Kazuma Yokoi

Effects of Gamma-Ray Damage on Energy Spectra and Polarization of a CdTe Detector With the Schottky Barrier for Imaging Equipment

Effects of gamma-ray radiation damage on energy spectra and polarization of the CdTe radiation detector with the Schottky barrier are demonstrated for application to the gamma-ray imaging equipment in diagnostic nuclear medicine. In particular, the polarization recovery mechanism by momentary bias-off (bias refreshment) is incorporated into the detector system, and the effect of radiation damage on the bias refreshment is quantitatively evaluated. The Schottky barrier is formed by using In as the anode of the CdTe detector. The gamma-ray irradiation source for the radiation damage test is 60Co. Gamma-ray count rate in an energy window and leakage current between the cathode and the anode are not affected until irradiation at 6.83 kGy. Energy resolution is not affected until irradiation at 66.8 Gy, and there is significant deterioration when irradiation is more than 1.29 kGy. Polarization of the CdTe detector is hardly affected and can be recovered by the bias refreshment for irradiation at 66.8 Gy. However, complete recovery becomes impossible at more than 1.29 kGy irradiation. In conclusion, it is demonstrated that the CdTe detector can be used as the gamma-ray detector in SPECT (Single Photon Emission Computed Tomography) equipment for more than 20 years, and continuous SPECT examinations lasting more than 120 min become possible with the bias refreshment

Simulation study of scatter correction in photon counting CT

To understand how the signals are affected by the radiation scattered by the test subject in Photon Counting CT system, the characteristics of the scattered photons were evaluated using Monte Carlo simulation “GEANT” (GEometry ANd Tracking). Cylinder water phantoms with diameters of 165 – 380 mm were examined, and the X-ray energy from 20 to 120 keV was divided into 5 ranges with a width of 20 keV in the detector. With the phantom with the diameter of 380 mm, the ratio of signals which are scattered in the phantom to those of the total X-rays incident on the detector turned out to be more than 50% in the 20 - 40 keV range, while it remained 2% in the 100 - 120 keV one. The profiles of this ratio were approximated by a quadratic function αx2 +β in each energy range where x corresponds to the longitudinal detector position. It was found that α and β can be described with the energy range and phantom size.