Scintillator Readout Research Articles

Secondary scintillation readout from GEM and THGEM with a large area avalanche photodiode

The readout of the scintillation produced in GEM and THGEM avalanches with large area avalanche photodiodes, as an alternative to Geiger-mode APDs was investigated in gaseous xenon and argon for pressures ranging from 1 to 2.5 bar, at room temperature. Using this scintillation readout, the amplification gains achieved in GEMs' and THGEMs' avalanches are more than two orders of magnitude higher, with improved energy resolution, than those achieved with the electron charge readout. This demonstrates a clear advantage of reading the secondary scintillation instead of the charge produced in the electron avalanches of micropattern electron multipliers. This is important for high-background and low-rate experiments, as is the case in direct Dark Matter search. Gains above 105 or even 106 can be achieved with a single- or double-GEM or THGEM operating in gaseous argon or xenon, resulting in an efficient single-electron detection setup, without gas aging.

Development of brain PET using GAPD arrays

In recent times, there has been great interest in the use of Geiger-mode avalanche photodiodes (GAPDs) as scintillator readout in positron emission tomography (PET) detectors because of their advantages, such as high gain, compact size, low power consumption, and magnetic field insensitivity. The purpose of this study was to develop a novel PET system based on GAPD arrays for brain imaging. The PET consisted of 72 detector modules arranged in a ring of 330 mm diameter. Each PET module was composed of a 4 × 4 matrix of 3 × 3 × 20 mm(3) cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled with a 4 × 4 array three-side tileable GAPD. The signals from each PET module were fed into preamplifiers using a 3 m long flat cable and then sent to a position decoder circuit (PDC), which output a digital address and an analog pulse of the interacted channel among 64 preamplifier signals transmitted from four PET detector modules. The PDC outputs were fed into field programmable gate array (FPGA)-embedded data acquisition (DAQ) boards. The analog signal was then digitized, and arrival time and energy of the signal were calculated and stored. The energy and coincidence timing resolutions measured for 511 keV gamma rays were 18.4 ± 3.1% and 2.6 ns, respectively. The transaxial spatial resolution and sensitivity in the center of field of view (FOV) were 3.1 mm and 0.32% cps/Bq, respectively. The rods down to a diameter of 2.5 mm were resolved in a hot-rod phantom image, and activity distribution patterns between the white and gray matters in the Hoffman brain phantom were well imaged. Experimental results indicate that a PET system can be developed using GAPD arrays and the GAPD-based PET system can provide high-quality PET imaging.

Time and position sensitive single photon detector for scintillator read-out

We have developed a photon counting detector system for combined neutron and γ radiography which can determine position, time and intensity of a secondary photon flash created by a high-energy particle or photon within a scintillator screen. The system is based on a micro-channel plate photomultiplier concept utilizing image charge coupling to a position- and time-sensitive read-out anode placed outside the vacuum tube in air, aided by a standard photomultiplier and very fast pulse-height analyzing electronics. Due to the low dead time of all system components it can cope with the high throughput demands of a proposed combined fast neutron and dual discrete energy γ radiography method (FNDDER). We show tests with different types of delay-line read-out anodes and present a novel pulse-height-to-time converter circuit with its potential to discriminate γ energies for the projected FNDDER devices for an automated cargo container inspection system (ACCIS).

Studies of Scintillator Tiles with SiPM Readout for Imaging Calorimeters

Imaging hadronic calorimeters with scintillator readout use small scintillator tiles individually read out by silicon photomultipliers to achieve the necessary granularity needed for sophisticated reconstruction algorithms at future collider detectors. For a second generation prototype of the CALICE analog HCAL new, 3 mm thick scintillator tiles with an embedded wavelength shifting fiber and new photon sensor from CPTA are being fabricated. The availability of blue-sensitive SiPMs also allows fiberless coupling of the photon sensor to the tile, a technique requiring modified geometries to achieve a high degree of response uniformity. We discuss results from test bench and from test beam measurements of different scintillator tile geometries as well as prospects for fiberless coupling of photon sensors.

Micromegas operation in high pressure xenon: charge and scintillation readout

The operational characteristics of a Micromegas operating in pure xenon at the pressure range of 1 to 10 bar are investigated. The maximum charge gain achieved in each pressure is approximately constant, around 4 × 102, for xenon pressures up to 5 bar and decreasing slowly above this pressure down to values somewhat above 102 at 10 bar. The MM presents the highest gains for xenon pressures above 4 bar, when compared to other micropattern gaseous multipliers. The lowest energy resolution obtained for X-rays of 22.1 keV exhibits a steady increase with pressure, from 13% at 1 bar to about 32% at 10 bar. The effective scintillation yield, defined as the number of photons exiting through the MM mesh holes per primary electron produced in the conversion region was calculated. This yield is about 2 × 102 photons per primary electron at 1 bar, increasing to about 6 × 102 at 5 bar and, then, decreasing again to 2 × 102 at 10 bar. The readout of this scintillation by a suitable photosensor will result in higher gains but with increased statistical fluctuations.

2D-sensitive hpxe gas proportional scintillation counter concept for nuclear medical imaging purposes

The operation and first images of a high pressure xenon Gas Proportional Scintillation Counter (GPSC) are presented. In this setup, primary electrons produced by the absorption of X- or γ-rays in the gas medium drift to a region where the electric field is set to a value above the gas scintillation threshold, the scintillation region. The primary ionization signal is amplified through the electroluminescence produced along the electron drift in this region. A Micro-Hole and Strip Plate covered with CsI (CsI-MHSP) is used as the photosensor for the scintillation readout. The 2D capability of the CsI-MHSP photosensor is achieved by means of two orthogonal resistive lines interconnecting the strips patterned on both surfaces of the MHSP. The interaction position of the incident radiation can be obtained by determining the centroid of the photosensor area irradiated by the electroluminescence pulse. This centroid is obtained from the amplitude of the charge pulses collected at both ends of the resistive lines. Preliminary analyses of the first images obtained with electroluminescence signals at xenon pressures up to 3 bar indicate a position resolution capability of about 1.2 mm at 2.9 bar, for 59.6 keV γ-photons.

High energy gamma-ray spectroscopy with LaBr 3 scintillation detectors

Lanthanum bromide scintillation detectors produce very high light outputs (∼60,000 ph/MeV) within a very short decay time (typically ∼20 ns) which means that high instantaneous currents can be generated in the photocathode and dynode chain of the photomultiplier tube (PMT) used for the scintillation readout. The net result is that signal saturation can occur long before the recommended PMT biasing conditions can be reached. In search of an optimized light readout system for LaBr 3, we have tested and compared two different PMT configurations for detection of gamma-rays up to 15 MeV. This range was chosen as being appropriate for gamma-ray remote sensing and medium energy nuclear physics applications. The experiments were conducted at two facilities: the Laboratori Nazionali del Sud (LNS) in Catania, Italy [1] and the High Intensity Gamma-ray Source (HIγS) at Triangle University Nuclear Laboratory, in Durham, North Carolina, USA [2]. The PMT configurations we have tested are (1) a standard dynode chain operated under-biased; (2) a 4-stage reduced chain operated at nominal inter-dynode bias. The results are that shortening the number of active stages, as in configuration (2), has advantages in preserving energy resolution and avoiding PMT saturation over a large energy range. However, the use of an under-biased PMT, configuration (1), can still be considered a satisfactory solution, at least in the case of PMTs manufactured by Photonis. The results of this study will be used in support of the Mercury Gamma-ray and Neutron Spectrometer (MGNS) on board of BepiColombo, the joint ESA/JAXA mission to Mercury, scheduled for launch in 2014.

Primary and secondary scintillation measurements in a Xenon Gas Proportional Scintillation Counter

NEXT is a new experiment to search for neutrinoless double beta decay using a 100 kg radio-pure high-pressure gaseous xenon TPC. The detector requires excellent energy resolution, which can be achieved in a Xe TPC with electroluminescence readout. Hamamatsu R8520-06SEL photomultipliers are good candidates for the scintillation readout. The performance of this photomultiplier, used as VUV photosensor in a gas proportional scintillation counter, was investigated. Initial results for the detection of primary and secondary scintillation produced as a result of the interaction of 5.9 keV X-rays in gaseous xenon, at room temperature and at pressures up to 3 bar, are presented. An energy resolution of 8.0% was obtained for secondary scintillation produced by 5.9 keV X-rays. No significant variation of the primary scintillation was observed for different pressures (1, 2 and 3 bar) and for electric fields up to 0.8 V cm−1 torr−1 in the drift region, demonstrating negligible recombination luminescence. A primary scintillation yield of 81 ± 7 photons was obtained for 5.9 keV X-rays, corresponding to a mean energy of 72 ± 6 eV to produce a primary scintillation photon in xenon.

Silicon drift photodetector arrays for the HICAM gamma camera

Silicon drift detectors (SDDs) have shown to be a competitive device for the readout of scintillators with respect to conventional photodetectors, thanks to their high quantum efficiency and low electronics noise. Recently, they have been successfully employed in first small prototypes of Anger cameras to achieve sub-millimeter spatial resolution in gamma-ray imaging. To cover larger formats of Anger cameras, in particular in the framework of the HICAM project, specially focused on human imaging, we have developed new SDD arrays of larger active areas. To assemble photodetector planes of several cm 2, we have designed a basic unit composed by a linear array of 5 SDDs of 1 cm 2 active area each. In this work, we present the results of the experimental characterization of these photodetector arrays in direct X-ray detection to evaluate the electronics noise, as well as gamma-ray detection with a scintillator.

Multi-pixel Geiger-mode avalanche photodiode and wavelength-shifting fibre-optics readout of plastic scintillator counters for the EMMA underground experiment

The results of a development of a scintillator counter with wavelength shifting (WLS) fibre and a multi-pixel Geiger-mode avalanche photodiode readout are presented. The photodiode has a metal-resistor-semiconductor layered structure and operates in the limited Geiger mode. The scintillator counter has been developed for the EMMA underground cosmic ay experiment.

GEM Operation in High-Pressure ${\rm CF}_{4}$: Studies of Charge and Scintillation Properties

Tetrafluoromethane (CF4) combined with Helium-3 is widely used as the filling gas for neutron detectors. CF4 is generally known as an efficient gas for proton and tritium stopping, as well as a medium with low gamma sensitivity. Filling pressures above 2.5 bar are needed to achieve detector position resolutions below 1 mm. In this work, we have studied the performance of a single-GEM operating in CF4 in the pressure range of 1.0 to 2.5 bar and shown that it is possible to achieve gains one order of magnitude higher than what is needed for thermal neutron detection. Charge gains of 1.6 times 104 and 400 are reached at pressures of 1.0 and 2.5 bar, respectively, indicating good properties for this purpose. In opposition to gases with higher scintillation efficiency such as xenon, GEM scintillation readout with VUV-sensitive avalanche photodiodes results in gains that are less than one order of magnitude higher than those obtained with charge readout, delivering similar energy resolutions.

Characterisation of a silicon photomultiplier device for applications in liquid argon based neutrino physics and dark matter searches

The performance of a silicon photomultiplier has been assessed at low temperature in order to evaluate its suitability as a scintillation readout device in liquid argon particle physics detectors. The gain, measured as 2.1 × 106 for a constant over-voltage of 4V was measured between 25°C and -196°C and found to be invariant with temperature, the corresponding single photoelectron dark count rate reducing from 1MHz to 40Hz respectively. Following multiple thermal cycles no deterioration in the device performance was observed. The photon detection efficiency (PDE) was assessed as a function of photon wavelength and temperature. For an over-voltage of 4V, the PDE, found again to be invariant with temperature, was measured as 25% for 460nm photons and 11% for 680nm photons. Device saturation due to high photon flux rate, observed both at room temperature and -196°C, was again found to be independent of temperature. Although the output signal remained proportional to the input signal so long as the saturation limit was not exceeded, the photoelectron pulse resolution and decay time increased slightly at −196°C.

Calorimeters With Scintillator at the International Linear Collider

The latest developments of calorimeters using scintillators as active medium for detectors at the ILC are reviewed. We mention those having already a prototype that was tested in the beam. Besides classical scintillator tile calorimeters with smallest tiles ever used, a completely new approach was proposed with independent measurement of scintillation and Cherenkov light produced in fibers. Special attention is given to new photodetectors - silicon photomultipliers - which have been extensively used for the scintillator readout. Their main advantages are a high gain ~106 at a low bias voltage 55-75 V, easy integration into a scintillator and a reasonable price.

GEM scintillation readout with avalanche photodiodes

The use of the scintillation produced in the charge avalanches in GEM holes as signal amplification and readout is investigated for xenon. A VUV-sensitive avalanche photodiode has been used as photosensor. Detector gains of about 4 × 104 are achieved in scintillation readout mode, for GEM voltages of 490 V and for a photosensor gain of 150. Those gains are more than one order of magnitude larger than what is obtained using charge readout. In addition, the energy resolutions achieved with the scintillation readout are lower than those achieved with charge readout. The GEM scintillation yield in xenon was measured as a function of GEM voltage, presenting values that are about a half of those achieved for the charge yield, and reach about 730 photons per primary electron at GEM voltages of 490 V.

Silicon drift photodetectors for scintillation readout in medical imaging

In the attempt to improve system performances of position-sensitive γ-ray detectors with contemporary ease of use and compactness, the use of silicon photodetectors for scintillation readout has become of increasing interest. With respect to the long-established photomultiplier tubes (PMTs), silicon photodiodes (PDs) have the advantages of higher quantum efficiency (QE), smaller dimensions and lower biasing voltages. Avalanche photodiodes (APDs) combine the high QE and compactness of PDs with the benefit of a moderate avalanche multiplication gain, which reduced the electronics noise contribution. However, the statistical component is again affected by the statistics of the multiplication itself (noise factor), and the noise component is still appreciable mainly in the high energy range. Moreover, the sensitivity of the gain to temperature and biasing variations represents a potential practical drawback in the use of APDs arrays for the application. As an alternative to the mentioned photodetectors, silicon drift detectors (SDDs) have recently shown to achieve excellent performances in scintillation light detection. The SDD is a photodetector characterized by an area-independent, low-value output capacitance (∼100 fF), which is one of the major factors limiting the noise performances of the other silicon detectors’ technologies. SDDs used for CsI(Tl) scintillation readout have already shown to achieve state-of-the-art energy resolution in γ-ray spectroscopy, and monolithic arrays of SDDs have been recently experimented for the development of Anger cameras for high-resolution γ-ray imaging. In this work, is the performances achievable with SDDs in both energy and spatial resolution are discussed together with a review of the most important results achieved with first prototypes of SDD-based γ-ray detectors.

Study of SiPM as a potential photodetector for scintillator readout

The Silicon PhotoMultiplier (SiPM) APD represents an interesting advance in photodetection and could soon be a rival to traditional PMTs in many applications. The SiPM is effectively a densely packed 2D array of Geiger-mode APD microcells, each having individual resistive quenching and multiplexed outputs. In this way the SiPM acts as a linear, high-gain photodetector for moderate photon flux ( N photon < N cells ) . The Metal-Resistor-Silicon (MRS) structure SiPM, produced by CPTA Russia, has been characterised and tested for scintillator light detection in medical applications such as PET. We present a summary of measurements of the device's primary operating characteristics and results of the application to scintillator readout.

First results of scintillator readout with silicon photomultiplier

A new type of silicon device has been realized that has many properties comparable to, or better than, a conventional PMT (Photomultiplier Tube). This paper presents the first results of using these photodetectors in place of a PMT in the readout of scintillators for possible PET (Positron Emission Tomography) applications. This device, the Silicon Photomultiplier (SiPM), is effectively an avalanche photodiode operated in Geiger mode. In Geiger-mode detectors, a very large current signal is produced regardless of the size of the input, giving just logical rather than proportional information. However, the SiPM is subdivided into a large number (1440) of microcells that act as independent and virtually identical Geiger-mode photodiodes. The outputs of all these individual microcells are connected so that the total output signal is the sum of the signals from all of the microcells that were activated. In this way proportional information can be obtained. As a consequence of their design, these detectors have potentially very fast timing, high gain (105-106) at low bias voltage (~50 V), a high quantum efficiency (35% at 500 nm), excellent single photoelectron resolution and are cheap to manufacture. Here we present results obtained with this new photodetector when used with pulsed LED and scintillator pixels

Readout of plastic scintillators with cooled large-area avalanche photodiodes

Time-of-flight measurements in multifragmentation of heavy, relativistic projectiles require very good time resolution and at the same time the detecting system must cope with a dynamic range of up to 8000:1. We look into the possibility of application of large-area avalanche photodiodes (LAAPDs) as alternative light sensors to meet the above requirements. The paper presents the results of our amplitude and time response studies of a plastic scintillator BC-408 readout with a φ16 mm LAAPD using radioactive sources. The measurements were performed using two different setups. The best time resolution has been obtained by exploiting LAAPD cooling to increase its gain beyond that accessible at room temperature. We reach 610 ps (FWHM) at −26°C for the corresponding LAAPD gain of ≈900 and the 90 Sr/ 90 Y β -ray source. We hope to reach below the desired 400 ps with the latter setup at the higher light levels available with multifragmentation products.

Development of a new photomultiplier tube with high sensitivity for a wavelength-shifter fiber readout

We have developed a new photomultiplier tube (PMT) with high sensitivity for a wavelength-shifter fiber readout of plastic scintillators. Improvements of the photoelectron yield were made on a 2-in. PMT (Hamamatsu R329) by adopting (1) a prism-shaped photocathode, (2) an extended-green photocathode material, and (3) an optical mirror surface of the electrodes. As a result, the number of photoelectrons obtained by the new PMT has increased to be 1.8-times that by R329. Other PMT characteristics, such as uniformity of sensitivity, time response and darkcurrent are not so much different from those of R329 for practical applications.

Avalanche photodiodes in scintillation detection

Abstract This paper summarizes our studies on performance of Large Area Avalanche Photodiodes (LAAPDs) in gamma rays scintillation detection. Produced by Advanced Photonix, Inc. LAAPDs, because of their high sensitivity, low excess noise and large active area can potentially replace small area photomultiplier tubes (PMTs) in readout of scintillators. Several inorganic scintillators have been used to evaluate performance of commercially available LAAPDs as light readout devices in gamma-ray spectroscopy and timing measurements. The main factors limiting energy resolution have been analyzed and the experimental results have been compared with those obtained with PMT readout. Notably better or comparable energy resolutions to those observed with a XP2020Q photomultiplier were obtained with the LAAPDs for a number of different scintillators. Particularly, the recorded energy resolutions of 4.3±0.2% and 4.8±0.14% measured with YAP and CsI(Tl) crystals, respectively, for the 662 keV γ - peak from a 137 Cs source belong to the best observed ever with these scintillators. Results of the study of timing with fast scintillators coupled to the LAAPDs showed subnanosecond time resolution of 570 ps for 60 Co γ -rays detected with LSO crystal.