K. Pauwels

Measurement of intrinsic rise times for various L(Y)SO and LuAG scintillators with a general study of prompt photons to achieve 10 ps in TOF-PET

The coincidence time resolution (CTR) of scintillator based detectors commonly used in positron emission tomography is well known to be dependent on the scintillation decay time (τd) and the number of photons detected (n), i.e. CTR proportional variant √τd/n. However, it is still an open question to what extent the scintillation rise time (τr) and other fast or prompt photons, e.g. Cherenkov photons, at the beginning of the scintillation process influence the CTR. This paper presents measurements of the scintillation emission rate for different LSO type crystals, i.e. LSO:Ce, LYSO:Ce, LSO:Ce codoped Ca and LGSO:Ce. For the various LSO-type samples measured we find an average value of 70 ps for the scintillation rise time, although some crystals like LSO:Ce codoped Ca seem to have a much faster rise time in the order of 20 ps. Additional measurements for LuAG:Ce and LuAG:Pr show a rise time of 535 ps and 251 ps, respectively. For these crystals, prompt photons (Cherenkov) can be observed at the beginning of the scintillation event. Furthermore a significantly lower rise time value is observed when codoping with calcium. To quantitatively investigate the influence of the rise time to the time resolution we measured the CTR with the same L(Y)SO samples and compared the values to Monte Carlo simulations. Using the measured relative light yields, rise- and decay times of the scintillators we are able to quantitatively understand the measured CTRs in our simulations. Although the rise time is important to fully explain the CTR variation for the different samples tested we determined its influence on the CTR to be in the order of a few percent only. This result is surprising because, if only photonstatistics of the scintillation process is considered, the CTR would be proportional to the square root of the rise time. The unexpected small rise time influence on the CTR can be explained by the convolution of the scintillation rate with the single photon time resolution (SPTR) of the photodetector and the photon travel spread (PTS) in the crystal. The timing benefits of prompt photons at the beginning of the scintillation process (Cherenkov etc) are further studied, which leads to the conclusion that the scintillation rise time, SPTR and PTS have to be lowered simultaneously to fully profit from these fast photons in order to improve the CTR significantly.

Effect of Aspect Ratio on the Light Output of Scintillators

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.

Single crystalline LuAG fibers for homogeneous dual-readout calorimeters

For the next generation of calorimeters, designed to improve the energy resolution of hadrons and jets measurements, there is a need for highly granular detectors requiring peculiar geometries. Heavy inorganic scintillators allow compact homogeneous calorimeter designs with excellent energy resolution and dual-readout abilities. These scintillators are however not usually suited for geometries with a high aspect ratio because of the important losses observed during the light propagation. Elongated single crystals (fibers) of Lutetium Aluminium garnet (LuAG, Lu3Al5O12) were successfully grown with the micropulling-down technique. We present here the results obtained with the recent fiber production and we discuss how the light propagation could be enhanced to reach attenuation lengths in the fibers better than 0.5 m.

A Systematic Study to Optimize SiPM Photo-Detectors for Highest Time Resolution in PET

We report on a systematic study of time resolution made with three different commercial silicon photomultipliers (SiPMs) (Hamamatsu MPPC S10931-025P, S10931-050P, and S10931-100P) and two LSO scintillating crystals. This study aimed to determine the optimum detector conditions for highest time resolution in a prospective time-of-flight positron emission tomography (TOF-PET) system. Measurements were based on the time over threshold method in a coincidence setup using the ultrafast amplifier-discriminator NINO and a fast oscilloscope. Our tests with the three SiPMs of the same area but of different SPAD sizes and fill factors led to best results with the Hamamatsu type of 50×50×μm2 single-pixel size. For this type of SiPM and under realistic geometrical PET scanner conditions, i.e., with 2×2×10×mm3 LSO crystals, a coincidence time resolution of 220 ±4 ps FWHM could be achieved. The results are interpreted in terms of SiPM photon detection efficiency (PDE), dark noise, and photon yield.

Measurement of LYSO Intrinsic Light Yield Using Electron Excitation

The determination of the intrinsic light yield (LYint) of scintillating crystals, i.e. number of optical photons created per amount of energy deposited, constitutes a key factor in order to characterize and optimize their energy and time resolution. However, until now measurements of this quantity are affected by large uncertainties and often rely on corrections for bulk absorption and surface/edge state. The novel idea presented in this contribution is based on the confinement of the scintillation emission in the central upper part of a 10 mm cubic crystal using a 1.5 MeV electron beam with diameter of 1 mm. A black non-reflective pinhole aligned with the excitation point is used to fix the light extraction solid angle (narrower than total reflection angle), which then sets a light cone travel path through the crystal. The final number of photoelectrons detected using a Hamamatsu R2059 photomultiplier tube (PMT) was corrected for the extraction solid angle, the Fresnel reflection coefficient and quantum efficiency (QE) of the PMT. The total number of optical photons produced per energy deposited was found to be 40000 ph/MeV ± 9% (syst) ±3% (stat) for LYSO. Simulations using Geant4 were successfully compared to light output measurements of 2 × 2 mm2 section crystals with lengths of 5-30 mm, in order to validate the light transport model and set a limit on Light Transfer Efficiency estimations.

Radiation Tolerance of LuAG:Ce and YAG:Ce Crystals Under High Levels of Gamma- and Proton-Irradiation

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 cm3 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 cm3) 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 1014 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.

Design and performance of detector modules for the endoscopic PET probe for the FP7-project EndoTOFPET-US

The detection of cancer in its early stage is known to be of key importance to improve cancer treatment and thus reduces mortality and cost. As a consequence, this has led to a growing interest in developing high performance multimodal imaging devices capable of detecting smallest possible tumors. As part of this approach, the EndoTOFPET-US project, a European FP7 program, aims to develop new biomarkers for the pancreatic and prostatic cancers. Testing such markers requires improving the detection of smaller tumors and performing biopsies based on combined anatomic and functional image information. The detection of small tumors using PET detectors also entails high sensitivity and high spatial resolution. One particular objective of this project is to develop a prototype of a novel bi-modal, TOFPET and ultrasound, endoscope for the detection of early stage pancreatic or prostatic tumors. It consists of two separate PET detectors, one a 23×23cm2 total area external plate (placed outside the body) with 256 arrays of 4×4 LYSO crystals of 3.1×3.1×15mm3 size coupled to Hamamatsu MPPC monolithic arrays of 3×3mm2 single sensors, and an internal (endoscopic) PET probe that consists of an array of 9×18 L YSO fibers with a size of 0.71×0.71×10mm3 coupled to a fully digital SiPM [1,2]. This PET tip is attached to a conventional ultrasound endoscope. Electronic collimation provided by time of flight measurements between the external PET plate and the internal PET probe allows the necessary sensitivity to efficiently reject background. This, however, requires a coincidence time resolution of 200ps FWHM. High spatial resolution of ≤ 1mm can be achieved due to the very high granularity of the endoscopic PET probe consisting of crystal pixel sizes of less than 800μm2 section. In this paper we present the design and current development of the PET modules for the endoscopic probe as well as their performance in terms of light yield (LY) and coincidence time resolution (CTR) made with different prototypes.

Progress on photonic crystals

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.

Study of the Angular Distribution of Scintillation Photons

This paper presents a characterization method to experimentally determine the angular distribution of scintillation light. By exciting LYSO crystals with a radioactive source, we measured the light angular profiles obtained with samples of different geometries in different conditions of wrapping. We also measured the angular distribution of light emitting in glue and compared it with the one emitting in air. Angular distribution of light output of photonic crystals is also provided. Consistency of the measurements is verified with conventional light output measurements.

Test beam results of a high granularity LuAG fibre calorimeter prototype

The progresses in the micropulling-down technique allow heavy scintillating crystals to be grown directly into a fibre geometry of variable shape, length and diameter. Examples of materials that can be grown with this technique are Lutetium Aluminum Garnets (LuAG, Lu3Al5O12) and Yttrium Aluminum Garnets (YAG, Y3Al5O12). Thanks to the flexibility of this approach, combined with the high density and good radiation hardness of the materials, such a technology represents a powerful tool for the development of future calorimeters. As an important proof of concept of the application of crystal fibres in future experiments, a small calorimeter prototype was built and tested on beam. A grooved brass absorber (dimensions 26cm×7cm×16cm) was instrumented with 64 LuAG fibres, 56 of which were doped with Cerium, while the remaining 8 were undoped. Each fibre was readout individually using 8 eightfold Silicon Photomultiplier arrays, thus providing a highly granular description of the shower development inside the module as well as good tracking capabilities. The module was tested at the Fermilab Test Beam Facility using electrons and pions in the 2–16 GeV energy range. The module performance as well as fibre characterization results from this beam test are presented.