Effect of a magnetic field generated by permanent magnets on the GPD polarization sensitivity

The Gas Pixel Detector (GPD) is an imaging X-ray polarimeter with a moderate spectral resolution and a very good position resolution.^{1, 2} The GPD derives this information from the true 2-d charge image of the photoelectron track produced in gas and collected by an ASIC CMOS chip after its drift and its multiplication. In this paper we report on the experimental results of the study of the effect of a strong magnetic field in reducing the diffusion and increasing the sensitivity for a GPD filled with one bar of He-DME 20-80. We generated a magnetic field of about 1600 Gauss by means of commercial magnets made of an alloy of Neodymium-Iron-Boron configured as one ring and one cylinder. We compared the pixel size distributions and the modulation curves with and without magnets at two different drift fields, corresponding to different nominal diffusion properties, with both polarized and unpolarized sources. The results obtained show that a not sensitive improvement is present at this fields implying that a much larger magnetic field is necessary with this mixture, albeit a shift on the position angle of the modulation curve, derived from a polarized source, is observed.