Background Reduction of Cosmic X-ray Detector By Means of High Speed Pulse Shape Processing

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We have studied digital processing techniques for the pulse shapes of the output signals from X-ray counters. Our first motivation was to reduce particle backgrounds in the proportional counter more efficiently than with usual rise time discrimination method. However, in the course of the study, we noticed that polarimetry of X-ray photons can be done by means of the pulse shape processing (analysis). The basic idea was due to the fact that photoelectrons are preferentially emitted to the direction parallel to the electric vector of the incoming photons. If energy of photoelectron is high enough to reflect its direction of emission to the elongation of the primary electron cloud, pulse shape would be different depending on the geometry between the emission direction and the anode wire. It is expected that for X-ray photons with electric vector parallel to the anode wire the output pulse rises more rapidly than for the vertical case, on average.To examine the idea of "pulse shape (or ri … More se time) X-ray polarimetor", we performed experiments at the synchrotron facility in the Institute for High Energy Physics, Tsukuba, Japan. We irradiated a Xe filled proportional counter against the monochromatic X-ray beam there. Output signals from the preamplifier of the counter were sampled using a Digital Storage Scope. The pulse shape data stored in a personal computer system were analyzed. Rise time of the pulse was investigated, and for X- ray energy from 20 keV to 40 keV, it was found that the average rise time (10%-50% of the pulse peak) was shorter for the parallel (electric vector // anode wire) case. One problem was that reproducibility of the absolute values of the rise time was not good.There are handful of techniques for X-ray poralimetry. To get higher sensitivity for polarization, scattering methods (e. g. Bragg crystal) would be better. However, as for X-ray astronomy use, polarimetry with proportional counter and pulse shape analysis have advantages of 1) photons in wide energy bands can be measured, 2) spectral & timing information is obtained at the same time. Thus further study will enable this technique for actual use. Less