an x - ray detector is formed with a geometry in the form of a spherical polygon , including an entrance window , a grid and an anode . the spherical polygonal entrance window and the grid form a spherical polygonal drift region between them . the electric field in this region is radial and eliminates parallax broadening . a spherical polygonal amplification region between a resistive anode on an insulating support and the grid allows very high gas amplification and good protection against spark discharges . a readout electrode on the back side of the anode insulator detects induced charges and protects the readout electronics against sparks .