Highest time resolution in scintillator based detectors is becoming more and more important. In medical detector physics L(Y)SO scintillators are commonly used for time of flight positron emission ...tomography (TOF-PET). Coincidence time resolutions (CTRs) smaller than 100ps FWHM are desirable in order to improve the image signal to noise ratio and thus give benefit to the patient by shorter scanning times. Also in high energy physics there is the demand to improve the timing capabilities of calorimeters down to 10ps. To achieve these goals it is important to study the whole chain, i.e. the high energy particle interaction in the crystal, the scintillation process itself, the scintillation light transfer in the crystal, the photodetector and the electronics. Time resolution measurements for a PET like system are performed with the time-over-threshold method in a coincidence setup utilizing the ultra-fast amplifier-discriminator NINO. With 2×2×3mm3 LSO:Ce codoped 0.4%Ca crystals coupled to commercially available SiPMs (Hamamatsu S10931-050P MPPC) we achieve a CTR of 108±5ps FWHM at an energy of 511keV. Under the same experimental conditions an increase in crystal length to 5mm deteriorates the CTR to 123±7ps FWHM, 10mm to 143±7ps FWHM and 20mm to 176±7ps FWHM. This degradation in CTR is caused by the light transfer efficiency (LTE) and light transfer time spread (LTTS) in the crystal. To quantitatively understand the measured values, we developed a Monte Carlo simulation tool in MATLAB incorporating the timing properties of the photodetector and electronics, the scintillation properties of the crystal and the light transfer within the crystal simulated by SLITRANI. In this work, we show that the predictions of the simulation are in good agreement with the experimental data. We conclude that for longer crystals the deterioration in CTR is mainly caused by the LTE, i.e. the ratio of photons reaching the photodetector to the total amount of photons generated by the scintillation whereas the LTTS influence is partly offset by the gamma absorption in the crystal.
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The renewal of interest in Time of Flight Positron Emission Tomography (TOF-PET), as well as the necessity to precisely tag events in high energy physics (HEP) experiments at future colliders are ...pushing for an optimization of all factors affecting the time resolution of the whole acquisition chain comprising the crystal, the photo detector, and the electronics. The time resolution of a scintillator-based detection system is determined by the rate of photo electrons at the detection threshold, which depends on the time distribution of photons being converted in the photo detector. The possibility to achieve time resolution of about 100 ps Full Width at Half Maximum (FWHM) requires an optimization of the light production in the scintillator, the light transport and its transfer from the scintillator to the photo detector. In order to maximize the light yield, and in particular the density of photons in the first nanosecond, while minimizing the rise time and decay time, particular attention must be paid to the energy transfer mechanisms to the activator as well as to the energy transition type at the activator ion. Alternatively other light emission mechanisms can be considered. We show that particularly Cerenkov emission can be used for this purpose. Special emphasis was put on the light transport within the crystal and at its interface with the photo detector. Since light is produced isotropically in the scintillator the detector geometry must be optimized to decrease the optical path-length to the photo detector. Moreover light bouncing within the scintillator, affecting about 70% of the photons generated in currently used crystals, must be reduced as much as possible. We also investigate photonics crystals that are specifically designed to favor specific light propagation modes at the limit of total reflection inside and outside of the crystal and how they might increase the light transfer efficiency to the photo detector and hence improve time resolution. Examples for the production and deposition of photonics crystals as layers on Lutetium Yttrium Ortho-Silicate (LYSO) and Lutetium Yttrium Aluminum Perovskite (LuYAP) crystals are shown here, as well as first results on an improved light extraction resulting from this method.
Scintillation crystals have a wide range of applications in detectors for high energy and medical physics. They are recquired to have not only good energy resolution, but also excellent time ...resolution. In medical applications, L(Y)SO crystals are commonly used for time of flight positron emission tomography (TOF-PET). This study aims at determining the experimental and theoretical limits of timing using L(Y)SO based scintillators coupled to silicon photomultipliers (SiPMs). Measurements are based on the time-over-threshold method in a coincidence setup utilizing the ultra-fast amplifier-discriminator NINO and a fast oscilloscope. Using a 2 x 2 x 3 mm super(3) LSO: Ce codoped 0.4% Ca crystal coupled to a commercially available SiPM (Hamamatsu S10931-050P MPPC), we achieve a coincidence time resolution (CTR) of 108 plus or minus 5ps FWHM measured at E=51 lkeV. We determine the influence of the data acquisition system to 27 plus or minus 2ps FWHM and thus negligible as compared to the CTR. This shows that L(Y)SO scintillators coupled to SiPM pho-todetectors are capable of achieving very good time resolution close to the desired lOOps FWHM for TOF-PET systems. To fully understand the measured values, we developed a simulation tool in MATLAB that incorporates the timing properties of the photodetector, the scintillation properties of the crystal and the light transfer within the crystal simulated by SLITRANI. The simulations are compared with measured data in order to determine their predictive power. Finally we use this model to discuss the influence of several important parameters on the time resolution like scintillation rise- and fall time and light yield, as well as single photon time resolution (SPTR) and the detection efficiency of the SiPM. In addition we find the influence of photon travel time spread in the crystal not negligible on the CTR, even for the used 2 x 2 x 3 mm super(3) geometry.
The influence of the geometry of the scintillators is presented in this paper. We focus on the effect of narrowing down the section of crystals that have a given length. The light output of a set of ...crystals with very similar scintillating properties but different geometries measured with several coupling/wrapping configurations is provided. We observe that crystals shaped in thin rods have a lower light output as compared to bulk or sliced crystals. The effect of unpolishing the crystal faces is also investigated, and it is shown that highest light outputs are not necessarily obtained with crystals having all faces polished. Simulation results based on a realistic model of the crystal that implements light scattering on the crystal edges are in agreement with the experimental data. Fine-tuning of this model would allow us to further explore the details of light propagation in scintillators and would be highly valuable to fast timing detection and highly granular detectors.
Photonic crystals (PhCs) and quantum optics phenomena open interesting perspectives to enhance the light extraction from scintillating media with high refractive indices as demonstrated by our ...previous work. By doing so, they also influence the timing resolution of scintillators by improving the photostatistics. The present contribution will demonstrate that they are actually doing much more. Indeed, photonic crystals, if properly designed, allow the extraction of fast light propagation modes in the crystal with higher efficiency, therefore contributing to increasing the density of photons in the early phase of the light pulse. This is of particular interest to tag events at future high-energy physics colliders, such as CLIC, with a bunch-crossing rate of 2 GHz, as well as for a new generation of time-of-flight positron emission tomographs (TOFPET) aiming at a coincidence timing resolution of 100 ps FWHM. At this level of precision, good control of the light propagation modes is crucial if we consider that in a 2 × 2 × 20-mm 3 LSO crystal, the time spread (peak to peak) of extracted photons can be as large as 400 ps considering simple light bouncing only. This paper presents a detailed analysis of the light propagation and extraction modes in a LSO crystal combining the LITRANI light ray tracing and the CAMFR PhC simulation codes. Ongoing measurement results are shown with an attempt to unfold the contribution from the improved photostatistics that result from the total enhanced light output on one side and from the improved contribution of fast propagation mode extraction on the other side. Some results are also shown on a new and more industrial process to produce PhCs by the use of nano-imprint technology.
A search for signatures of extra spatial dimensions in the diphoton invariant-mass spectrum has been performed with the CMS detector at the LHC. No excess of events above the standard model ...expectation is observed using a data sample collected in proton-proton collisions at √s=7 TeV corresponding to an integrated luminosity of 2.2 fb(-1). In the context of the large-extra-dimensions model, lower limits are set on the effective Planck scale in the range of 2.3-3.8 TeV at the 95% confidence level. These limits are the most restrictive bounds on virtual-graviton exchange to date. The most restrictive lower limits to date are also set on the mass of the first graviton excitation in the Randall-Sundrum model in the range of 0.86-1.84 TeV, for values of the associated coupling parameter between 0.01 and 0.10.
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SiPM angular response and enhanced light extraction Nemallapudi, M. V.; Gundacker, S.; Knapitsch, A. ...
2013 IEEE Nuclear Science Symposium and Medical Imaging Conference (2013 NSS/MIC),
10/2013
Conference Proceeding
Optical coupling of scintillators with photodetectors is important to increase the achievable light output. Understanding the angular response of photodetectors is crucial to enhance the light ...extraction. Losses induced by Fresnel reflections on a Silicon Photomultiplier (SiPM) surface result in an angular dependence of the detected signal on incident light. We built a dedicated test bench at CERN to make a systematic study on samples of various specifications and manufacturers. We present the results of this study.
A major challenge for future particle physics experiments and nuclear medicine imaging applications will be the improvement of energy and time resolution of the detector systems. Both parameters are ...strongly correlated with the number of photoelectrons which can be registered after a particle has deposited its energy in the scintillator. One problem in heavy scintillating materials is that a large fraction of the light produced inside the bulk material is trapped inside the crystal due to total internal reflection. Recent developments in the area of nanophotonics show that those limitations can be overcome by introducing a photonic crystal (PhC) slab at the outcoupling surface of the scintillator. Photonic crystals are optical materials which can affect the propagation of light in multiple ways. In this work, the PhC is used for the extraction of photons which are otherwise reflected within the scintillator. Our simulations show light output improvements for a wide range of scintillating materials due to light scattering effects of the photonic grating. In the practical part of the work we show how we were producing first samples of PhC slabs on top of different scintillators to confirm the simulation results by measurements. Through the deposition of an auxiliary layer of silicon nitride and the adaptation of the standard electron beam lithography (EBL) parameters we could successfully produce several PhC slabs on top of 1.2 mm × 2.6 mm × 5 mm lutetium oxyorthosilicate (LSO) scintillators. In the characterization process we show a 30-60% light yield improvement of the different PhC designs when compared to an unstructured reference scintillator, which is also in close accordance with our simulation results.
Progress on photonic crystals Lecoq, P; Auffray, E; Gundacker, S ...
IEEE Nuclear Science Symposuim & Medical Imaging Conference,
2010-Oct.
Conference Proceeding
The renewal of interest for Time of Flight Positron Emission Tomography (TOF PET) has highlighted the need for increasing the light output of scintillating crystals and in particular for improving ...the light extraction from materials with a high index of refraction. One possible solution to overcome the problem of total internal reflection and light losses resulting from multiple bouncing within the crystal is to improve the light extraction efficiency at the crystal/photodetector interface by means of photonic crystals, i.e. media with a periodic modulation of the dielectric constant at the wavelength scale. After a short reminder of the underlying principles this contribution proposes to present the very encouraging results we have recently obtained on LYSO pixels and the perspectives on other crystals such as BGO, LuYAP and LuAG. These results confirm the impressive predictions from our previously published Monte Carlo simulations. A detailed description of the sample preparation procedure is given as well as the methodology and different characterization steps to control the process and evaluate the results. Pictures and quantitative results are shown, which confirm that significant light output gain factors (50% and more) can be obtained with this approach. Finally an interesting feature of photonic crystals to collimate light in some privileged directions is highlighted.
Future high-energy physics (HEP) experiments as well as next generation medical imaging applications are more and more pushing towards better scintillation characteristics. One of the problems in ...heavy scintillating materials is related to their high electronic density, resulting in a large index of refraction. As a consequence, most of the scintillation light produced in the bulk material is trapped inside the crystal due to total internal reflection. The same problem also occurs with light emitting diodes (LEDs) and has for a long time been considered as a limiting factor for their overall efficiency. Recent studies have shown that those limits can be overcome by means of light scattering effects of photonic crystals (PhCs). In our simulations we could show light yield improvements between 90% and 110% when applying PhC structures to different scintillator materials. To evaluate the results, a PhC modified scintillator was produced in cooperation with the NIL (Nanotechnology Institute of Lyon). By using silicon nitride (Si
3N
4) as a transfer material for the PhC pattern and a 70
nm thick Indium Tin Oxide (ITO) layer for the electrical conductivity during the lithography process, we could successfully fabricate first samples of PhC areas on top of LYSO crystals.
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