Investigation of electron multiplication effect in optical property modulation-based radiation detection method for PET

In this paper, we further explore the optical property modulation-based method for ionizing radiation photon detection in PET as a potential new direction to dramatically improve the coincidence time resolution. We compare the performance of three detector crystals for this method including two types of cadmium telluride (CdTe) crystals and one bismuth silicon oxide (BSO) crystal under high bias voltages up to 3500V. We first show that the induced current flow in the detector crystal determines the strength of the optical property modulation signal due to ionization. A larger resistivity is favorable for reducing the dark current (noise) in the crystal and facilitates the detection of weak optical property modulation signals. In addition, we show that BSO is a potential candidate detector material. When biased at 3500 V, it has comparable modulation signal sensitivity as CdTe biased at 1000V, but with higher resistivity (lower noise), larger 511 keV photon attenuation coefficient, lower price, better crystal surface finish quality, and less toxicity. By studying the dependence of modulation signal amplitude on crystal bias voltage, we show that the modulation signal amplitude (induced by both UV laser diode and Ge-68 source) is linearly proportional to crystal bias voltage with a linear fit R factor of around 0.95. The modulation signal amplitude induced by UV laser diode irradiation increases from 0% to 2% (normalized to the average signal level) for both CdTe and BSO under crystal bias voltage from 0V to 3500V. The modulation signal amplitude induced by Ge-68 irradiation increases from 0% to 12% for CdTe under crystal bias voltage from 0V to 1500 V, and increases from 0% to 10% for BSO under crystal bias voltage from 0V to 3500 V. Therefore the electron multiplication effect (with high crystal bias) shows promise to significantly boost the modulation signal amplitude with the ultimate goal to achieve single 511 keV photon detection.