Scintillator Readout Research Articles

Characterization of the Detection Module of the INSERT SPECT/MRI Clinical System

Within the vast panorama of multimodal systems for clinical diagnosis, PET/MRI has been receiving significant attention. In parallel, new technological solutions have been paving the way to simultaneous SPECT/MRI systems, whose development has been so far delayed due to the challenging compatibility between MRI, gamma-ray detectors, and collimators. This paper presents an silicon photomultiplier (SiPM)-based Anger camera designed for preclinical and clinical SPECT static inserts for standard MR scanners. The gamma-ray detector is based on a continuous CsI(Tl) scintillator readout by arrays of SiPMs and custom ASICs. The design has been adapted to fit with the limited space conditions (23 mm thickness), but also considering mutual compatibility issues. Despite these constraints and thanks to the adoption of an iterative statistical event estimation method simply requiring a flood image, the detector maintains state-of-the-art performance in terms of intrinsic spatial resolution (1.0 mm FWHM within the usable field of view) and an energy resolution below 14% FWHM at 140 keV. The experimental studies here performed show that the camera, designed to operate at 0 °C to reduce the dark current of SiPMs, can operate up to 10 °C without significant worsening of imaging performance.

Performance evaluation of the MOLECUBES β-CUBE—a high spatial resolution and high sensitivity small animal PET scanner utilizing monolithic LYSO scintillation detectors

The MOLECUBES β-CUBE scanner is the newest amongst commercially available preclinical PET scanners for dedicated small animal imaging. The scanner is compact, lightweight and utilizes a small footprint to facilitate bench-top imaging. It can be used individually, or in combination with the X-CUBE CT scanner, which provides the ability to perform all necessary PET data corrections and provide fully quantitative PET images. The PET detector comprises of an 8 mm thick monolithic LYSO scintillator read-out by an array of 3 mm × 3 mm Hamamatsu silicon photomultipliers. The monolithic scintillator provides the ability to measure depth-of-interaction which aids in the development of such a compact scanner. With a scanner diameter of 7.6 cm and axial length of 13 cm it is suitable for imaging both whole-body mice and rats. This paper presents the design and imaging performance of the β-CUBE scanner. NEMA NU4-2008 characterization and a variety of phantom and animal imaging studies to demonstrate the quantitative imaging performance of the PET scanner are presented. Spatial resolution of 1 mm is measured with a filtered-back projection reconstruction algorithm at the center of the scanner and DOI measurement helps maintain the excellent spatial resolution over the entire imaging FOV. An absolute peak sensitivity of 12.4% is measured with a 255–765 keV energy window. The scanner demonstrates good count-rate performance, with a peak NEC of 300 kcps and 160 kcps measured with ~900 µCi in the NEMA mouse and rat phantoms, respectively. Imaging data with the NEMA image quality phantom and Micro Derenzo phantoms demonstrate the ability to achieve good image quality and accurate quantitative data. Image uniformity of 7.4% and spill-over ratio of 8% were measured. The superior spatial resolution, excellent energy resolution and sensitivity also provide superior contrast recovery, with ~70% recovery for the 2 mm rods. While current commercial preclinical PET scanners have spatial resolution in the 1–2 mm range, the 1 mm3 volumetric resolution presents significant improvement over current commercially available preclinical PET scanners. In combination with the X-CUBE scanner it provides the ability to perform fully quantitative imaging with spatially co-registered high-resolution 3D PET-CT images.

Silicon photomultipliers as readout for a segmented Time-of-Flight plastic detector

Particle identification at radioactive ion beam facilities requires Time-of-Flight detectors with an optimal time resolution and high rate capability. Segmented plastic scintillators are a common option for such detectors. A promising approach to cope with the high segmentation of modern devices is the use of silicon photomultipliers (SiPMs) as readout of the Time-of-Flight detectors. A prototype device consisting of strips of plastic scintillator of 4.4 × 100 × 1 mm3 readout at both ends by SiPMs of two different sizes was tested using a 124Xe beam at 600 MeV/nucleon. Timing and time-over-threshold information were extracted. The time resolution of the prototype was found to vary with the applied voltage and the SiPM size. After optimizing the voltage, resolutions of 14.3(10) ps and 10.4(1) ps were obtained for SiPMs of 1 × 1 mm2 and 3× 3 mm2 respectively. These results point to SiPMs being suitable as readout of plastic scintillators at radioactive ion beam facilities.

The comparison of detection parameters of selected photomultipliers depending on the shape of the scintillator

The manuscript clarifies the issues concerning the effective miniaturization of readout of the plastic scintillators while maintaining their high detection efficiency and sensitivity. Values obtained from the measurements of the chosen gamma emitters (60Co, 137Cs, 241Am), at various distances, were used to compare the detection efficiencies. The organic plastic scintillators, with the ternary system of different shapes and volumes, were chosen for the measurement. The detection parameters for the examined 1" PMT, with variable photocathode geometry, were experimentally found and compared to the normally used 2" PMT, with the circular type of photocathode. The primary aim of this work was to verify whether, in the case of mobile applications, such as UAV, it is possible to replace the currently bulky and quite heavy electronics with a miniature version and simultaneously preserve their detection parameters.

Application of large area SiPMs for the readout of a plastic scintillator based timing detector

In this study an array of eight 6 mm × 6 mm area SiPMs was coupled to the end of a long plastic scintillator counter which was exposed to a 2.5 GeV/c muon beam at the CERN PS. Timing characteristics of bars with dimensions 150 cm × 6 cm × 1 cm and 120 cm × 11 cm × 2.5 cm have been studied. An 8-channel SiPM anode readout ASIC (MUSIC R1) based on a novel low input impedance current conveyor has been used to read out and amplify SiPMs independently and sum the signals at the end. Prospects for applications in large-scale particle physics detectors with timing resolution below 100 ps are provided in light of the results.

A CMOS Frontend for Scintillators Readout by Photodiodes for Nuclear Physics Experiments

We designed a CMOS frontend for the readout of scintillators coupled with silicon photodiodes for nuclear physics experiments. The preamplifier features a continuous-reset feedback and an input pMOS transistor. The designed input dynamic range is 90-MeV silicon equivalent. The frontend has been integrated in single channel and multichannel versions up to 16-channel application specific integrated circuits (ASICs) in the C35B4C3 AMS technology. The measured integral-non-linearity over the whole dynamic range is below 0.5%. The intrinsic rise time (10%–90%) of the preamplifier is of the order of 3 ns over the entire dynamic range and is below 50 ns when coupled with a 130-pF photodiode. We qualified the spectroscopic performance by acquiring spectra of a mixed nuclei $\alpha $ source and of different $\gamma $ sources (137Cs and 60Co) with a CsI(Tl) scintillator crystal $3.2~\text {cm}\times 3.2~\text {cm}\times 6~\text {cm}$ coupled with a $18~\text {mm}\times 18~\text {mm}$ silicon p-i-n diode 300- $\mu \text{m}$ -thick readout by the developed ASIC.

Depth of interaction determination in monolithic scintillator with double side SiPM readout

BackgroundMonolithic scintillators read out by arrays of photodetectors represent a promising solution to obtain high spatial resolution and the depth of interaction (DOI) of the annihilation photon. We have recently investigated a detector geometry composed of a monolithic scintillator readout on two sides by silicon photomultiplier (SiPM) arrays, and we have proposed two parameters for the DOI determination: the difference in the number of triggered SiPMs on the two sides of the detector and the difference in the maximum collected signal on a single SiPM on each side. This work is focused on the DOI calibration and on the determination of the capability of our detector. For the DOI calibration, we studied a method which can be implemented also in detectors mounted in a full PET scanner. We used a PET detector module composed of a monolithic 20 × 20 × 10 mm3 LYSO scintillator crystal coupled on two opposite faces to two arrays of SiPMs. On each side, the scintillator was coupled to 6 × 6 SiPMs. In this paper, the two parameters previously proposed for the DOI determination were calibrated with two different methods. The first used a lateral scan of the detector with a collimated 511 keV pencil beam at steps of 0.5 mm to study the detector DOI capability, while the second used the background radiation of the 176Lu in the scintillator. The DOI determination capability was tested on different regions of the detector using each parameter and the combination of the two.ResultsWith both parameters for the DOI determination, in the lateral scan, the bias between the mean reconstructed DOI and the real beam position was lower than 0.3 mm, and the DOI distribution had a standard deviation of about 1.5 mm. When using the calibration with the radioactivity of the LYSO, the mean bias increased of about 0.2 mm but with no degradation of the standard deviation of the DOI distribution.ConclusionsThe two parameters allow to achieve a DOI resolution comparable with the state of the art, giving a continuous information about the three-dimensional interaction position of the scintillation. These results were obtained by using simple estimators and a detector scalable to a whole PET system. The DOI calibration obtained using lutetium natural radioactivity gives results comparable to the other standard method but appears more readily applicable to detectors mounted in a full PET scanner.

Applications for New Scintillator Technologies in Gamma Ray Astronomy

Scintillators have long been used for probing the high energy universe. The reliability and low cost of scintillator-PMT detectors have made them the de facto standard for experiments on high altitude balloons and in orbiting satellites. New scintillators and new readout technologies offer important opportunities for more capable experiments. Recent scintillator developments include faster signals, increased light output, improved energy resolution, and better handling characteristics. Although PMTs remain effective for scintillator readout, new technologies offer more compact, rugged devices with much lower operational voltages. The adoption of these technologies is not without its difficulties, especially for space applications, where the technology readiness level can be an important consideration. To illustrate these issues, we will discuss the use of scintillators in Compton imaging experiments. At energies from about 500 keV to 30 MeV, Compton telescopes are the most effective means of imaging the gamma ray sky. To date, the only Compton telescope that has flown in space was the COMPTEL instrument on NASA's Compton Gamma Ray Observatory (CGRO). CGRO, launched in 1991 and de-orbited in 2000, was based entirely on the use of technologies from the 1980’s. We have been working on an improved Compton telescope design, called the Advanced Scintillator Compton Telescope (ASCOT). It is much like COMPTEL, but utilizes up-to-date scintillator and readout technologies.

Liquid xenon calorimeter for MEG II experiment with VUV-sensitive MPPCs

The MEG II experiment is an upgrade of the MEG experiment to search for the charged lepton flavor violating decay of muon, μ+→e+γ. The MEG II experiment is expected to reach a branching ratio sensitivity of 4×10−14, which is one order of magnitude better than the sensitivity of the current MEG experiment. The performance of the liquid xenon (LXe) γ-ray detector will be greatly improved with a highly granular scintillation readout realized by replacing 216 photomultiplier tubes (PMTs) on the γ-ray entrance face with 4092 Multi-Pixel Photon Counters (MPPCs). For this purpose, we have developed a new type of MPPC which is sensitive to the LXe scintillation light in vacuum ultraviolet (VUV) range, in collaboration with Hamamatsu Photonics K.K. We have measured the performance of the MPPC in LXe, and an excellent performance has been confirmed including high photon detection efficiency (>15%) for LXe scintillation light. An excellent performance of the LXe detector has been confirmed by Monte Carlo simulations based on the measured properties of the MPPC. The construction of the detector is in progress, aiming to start physics data taking in 2017.

Time resolution of time-of-flight detector based on multiple scintillation counters readout by SiPMs

A new timing detector measuring ∼50MeV/c positrons is under development for the MEG II experiment, aiming at a time resolution σt∼30ps. The resolution is expected to be achieved by measuring each positron time with multiple counters made of plastic scintillator readout by silicon photomultipliers (SiPMs). The purpose of this work is to demonstrate the time resolution for ∼50MeV/c positrons using prototype counters. Counters with dimensions of 90×40×5mm3 readout by six SiPMs (three on each 40×5mm2 plane) were built with SiPMs from Hamamatsu Photonics and AdvanSiD and tested in a positron beam at the DAΦNE Beam Test Facility. The time resolution was found to improve nearly as the square root of the number of counter hits. A time resolution σt=26.2±1.3ps was obtained with eight counters with Hamamatsu SiPMs. These results suggest that the design resolution is achievable in the MEG II experiment.

Front-end electronics for the FAZIA experiment

FAZIA is a multidetector specifically designed to optimize A and Z reaction product identification in heavy-ion collision experiments. This multidetector is modular and based on three-layer telescopes made of two silicon detectors followed by a thick (10 cm) CsI(Tl) scintillator read-out by a photodiode. Its electronics is fully digital. The goal to push at maximum identification capability while preserving excellent energy resolution, can be achieved by using pulse-shape analysis techniques and by making an intensive use of high-speed flash ADCs. This paper presents the front-end part of the electronics.

A mini-phoswich scintillator as a possible stop detector for the NUMEN project

In the framework of the NUMEN project, aimed at the investigation of the nuclear matrix elements connected to the neutrinoless double beta decay by means of the Double Charge Exchange nuclear reactions (Cappuzzello et al., 2015), a high granularity stop detector for heavy ions is needed. It has to allow the identification of ions up to Z≈10 while maintaining a total energy resolution around 2%. As the use of silicon detectors is not possible, due to their poor radiation hardness, scintillators are being investigated as possible candidates. In this paper we show a promising result obtained using a plastic+inorganic phoswich scintillator readout by means of a Silicon Photo Multiplier.

Development of Low-Noise / Low-Power Preamplifier for the Readout of Inorganic Scintillators and their Mass Production Test System

The development of the preamplifer and all test procedures done by our group during the past several years at the University of Basel will be discussed. In addition, we present the results from the testing of the first 1500 channels of the preamplifiers for the Crystal Barrel experiment using a specially developed automatized test system.

Development of deep-UV sensitive MPPC for liquid xenon scintillation detector

The liquid xenon (LXe) γ-ray detector for the MEG II experiment is based on a highly granular scintillation readout with Multi-Pixel Photon Counters (MPPCs). Here we report on the development of a large-area MPPC sensitive to LXe scintillation light in deep-UV range for the MEG II LXe detector. A prototype model of the deep-UV MPPC with an active area of 12×12mm2 was successfully tested in LXe, showing an excellent performance such as a high photon detection efficiency for the LXe scintillation light (about 20% at ΔV=2.5V), a high internal gain (about 106 at ΔV=2.5V) and an excellent single photoelectron resolution. The sensor chip of the MPPC is segmented into four sectors, which are then connected in series in order to reduce the overall sensor capacitance. The signal fall time of about 135ns for the non-segmented sensor was significantly reduced down to 25ns with the series-connected sensor segments. The preliminary results on the performance of the deep-UV MPPC are presented.

Simultaneous readout of charge and scintillation pulses from electron avalanches for improving the response of micropattern gaseous detectors

Electroluminescence production and readout as amplification process for primary ionisation in time projection chambers (TPCs), are under study for application namely to rare event detection, where low-rate and high-radiation background are factors of ultimate importance, implying the highest achievable gain and best energy resolution in the detector. The use of the electroluminescence signal rather than the secondary charge signal provides these with additional advantages. However, the simplicity and low cost of charge readout electronics over scintillation readout systems imply that alternatives to electroluminescence production and readout as amplification process for primary ionisation are being investigated to apply to large scale detectors. The use of the secondary avalanche ionisation produced in micropattern electron multipliers like Large Electron Multipliers and also to gaseous detectors with pixelized readout for imaging is, therefore, investigated. Charge readout is simple and straightforward and is by far the most used readout method. A drawback is that the charge amplification method has energy resolutions that are typically worse than the scintillation amplification method, thus exploring methods to improve the energy resolution of the charge readout are always interesting.In this work, we implement simultaneous readout of charge and scintillation produced in the avalanches of a Thick Electron Multiplier (THGEM) to explore the improvement of the detector response to ionising radiation in the charge readout channel. The charge readout could be used predominantly for position determination, while the scintillation readout would be used for energy measurement.

Development of a high-resolution detection module for the INSERT SPECT/MRI system.

A new multi-modality imaging tool is under development in the framework of the INSERT (Integrated SPECT/MRI for Enhanced Stratification in Radio-chemo Therapy) project, supported by the European Community. The final goal is to develop a custom SPECT apparatus that can be used as an insert for commercially available MRI systems. INSERT is expected to offer more effective and earlier diagnosis with potentially better outcome in survival for the treatment of brain tumors, primarily glioma. Two SPECT prototypes are being developed, one dedicated to preclinical imaging (7 and 9.4 T), the second one dedicated to clinical imaging (3 T). The fundamental unit is a 5 cm x 5 cm gamma camera, based on the well-established Anger architecture with a continuous CsI:Tl scintillator readout by an array of silicon photomultipliers (SiPMs). The photodetector matrix will be composed by 12x12 SiPMs (FBK), each one with an active area of 4x4 mm2, for an overall field of view of 50.40x51.70 mm2, considering also insensitive areas between different detectors. In order to reduce complexity and costs the 144 channels are shortcut in group of 4 and readout by a custom-designed 36 channels ASIC. Each electronic channel features a fast current conveyor stage, followed by an RC filter with selectable peaking times and the electronics necessary to provide an appropriate output for the data acquisition system. Preliminary Monte Carlo simulations suggest a spatial resolution between 0.8 and 1 mm and an energy resolution between 11% and 15% (140 keV), depending on the dark count rate of the SiPM technology (100-500 kHz/mm2). Experimental measurements are under development to confirm these results. For example, a single 4x4 SiPM (FBK, RGB-HD), coupled to a CsI:Tl scintillator, has been readout by a single channel version of the ASIC, providing an energy resolution close to 12% at 122 keV at room temperature.

Forward scintillation hodoscope for nuclear fragment detection at the high acceptance dielectron spectrometer (HADES) setup

The main characteristics of the forward scintillation hodoscope and readout electronics of the high acceptance dielectron spectrometer (HADES) at GSI (Darmstadt, Germany) are presented. Methods for adjustment and amplitude-time calibration of hodoscope detectors using cosmic rays and results of measurements of the Au + Au reaction at a beam energy of 1.24 GeV/nucleon are described. The use of the forward hodoscope for tagging the n + p reaction by means of the deuteron beam incident on a liquid hydrogen target is discussed. A procedure for determining the reaction plane angle in event-by-event analysis of the Au + Au reaction data (at 1.24 GeV/nucleon) is presented.

Simulation of the expected performance of INSERT: A new multi-modality SPECT/MRI system for preclinical and clinical imaging

A new multi-modality imaging tool is under development in the framework of the INSERT (INtegrated SPECT/MRI for Enhanced Stratification in Radio-chemo Therapy) project, supported by the European Community. The final goal is to develop a custom SPECT apparatus, that can be used as an insert for commercially available MRI systems such as 3T MRI with 59cm bore diameter. INSERT is expected to offer more effective and earlier diagnosis with potentially better outcome in survival for the treatment of brain tumors, primarily glioma. Two SPECT prototypes will be developed, one dedicated to preclinical imaging, the second one dedicated to clinical imaging.The basic building block of the SPECT detector ring is a small 5cm×5cm gamma camera, based on the well-established Anger architecture with a continuous scintillator readout by an array of silicon photodetectors. Silicon Drift Detectors (SDDs) and Silicon PhotoMultipliers (SiPM) are being considered as possible scintillator readout, considering that the detector choice plays a predominant role for the final performance of the system, such as energy and spatial resolution, as well as the useful field of view of the camera. Both solutions are therefore under study to evaluate their performances in terms of field of view (FOV), spatial and energy resolution. Preliminary simulations for both the preclinical and clinical systems have been carried out to evaluate resolution and sensitivity.

Degradation of resolution in a homogeneous dual-readout hadronic calorimeter

If the scintillator response to a hadronic shower in a semi-infinite uniform calorimeter structure is S relative to the electronic response, then S/E=[fem+(1−fem)(h/e)], where E is the incident hadron energy, fem is the electronic shower fraction, and h/e is the hadron/electron response ratio. If there is also a simultaneous readout with a different h/e, say a Cherenkov signal C, then a linear combination of the two signals provides an estimator of E that is proportional to the incident energy and whose distribution is nearly Gaussian—even though the S and C distributions are non-linear in E, wide, and skewed. Since an estimator of fem is also obtained, it is no longer a stochastic variable. Much of the remaining resolution variance is due to sampling fluctuations. These can be avoided in a homogeneous calorimeter. The energy resolution depends upon the contrast in h/e between the two channels. h/e is small in the Cherenkov channel. Mechanisms that increaseh/ein sampling calorimeters with organic scintillator readout are not available in a homogeneous inorganic scintillator calorimeter. The h/e contrast is very likely too small to provide the needed energy resolution.

Improved EMCCD gamma camera performance by SiPM pre-localization

High spatial resolution γ-imaging can be achieved with scintillator readout by low-noise, fast, electron-multiplying charge-coupled devices (EMCCDs). Previously we have shown that false-positive events due to EMCCD noise can be rejected by using the sum signal from silicon photomultipliers (SiPMs) mounted on the sides of the scintillator. Here we launch a next generation hybrid CCD-SiPM camera that utilizes the individual SiPM signals and maximum likelihood estimation (MLE) pre-localization of events to discriminate between true and false events in CCD frames. In addition, SiPM signals are utilized for improved energy discrimination. The performance of this hybrid detector was tested for a continuous CsI:Tl crystal at 140 keV. With a pre-localization accuracy of 1.06 mm (full-width-at-half-maximum) attained with MLE the signal-to-background ratio (SBR) was improved by a factor of 5.9, 4.0 or 2.2 compared to the EMCCD-only readout, at the cost of rejecting, respectively, 47%, 9% or 4% of the events. Combining the pre-localization and SiPM energy estimation improved the energy resolution from 50% to (19 ± 3)% while maintaining the spatial resolution at 180 µm.