Spatial uniformity and UV sensitivity of microsphere plates (MSPs)

We have recently reported on a detailed study of basic microsphere plate (MSP) operation [A.S. Tremsin et al., Nucl. Instr. Meth. A 368 (1996) 719]. In this paper, we describe, with specific regard to imaging applications, the spatial uniformity of MSP quantum efficiency and gain. Measurements were made using 0.7 and 1.4 mm thick MSPs, both ''bare'' (nichrome-electroded) and coated with an Al layer for enhanced UV detection efficiency. As might be expected from the granular structure of the MSP, the relative variation in pulse gain dG/G was found to decrease with scale size, falling from similar to 12% when averaged over 0.18 mm areas to similar to 7% for 1 mm square cells. The efficiency of the Al-coated MSP to 2540 Angstrom radiation was one order of magnitude higher than that of the bare plate. The details of image transfer from the input to the output face of the MSP were measured by scanning a fine slit across the working diameter of the detector.