Speaker
Description
Photon-counting computed tomography (PCCT) has attracted considerable interest as a next-generation medical X-ray imaging modality because of its high spatial resolution, intrinsic spectral sensitivity, and improved dose efficiency [1]. While current commercial PCCT systems mainly employ direct-conversion semiconductor detectors, indirect photon-counting detectors based on scintillators and silicon photomultipliers are also being investigated as future alternatives [1,2]. In such high-flux X-ray imaging systems, nanosecond-scale scintillation decay times are essential for suppressing pulse pile-up and maintaining high count-rate capability [2]. Therefore, a measurement framework that enables precise and quantitative evaluation of short scintillation decay constants under X-ray excitation is required. In this work, we numerically investigate a frequency-domain approach for evaluating scintillation decay time constants under sinusoidally modulated X-ray excitation.
Frequency-domain lifetime measurements based on modulation depth and phase lag are well established in luminescence spectroscopy [3], and their applicability to scintillator-related materials has been demonstrated for ZnSe-based scintillators [4]. In contrast, radioluminescence decay under X-ray excitation has mainly been studied in the time domain, for example by time-resolved X-ray excited optical luminescence techniques [5].
In this study, the scintillation process was modeled as a linear response system, and the amplitude ratio $|H(\omega)|$ and phase lag $\angle H(\omega)$ of the radioluminescence signal relative to the X-ray modulation were calculated over a modulation-frequency range from 1 kHz to 100 MHz. Representative scintillators, including NaI(Tl), CsI(Tl), Gd$_3$(Al, Ga)$_5$O$_{12}$(Ce) (GAGG), Bi$_{4}$Ge$_3$O$_{12}$ (BGO), CdWO$_4$ (CWO), and CeBr$_3$, were compared. A triple-component decay model was also examined using $\beta$-Ga$_2$O$_3$.
As shown in Fig. 1, the calculated frequency responses clearly depend on the decay time constant. Faster scintillators maintain higher amplitude ratios and smaller phase lags up to higher frequencies, whereas slower scintillators exhibit earlier roll-off and larger phase lags. Distinct separation among representative scintillators was obtained in both amplitude and phase responses, indicating that decay constants can be estimated by combined fitting of these quantities. In the multi-component case, non-monotonic phase behavior was observed, suggesting the possibility of identifying multiple decay channels in the frequency domain.
These results support the feasibility of lock-in-based radioluminescence measurements using a modulated X-ray source and provide a design basis for experimental validation of scintillation lifetime metrology for high-count-rate X-ray imaging applications.
[1] F. R. Schwartz et al., Radiology 314 (2025), e240662.
[2] J. J. van Blaaderen et al., Chem. Mater. 37 (2025), 1716–1740.
[3] N. Boens et al., Anal. Chem. 79 (2007), 2137–2149.
[4] J. Mickevičius et al., Nucl. Instrum. Methods Phys. Res. A 610 (2009), 321–324.
[5] M. J. Ward et al., J. Phys.: Conf. Ser. 425 (2013), 092006.