Monte Carlo simulation of the materials and filling gas of the imaging proportional counters for the stellar x-ray polarimeter

Four imaging proportional counters are to be flown aboard the Russian mission Spectrum-XGamma as a part of the Stellar X-Ray Polanmeter. The gas mixture will be based on Xe-Ar. We have performed some Monte Carlo calculations to investigate the impact of gas mixture and material choices with respect to gamma and neutron induced background. We have used the latest version of MCNP code for the neutron and gamma transport.