AN ISPA-CAMERA FOR GAMMA-RAYS WITH IMPROVED ENERGY RESOLUTION

An Imaging Silicon Pixel Array (ISPA)-tube with nonfiber optic quartz window has been tested with respect to spatial and energy resolutions. The 122-keV gamma-quanta of a Co-57 source was converted in a planar YAP-disc or in optically separated YAP-pillars of 0.6 x 0.6-mm(2) or 0 .3 x 0.3-mm(2) cross sections. The achieved intrinsic spatial resoluti on is worsened when compared to the ISPA-version with fiber optic glas s window but is still in the submillimeter region. As the YAP emission spectrum is fully covered by the spectral transmission of the quartz window and well adapted to the photocathode quantum efficiency spectru m, the resulting improved energy resolution allows for triggering of I SPA-events within a selected energy window at the 122-keV total absorp tion peak. This mode of operation, since it strongly reduces the numbe r of unwanted events, is essential for the application of a gamma came ra in environmental conditions where a wider energy range from other b ackground sources may be present.