The planispherical chamber: A parallax-free gaseous X-ray detector for imaging applications

Abstract

Crystallography or X-ray fluorescence experiments which require good signal to noise ratios and high position resolution can take advantage of the outstanding signal amplification capabilities of MicroPattern Gaseous Detectors (MPGDs) such as Gaseous Electron Multipliers (GEMs) coupled with the position resolution achieved by optical readout realized with CCD or CMOS cameras. Increasing the detection probability of incident radiation with thicker drift volumes in these detectors leads to a spatial resolution-limiting parallax error when employing parallel electric field lines in the drift region.We describe a new GEM-based detector concept, consisting of a cathode, GEM electrodes and field shaping rings suitably segmented and powered to create a radial electric field, thus minimizing the parallax error. A CCD camera is used to record scintillation light originating from charge multiplication in the high field of the GEM holes in an Ar/CF4 (80/20%) gas mixture. Assembled as pinhole camera, the device permits to obtain high detection efficiencies for soft X-rays, exempt from the parallax error intrinsic in the use of standard gaseous detectors with thick conversion layers. The use of several GEMs in cascade allows for high charge multiplication factors. Switching from straight to radially focused drift field lines, a significant reduction of the parallax error as well as an increased signal-to-noise ratio were achieved, effectively paving the way for applications such as X-ray crystallography realized with optically read out GEMs.