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Dark-count-less x-ray photon counting using an LSO-MPPC detector and its application to computed tomography

X-ray photons are detected using an Lu2(SiO4)O [LSO] single-crystal scintillator with a decay time of 40 ns and a multipixel photon counter (MPPC). The photocurrent from the MPPC is amplified by a high-speed current-voltage amplifier with an 80 MHz-gain-band operational amplifier, and the 200-ns-width event pulses are sent to a multichannel analyzer to measure X-ray spectra. The MPPC is driven in the pre-Geiger mode at a bias voltage of 70.7 V and a temperature of 23°C. Photon-counting computed tomography (PC-CT) is accomplished by repeated linear scans and rotations of an object, and projection curves of the object are obtained by linear scanning. The exposure time for obtaining a tomogram was 10 min with scan steps of 0.5 mm and rotation steps of 1.0°. At a tube voltage of 100 kV, the maximum count rate was 350 kcps/pixel. We carried out PC-CT using gadolinium media and confirmed the energydispersive effect with changes in the lower level voltage of event pulses using a comparator.