Position sensitive photon counting with an ISPA tube

The newly developed Imaging Silicon Pixel Array (ISPA) tube consists of a photocathode viewed at a 30-mm distance by a silicon chip, which contains 1024 pixels with 75 /spl mu/m/spl times/500 /spl mu/m edges. With this tube we imaged, as an example of a weak light source, /spl beta/-tracks (/sup 90/Sr) traversing a fused square bundle (2.5 mm edges), which contains 1600 individual scintillating fibers of 60-/spl mu/m transverse dimension. Simultaneously we counted the number of photoelectrons/mm (hit density) at different source positions along the 2-m fiber bundle, with potential differences varying from 10 kV to 26 kV between photocathode and pixel anode of the ISPA tube, and at different threshold settings of the pixel chip. The obtained hit densities are compared with those measured with a Hybrid Photomultiplier Tube (HPMT), which contains a silicon pin diode as anode instead of a pixel chip. >