The dependence of the energy resolution of gas proportional scintillation counters on the scintillation region to photomultiplier distance

The energy resolution for broad X-ray beams of gas proportional scintillation counters (GPSC) with medium to large size windows, parallel grids, and no focusing system is discussed. The variation of the pulse amplitude with the radial distance is calculated for several distances, d, (taken from the bottom of the scintillation region to the photomultiplier´s photocathode), in the 0-50 mm range and compared with experimental results. It is shown that for a 2-in photomultiplier d distances from 10-20 mm, commonly used in the detector designs, represent a worst-case situation. The best design performance for GPSC is obtained bringing the scintillation region closer to the photomultiplier window. The advantages of this design consist not only in a light collection improvement but also, and more importantly, in a small dependence of the detector amplitude on the X-ray radial interaction position, a factor that limits the detector resolution for broad X-ray beams.<>