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 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 to 50 mm range), and compared with experimental results. It is shown that for a two-inch photomultiplier, d distances from 10 to 20 mm, commonly used in the detector designs, represent a worst-case situation