Characterization of multi-pixel HPD tubes

Both commercial 7-pixel and experimental 19-pixel HPD tubes have been tested for response uniformity. The integrated charge response indicates a uniformity close to the percent level both across the surface of each pixel and the pixel-to-pixel borders. Full response uniformity is obtained only well above the Si-diode full-depletion voltage. In order to better understand this fact, the charge collection process has been thoroughly studies as a function of bias and position, thereby offering an explanation to some of the nonuniformities. Occasionally small dimples and scars are seen to spoil the HPD uniformity. These dimples were found to be caused by high power, strongly focused light bursts produced during the setup time and were observed to fade and disappear in the time scale of days. Finally the seven pixel HPDs were used to localize an optical fiber emitting light flashes of variable amplitude; 150 μm position resolution with 100 photoelectron light pulse amplitude were achieved.