Detection of Ionization-Induced Charge Carriers by Probing with Sub-Terahertz Electromagnetic Waves

To realize sub 10 picosecond coincidence time resolution in positron emission tomography, we propose to use methods of “pump-probe spectroscopy” on the ionization produced by 511 keV photon interactions. The enhancement of light-matter interaction between probe light and ionization charge carriers induced by 511 keV annihilation photon interactions is the key to improving the sensitivity to a single photon detection level. The optical modulation caused by ionization charge carriers can be maximized in pump-probe methodology with terahertz wave. The energy level of the photon in this range is suitable to directly interact with free charge carriers so that the optical property modulation can be directly enhanced. We observed a 9.0% of transmission change with and without beta irradiation using terahertz time domain spectroscopy with near-field antenna. The change in optical properties such as refractive index or absorption for a given density of charge carriers is described by what is known as the “Drude Model.” The measured conductance change induced by beta irradiation in terahertz frequency range fits to the Drude-like dispersion of free charge carriers with root mean square error of 0.70 µΩ−1. The optical intensity modulation simulated at 0.2 THz was 68.40%, 18.69 times larger than the mean optical intensity modulation with probe laser wavelength of 1.06 µm, These results show the feasibility of detecting single annihilation photons and layout a road map using sub-THz probes.