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We study the self-collimation phenomenon in photonic crystals (PhC) of wide bandgap materials for ultra-fast and high spatial resolution x-ray detection. We work on various heavy inorganic scintillators: BaF2, GaN, ZnO, CsI:Tl, NaI:Tl, LYSO, WO4 compounds, and plastic scintillators. Conventional scintillator detectors do not rely on a direct detection mechanism; hence, they require intricate design and fabrication processes. We offer a PhC design to observe self-collimation phenomena and overcome the ongoing spatial resolution challenges with these types of materials. We investigate the photonic band diagrams and iso-frequency contours. Fourier transforms based on finite-difference time-domain and frequency domain simulations are done for verifying and analyzing the self-collimation with the selected material. Light extraction efficiency at the PhC–air interface, depending on the truncation distance from the excitation point, is measured. Beam divergence values are calculated at 1 mm propagation distance. The vertical field profiles are obtained to observe the confinement. For the spatial resolution analysis, cross-sectional beam profiles have been examined. Gaussian envelopes are fitted to beam profiles for a consistent data analysis, and full-width-at-half-maximum values are considered. As a result, we theoretically prove and demonstrate the spatially resolved x-ray detection at the sub-micrometer level for a wide range of scintillator materials. [ABSTRACT FROM AUTHOR]

Solid state photodetectors like silicon photomultipliers (SiPMs) are playing an important role in several fields of medical imaging, life sciences and high energy physics. They are able to sense optical photons with a single photon detection time precision below 100 ps, making them ideal candidates to read the photons generated by fast scintillators in time of flight positron emission tomography (TOF-PET). By implementing novel high-frequency readout electronics, it is possible to perform a completely new evaluation of the best timing performance achievable with state-of-the-art analog-SiPMs and scintillation materials. The intrinsic SiPM single photon time resolution (SPTR) was measured with Ketek, HPK, FBK, SensL and Broadcom devices. Also, the best achieved coincidence time resolution (CTR) for these devices was measured with LSO:Ce:Ca of mm3 and mm3 size crystals. The intrinsic SPTR for all devices ranges between 70 ps and 135 ps FWHM when illuminating the entire mm2 or mm2 area. The obtained CTR with LSO:Ce:Ca of mm3 size ranges between 58 ps and 76 ps FWHM for the SiPMs evaluated. Bismuth Germanate (BGO), read out with state of-the-art NUV-HD SiPMs from FBK, achieved a CTR of 158 ps and 277 ps FWHM for mm3 and mm3 crystals, respectively. Other BGO geometries yielded 167 3 ps FWHM for mm3 and 235 5 ps FWHM for mm3 also coupled with Meltmount (n = 1.582) and wrapped in Teflon. Additionally, the average number of Cherenkov photons produced by BGO in each 511 keV event was measured to be 17 3 photons. Based on this measurement, we predict the limits of BGO for ultrafast timing in TOF-PET with Monte Carlo simulations. Plastic scintillators (BC422, BC418), BaF2, GAGG:Ce codoped with Mg and CsI:undoped were also tested for TOF performance. Indeed, BC422 can achieve a CTR of 35 2 ps FWHM using only Compton interactions in the detector with a maximum deposited energy of 340 keV. BaF2 with its fast cross-luminescence enables a CTR of 51 5 ps FWHM when coupled to VUV-HD SiPMs from FBK, with only ∼22% photon detection efficiency (PDE). We summarize the measured CTR of the various scintillators and discuss their intrinsic timing performance. [ABSTRACT FROM AUTHOR]

A systematic study is conducted to understand the digital time of flight (TOF) response for a pair of fast scintillator detectors; BaF 2 –LaBr 3 , BaF 2 -BC501A, and LaBr 3 -BC501A respectively. Coincidence signals from each detector pair, irradiated by 22Na source, are acquired by LeCroy HDO5000A digital oscilloscope. While operating the oscilloscope at 2.5 giga samples per second (GSPS) and 500 mega samples per second (MSPS) of sampling rates, over 10 k coincidence signals were collected for each pair of detectors. Data at different sampling rates; 1.66 GSPS, 1.25 GSPS, 833 MSPS, 625 MSPS, 333 MSPS, and 250 MSPS were generated by down-sampling method. Using anode signal, the photon arrival time marker is determined by digital constant fraction (DCF) algorithm for each detector pair on event-by-event basis. For a given rate, the algorithm is optimized at various delay and fraction values to get the minimum TOF dispersion. Mentioned pairs reveals the TOF resolution (FWHM) as; 0.58 ns, 0.79 ns, 0.907 ns and 0.53 ns, 0.94 ns, 1.08 ns at 250 MSPS and 2.5 GSPS respectively. While analysing the events, a saturation in the TOF width is observed from 500 MSPS onward. The saturated dispersion values for the aforementioned pairs are found to be 0.53 ns, 0.94 ns, and 1 ns respectively. Effect is being understood by calculating the optimum DCF transition region distribution for BC501A and LaBr 3 detectors, reveals a constant curvature profile at different rates. To further explain the saturation effect on computational ground, a simple pulse fitting approach is adopted using Landau distribution. Event-by-Event processing of fitted pulses followed by TOF resolution calculation reproduces the experimental numbers. [ABSTRACT FROM AUTHOR]