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 γ-quanta of a Co57 source was converted in a planar YAP-disc or in optically separated YAP-pillars of 0.6 x 0.6-mm2 or 0.3 x 0.3-mm2 cross sections. The achieved intrinsic spatial resolution is worsened when compared to the ISPA-version with fiber optic glass 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 spectrum, the resulting improved energy resolution allows for triggering of ISPA-events within a selected energy window at the 122-keV total absorption peak. This mode of operation, since it strongly reduces the number of unwanted events, is essential for the application of a γ camera in environmental conditions where a wider energy range from other background sources may be present