Large-area silicon drift detectors for new applications in nuclear medicine imaging

In this paper, we have investigated the performance of a large-area (1 cm²) square-shaped silicon drift detector (SDD) operating as an X-ray detector in direct conversion mode, as well as a photodetector in combination with a single crystal CsI:Tl scintillator (10 mm × 10 mm × 7 mm). An external FET, along with a low-noise charge sensitive preamplifier, was used for testing the prototype detector in the temperature range of -50 to +26°C. In direct conversion mode, the SDD exhibited an energy resolution of 195 eV FWHM (at 5.9 keV, -23.5°C, 4 μs shaping time) and 170 eV FWHM (at 5.9 keV, -50°C, 8 μs shaping time). Pulse height spectra and signal rise times were recorded at different X-ray interaction locations across the SDD surface with the aid of a moveable collimator. Depending on collimator position, we measured signal rise times between 100 and 750 ns. After coupling the CsI-scintillator directly to the SDD surface, we obtained an excellent energy resolution of 7.0% FWHM with an uncollimated ⁵⁷Co source (122 keV, -10°C, 12 μs shaping time). At -23.5°C, we measured a signal rise time of approximately 25 μs for this scintillator-SDD unit. Using a pulsed LED as excitation source, we were able to evaluate the electron drift characteristics of the tested SDD and to identify the temporal behavior of the CsI:Tl scintillator to be the dominant component of this long rise time. A short analysis regarding the different SDD signal rise times with and without scintillator will be presented.