Low Light Output Research Articles

Luminescence properties of CsI:Eu crystals

The work is aimed to research of CsI:Eu single crystals’ luminescence at different excitations in the wide temperature range (from 8 to 300K) and admixture concentration from 10−4% up to 10−1% of Eu2+ ions. This study was directed to separation of different luminescence mechanisms and luminescence quenching explore. It is found that due to the temperature quenching the room temperature yield is decreased in order of magnitude even for heavy Eu doped CsI crystals. Intense low temperature luminescence band at 441–448nm connected with Eu2+ 4f6 5d→4f7 radiative transition is observed. The results obtained show that the excitonic mechanism plays the main role in energy transfer process to Eu2+ ions and this is the reason for relatively low light yield of this emission at RT. The nature of the additional emission bands (410, 450, 480 and 500nm) are apparently caused by the significant non-isomorphism of cations and/or by the presence of oxygen-containing admixtures.

Development of a pixelated GSO gamma camera system with tungsten parallel hole collimator for single photon imaging

In small animal imaging using a single photon emitting radionuclide, a high resolution gamma camera is required. Recently, position sensitive photomultiplier tubes (PSPMTs) with high quantum efficiency have been developed. By combining these with nonhygroscopic scintillators with a relatively low light output, a high resolution gamma camera can become useful for low energy gamma photons. Therefore, the authors developed a gamma camera by combining a pixelated Ce-doped Gd(2)SiO(5) (GSO) block with a high quantum efficiency PSPMT. GSO was selected for the scintillator, because it is not hygroscopic and does not contain any natural radioactivity. An array of 1.9 mm × 1.9 mm × 7 mm individual GSO crystal elements was constructed. These GSOs were combined with a 0.1-mm thick reflector to form a 22 × 22 matrix and optically coupled to a high quantum efficiency PSPMT (H8500C-100 MOD8). The GSO gamma camera was encased in a tungsten gamma-ray shield with tungsten pixelated parallel hole collimator, and the basic performance was measured for Co-57 gamma photons (122 keV). In a two-dimensional position histogram, all pixels were clearly resolved. The energy resolution was ∼15% FWHM. With the 20-mm thick tungsten pixelated collimator, the spatial resolution was 4.4-mm FWHM 40 mm from the collimator surface, and the sensitivity was ∼0.05%. Phantom and small animal images were successfully obtained with our developed gamma camera. These results confirmed that the developed pixelated GSO gamma camera has potential as an effective instrument for low energy gamma photon imaging.

Neutron to Gamma Pulse Shape Discrimination in Liquid Argon Detectors with High Quantum Effciency Photomultiplier Tubes

Abstract A high Light Yield Liquid Argon chamber has been radiated with an Am/Be source for signal-to-background separation level characterization in a Dark Matter Liquid Argon based detector. Apart from the standard nuclear recoil and electron events, from neutron elastic interactions and gamma conversions respectively, an intermediate population has been observed which is attributed to inelastic neutron scatters on Argon nuclei producing Argon recoil and simultaneous gammas from nuclear de-excitation. Taking account of these events results in a better determination of the recoil-like to electron-like separation based on the shape of the scintillation pulse. The results of this recent study as well as from a previous study with a chamber with a lower Light Yield are presented.

Characterization of BaCl 2 scintillation crystal at low temperature

A BaCl 2 scintillation crystal was grown by the Czochralski method. The grown crystal was cut to a size of 10×10×5 mm 3. The scintillation properties of the crystal such as pulse height spectra, energy resolution, and fluorescence decay time were measured with a 137Cs (662 keV) γ-ray source at room temperature. We measured the temperature dependence of the scintillation light yield and decay time with a bi-alkali photomultiplier tube for the BaCl 2 crystal. The BaCl 2 crystal was cooled down with compressed helium gas from room temperature to 10 K. We measured the light yield and decay time changes of the BaCl 2 crystal from 10 K to room temperature. The light yield of the BaCl 2 at 200 K was four times higher than that at room temperature. The decay time increases as temperature decreases. The BaCl 2 scintillation crystal has a low light yield but a fast decay time so that it can be a calorimeter candidate for high energy physics experiments.

Trapezoidal-shaped detector to reduce edge effects in small gamma camera

Abstract In recent years, there has been a growing interest in compact and high resolution small gamma cameras for the early detection of breast cancer and thyroid diseases. We proposed a new detector consisting of a trapezoidal-shaped crystal and a position-sensitive photomultiplier tube (PSPMT) to reduce the edge effect. In this study, the imaging performance of the proposed detector was evaluated by DETECT2000 simulation. Trapezoidal-shaped NaI(Tl) and CsI(Tl) crystals were modeled and the 2-dimensional event positions were calculated using Anger-logic. 99mTc (140 keV) and 131I (364 keV) gamma rays were generated on evenly spaced points with 3.0 mm spacing in the X–Y plane starting 1.0 mm away from the corner surface and 10,000 gamma events were simulated at each location. The simulated results demonstrated that all the 99mTc and 131I point sources were clearly identified in the NaI(Tl) crystal. CsI(Tl) crystal could image 131I sources without edge effect but did not distinguish 99mTc points at the periphery region due to low light yield. In conclusion, our new detector with an enlarged FOV without increasing crystal size could be a useful tool in breast as well as thyroid imaging.

Basic study of single crystal fibers of Pr:Lu3Al5O12 scintillator for gamma-ray imaging applications

Abstract Single-crystalline fibers were grown from 0.25, 0.70, and 1.50 mol% Pr-doped Lu 3 Al 5 O 12 (LuAG) melts by the micro-pulling down (μ-PD) method with a diameter of 0.3–0.5 mm and a length of about 200 mm. They were cut to 10 mm long specimens, and their scintillation properties, including light yield and decay time profile, were examined. These results were compared with corresponding properties of the specimens (0.8×0.8×10 mm 3 ) cut from the bulk crystals produced by conventional Czochralski (CZ) growth. The μ-PD-grown fibers demonstrated relatively low light yield and had the same decay time constant when compared with those of the samples cut from the CZ-grown crystals. The fiber crystals were used to assemble scintillating arrays with dimensions of O 0.5×10 mm 2 ×20 pixels and O 0.3×10 mm 2 ×30 pixels coated by a BaSO 4 reflector. After optical coupling with a position sensitive photomultiplier tube, the fiber-based arrays demonstrated acceptable imaging capability with a spatial resolution of about 0.5 mm.

Investigation of ZnO-Based Polycrystalline Ceramic Scintillators for Use as $\alpha$-Particle Detectors

ZnO-based scintillators are particularly well suited for use as the associated particle detector in a deuterium-tritium (D-T) neutron generator. Application requirements include the exclusion of organic materials, outstanding timing resolution, and high radiation resistance. ZnO, ZnO:Ga, ZnO:In, ZnO:In,Li, and ZnO:Er,Li have demonstrated fast (sub-nanosecond) decay times with relatively low light yields. ZnO:Ga has been used in a powder form as the associated particle detector for a D-T neutron generator. Unfortunately, detectors using powders are difficult to assemble and the light yield from powders is less than satisfactory. Single-crystal ZnO of sufficient size has only recently become available. New applications for D-T neutron generators require better timing resolution and higher count rates than are currently available with associated particle detectors using YAP:Ce as the scintillator. Recent work suggests that ZnO-based scintillators can provide alpha-particle-excited light yields comparable to YAP:Ce scintillators. ZnO-based polycrystalline ceramic scintillators offer the advantages of high light yield, ease of fabrication, low cost, and robust mechanical properties. Precursor powders used in these studies include ZnO and ZnO:Ga powders synthesized using solution-phase, urea precipitation, and combustion synthesis techniques as well as ZnO powder from a commercial vendor. Precursor powders have been sintered using uniaxial hot pressing and spark plasma sintering techniques. Photoluminescence measurements have confirmed that, for most samples, the emissions from these sintered bodies consist primarily of slow, visible emissions rather than the desired sub-nanosecond near-band-edge emissions. Subsequent hydrogen treatments have shown significant improvements in the luminescence characteristics of some ceramic bodies, while other samples have shown no change in luminescence.

Luminescence of Heavily Ce-Doped Alkaline-Earth Fluorides

Luminescent and structural properties of M1-xCexF2+x (M=Ca, Sr, Ba, 0.15<x<0.35) single crystals were studied. It was found that the solid solutions have fluorite crystal structure with the lattice constant noticeably different from that of MF2 crystal. A partial deficiency in the fluorine positions as well as excessive fluorine in interstitial sites was revealed. Crystal lattice modification does not strongly affect the basic absorption and emission properties as compared with weakly Ce-doped MF2. The spectral-kinetic parameters of solid solutions luminescence are similar to those of Ce3+ ion in perturbed site in CeF3 lattice. Low light yield of mixed compounds is connected with the small Stokes shift and reabsorption. The probable model of perturbed Ce3+ center in M1-xCexF2+x crystals assumes the presence of interstitial fluorine in the nearest or next nearest neighborhood of the cerium ion.

Time Discrimination Techniques Using Artificial Neural Networks for Positron Emission Tomography

Relevant information in positron emission tomography is currently being obtained mostly by analog signal-processing methods. New digital PET scanner architectures are now becoming available, which offer greater flexibility and easier reconfiguration capability as compared to previous PET designs. Moreover, new strategies can be devised to extract more information with better accuracy from the digitized detector signals. Trained artificial neural networks (ANN) have been investigated to improve coincidence timing resolution with different types of Avalanche PhotoDiode (APD)-based detectors. The signal at the output of a charge-sensitive preamplifier was digitized with an off-the-shelf, free-running 100-MHz, 8-bit analog-to-digital converter and time discrimination was performed with ANNs implemented in field-programmable gate array (FPGA). Results show that ANNs can be particularly efficient with slow and low light output scintillators, such as BGO, but less so with faster luminous crystals, such as LSO. In reference to a fast PMT-plastic detector, a time resolution of 6.5 ns was achieved with a BGO-APD detector. With LSO, the ANN was found to be competitive with other digital techniques developed in previous works. ANNs implemented in FPGAs provide a fast and flexible circuit that can be easily reconfigured to accommodate various detectors under different signal/noise conditions.

Future use of silicon photomultipliers for K aos at MAMI and P¯ANDA at FAIR

A characterization of scintillating fibres with silicon photomultiplier read-out was performed in view of their possible application in fibre tracking detector systems. Such a concept is being considered for the K aos spectrometer at the Mainz Microtron MAMI and as a time-of-flight start detector for the hypernuclear physics programme at the P¯ANDA experiment of the FAIR project. Results on particle detection efficiency and time resolution are discussed. In summary, the silicon devices are very suitable for the detection of the low light yield from scintillating fibres insofar a trigger scheme is found to cope with the noise rate characteristics.

Gamma ray spectroscopy at high energy and high time resolution at JET

In fusion plasmas gamma ray emission is caused by reactions of fast particles, such as fusion alpha particles, with impurities. Gamma ray spectroscopy at JET has provided valuable diagnostic information on fast fuel as well as fusion product ions. Improvements of these measurements are needed to fully exploit the flux increase provided by future high power experiments at JET and ITER. Limiting aspects are, for instance, the count rate capability due to a high neutron/gamma background combined with slow detector response and a modest energy resolution due to the low light yield of the scintillators. This paper describes the solutions developed for achieving higher energy resolution, signal to background, and time resolution. The detector design is described based on the new BrLa3 scintillator crystal. The paper will focus on hardware development, including a photomultiplier tube capable of stable operation at counting rate as high as 1 MHz, the magnetic shielding, and the fast digital data acquisition system.

Crystal growth and scintillation properties of YAlO 3:Pr co-doped with Mo 3+ and Ga 3+ ions

In this work, we present results of studies aimed at optimization of scintillation performance of fast but low-light yield YAlO 3:Pr scintillator. We applied co-doping with trivalent ions Mo 3+ and Ga 3+ in attempts to reduce the concentration of charge trapping centers that may exist in this material. Systematic study was performed by using the micro-pulling-down method for single crystal growth. Square-shaped YAlO 3:(Pr,Mo) and YAlO 3:(Pr,Ga) crystals with selected co-dopant concentrations were grown. Experimental results including absorption spectra, luminescence under X-ray excitation and scintillation decay are presented. No positive contribution of the co-dopant ions was found. Complicated co-dopant behavior makes the improvement of scintillation parameters difficult.

SAPHIRE (scintillator avalanche photoconductor with high resolution emitter readout) for low dose x-ray imaging: Spatial resolution

An indirect flat panel imager (FPI) with programmable avalanche gain and field emitter array (FEA) readout is being investigated for low-dose and high resolution x-ray imaging. It is made by optically coupling a structured x-ray scintillator, e.g., thallium (Tl) doped cesium iodide (CsI), to an amorphous selenium (a-Se) avalanche photoconductor called high-gain avalanche rushing amorphous photoconductor (HARP). The charge image created by the scintillator/HARP (SHARP) combination is read out by the electron beams emitted from the FEA. The proposed detector is called scintillator avalanche photoconductor with high resolution emitter readout (SAPHIRE). The programmable avalanche gain of HARP can improve the low dose performance of indirect FPI while the FEA can be made with pixel sizes down to 50 microm. Because of the avalanche gain, a high resolution type of CsI (Tl), which has not been widely used in indirect FPI due to its lower light output, can be used to improve the high spatial frequency performance. The purpose of the present article is to investigate the factors affecting the spatial resolution of SAPHIRE. Since the resolution performance of the SHARP combination has been well studied, the focus of the present work is on the inherent resolution of the FEA readout method. The lateral spread of the electron beam emitted from a 50 microm x 50 microm pixel FEA was investigated with two different electron-optical designs: mesh-electrode-only and electrostatic focusing. Our results showed that electrostatic focusing can limit the lateral spread of electron beams to within the pixel size of down to 50 microm. Since electrostatic focusing is essentially independent of signal intensity, it will provide excellent spatial uniformity.

Evaluation of Melt-Grown, ZnO Single Crystals for Use as Alpha-Particle Detectors

As part of an ongoing investigation of the scintillation properties of zinc-oxide-(ZnO)-based scintillators, several melt-grown, ZnO single crystals have been characterized using alpha-particle excitation, infrared reflectance, and room temperature photoluminescence. The crystals, grown by Cermet, Inc., using an oxygen-pressurized melt-growth process, were doped with Group 1 elements (Li), Group 2 elements (Mg), Group 3 elements (Ga, In) and lanthanides (Gd, Er, Tm). The goals of these studies are to better understand the scintillation mechanisms associated with various members of the ZnO scintillator family and to then use this knowledge to improve the radiation detection capabilities of ZnO-based scintillators. One application for which ZnO is particularly well suited as a scintillator is as the associated particle detector in a deuterium-tritium (D-T) neutron generator. Application requirements include the exclusion of organic materials, outstanding timing resolution, and high radiation resistance. ZnO:Ga and ZnO:In have demonstrated fast (subnanosecond) decay times with relatively low light yields, and ZnO(Ga) has been used in a powder form as the associated particle detector for a D-T neutron generator. Four promising candidate materials, ZnO, ZnO:Ga, ZnO:In,Li, and ZnO:Er,Li, were identified in this study. These four samples demonstrated sub-nanosecond decay times and alpha-particle-excited- luminescence comparable to BC-400 fast plastic scintillator. The ZnO:Mg,Ga, ZnO:Gd, and ZnO:Li samples demonstrated appreciable slow (microsecond) decay components that would be incompatible with high-counting-rate applications.

Transparent Top-Emitting Organic Light-Emitting Diodes Using a Cs/Al/Ag/ITO Semi-Transparent Cathode

A top-emitting organic light-emitting diode (TEOLED) was fabricated with a semi-transparent multilayer cathode composed of Cs/Al/Ag and an ITO capping layer. The optical properties of Cs (0.5 nm)/Al (2.0 nm)/Ag (20 nm) at a wavelength of 533 nm exhibited a transmittance of 50 %. The light-output of the TEOELD with the Cs/Al/Ag/ITO showed a maximum luminance of 51,000 cd/m at a current density of 2.32 A/m (corresponding to the forward bias voltage of 11.2 V); the device structure consisted of glass/Ag (100 nm)/ITO (125 nm)/2-TNATA (30 nm)/NPB (15 nm)/Alq3 (55 nm)/Cs (1 nm)/Al (2 nm)/Ag (20 nm)/ITO (100 nm). Also, the external quantum efficiency and the power efficiency at a luminance of 1000 cd/m (corresponding to the forward bias voltage of 6.8 V) were 1.7 % and 2.0 lm/W, respectively. This TEOLED exhibited similar electrical properties, but a little lower light output, compared with a reference TEOLED of the same structure using Alq3 instead of ITO as the capping layer. These characteristics are strongly governed by the thickness, the electrical properties, and optical properties of the semi-transparent multilayer cathode composed of Cs/Al/Ag and by the optical properties of the top capping layer.

Thermal quenching of Ce3+ emission in PrX3 (X = Cl, Br) by intervalence charge transfer

The cause of the relatively low scintillation light yield ofPrBr3:Ce3+ is investigated by means of optical spectroscopy, the temperature dependence of scintillationproperties and the temperature dependence of optically excited decay curves of undoped andCe3+-dopedPrCl3 and PrBr3. The integrated intensity of x-ray excited luminescence ofPrBr3:5% Ce3+ shows that the light yield at room temperature (RT) is two times less than at 80 K. The decay timeof Ce3+ emission optically excited to its 5d band inPrBr3:5% Ce3+ has a singleexponential decay of 11.0 ± 1.1 and 6.0 ± 0.6 ns at 10 K and RT, respectively. It is proposed thatCe3+ emission is quenched by a metal-to-metal charge transfer of followed by emission of Pr3+ which is strongly concentration quenched.

Luminescence of Ce3+ activated fluoro-apatites M5(PO4)3F (M = Ca,Sr, Ba) under VUV–UV and x-ray excitation

Fluoro-apatite M5(PO4)3F (M = Ca, Sr, Ba) phosphorsactivated with Ce3+ ions were prepared by a solid state reaction technique at high temperature. The excitationspectra in the VUV (vacuum ultraviolet)–UV range and the emission spectra in theUV–vis range together with decay time spectra are investigated and discussed.X-ray excited emission spectra were measured and light yields are calculated.The results revealed that Ca(II) sites are preferentially occupied at low dopingconcentration, and then Ca(I) sites are dominant with the increasing of concentration forCe3+ inCa5(PO4)3F. Incontrast, Ce3+ ions mainly occupied the Sr(II)/Ba(II) sites inSr5(PO4)3F and Ba5(PO4)3F even at higher doping concentration. The low light output seems to suggest that thematerials are not suitable x-ray or gamma-ray phosphors.

PrBr$_3$: Ce$^3+$: A New Fast Lanthanide Trihalide Scintillator

We present PrBr3: Ce3+ as a new member of the lanthanide trihalide scintillator family. This compound has the same crystal structure as its cousin LaBr3: Ce3+. The calculated density rhor (5.33 g/cm3) and the effective atomic number Zeff (48.27) are higher than those of LaBr3 : Ce3+ (rhor=5.07 g/cm3 and Z eff=46.90). This results in a shorter total attenuation length of 2.09 cm for 511 keV gamma-ray excitation. Lower light yield than that of LaBr3: Ce3+ is observed for prBr3:5%Ce3+ (16 000 photons/MeV) and 20% Ce3+ (21 000 photons/MeV) versus 70 000 photons/MeV in LaBr 3: Ce3+. Scintillation decay curves of those crystals are non exponential and their decay times are faster than 10 ns. The scintillation properties of undoped PrBr3 are also presented

The influence of acceptor anneal temperature on the performance of InGaN/GaN quantum well light-emitting diodes

Temperature-dependent measurements of the pulsed light-current characteristics of InGaN light-emitting diodes that were thermally annealed at different temperatures have been investigated. A distinct light output, at a fixed current density, with operating temperature arises where the light output increases as the operating temperature is reduced from 300 K, reaches a maximum, and then decreases with subsequent reductions of the operating temperature. We observe that light-emitting diodes thermally annealed at higher temperatures, which is believed to increase the number of electrically activated acceptors in the p layers, have a lower light output below 300 K and the maximum light output shifts to higher operating temperatures. Measured absorption and emission spectra show that the thermal anneal process has not affected the structure of the quantum wells within these samples. The light output, for a fixed current density, has been simulated as a function of operating temperature, and we find that by changing the concentration of acceptor atoms, compensating donor atoms, and the hole mobility in the p layers, the trends observed experimentally can be reproduced. On the basis of the simulations we find that the distinct behavior of the light output with operating temperature is due to the combination of Shockley-Reed-Hall recombination, at operating temperatures around 300 K, and electron drift leakage, at operating temperature below 300 K, and the increase of the acceptor concentration results in an increased electron drift leakage due to the change of the concomitant hole mobility. The simulations support the view that the experimental observations can be explained through changes of the acceptor concentration in the p layers when the thermal anneal temperature is increased.

Perspectives for high resolution and high light output LuAP:Ce crystals

After its introduction in 1994, LuAlO/sub 3/:Ce (LuAP:Ce) encountered problems of 1) low light output, 2) self-absorption of scintillation light, 3) nonproportionality of light output versus energy, and 4) poor energy resolution, and 5) small non-uniform crystals. Studying newly available LuAP:Ce-doped and undoped crystals (from Photonic Materials), we have found that self-absorption is chiefly dependent on the host crystal structure and not on Ce doping. We examined the scintillation properties of these crystals and found 6.8/spl plusmn/0.3% energy resolution for 662 keV for 2 mm/spl times/2 mm /spl times/10 mm pixels. The intrinsic resolution is equal to 2.8/spl plusmn/0.4% at 2360/spl plusmn/120phe/MeV (9800/spl plusmn/500 ph/MeV) light yield. These crystals have nonproportionality of 93/spl plusmn/4% at 16 keV and intrinsic energy resolution close to that of YAP:Ce. Fast components of the scintillation comprise only about half of the light that is emitted in 100 microseconds.