Performance of the GSC engineering counter for MAXI/ISS

We are developing Monitor of All Sky X-ray Image (MAXI) which will be mounted on the Japanese Experiment Module of the International Space Station. MAXI is an all-sky X-ray monitor which scans the sky in every 90 minutes. The sensitivity will be as high as 7 mCrab (5 (sigma) level) in one scan and 1 mCrab in one-week accumulation. The GSC (Gas Slit Camera) instrument consists of twelve one-dimensional position sensitive proportional counters using the Xe++CO2 gas and the carbon fiber anodes of 10micrometers diameter. The window size is 272 x 190 mm. The position is obtained by the charge division method. It is used to identify the source in the long rectangular field-of-view (1.5 x 80 degrees). Three cameras will be set to cover the 1.5 x 160 degrees arc. The position resolution is essentially important, which becomes better in the higher gas gain. We have tested gas mixtures of Xe+CO2with CO2 equals 0.2\%, 0.5\%, 1\%, 3%. The CO2 equals 0.5\% showed the most uniform gas gain, but has a little after pulses. We chose the Xe (99%) + CO2 (1%) combination for the flight counters. It can achieve the uniform gas gain in the cell and negligible after-pulse in high operating voltage. The engineering model of the counter (EM1) was build. We have tested the position resolution and the energy resolution across the counter. The position resolution and the energy resolution depend on the X-ray energy. On the basis of these results, together with the collimator response, we performed a realistic simulation.