Low Light Output Research Articles

GaAs as a Bright Cryogenic Scintillator for the Detection of Low-Energy Electron Recoils From MeV/c2 Dark Matter

This article presents the measurements of the luminescence and scintillation under X-ray of undoped, Si-doped, and Si, B codoped gallium-arsenide (GaAs) samples at cryogenic temperature over a wide infrared (IR) region using Si and InGaAs photodetectors. The undoped GaAs has a narrow emission band at 838 nm (1.48 eV) and a low light output of about 2 ph/keV. The GaAs:Si has three broad luminescence bands at 830 nm (1.49 eV), 1070 nm (1.16 eV), and 1335 nm (0.93 eV) and a light output of about 67 ph/keV. GaAs:(Si, B) has four luminescence bands at 860 nm (1.44 eV), 930 nm (1.33 eV), 1070 nm (1.16 eV), and 1335 nm (0.93 eV) with a light yield of approximately 119 ph/keV. With advances in photodetection, GaAs promises to be a useful cryogenic scintillator for the detection of electron recoils from MeV/c2 dark matter.

Characterization of CsI(Tl) and LYSO(Ce) scintillator detectors by measurements and Monte Carlo simulations

In this study, we investigated the performance of two cuboid scintillation detectors: thallium-activated cesium iodide [CsI(Tl)] and cerium-doped lutetium yttrium orthosilicate [LYSO(Ce)]. The CsI and LYSO crystals were 5 mm thick with an active area of 10 mm × 10 mm. The LYSO scintillator is characterized by its high stopping power and non-hygroscopicity (neither packaging nor light guide are required). Nevertheless, the main disadvantages of LYSO detectors are a lower light output (32 photons/keV) and an intrinsic radioactivity caused by the β− decay of 176Lu (half-life 3.78 × 1010 years). In contrast, CsI crystals present a relatively high light output (54 photons/keV) allowing better energy resolution; however, the drawback of a CsI detector is low detection efficiency.First, we measured the photon detection efficiency of each scintillator in the photon energy range of 60–1332 keV using radioactive sources. Second, we computed detector efficiencies using the Monte Carlo codes MCNP and Geant4. A comparison between simulated and measured efficiencies showed a good agreement. This comparison confirmed the validity of the mathematical models developed for the two scintillation detectors CsI(Tl) and LYSO(Ce) under investigation.

Monte Carlo simulations of novel optical imaging methods for 131I radiometabolic therapy dosimetry

A new method for dosimetry and imaging (nodule size detection) in metabolic radiotherapy of thyroid gland with 131I is investigated using Monte Carlo simulations. Two approaches are compared, based on Cerenkov effect in human tissues and scintillation effect in a scintillator material placed on the detector. The feasibility is evaluated in terms of dosimetry and imaging performances, taking into account the patient variability and the geometrical configuration for the proposed detection device. The advantages and limitations are discussed with respect to the conventional approach. A good linear correlation between the number of photons and the absorbed dose was found for thyroid sick lobe (R2>0.95) and thyroid healthy lobe (R2>0.95) after gamma attenuation and optical absorption correction and for muscle (R2>0.94), while poorer results were found for skin (R2>0.85). No significant differences were found for Cerenkov and scintillation approach. The standard deviation of the nodule diameter was estimated 9.2 mm for Cerenkov approach and 7.4 nm for scintillation approach. The scintillator detector showed a definitely greater efficiency (a factor 104), as expected. The proposed geometry was combined with a Silicon Photomultiplier detector in order to overcome the low light yield in Cerenkov effect and the estimated total number of photons detected at the average of patient parameters was estimated 1.8× 108 for scintillation and 1.3 × 103 for Cerenkov.

Charge-to-heat transducers exploiting the Neganov-Trofimov-Luke effect for light detection in rare-event searches

In this work we present how to fabricate large-area (15 cm2), ultra-low threshold germanium bolometric photo-detectors and how to operate them to detect few (optical) photons. These detectors work at temperatures as low as few tens of mK and exploit the Neganov-Trofimov-Luke (NTL) effect. They are operated as charge-to-heat transducers: the heat signal is linearly increased by simply changing a voltage bias applied to special metal electrodes, fabricated onto the germanium absorber, and read by a (NTD-Ge) thermal sensor. We fabricated a batch of five prototypes and ran them in different facilities with dilution refrigerators. We carefully studied how impinging spurious infrared radiation impacts the detector performances, by shining infrared photons via optical-fiber-guided LED signals, in a controlled manner, into the bolometers. We hence demonstrated how the radiation-tightness of the test environment tremendously enhances the detector performances, allowing to set electrode voltage bias up to 90 volts without any leakage current and signal-to-noise gain as large as a factor 12 (for visible photons). As consequence, for the first time we could operate large-area NTD-Ge-sensor-equipped NTL bolometric photo-detectors capable to reach sub 10-eV baseline noise (RMS). Such detectors open new frontiers for rare-event search experiments based on low light yield Ge-NTD equipped scintillating bolometers, such the CUPID neutrinoless double-beta decay experiment.

Qualification and development of fast neutron imaging scintillator screens

Abstract We have performed extensive testing and qualification of different commercial fast neutron scintillator screens in camera-based imaging detectors. These include organic scintillators and a ZnS:Cu scintillator mixed in a polypropylene (PP) matrix. Furthermore, we have developed simple and inexpensive in-house ZnS-based fast neutron imaging screens and their performance has been tested and compared to the aforementioned commercial ones. Furthermore, ZnS:Ag was mixed with high viscosity glycerol to create suspension type imaging screen. The ZnS concentration and the screen thickness have been optimized experimentally using sample screen pieces. The screens have been tested in spallation and fission spectra. The plastic scintillator performed the worst mainly due to its low light output, while the PP/ZnS screen the best. The in-house ZnS–epoxy screens produced about 60% of light intensity of its commercial counterpart, which is mainly due to the lower hydrogen density of the optical epoxy compared to polypropylene. A simple model of the converter/ZnS screen type was developed to help understand its properties and to optimize its performance. Some fast neutron radiographic images are shown to illustrate the capabilities of the screens.

Core-shell ZnO:Ga-SiO2 nanocrystals: limiting particle agglomeration and increasing luminescence via surface defect passivation.

Heat treatment is needed to increase the luminescence intensity of ZnO:Ga particles, but it comes at the cost of higher particle agglomeration. Higher agglomeration results in low transparency of scintillating powder when embedded in a matrix and constitutes one of the biggest disadvantages, besides low light yield and low stopping power, of ZnO:Ga powder. Limiting ZnO:Ga particle size is therefore a key step in order to prepare highly luminescent and transparent composites with prospects for optical applications. In this work, SiO2 coating was successfully used to improve luminescence intensity or limitation of crystallite size growth during further annealing. Furthermore, ZnO:Ga and ZnO:Ga-SiO2 core–shells were embedded in a polystyrene matrix.

Distance metrics for digital pulse-shape discrimination of scintillator detectors

Digital pulse-shape discrimination (PSD) of scintillator detectors has gained popularity as an efficient technique for the discrimination between different types of particles. In this paper, we have investigated the use of simple distance metrics for digital PSD of neutron and gamma-rays in liquid scintillator detectors. The distance metrics are commonly used in the field of pattern recognition and offer the advantage of a simple and effective performance. Our method is based on quantifying the difference between a sampled detector pulse and an ideal step pulse. For this purpose, some of the common distance metrics including Euclidean, City-block, Canberra, root mean square (RMS) and Hellinger were experimentally examined through the discrimination of neutrons and gamma-rays in a liquid scintillator detector and their performances were compared to that of the conventional charge-comparison method. It was found that the City-Block distance metric exhibits a very simple and effective PSD approach by producing a figure-of-merit (FOM) of 0.72 against a FOM = 0.54 for the charge-comparison method in the very low light output range of below 160 keVee (electron equivalent energy) and a mean FOM value of 1.23 over the whole examined light output range of 60–1000 keVee.

Improved neutron–gammadiscrimination at low-light output events using conical trans-stilbene

Scintillator geometry affects light-collection efficiency (LCE) and, as a consequence, detector-performance metrics that depend on LCE, such as particle-type identification. This work compares the neutron–gammapulse-shape discrimination (PSD) performance of two trans-stilbene crystals of different geometries: a cylinder and cone. Both geometries have a base diameter and height of 50 mm. Results demonstrate that the cone achieved a higher figure of merit over the cylinder by approximately 22.6% for pulses ranging from 25 keVee to 100 keVee. This improvement can be especially helpful for applied detection systems to better resolve neutrons and gamma rays at low-light output events (<100 keVee), where PSD can suffer.

Luminescence and Energy Transfer between Ce3+ and Pr3+ in BaY2Si3O10 under VUV-vis and X-ray Excitation.

A detailed investigation on photoluminescence properties and energy transfer (ET) dynamics of Ce3+, Pr3+-doped BaY2Si3O10 is provided along with the potential X-ray excited luminescence application. The luminescence properties of Pr3+ are studied in VUV-UV-vis spectral range at low temperature, and the spectral profiles of Pr3+3P0 and 1D2 emission lines are determined using time-resolved emission spectra. Upon 230 nm excitation, the electron population from Pr3+ 4f5d state to its 4f2 excited state is discussed in detail. As Pr3+ concentration rises, Pr3+3P0 and 1D2 luminescence possess different concentration-related properties. The incorporation of Ce3+ in the codoped sample produces the strong Ce3+ luminescence under 230 nm excitation, which is the combined result of Pr3+ 4f5d → Ce3+ 5d ET and Ce3+ intrinsic excitation. On the other hand, the increasingly strong ET of Ce3+ 5d → Pr3+ 4f2 results in the decrease of Ce3+ emission intensity and the gradual deviation of Ce3+ luminescence decay from the single exponential in the system. By employing the Inokuti-Hirayama model, the dipole-dipole interaction is confirmed as the predominant multipolar effect in controlling this ET process, and the value of C DA is determined to be 9.97 × 10-47 m6·s-1. Finally, the relatively low scintillation light yield of Ce3+-doped BaY2Si3O10 material impedes its application potential in the scintillator field, and the cosubstitution of Pr3+ results in the observable decline of scintillation performance.

Crystal structure, electronic structure, optical and scintillation properties of self-activated Cs4YbI6

A self-activated Cs4YbI6 single crystal was grown by the vertical Bridgman method. Crystal structure refinements verified the phase purity and the trigonal crystal system with a space group of R3̅c. By using differential scanning calorimetry, the melting and crystallization points were determined to be 550 and 510 °C, respectively. Luminescence and scintillation properties were systematically studied. Upon ultraviolet light (360 nm) excitation, the Cs4YbI6 crystal exhibits bluish-green emission centered at 450 and 480 nm due to spin-allowed and spin-forbidden transitions of Yb2+ activators. The lifetimes of the corresponding emission bands at room temperature are tens and hundreds of nanoseconds, respectively. X-ray excited radioluminescence spectrum is dominated by the spin-forbidden transition of Yb2+ at 480 nm. The absolute light yield is 2700 ± 200 photons/MeV with a principal scintillation decay time of 33 ns. The physical explanation for the low light yield observed is proposed from experimental and theoretical insights.

Luminescent and scintillation properties of Ce3+ doped Ca2RMgScSi3O12 (R = Y, Lu) single crystalline films

The work is dedicated to the growth and investigation of the luminescent and scintillation properties of single crystalline films (SCFs) of Ca2−xR1+xMg1+xSc1−xSi3O12:Ce (R = Y, Lu) mixed garnets with x = 0–0.25, grown using the liquid phase epitaxy method onto Y3Al5O12 substrates from PbO-B2O3 based flux. The absorption, luminescent and scintillation properties of Ca2−xY1+x Mg1+xSc1−xSi3O12:Ce and Ca2−xLu1+xMg1+xSc1−xSi3O12:Ce SCFs with x = 0 and 0.25 were investigated and compared with the reference YAG:Ce and LuAG:Ce SCFs. Using the Ca2+, Mg2+ and Si4+ alloying, the Ce3+ emission spectra in Ca2−xR1+xMg1+xSc1−xSi3O12:Ce (R = Y, Lu; x = 0–0.25) SCFs can be notably extended in the red range in comparison with YAG:Ce and LuAG:Ce SCFs due to the increase of crystal field strength and Ce3+ multicenter creation in the dodecahedral positions of the lattices of these mixed garnet compounds. Due to the formation of Ce4+ ions, the as-grown Ca2−x R1+xMg1+xSc1−xSi3O12:Ce (R = Y, Lu) SCFs at x = 0 and 0.25 show relatively low light yield. However, after annealing in reducing atmosphere (95% N2 + 5% H2) at T > 1000°C, a recharging Ce4+→Ce3+ takes place. After that, these SCFs possess the light yield about of 30% and 31% in comparison with the reference YAG:Ce and LuAG:Ce SCFs, respectively, and a fast scintillation response with the decay times in the ns range under α–particles excitation by 239Pu (5.15MeV) source.

Enhanced scintillation of Ba3In(B3O6)3 based on nitrogen doping

Scintillating materials, as a class of luminescent materials, are highly demanded for practical use in the high-energy detection. However, the applications are often hampered by their low light yield (LY) or long decay time for many traditional scintillators. In this work, upon nitrogen anion doping, scintillation performance in layered borate Ba3In(B3O6)3 (BIB) has been excellently enhanced with high XEL intensity of ～3 times as large as that of commercial Bi4Ge3O12 (BGO) and ultra-fast fluorescent decay time of ～1.25ns. To shed light on origins of the intrinsic violet-blue emission, we measured the in-situ vacuum ultraviolet excited (VUV) emission spectra of N-BIB ceramic. Combined with experiments and first principles calculations, the band-gap reduction and donor-acceptor density increasing by nitrogen (N) doping is responsible for the enhancement of scintillation performance for N-doped Ba3In(B3O6)3. Moreover, nitrogen anion doping rather than conventional cation doping is found to be also applicable to other intrinsic luminescent materials for enhancing performance.

Highly improved operation of monolithic BGO-PET blocks

In PET scanners both scintillation crystals and photosensors are key components defining the system's performance and cost. Original PET systems used BGO or NaI(Tl) scintillators but achieved limited performance due to its slow decay and relatively low light output. Moreover, NaI(Tl) has low stopping power for 511 keV annihilation photons. In this study we report the possibility to reintroduce BGO crystals, and in particular in the form of monolithic blocks, especially suitable for low-dose large-size PET scanners, offering significantly improved sensitivity at a highly reduced cost compared to LYSO type fast scintillators. We have studied the performance of a monolithic BGO block as large as 50 × 50 × 15 mm3 with black-painted lateral walls to reduce lights spread, enabling accurate photon depth of interaction (DOI) measurements. A directional optical layer, called retro-reflector, was coupled to the entrance face bouncing back the scintillation light in the direction of the emission source and, therefore, adding to the light signal while preserving the narrow light cone distribution. Four configurations namely 12 × 12 and 16 × 16 SiPM arrays (3 mm × 3 mm each) as photosensors, with or without a nanopattern treatment at the crystal exit face, have been studied. This structure consisted of a thin layer of a specific high refractive index material shaped with a periodic nanopattern, increasing the scintillation light extraction. The readout returned information for each SiPM row and column, characterizing the X-Y light distribution projections. We have studied the detector spatial resolution using collimated 22Na sources at normal incidence. The DOI resolution was evaluated using collimated gamma beams with lateral incidence. The overall best detector performance was obtained for the 16× 16 SiPM array offering higher readout granularity. We have determined the spatial resolution for 3 separated DOI layers, obtaining the best results for the DOI region near to the photosensor.

Solution growth and scintillation properties of 9-phenylcarbazole

In this paper we report on the crystal growth and characterization of 9-phenylcarbazole (PCz). Crystals of PCz were obtained by the solution growth technique. Several organic solvents were investigated including heptane, cyclohexane, toluene, xylene, methanol, ethanol, and acetone. The largest crystals of the highest quality were grown from acetone.Radioluminescence spectra of pure PCz crystals exhibit a broad emission band peaking at 381 nm. Adding a small amount of an impurity (dopant) shifts the emission wavelength to 446 nm which is a better match to standard bialkali photomultiplier tubes. Single crystals of pure PCz have low light yields (typically < 5,000 ph/MeV), but doped crystals show high light yields of up to 25,000 ph/MeV and have a fast scintillation decay of less than 20 ns. Pulse shape discrimination with doped PCz crystals using neutron and gamma-rays from a 252Cf source yields a Figure of Merit (FOM) of about 2.7 at 1.0 MeVee cut-off energy.As far as we are aware, doped PCz is one of the brightest organic single crystal scintillators in existence and is a very promising organic neutron detector for a multitude of applications including homeland security, nuclear non-proliferation, nuclear physics, and non-destructive testing.

A Low Noise Front End for the Belle II Forward Electromagnetic Calorimeter Upgrade

The Belle II experiment will operate at the SuperKEKB e+ e− collider, designed to reach a top luminosity of $8 \times 10^{35}$ at the $\Upsilon $ (4s) resonance. The high background environment of the accelerator poses serious challenges to the design of the detector. In particular, the Belle II collaboration is developing an ambitious upgrade program that involves the forward electromagnetic calorimeter. We will use pure CsI crystals, since they have less scintillation time but unfortunately much lower light yield. The electromagnetic calorimeter upgrade is subject to the same detector constraints as the first design; an intense research and development program on photon-detectors and front-end electronics has been initiated by the Italian collaboration. Our preliminary results show that a readout chain using avalanche photodiodes (APDs) to match the detector constraints and a dedicated front-end card for the readout meet the Belle II collaboration requirements. In this paper, we will show the experimental setup and detail the main characteristics of the read-out and power distribution system since the front-end card hosts both a low noise preamplifier and a power regulator to set the single APD bias voltage. The main HV module has been developed for this purpose to deliver a low noise power distribution to the front-end cards. All the relevant parameters of the front-end system can be set and monitored via Ethernet using the Experimental Physics and Industrial Control System. Slow control and monitoring functions in Belle II are performed by a dedicated hardware. The platform has been designed as an uP-based service-oriented platform, capable to offer an integrated solution for all the needs of data acquisition, analysis, and network functions.

Bismuth germanate coupled to near ultraviolet silicon photomultipliers for time-of-flight PET

Bismuth germanate (BGO) was a very attractive scintillator in early-generation positron emission tomography (PET) scanners. However, the major disadvantages of BGO are lower light yield and longer rise and decay time compared to currently popular scintillators such as LSO and LYSO. This results in poorer coincidence timing resolution and it has generally been assumed that BGO is not a suitable scintillator for time-of-flight (TOF) PET applications. However, when a 511 keV photon interacts in a scintillator, a number of Cerenkov photons are produced promptly by energetic electrons released by photoelectric or Compton interactions. If these prompt photons can be captured, they could provide a better timing trigger for PET. Since BGO has a high refractive index (increasing the Cerenkov light yield) and excellent optical transparency down to 320 nm (Cerenkov light yield is higher at shorter wavelengths), we hypothesized that the coincidence timing resolution of BGO can be significantly improved by efficient detection of the Cerenkov photons. However, since the number of Cerenkov photons is far less than the number of scintillation photons, and they are more abundant in the UV and blue part of the spectrum, photosensors need to have high UV/blue sensitivity, fast temporal response, and very low noise in order to trigger on the faint Cerenkov signal. In this respect, NUV-HD silicon photomultipliers (SiPMs) (FBK, Trento, Italy) are an excellent fit for our approach. In this study, coincidence events were measured using BGO crystals coupled with NUV-HD SiPMs. The existence and influence of Cerenkov photons on the timing measurements were studied using different configurations to exploit the directionality of the Cerenkov emissions. Coincidence resolving time values (FWHM) of ~270 ps from 2 × 3 × 2 mm3 BGO crystals and ~560 ps from 3 × 3 × 20 mm3 BGO crystals were obtained. To our knowledge, these are the best coincidence resolving time values reported for BGO to date. With these values, BGO can be considered as a relevant scintillator for TOF PET scanners, especially if photodetectors with even better near UV/blue response can be developed to further improve the efficiency of Cerenkov light detection.

Influence of peculiarities of electronic excitation relaxation on luminescent properties of MgWO4

Luminescent properties of magnesium tungstate monocrystals grown by two different methods are studied. Only the exciton luminescence of these crystals themselves is observed. Temperature dependence of the low-energy range in the luminescence excitation spectra is described by the Urbach rule. Slope coefficient σ0 = 0.74 obtained from this dependence implies autolocalization of the excitons in MgWO4. The processes of electronic excitations relaxation are considered depending on the structure of valence band in MgWO4 and in other wolframites, ZnWO4 and CdWO4. In contrast to ZnWO4 and CdWO4, the d-states of the cation do not participate in formation of the MgWO4 valence band. Using the excitation spectra measured in the range of the fundamental absorption (4–20 eV), it is shown that this difference manifests itself in relaxation of electronic excitations and may be the cause of the relatively low light yield of MgWO4.

The FLARES project: An innovative detector technology for rare events searches

FLARES is an innovative project in the field of rare events searches, such as the search for the neutrinoless double beta decay. It aims at demonstrating the high potential of a technique that combines ultra-pure scintillating crystals with arrays of high performance silicon drift detectors, operated at about 120K, to reach a 1% level energy resolution. The proposed technique will combine in a single device all the demanding features needed by an ideal experiment looking for rare events. The performance of a first production of matrices of silicon drift detectors as well as first measurements of the low temperature light yield of a selection of high purity scintillating crystals will be presented and discussed.

The Design of LED Fiber Automotive Headlamp

It's more and more common to use LED as the light source in automotive lighting. But heat dissipation and low light output of single die light source have restricted the LED to be the mainstream. To use fiber as light-transmitting component could perfectly resolve above weakness. In the paper a design method of LED fiber automotive headlamp is mentioned. And for COB (chip on board) LED, a mathematic model of TIR (total internal reflected) lens is proposed. It could effectively focus the rays emitted from surface light source.

Principal Component Analysis for pulse-shape discrimination of scintillation radiation detectors

In this paper, we report on the application of Principal Component analysis (PCA) for pulse-shape discrimination (PSD) of scintillation radiation detectors. The details of the method are described and the performance of the method is experimentally examined by discriminating between neutrons and gamma-rays with a liquid scintillation detector in a mixed radiation field. The performance of the method is also compared against that of the conventional charge-comparison method, demonstrating the superior performance of the method particularly at low light output range. PCA analysis has the important advantage of automatic extraction of the pulse-shape characteristics which makes the PSD method directly applicable to various scintillation detectors without the need for the adjustment of a PSD parameter.