The emergence of new solid-state avalanche photodetectors, e.g. SiPMs, with unprecedented timing capabilities opens new ways to profit from ultrafast and prompt photon emission in scintillators. In ...time of flight positron emission tomography (TOF-PET) and high energy timing detectors based on scintillators the ultimate coincidence time resolution (CTR) achievable is proportional to the square root of the scintillation rise time, decay time and the reciprocal light yield, CTR∝τrτd∕LY. Hence, the precise study of light emission in the very first tens of picoseconds is indispensable to understand time resolution limitations imposed by the scintillator. We developed a time correlated single photon counting setup having a Gaussian impulse response function (IRF) of 63ps sigma, allowing to precisely measure the scintillation rise time of various materials with 511keV excitation. In L(Y)SO:Ce we found two rise time components, the first below the resolution of our setup <10 ps and a second component being ∼380 ps. Co-doping with Ca2+ completely suppresses the slow rise component leading to a very fast initial scintillation emission with a rise time of <10ps. A very similar behavior is observed in LGSO:Ce crystals. The results are further confirmed by complementary measurements using a streak-camera system with pulsed X-ray excitation and additional 511 keV excited measurements of Mg2+ co-doped LuAG:Ce, YAG:Ce and GAGG:Ce samples.
This paper presents new developments in inorganic scintillators widely used for radiation detection. It addresses major emerging research topics outlining current needs for applications and material ...sciences issues with the overall aim to provide an up-to-date picture of the field. While the traditional forms of scintillators have been crystals and ceramics, new research on films, nanoparticles, and microstructured materials is discussed as these material forms can bring new functionality and therefore find applications in radiation detection. The last part of the contribution reports on the very recent evolutions of the most advanced theories, methods, and analyses to describe the scintillation mechanisms.
Achieving fast timing in positron emission tomography (PET) at the level of few tens of picoseconds of picoseconds is limited by the photon emission rate of existent materials with standard ...scintillation mechanisms. This has led to consider quantum confined excitonic sub-1 ns emission in semiconductors as a viable solution to enhance the amount of fast-emitted photons produced per gamma event. However the introduction of nanocrystals and nanostructures into the domain of radiation detectors is a challenging problem. In order to move forward along this line, the standard bulk detector geometry and readout should be updated to allow for the implementation of new materials and within others, compensate for some of their intrinsic limitations. In this paper we will cover two study cases in which a fast emitter is combined with state-of-the-art scintillators in a sampling geometry designed to provide better timing for a fraction of the 511 keV events. For this test, we use a fast plastic scintillator BC-422 able to deliver a detector time resolution (DTR) of 25 ps FWHM (equivalent coincidence time resolution CTR of 35 ps) and we combined it with LYSO or BGO 200 m thick plates building a sampling pixel composed by two active scintillating materials. We develop a new proof of concept readout that allows for the identification of different types of events, carrying standard or improved timing information. Results are showing a DTR of 67 ps FWHM (equivalent to a CTR of 95 ps) for one third of the events depositing 511 keV in the BGO + BC-422 mm3 sampling pixel. The other two third of the 511 keV events perform like standard bulk 3 mm long BGO crystals with a time resolution of around 117 ps (equivalent to a CTR of 165 ps). For the case of LYSO + BC-422 sampling pixel, shared 511 keV events reach a DTR of 39 ps (CTR of 55 ps) in comparison to 57 ps (CTR of 83 ps) for 511 keV events fully contained in LYSO of the same size. This work is a step forward in the integration of fast semiconductor nanocrystals and nanostructures with present detector technologies.
Abstract
Large Stokes shift fast emitters show a negligible reabsorption of their luminescence, a feature highly desirable for several applications such as fluorescence imaging, solar-light managing, ...and fabricating sensitive scintillating detectors for medical imaging and high-rate high-energy physics experiments. Here we obtain high efficiency luminescence with significant Stokes shift by exploiting fluorescent conjugated acene building blocks arranged in nanocrystals. Two ligands of equal molecular length and connectivity, yet complementary electronic properties, are co-assembled by zirconium oxy-hydroxy clusters, generating crystalline hetero-ligand metal-organic framework (MOF) nanocrystals. The diffusion of singlet excitons within the MOF and the matching of ligands absorption and emission properties enables an ultrafast activation of the low energy emission in the 100 ps time scale. The hybrid nanocrystals show a fluorescence quantum efficiency of ~60% and a Stokes shift as large as 750 meV (~6000 cm
−1
), which suppresses the emission reabsorption also in bulk devices. The fabricated prototypal nanocomposite fast scintillator shows benchmark performances which compete with those of some inorganic and organic commercial systems.
The performance of a light sharing and recirculation mechanism that allows the extraction of depth of interaction (DOI) are investigated in this paper, with a particular focus on timing. In parallel, ...a method to optimize the coincidence time resolution (CTR) of PET detectors by use of the DOI information is proposed and tested. For these purposes, a dedicated 64-channels readout setup has been developed with intrinsic timing resolution of 16 ps FWHM. Several PET modules have been produced, based on LYSO:Ce scintillators and commercial silicon photomultiplier (SiPM) arrays, with mm2 individual SiPM size. The results show the possibility to achieve a timing resolution of 157 ps FWHM, combined with the already demonstrated spatial resolution of 1.5 mm FWHM, DOI resolution of 3 mm FWHM, and energy resolution of 9% FWHM at 511 keV, with 15 mm long crystals of section mm2 and mm2. At the same time, the extraction of the DOI coordinate has been demonstrated not to deteriorate the timing performance of the PET module.
A new method for obtaining depth of interaction (DOI) information in PET detectors is presented in this study, based on sharing and redirection of scintillation light among multiple detectors, ...together with attenuation of light over the length of the crystals. The aim is to obtain continuous DOI encoding with single side readout, and at the same time without the need for one-to-one coupling between scintillators and detectors, allowing the development of a PET scanner with good spatial, energy and timing resolutions while keeping the complexity of the system low. A prototype module has been produced and characterized to test the proposed method, coupling a LYSO scintillator matrix to a commercial SiPMs array. Excellent crystal separation is obtained for all the scintillators in the array, light loss due to depolishing is found to be negligible, energy resolution is shown to be on average 12.7% FWHM. The mean DOI resolution achieved is 4.1 mm FWHM on a 15 mm long crystal and preliminary coincidence time resolution was estimated in 353 ps FWHM.
The extremely harsh conditions, in which the detectors will have to operate during the High Luminosity phase of the Large Hadron Collider at CERN, set stringent requirements on the properties of the ...scintillators which can be used. Among different scintillating materials under study, inorganic crystals such as LuAG:Ce and YAG:Ce represent good candidates for such application. A detailed investigation of the radiation hardness of LuAG:Ce and YAG:Ce crystal samples (1 ×1 ×1 cm 3 cubes) produced by Crytur is presented in this study. Given their potential in many calorimeter designs, YAG:Ce samples with high aspect ratio ( 1 ×1 ×14 cm 3 ) have also been tested. Optical and scintillating properties of the samples were studied before and after irradiation with different sources and at different intensities. Irradiation with gamma-rays to the doses of 1 and 100 kGy and with 24 GeV protons up to an integrated fluence of 10 14 cm -2 were performed at CERN. The scintillating properties of the crystals, as emission and excitation spectra and light yield remained unchanged after irradiation and only small levels of induced absorption were observed. The results obtained in this test confirm the potential of LuAG:Ce and YAG:Ce crystals as good candidates for calorimetry applications in future high energy physics experiments.
Particle detectors at future collider experiments will operate at high collision rates and thus will have to face high pile up and a harsh radiation environment. Precision timing capabilities can ...help in the reconstruction of physics events by mitigating pile up effects. In this context, radiation tolerant, scintillating crystals coupled to silicon photomultipliers (SiPMs) can provide a flexible and compact option for the implementation of a precision timing layer inside large particle detectors. In this paper, we compare the timing performance of aluminum garnet crystals (YAG: Ce, LuAG: Ce, GAGG: Ce) and the improvements of their time resolution by means of codoping with Mg2+ ions. The crystals were read out using SiPMs from Hamamatsu glued to the rear end of the scintillator and their timing performance was evaluated by measuring the coincidence time resolution (CTR) of 150GeV charged pions traversing a pair of crystals. The influence of crystal properties, such as density, light yield and decay kinetics on the timing performance is discussed.
The best single detector time resolutions are in the range of 23–30ps (sigma) and only achieved by codoping the garnet crystals with divalent ions, such as Mg2+. The much faster scintillation decay in the co-doped samples as compared to non co-doped garnets explains the higher timing performance. Samples of LSO: Ce, Ca and LYSO:Ce crystals have also been used as reference time device and showed a time resolution at the level of 17ps, in agreement with previous results.
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