A curved-grid gas proportional scintillation counter instrumented with a 25-mm active-diameter photosensor

The curved grid technique has been shown to improve the performance of large-area gas proportional scintillation counters (GPSC) without the need for complex, bulky focusing systems or large-area photomultiplier tubes (PMTs). The technique provides a simple method to compensate for the variation in solid angle viewed by the PMT as a function of the radial distance from the cylindrical axis of the detector. The two parallel grids used in conventional GPSCs are replaced by a curved grid and a planar grid to delimit the scintillation region. The shape of the curved grid is calculated to produce a radially increasing scintillation yield that compensates for the variation in the solid angle viewed by the photosensor. We applied this technique to a GPSC instrumented with a PMT having a 25-mm diameter photocathode. In contrast to previous studies, in this work we used a detector window with a larger diameter than that of the PMT, in order to access the full capability of the technique. The maximum value of the detector entrance window to photocathode diameter ratio thus far achieved is 1.