In this work, we continue the study of an optical based method for annihilation photon detection with the potential for a dramatic improvement in time resolution for time-of-flight positron emission tomography (ToF-PET). Previous work has shown that the refractive index of materials such as bismuth silicon oxide (BSO) and cadmium telluride (CdTe) can be modulated by the charge cloud created by annihilation photon interactions, though the ultrafast nature of the index modulation process remains untested. At the Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory, the arrival time of X-ray pulses with photon energies between 0.5–10 keV is routinely detected with femtosecond scale time resolution. The ionizing interactions alter the local band structure in optically transparent insulators, changing the refractive index. Using a frequency chirped visible continuum probe pulse for a monotonic wavelength-to-time mapping, we measured the induced refractive index modulation, with interferometric sensitivity, and a sub-picosecond time resolution. In this work, we show that femtosecond scale resolution can be achieved for photon arrival time measurement using the refractive index modulation mechanism. This new detection concept has the potential to also achieve significantly improved timing capability for ToF-PET.
We compare the performance of two detector materials, cadmium telluride (CdTe) and bismuth silicon oxide (BSO), for optical property modulation-based radiation detection method for positron emission tomography (PET), which is a potential new direction to dramatically improve the annihilation photon pair coincidence time resolution. We have shown that the induced current flow in the detector crystal resulting from ionizing radiation determines the strength of optical modulation signal. A larger resistivity is favorable for reducing the dark current (noise) in the detector crystal, and thus the higher resistivity BSO crystal has a lower (50% lower on average) noise level than CdTe. The CdTe and BSO crystals can achieve the same sensitivity under laser diode illumination at the same crystal bias voltage condition while the BSO crystal is not as sensitive to 511-keV photons as the CdTe crystal under the same crystal bias voltage. The amplitude of the modulation signal induced by 511-keV photons in BSO crystal is around 30% of that induced in CdTe crystal under the same bias condition. In addition, we have found that the optical modulation strength increases linearly with crystal bias voltage before saturation. The modulation signal with CdTe tends to saturate at bias voltages higher than 1500 V due to its lower resistivity (thus larger dark current) while the modulation signal strength with BSO still increases after 3500 V. Further increasing the bias voltage for BSO could potentially further enhance the modulation strength and thus, the sensitivity.
Using conventional scintillation detection, the fundamental limit in positron emission tomography (PET) annihilation photon pair coincidence time resolution is strongly dependent on the inherent temporal variances generated during the scintillation process, yielding an intrinsic physical limit of around 100 ps. On the other hand, modulation mechanisms of a material's optical properties as exploited in the optical telecommunications industry can be orders of magnitude faster. In this paper we borrow from the concept of optics pump-probe measurement to study whether ionizing radiation can also produce fast modulations of optical properties, which can be utilized as a novel method for radiation detection. We show that a refractive index modulation of approximately 5x10-6 is induced by interactions in a cadmium telluride (CdTe) crystal from a 511 keV photon source. Furthermore, using additional radionuclide sources, we show that the amplitude of the optical modulation signal varies linearly with both the radiation source flux rate and average photon energy.
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