projected mask pixel. Background rejection at higher energies can be improved by using fluorescence gating. However, this technique requires the position readout to be capable of detecting the simultaneous double event signature. The scheme we propose combines both excellent position resolution with the ability to resolve simultaneous events. The readout scheme consists of an array of charge measurement electronic channels connected to groups of cathode strips. The particular cathode grouping arrangement allows a large reduction (approximately 1/6) in the number of channels required compared to the fully parallel scheme, with one channel per electrode. However, the new design still retains the charge centroiding and parallel processing capabilities of the fully parallel scheme, enabling it to provide high spatial resolution and resolve multiple simultaneous events. We present results of a Monte Carlo simulation of the detector and readout. The simulation models the physics involved in each x ray interaction and predicts the primary ionization distribution. Simulation of electron diffusion and gas multiplication are used to predict the charge induced on each cathode strip. Electronic noise and other signal degradation factors are included for a realistic assessment of readout performance. Thus far, the position readout is modeled in one axis only. The success of the new scheme is assessed by comparison with the fully parallel readout. !13