Future experiments in high energy physics and medical imaging require radiation detectors having properties which are not presently available. The main limitations arise from a lack of suitable scintillation crystals. This dilemma prompts the need for research leading to the discovery of new fast and bright scintillator materials that combine unique properties to fulfil modern experiment requirements without compromises. In this work, single crystals of Cs2MgCl4 and Cs3MgCl5 up to 12 mm in diameter are grown via the vertical Bridgman method. Scintillation properties are reported for the first time, and core valence luminescence is observed for both compounds. X-ray excited radioluminescence emission of Cs2MgCl4 is centered at 295 nm, with a scintillation decay time of 2.25 +/- 0.05 ns and relatively high core-valence light yield of 2200 +/- 110 ph per MeV. Cs3MgCl5 has two main emission peaks centered at 242 nm and 302 nm, a decay time of 1.46 +/- 0.05 ns, and a light yield of 1340 +/- 70 ph per MeV. The better coincidence time resolution (CTR) is obtained with Cs2MgCl4, which is measured to be 129 +/- 4 ps FWHM. Density functional theory (DFT) calculations are also performed and provide supporting evidence that the observed scintillation originates from core valence luminescence. The combination of speed and brightness of these new scintillators could be useful for fast timing applications in which moderately dense materials are required.

Objective. Modern PET scanners offer precise TOF information, improving the SNR of the reconstructed images. Timing calibrations are performed to reduce the worsening effects of the system components and provide valuable TOF information. Traditional calibration procedures often provide static or linear corrections, with the drawback that higher-order skews or event-to-event corrections are not addressed. Novel research demonstrated significant improvements in the reachable timing resolutions when combining conventional calibration approaches with machine learning, with the disadvantage of extensive calibration times infeasible for a clinical application. In this work, we made the first steps towards an in-system application and analyzed the effects of varying data sparsity on a machine learning timing calibration, aiming to accelerate the calibration time. Furthermore, we demonstrated the versatility of our calibration concept by applying the procedure for the first time to analog readout technology. Approach. We modified experimentally acquired calibration data used for training regarding their statistical and spatial sparsity, mimicking reduced measurement time and variability of the training data. Trained models were tested on unseen test data, characterized by fine spatial sampling and rich statistics. In total, 80 decision tree models with the same hyperparameter settings, were trained and holistically evaluated regarding data scientific, physics-based, and PET-based quality criteria. Main results. The calibration procedure can be heavily reduced from several days to some minutes without sacrificing quality and still significantly improving the timing resolution from ( 304 ± 5 ) ps to ( 216 ± 1 ) ps compared to conventionally used analytical calibration methods. Significance. This work serves as the first step in making the developed machine learning-based calibration suitable for an in-system application to profit from the method’s capabilities on the system level. Furthermore, this work demonstrates the functionality of the methodology on detectors using analog readout technology. The proposed holistic evaluation criteria here serve as a guideline for future evaluations of machine learning-based calibration approaches.

Scintillators with faster timing capabilities are currently in high demand for use in radiation detection systems in the fields of nuclear and medical physics. The limited number of suitable materials that meet the performance criteria of next generation detection systems presents an opportunity for discovery of new fast scintillator materials. In this work, the effects of doping several ultrafast core-valence luminescent (CVL) scintillators with divalent Zn is explored. Three compounds are investigated – CsMgCl3, Cs2MgCl4, and Cs3MgCl5 – and single crystals of each doped with 5 mol% Zn are grown via the Bridgman method. Additionally, mixing across the full range of concentrations (from 0% to 100% Zn) is explored in the Cs2Mg1-xZnxCl4 and Cs3Mg1-xZnxCl5 systems. For low concentrations of Zn, light yields of all three compounds are enhanced (by up to ∼60%) compared to the pure crystals, achieving what we believe to be the brightest known CVL, CsMgCl3:Zn 5% (3,400 ± 170 ph/MeV light yield). More importantly, Zn doping does not affect the ultrafast timing properties, with each composition maintaining a single-component decay time around 1-3 nanoseconds. A sub-100 ps coincidence time resolution (CTR) is also achieved with CsMgCl3:Zn 5%. The results of this work reveal a new avenue towards obtaining brighter CVL materials, which could open up possibilities for more advanced ultrafast scintillators to be discovered moving forward.

Due to their reported fast decay times, Cs2ZnCl4 and Cs3ZnCl5 are promising candidates for detection of gamma rays and X-rays in high count rate and fast timing applications. In this work, we show that single crystals with better optical quality than previously demonstrated - and larger in size - can be grown via the vertical Bridgman method. Highly transparent (sic)7 mm crystals of undoped Cs2ZnCl4 and Cs3ZnCl5 are grown and measured to have light yields surpassing those previously reported, achieving 1980 +/- 100 ph/MeV and 1460 +/- 70 ph/MeV at 662 keV - a 55% and 232% improvement, respectively. We observe single-component scintillation decay times for both Cs2ZnCl4 (1.66 ns) and Cs3ZnCl5 (0.82 ns) and radioluminescence emission with maximum intensity at similar to 290 nm. Scalability of these materials is also evaluated based on growth of (sic)22 mm crystals. Minimal cracking is observed, and the fast decay times are maintained at this size. Coincidence time resolution of 3 x 3 x 5 mm3 and 7 x 7 x 10 mm(3) pixels cut from (sic)22 mm Cs2ZnCl4 are measured to be 148 +/- 1 ps FWHM and 175 +/- 1 ps FWHM, respectively. The improved performance and ability to be fabricated in large sizes now place Cs2ZnCl4 and Cs3ZnCl5 on the map as potential contenders for radiation detection applications where BaF2 - the most commonly used ultrafast inorganic scintillator - is typically considered.

Compared to photon therapy, proton therapy allows a better conformation of the dose to the tumor volume with reduced radiation dose to co-irradiated tissues. In vivo verification techniques including positron emission tomography (PET) have been proposed as quality assurance tools to mitigate proton range uncertainties. Detection of differences between planned and actual dose delivery on a short timescale provides a fast trigger for corrective actions. Conventional PET-based imaging of 15O (T1/2 = 2 min) and 11C (T1/2 = 20 min) distributions precludes such immediate feedback. We here present a demonstration of near real-time range verification by means of PET imaging of 12N (T1/2 = 11 ms). PMMA and graphite targets were irradiated with a 150 MeV proton pencil beam consisting of a series of pulses of 10 ms beam-on and 90 ms beam-off. Two modules of a modified Siemens Biograph mCT PET scanner (21 × 21 cm2 each), installed 25 cm apart, were used to image the beam-induced PET activity during the beam-off periods. The modifications enable the detectors to be switched off during the beam-on periods. 12N images were reconstructed using planar tomography. Using a 1D projection of the 2D reconstructed 12N image, the activity range was obtained from a fit of the activity profile with a sigmoid function. Range shifts due to modified target configurations were assessed for multiples of the clinically relevant 108 protons per pulse (approximately equal to the highest intensity spots in the pencil beam scanning delivery of a dose of 1 Gy over a cubic 1 l volume). The standard deviation of the activity range, determined from 30 datasets obtained from three irradiations on PMMA and graphite targets, was found to be 2.5 and 2.6 mm (1σ) with 108 protons per pulse and 0.9 and 0.8 mm (1σ) with 109 protons per pulse. Analytical extrapolation of the results from this study shows that using a scanner with a solid angle coverage of 57%, with optimized detector switching and spot delivery times much smaller than the 12N half-life, an activity range measurement precision of 2.0 mm (1σ) and 1.3 mm (1σ) within 50 ms into an irradiation with 4 × 107 and 108 protons per pencil beam spot can be potentially realized. Aggregated imaging of neighboring spots or, if possible, increasing the number of protons for a few probe beam spots will enable the realization of higher precision range measurement.

Real-time range verification of particle beams is important for optimal exploitation of the tissue-sparing advantages of particle therapy. Positron Emission Tomography (PET) of the beam-induced positron emitters such as 15O (T1/2 = 122 s) and 11C (T1/2 = 1223 s) has been used for monitoring of therapy in both clinical and preclinical studies. However, the half-lives of these nuclides preclude prompt feedback, i.e., on a sub-second timescale, on dose delivery. The in vivo verification technique relying on the in-beam PET imaging of very short-lived positron emitters such as 12N (T1/2 = 11 ms), recently proposed and investigated in feasibility experiments with a proton beam, provides millimeter precision in range measurement a few tens of milliseconds after the start of an irradiation. With the increasing interest in helium therapy, it becomes relevant to study the feasibility of prompt feedback using PET also for helium beams. A recent study has demonstrated the production of very short-lived nuclides (T1/2 = 10 ms attributed to 12N and/or 13O) during irradiation of water and graphite with helium ions. This work is aimed at investigating the range verification potential of imaging these very short-lived nuclides. PMMA targets were irradiated with a 90 AMeV 4He pencil beam consisting of a series of pulses of 10 ms beam-on and 90 ms beam-off. Two modules of a modified Siemens Biograph mCT PET scanner (21 × 21 cm2), installed 25 cm apart, were used to image the beam-induced PET activity during the beam-off periods. For the irradiation of PMMA, we identify the very short-lived activity earlier observed to be 12N (T1/2 = 11.0 ms). The range precision determined from the 12N activity profile that is measured after just one beam pulse was found to be 9.0 and 4.1 mm (1σ) with 1.3 × 1074He ions per pulse and 6.6 × 1074He ions per pulse, respectively. When considering 4.0 × 1074He ions, which is about the intensity of the most intense distal layer spot in a helium therapy plan, a range verification precision in PMMA of 5.7 mm (1σ) can be realized. The range precision scales approximately with the inverse square root of the number of 4He ions, i.e., the relative statistical accuracy of the number of coincidence events. Thus, when summing data over about 10 distal layer spots, this study shows good prospects for obtaining 1.8 mm (1σ) precision in range verification, within 50 ms after the start of a helium irradiation by in-beam PET imaging (scanner 29% solid angle) of 12N.

Here, we evaluated fast Lu-based scintillation crystals for construction of a PET system with 100-ps coincidence time resolution (CTR). Four 3×3×10 mm 3 LGSO crystals with decay times ranging from 33-38 ns, and 3×3×10 mm 3 and 1.9×1.9×10 mm 3 LSO crystals with decay times of 34-36 ns were tested. In order to achieve excellent CTR, we used a novel side-readout detector configuration where the long face of each crystal rod is coupled to multiple photo sensors. The light output, decay time, and energy performance were measured with a fast PMT. CTR was measured with crystals coupled side-on to SiPMs and waveforms were digitally sampled. In all of these studies we achieved ≤100 ps CTR.

We describe a long axial field-of-view (FOV) PET scanner for high-sensitivity and total-body imaging of nonhuman primates and present the physical performance and first phantom and animal imaging results. Methods: The mini-EXPLORER PET scanner was built using the components of a clinical scanner reconfigured with a detector ring diameter of 43.5 cm and an axial length of 45.7 cm. National Electrical Manufacturers Association (NEMA) NU-2 and NU-4 phantoms were used to measure sensitivity and count rate performance. Reconstructed spatial resolution was investigated by imaging a radially stepped point source and a Derenzo phantom. The effect of the wide acceptance angle was investigated by comparing performance with maximum acceptance angles of 14°–46°. Lastly, an initial assessment of the in vivo performance of the mini-EXPLORER was undertaken with a dynamic 18F-FDG nonhuman primate (rhesus monkey) imaging study. Results: The NU-2 total sensitivity was 5.0%, and the peak noise-equivalent count rate measured with the NU-4 monkey scatter phantom was 1,741 kcps, both obtained using the maximum acceptance angle (46°). The NU-4 scatter fraction was 16.5%, less than 1% higher than with a 14° acceptance angle. The reconstructed spatial resolution was approximately 3.0 mm at the center of the FOV, with a minor loss in axial spatial resolution (0.5 mm) when the acceptance angle increased from 14° to 46°. The rhesus monkey 18F-FDG study demonstrated the benefit of the high sensitivity of the mini-EXPLORER, including fast imaging (1-s early frames), excellent image quality (30-s and 5-min frames), and late-time-point imaging (18 h after injection), all obtained at a single bed position that captured the major organs of the rhesus monkey. Conclusion: This study demonstrated the physical performance and imaging capabilities of a long axial FOV PET scanner designed for high-sensitivity imaging of nonhuman primates. Further, the results of this study suggest that a wide acceptance angle can be used with a long axial FOV scanner to maximize sensitivity while introducing only minor trade-offs such as a small increase in scatter fraction and slightly degraded axial spatial resolution.

In spite of the excellent performance of SiPMs in the application of time-of-flight PET detectors, the development of better photomultipliers for PET detectors is still worth considering. Fast timing with detectors based on the PMT light readout depends mainly on the time jitter and the quantum efficiency of the PMTs. Several studies in the past showed that a screening grid at the anode significantly improves the time resolution when measured with scintillation detectors. ADIT Co. has started a development of a 1 '' diameter fast PMT with the screening grid at the anode to be applied in the block detectors in TOF PET scanners. In the presented studies, we report the first comparative study of the L25, a classical PMT, with one that is equipped with a screening grid. Adding the screening grid yielded a large improvement in the time resolution, up to a factor of 1.2.

PURPOSE:In support of a project to build a total-body PET scanner with an axial field-of-view of 2 m, the authors are developing simple, cost-effective block detectors with combined time-of-flight (TOF) and depth-of-interaction (DOI) capabilities.METHODS:This work focuses on investigating the potential of phosphor-coated crystals with conventional PMT-based block detector readout to provide DOI information while preserving timing resolution. The authors explored a variety of phosphor-coating configurations with single crystals and crystal arrays. Several pulse shape discrimination techniques were investigated, including decay time, delayed charge integration (DCI), and average signal shapes.RESULTS:Pulse shape discrimination based on DCI provided the lowest DOI positioning error: 2 mm DOI positioning error was obtained with single phosphor-coated crystals while 3-3.5 mm DOI error was measured with the block detector module. Minimal timing resolution degradation was observed with single phosphor-coated crystals compared to uncoated crystals, and a timing resolution of 442 ps was obtained with phosphor-coated crystals in the block detector compared to 404 ps without phosphor coating. Flood maps showed a slight degradation in crystal resolvability with phosphor-coated crystals; however, all crystals could be resolved. Energy resolution was degraded by 3%-7% with phosphor-coated crystals compared to uncoated crystals.CONCLUSIONS:These results demonstrate the feasibility of obtaining TOF-DOI capabilities with simple block detector readout using phosphor-coated crystals.

The main aim of this work was characterization of the newest MPPC arrays (with 12×12mm and 24×24mm active area) made using through silicon via (TSV) technology in gamma-ray spectrometry with five different scintillators: CsI:Tl, NaI:Tl, LSO/LYSO, BGO, LaBr3. The results of the study are compared with that obtained previously with the older sample of the 12×12mm MPPC array made as a monolithic device. TSV MPPC array with the size of 24×24mm is one of the first commercially available SiPM with so large active area and with the dead space between channels minimized to only 0.2mm. Moreover in these devices Hamamatsu introduced resistors made with a new type of material. Such a change allowed lowering of crosstalks and afterpulses. These improved TSV MPPC arrays can be commercially used for scintillation light readout of “large” crystals with diameter of 1×1inch or 2×2inch, suitable for gamma spectrometry. Such combination may allow replacement of PMTs in monitoring handheld devices. The presented studies with MPPCs were carried out in the two main experimental conditions: direct illumination of the SiPM using a LED pulser and measurements with scintillators. The first method allowed the measurements of basic parameters of MPPCs such as breakdown voltage and excess noise factor (ENF). The second method allowed the measurements of the full scintillation detector parameters, such as: optimum operating voltage, number of photoelectrons (number of fired APD cells), photon detection efficiency (PDE), linearity and energy resolution.

The Wide-Angle Compton Coincidence (WACC) technique was adapted to measure the electrons produced in cadmium zinc telluride (CZT) detector by Compton scattered gamma rays of 662 keV energy emitted by a 137Cs source. The measurements were carried out with a 4 × 4 pixel anode array build on a 5 mm thick CZT crystal with 1.1 mm pitch. A High Purity Germanium (HPGe) detector operating in coincidence mode measured energy of the scattered photons. The coincidence pulse height data from the CZT and HPGe detectors were recorded in a list mode. The CZT electron spectra were determined by subtracting the energy of the scattered photons from the incident photon energy. The scattering angle was determined by selecting a narrow energy window in the collected HPGe spectra. The CZT electron linearity and resolution were measured in the energy range of 100 keV-300 keV corresponding to the scattering angle range of 31°-66°.

Compton cameras are of general interest in various fields of operation. Because of the ability to locate and identify remote sources, homeland security supports the development of such devices in a rugged and reliable form. The decisions upon appropriate materials for the scatter- and absorber plane depend on performance and economical trade-offs. In order to estimate the expected performance of the Compton camera, simulations are necessary. Certain experimentally determined parameters have to be fed into simulations, such as the energy resolution of the detector.Two materials with low effective atomic number (Z(eff)), CaF2 and plastic, promise to be good candidates for the scattering plane. Those scintillators are known for quite some time, but not very well characterized with respect of energy resolution and nonlinearity. A modified Compton coincidence technique using a high purity Germanium (HPGe) detector in coincidence with the investigated scintillator is discussed in this paper: The wide-angle Compton-coincidence (WACC) setup provides a fast and reliable means for characterization of low-Z scintillators. For quality control purposes, the actual scatter detector can be monitored inhouse using the WACC technique. This work presents results of different scintillator materials and sizes for validation and exploration of this method.

A development of silicon photomultipliers (SiPM), with a large number of micro-pixels and improved linearity of the pulse height response, allow discussing their possible application to gamma spectrometry with scintillators. Most of the measurements were done with Hamamatsu MPPCs sensors equipped with 3600 and 14400 pixels because of their well pronounced single photoelectron spectra. It allowed precisely measure the photoelectron (phe) numbers/fired pixels and then to discuss in a quantitative manner an obtainable energy resolution. The studied detectors were characterized first by means of a laser light pulser and then by 3×3×3 mm 3 LFS and CsI(Tl) crystals coupled to 3×3 mm 2 Hamamatsu MPPC. In the study with the light pulser, the linearity of the MPPC response, expressed in the photoelectron number measured in relation to the first photoelectron, versus a light pulse intensity observed with PMT was checked and then the pulse height resolution was measured. Through analysis of the energy resolution the lower limit of the phe number was derived, assuming Poisson statistic. Further on, based on a directly measured phe number, the discussion of an excess noise factor of MPPC was done. In scintillation tests the measurements of the energy resolution and non-proportionality of the light yield were done with LFS and CsI(Tl) and discussed in a quantitative way, based on the measured PHE number, for both types of 3×3 mm 2 MPPC detectors.

Silicon photomultipliers (SiPM) are relatively new photodetectors studied presently by many groups in the world. Experiments requiring good time resolution are one of the areas of interest for application of SiPMs. Experiments with high photon statistics and fast picosecond lasers showed that SiPMs time resolution can be extremely good, much below 100 ps. Unfortunately, in case of a full detector with a scintillation crystal like LSO, a high capacitance of this type of devices deteriorates the rise time of the output pulse. In consequence, the timing resolution measured with the slow pulses from SiPM could be worse than results obtained with photomultipliers. In this work a detailed studies of the time resolution with scintillation detectors based on a 3×3 mm 2 Multi Pixel Photon Counters (MPPCs) from Hamamatsu and LSO or LFS-3 crystals are presented. Most of the measurements were done using a detector with a pixel size of 50 μm (S10362–33–050C). The results of coincidence experiments with an 22 Na gamma source are analyzed in terms of number of photoelectrons, time jitter, excess noise factor and output pulse characteristics. The aim of the work is to present timing capabilities of scintillation detectors based on MPPC in comparison with detectors based on classic timing photomultipliers.

The observed discrepancy in the scintillator light output, measured by different PMTs, triggered studies to understand the problem. In that purpose the photoelectrons number were measured by two different methods: the classical one based on comparison of the full energy peak to that of single photoelectron and by a method based on the pulse height resolution of the peak due to the light pulser. Under the test significant number of different PMTs from Photonis and Hamamatsu were used. We concluded that the number of phe obtain by means of classical method was higher than the number of phe calculated from the pulse height resolution of the light pulser peak for all of the PMTs but XP2020Q. It leads to large dispersion in the estimated light output for a given scintillator. In details, the light output of BGO and LSO determined with R6231 and R2095 is comparable to those measured with XP2020Q and S3590-18 pin photodiode, when photoelectron number calculated from the pulse height resolution is used. Further carried on in-depth studies of the photoelectron number at different HV suggested that the effect is related to the space charge created over the dynode structure of PMTs. Operation of PMTs at lower HV/gain minimizes this effect, thus low noise electronics is recommended to get the single photoelectron peak at this conditions. Moreover, the absolute light output of scintillators is affected by differences in the quantum efficiency calibration at Photonis and Hamamatsu.

The new 1 inch and 1.5 inch in diameter photomultipliers for timing applications from Photonis and Hamamatsu have been tested. Time resolution of XP1020, XP3060, R9800 and R9420 was measured with a 10 x 10 x 5 mm(3) LSO crystal in coincidence experiments with 511 keV annihilation quanta from Na-22 gamma source. Results were discussed in terms of measured photoelectron number and time jitter. Single photoelectron spectra were recorded and excess noise factor for each tube was also calculated. The final comparison of the tested tubes and their timing properties were presented in relation to large amount of experimental data of various types of PMTs collected during last few years. Especially, observed linear dependency between the time resolution normalized to the number of photoelectrons and time jitter was pointed out. Inconsistency of the data collected with the Hamamatsu R9420 PMT resulting from the overestimated photoelectron number was reported and further studied. Additional experiments with LED light source and different experimental set-ups were discussed and comparison of the two methods of the photoelectron number measurements was performed.

A growing interest in the development of dual modality PET/CT scanners prompts the comparative study of numerous scintillators to select the best one, which could be used simultaneously in PET detectors working in the pulsing mode and in the CT detectors working in the current mode. In the comparative measurements, done in the same experimental conditions, various samples of BGO, GSO, GSO:Ce,Zr, LGSO, LSO, LYSO, MLS, LaCl3, LaBr3 and CWO scintillators were tested. The measurements covered a determination of the light output, energy resolution, non-proportionality of the light yield, decay times of the light pulses and for the selected crystals their time resolution for 511 keV annihilation quanta. Moreover, a comparative study of afterglow, induced by 60 keV gamma-rays from a strong 241Am source (13.9 GBq), was done in the second range