Approaches for Measuring Fast Modulation of Optical Properties Induced by 511keV Photon Interactions

With a goal to achieve less than 10 picosecond (ps) coincidence time resolution for advancing time-of-flight positron emission tomography (TOF-PET), we seek to leverage the effects of transient free carriers generated from 511 keV interactions in a material. Most ultrafast optics methods rely on many excitation particles with a priori information on their arrival times. However, in PET it is critical to find a way to sensitively detect single, randomly emitted 511 keV annihilation photons from the ionization it produces. We investigate two ways to increase the signal: employing probe laser wavelength in the mid-IR and confining the probe beam to match the size of ionization tracks created by the 511 keV photons. We observed a continued decrease of 2% in transmission of the mid-IR probe beam (8μm) in a 10 X 10 X 1 mm3 Cadmium Telluride crystal with 511 keV annihilation irradiation over 300 seconds. The trend was reversed when the radiation source is removed, and reached the previous baseline transmission in 800 seconds. For confining the interaction volume to match the probe beam diameter, we revised a previously investigated interferometry approach to use a focused beam and did not apply a bias voltage across the detector crystal. With a 405nm laser diode as an excitation source, a shift of 43% in transmitted signal voltage was observed, and with Am-241 alpha source irradiation, a 1% shift was measured. With the improved sensitivity, these findings provide insights into possible methods of detecting single 511 keV events.