Ce Scintillator Research Articles

Three-dimensional SPECT imaging with LaBr3:Ce scintillator for characterization of nuclear waste

Characterization of nuclear waste in terms of radioactivity distribution is important not only for their safe disposal but also for nuclear material accounting. Single Photon Emission Computed Tomography (SPECT) provides a non-invasive technique for the characterization and activity distribution of the gamma-emitting sources in a matrix. Sodium iodide scintillators, which are most commonly used, suffer from poor energy resolution and do not provide accurate peak discrimination for radioisotopes like 239Pu which have overlapping peaks. Cerium-activated lanthanum bromide (LaBr3:Ce) scintillators have better energy resolution and provide better peak discrimination. In this paper, experimental studies using LaBr3:Ce for 3D SPECT imaging of dummy waste drum has been discussed. The reconstruction has been done using the Filtered Backprojection scheme with attenuation compensation based on Novikov's inversion formula.

Optical properties and defect structure of Sr2+ co-doped LaBr3:5%Ce scintillation crystals

Sr2+ co-doped LaBr3:5%Ce scintillators show a record low energy resolution of 2% at 662keV and a considerably better proportional response compared to standard LaBr3:5%Ce. This paper reports on the optical properties and time response of Sr co-doped LaBr3:5%Ce. Multiple excitation and emission bands were observed in X-ray and optically excited luminescence measurements. Those bands are ascribed to three different Ce3+ sites. The first is the unperturbed site with the same luminescence properties as those of standard LaBr3:Ce. The other two are perturbed sites with red-shifted 4f-5d1 Ce3+ excitation and emission bands, longer Ce3+ decay times, and smaller Stokes shifts. The lowering of the lowest 5d level of Ce3+ was ascribed to larger crystal field interactions at the perturbed sites. Two types of point defects in the LaBr3 matrix were proposed to explain the observed results. No Ce4+ ions were detected in Sr co-doped LaBr3:5%Ce by diffuse reflectance measurements.

Characterization of large volume 3.5″×8″ LaBr3:Ce detectors

The properties of large volume cylindrical 3.5″×8″ (89mm×203mm) LaBr3:Ce scintillation detectors coupled to the Hamamatsu R10233-100SEL photo-multiplier tube were investigated. These crystals are among the largest ones ever produced and still need to be fully characterized to determine how these detectors can be utilized and in which applications. We tested the detectors using monochromatic γ–ray sources and in-beam reactions producing γ rays up to 22.6MeV; we acquired PMT signal pulses and calculated detector energy resolution and response linearity as a function of γ-ray energy. Two different voltage dividers were coupled to the Hamamatsu R10233-100SEL PMT: the Hamamatsu E1198-26, based on straightforward resistive network design, and the “LABRVD”, specifically designed for our large volume LaBr3:Ce scintillation detectors, which also includes active semiconductor devices. Because of the extremely high light yield of LaBr3:Ce crystals we observed that, depending on the choice of PMT, voltage divider and applied voltage, some significant deviation from the ideally proportional response of the detector and some pulse shape deformation appear. In addition, crystal non-homogeneities and PMT gain drifts affect the (measured) energy resolution especially in case of high-energy γ rays. We also measured the time resolution of detectors with different sizes (from 1″×1″ up to 3.5″×8″), correlating the results with both the intrinsic properties of PMTs and GEANT simulations of the scintillation light collection process. The detector absolute full energy efficiency was measured and simulated up to γ-rays of 30MeV

Silicon Drift Detectors for Readout of Scintillators in Gamma-Ray Spectroscopy

In this work, we report on a new development of Silicon Drift Detectors (SDDs) for gamma-ray spectroscopy with space science applications. The research is supported by the European Space Agency through the Technology Research Programme (TRP). The final goal of the development is the realization of monolithic arrays of SDDs which will be assembled to readout large (2” and 3”) LaBr3(Ce) scintillators. We present here the results of the development of a single SDD prototype, with 8 × 8 mm2 active area, produced at Fondazione Bruno Kessler (FBK) semiconductor laboratories. We discuss the design issues related to the specific use of this device as a photo-detector for scintillators. Then, we focus on the read-out electronics. Since this SDD does not include a front-end transistor on the silicon chip, we have adopted a CMOS charge preamplifier (CUBE) recently developed at Politecnico di Milano. This preamplifier has allowed the achievement of state-of-the-art noise performance using a SDD technology process without the integration of the FET (Field Effect Transistor) on the detector chip. A quantum efficiency of about 80% has been measured for the SDD at the emission wavelength band of LaBr3 (360-380 nm). First experimental measurements consisting of direct 55 Fe irradiation of the SDD without scintillator, have demonstrated energy resolution of 140 eV and 129 eV at -20°C and -43°C respectively. By coupling the SDD with a LaBr3(Ce) scintillator (9 mm diameter), we have measured energy resolution of 5.6% FWHM and 2.6% FWHM at 122 keV and 662 keV respectively.

On the response of GdAlO3:Ce powder scintillators

The aim of the present study was to investigate the luminescence efficiency (XLE) of gadolinium aluminum perovskite (GdAlO3:Ce) powder scintillator. This powder phosphor, also known as GAP:Ce scintillator, is a non-hygroscopic material, emitting blue light with short decay time. For the purposes of this study, five scintillating screens with coating thicknesses, 14.7, 31.0, 53.7, 67.2 and 121.1mg/cm2, were prepared in our laboratory from GdAlO3:Ce powder (Phosphor Technology, Ltd) by sedimentation on silica substrates. The light emitted by the phosphors under investigation was evaluated by performing measurements of the absolute luminescence efficiency (AE), X-ray luminescence efficiency and detector quantum gain (DQG) under X-ray exposure conditions with tube voltages ranging from 50 to 140kV. The quantum detection efficiency (QDE) and energy absorption efficiency (EAE) were also evaluated. The spectral compatibility of GdAlO3:Ce, with various existing optical detectors, was investigated after emission spectra measurements. A theoretical model, describing radiation and light transfer, was used to fit experimental AE data. This has allowed the estimation of optical attenuation coefficients of the scintillator. GdAlO3:Ce exhibited higher QDE and EAE values, compared to aluminium perovskite (YAlO3:Ce) but lower absolute efficiency values. Absolute efficiency was found to increase with increasing X-ray tube voltage, although for values higher than 120kVp a decrease was observed.

A Dual-energy X-ray detector on the basis of ZnSe(Al) and LGSO(Ce) composite scintillators

The results of tests of an experimental prototype of a two-energy scintillation detector, which is based on a two-layer composite system of fine-crystalline ZnSe(Al) and LGSO(Ce) scintillators, are presented and its properties are determined. The X-rays spectra for each element of the detector were obtained. The analysis of these spectra allowed determination of the energy efficiency of the X-ray absorption for each element of the detector in an energy range of 30–70 keV. The dependences of proper signals from photodetectors under the action of X rays (the tube voltage varied from 40 to 90 kV) were restored. The corresponding X-ray spectra are presented as well. The dependences of scintillation signals from disperse ZnSe(Al) and LGSO(Ce) scintillators on the X-ray tube voltage are presented (the voltage varied from 40 to 70 keV). Due to the high registration efficiency of relatively low-energy X rays (up to 70 keV), this detector can be used not only in engineering introscopy (customs baggage control, mail, cargo-container, and vehicle checks, etc.), but also in various fields of medical radiology.

Improved event positioning in a gamma ray detector using an iterative position-weighted centre-of-gravity algorithm

An iterative position-weighted centre-of-gravity algorithm was developed and tested for positioning events in a silicon photomultiplier (SiPM)-based scintillation detector for positron emission tomography. The algorithm used a Gaussian-based weighting function centred at the current estimate of the event location. The algorithm was applied to the signals from a 4 × 4 array of SiPM detectors that used individual channel readout and a LYSO:Ce scintillator array. Three scintillator array configurations were tested: single layer with 3.17 mm crystal pitch, matched to the SiPM size; single layer with 1.5 mm crystal pitch; and dual layer with 1.67 mm crystal pitch and a ½ crystal offset in the X and Y directions between the two layers. The flood histograms generated by this algorithm were shown to be superior to those generated by the standard centre of gravity. The width of the Gaussian weighting function of the algorithm was optimized for different scintillator array setups. The optimal width of the Gaussian curve was found to depend on the amount of light spread. The algorithm required less than 20 iterations to calculate the position of an event. The rapid convergence of this algorithm will readily allow for implementation on a front-end detector processing field programmable gate array for use in improved real-time event positioning and identification.

Improvement of LaBr3:5%Ce scintillation properties by Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+ co-doping

This paper reports on the effects of Li+, Na+, Mg2+, Ca2+, Sr2+, and Ba2+ co-doping on the scintillation properties of LaBr3:5%Ce3+. Pulse-height spectra of various gamma and X-ray sources with energies from 8 keV to 1.33 MeV were measured from which the values of light yield and energy resolution were derived. Sr2+ and Ca2+ co-doped crystals showed excellent energy resolution as compared to standard LaBr3:Ce. The proportionality of the scintillation response to gamma and X-rays of Ca2+, Sr2+, and Ba2+ co-doped samples also considerably improves. The effects of the co-dopants on emission spectra, decay time, and temperature stability of the light yield were studied. Multiple thermoluminescence glow peaks, decrease of the light yield at temperatures below 295 K, and additional long scintillation decay components were observed and related to charge carrier traps appearing in LaBr3:Ce3+ with Ca2+, Sr2+, and Ba2+ co-doping.

LaBr3 scintillator response to admixed neutron and [formula omitted] fluxes

The γ-ray spectroscopy is a promising method for diagnosing fast ions and confined α particles in a fusion plasma device. This application requires γ-ray detectors with high energy resolution (say a few percent for γ-ray energies in the range 1–5MeV), high efficiency and high count rate capability, ideally up to a few MHz. Furthermore, the detector will have to withstand the high 14MeV and 2.45MeV neutron fluxes produced by the main fusion reactions between deuterium and tritium. Experimental results demonstrate that the requirements on energy resolution, efficiency and count rate can be met with a LaBr3(Ce) scintillator detector equipped with fast digital data acquisition. The measured response of the detector to 2.45MeV neutrons is presented in this paper and discussed in terms of the interaction mechanism between neutrons and detector.

Position-resolved Positron Annihilation Lifetime Spectroscopy

A new method which allows for position-resolved positron lifetime spectroscopy studies in extended volume samples is presented. In addition to the existing technique of in-situ production of positrons inside large (cm3) bulk samples using high-energy photons up to 16 MeV from bremsstrahlung production, granular position-sensitive photon detectors have been employed. A beam of intense bremsstrahlung is provided by the superconducting electron linear accelerator ELBE (Electron Linear Accelerator with high Brilliance and low Emittance) which delivers electron bunches of less than 10 ps temporal width and an adjustable bunch separation of multiples of 38 ns, average beam currents of 1 mA, and energies up to 40 MeV. Since the generation of bremsstrahlung and the transport to the sample preserves the sharp timing of the electron beam, positrons generated inside the entire sample volume by pair production feature a sharp start time stamp for positron annihilation lifetime studies with high timing resolutions and high signal to background ratios due to the coincident detection of two annihilation photons. Two commercially available detectors from a high-resolution medial positron-emission tomography system are being employed with 169 individual Lu2SiO5:Ce scintillation crystals, each. In first experiments, a positron-lifetime gated image of a planar Si/SiO2 (pieces of 12.5 mm × 25 mm size) sample and a 3-D structured metal in Teflon target could be obtained proving the feasibility of a three dimensional lifetime-gated tomographic system.

The generalized centroid difference method for picosecond sensitive determination of lifetimes of nuclear excited states using large fast-timing arrays

A novel method for direct electronic “fast-timing” lifetime measurements of nuclear excited states via γ–γ coincidences using an array equipped with N∈N equally shaped very fast high-resolution LaBr3(Ce) scintillator detectors is presented. Analogous to the mirror symmetric centroid difference method, the generalized centroid difference method provides two independent “start” and “stop” time spectra obtained by a superposition of the N(N−1)γ–γ time difference spectra of the N detector fast-timing system. The two fast-timing array time spectra correspond to a forward and reverse gating of a specific γ–γ cascade. Provided that the energy response and the electronic time pick-off of the detectors are almost equal, a mean prompt response difference between start and stop events is calibrated and used as a single correction for lifetime determination. These combined fast-timing arrays mean γ–γ time-walk characteristics can be determined for 40keV<Eγ<1.3MeV with an accuracy less than 5ps using a 152Eu γ-ray source. Due to reduction and cancellation of many possible systematic errors, the lifetime determination limit of the method over the total dynamic range is mainly determined by the statistics. The setup of an N=4 detector fast-timing array delivered an absolute time resolving power of 3ps for 10000 γ–γ events per total fast timing array start and stop time spectrum. The new method is tested over the total dynamic range by the measurements of known picosecond lifetimes in standard γ-ray sources.

SU‐E‐I‐80: Development of a Compton Camera for Online Range Monitoring of Laser‐Accelerated Proton Beams

Purpose: Development of a versatile detector system primarily designed for online range monitoring of proton beams generated via the novel technique of particle acceleration from high‐power, short‐pulse lasers to be used in bio‐medical applications. Methods: While so far mostly offline diagnostics tools are used in this context, we aim at developing a technique based on the position‐sensitive detection of prompt gamma rays emitted from nuclear reactions between the proton beam and the biological sample. For this purpose, we develop a Compton camera capable to track not only the Compton scattered primary photon, but also the secondary Compton electron. Results: Extensive simulation studies resulted in an optimized design of the Compton camera based on a LaBr3(Ce) scintillation crystal (50×50×30 mm3 block crystal, read out by a multi‐anode PMT) acting as absorber, preceded by a stack of 6 double‐sided silicon strip detectors as scatterers (500 μm thick, 128 strips/side, pitch 390 μm). From the design simulations an angular resolution of &lt;= 2° (at 2–6 MeV) and a source image reconstruction efficiency of 10−3 −10−3 (at 1–5 MeV) can be expected. The LaBr3 crystal has been characterized with laboratory sources, resulting in a time resolution of 540 ps and a position‐dependent energy resolution (best in the central region). Using a collimated source the point‐spread function was determined from the pixelated light distribution, yield a (preliminary and by software optimization improving) spatial resolution of &lt; 3mm. Conclusion: The Compton camera based approach for prompt‐γ detection from nuclear interactions of therapeutic ion beams offers very promising perspectives for online range monitoring. An arrangement of several camera modules could even be used in a γ‐PET mode to additionally detect delayed annihilation radiation from positron emitters in the irradiation interrupts (with improved performance in the presence of an additional third (prompt) γ (as e.g. in 10C, 14O). Funding support via German Science Foundation: DFG Cluster of Excellence MAP (Munich Centre for Advanced Photonics)

TH‐C‐144‐02: Experimental Study of Discrete Prompt Gamma Lines for In‐Vivo Proton Range Verification

Purpose: A novel method for in vivo proton beam range verification is presented, using energy‐resolvable proton‐induced prompt gamma lines. Methods: Prompt gamma‐rays emitted near the end‐of‐range of a clinical proton beam originate from transitions between the lower states of excited nuclear reaction products. We developed a prompt gamma‐ray detection system with the aim of resolving the resulting discrete gamma lines. The ability to quantify the strength of these lines can provide valuable information for in vivo range verification: known cross sections may be used as prior knowledge, and uncertainties resulting from differences in tissue composition could be reduced. A LaBr3:Ce scintillation detector was used, which provides high energy resolution. The pulses from the photomultiplier tube were acquired with analog‐to‐digital converters at high sampling rates, synchronized to the cyclotron radiofrequency to separate the quasi‐continuous neutron‐induced background from the prompt gamma signals of interest. Results: Preliminary measurements were performed with a 150 MeV proton pencil‐beam targeted on the center of a water phantom with a 15 cm diameter. A 1 cm lead slit collimator was used perpendicular to the beam direction, centered at the end‐of‐range. The acquired gamma spectra show the 6.13 MeV gamma line from oxygen‐16, and its escape peak, can be clearly resolved with an energy resolution of ∼1% FWHM. Conclusion: Our results show the feasibility of measuring discrete prompt gamma lines using a practical detection system. These gamma lines can be attributed to specific nuclear reactions with established properties, potentially enabling accurate proton range verification using a small number of measurement points. Experiments with different collimators and proton energies are currently being performed to determine an optimal configuration for range verification.

New approaches for improvement of TOF-PET

We present results of simulations on the influence of photon propagation and the Cherenkov effect on the time resolution of LSO:Ce scintillators. The influence of the scintillator length on the coincidence time resolution is shown. Furthermore, the impact of the depth of interaction on the time resolution, the light output and the arrival time distribution at the photon detector is simulated and it is shown how these information can be used for time walk correction.

Improving of LSO(Ce) Scintillator Properties by Co-Doping

The influence of the co-doping of cerium-doped lutetium oxyorthosilicate (LSO) with ytterbium on the output characteristics of this scintillator material were investigated by studying X-ray luminescence spectra, emission decay kinetics, afterglow level, Vickers indentation hardness, thermally stimulated luminescence, and radiation stability of luminescence parameters. It has been demonstrated that the optimal doping contents are ~ 0.1 mol.% and ~ 0.5 mol.% for cerium and ytterbium, respectively. A considerable increase in the radiation stability of light output, decrease in the afterglow level by more than two orders of magnitude and a 20% decrease in indentation hardness at the optimal concentration of Yb have been observed, however, at the expense of the light output.

Integrated Neutron Detector for Handheld Systems

This paper describes detectors comprised of Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiYCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> :Ce (CLYC) scintillators coupled to silicon photomultipliers (MPPC, Hamamatsu). CLYC has been developed for dual gamma ray and thermal neutron detection. MPPCs are compact detectors with a very thin profile (~2 mm), but also a small active area (6 mm × 6 mm). The combination of both can create a compact replacement of a He-3 tube. Three different crystal sizes were tested: a 5 mm × 5 mm × 5 mm cube, a 1 cm × 1 cm × 4 cm cuboid, and a Ø1 in × 1 in cylinder. All three detectors showed a well-resolved thermal neutron peak with energy resolution of 5.4%, 7%, and 11% (FWHM), respectively. The degradation in larger crystals is due to a mismatch between the coupled crystal face and the active area of the light detector. All detectors showed the 662 keV gamma ray peak from a Cs-137 source, indicating capability for limited gamma ray spectroscopy, with energy resolutions of 10%, 17%, and 26%, respectively. The capability for pulse shape discrimination was shown as well. The efficiency of the detectors was measured against a 10 atm He-3 tube (9 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> volume). The best absolute efficiency was shown by the cylinder detector, next by the He-3 tube (almost the same as the cylinder detector), and finally by the cuboid detector. The latter showed the best neutron count per cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , twice as high as the two other detectors.

Fast Neutron Spectroscopy Using${\rm Cs}_{2}{\rm LiYCl}_{6}{:}{\rm Ce}$ (CLYC) Scintillator

The response of Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiYCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> :Ce (CLYC) scintillator material to fast neutrons has been measured using a Van de Graaff accelerator. Beams of monoenergetic neutrons in the energy range 0.359 MeV to 4.703 MeV were used to irradiate a 9 mm × 9.5 mm × 12 mm CLYC crystal. Following pulse-shape discrimination to separate neutron and gamma-ray events, peaks are observed in the neutron spectrum and assigned to fast-neutron events in the scintillator. One of the peaks is interpreted as being due to the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">35</sup> Cl(n, p) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">35</sup> S reaction, and it is shown that the proton energy released in this reaction varies linearly with the energy of the incoming neutron. The linearity of the response may enable CLYC to be used for fast-neutron spectroscopy with well defined spectral peaks. The response of CLYC to thermal neutrons and gamma rays is well known, and the material has potential for simultaneous thermal-neutron detection, fast-neutron spectroscopy, and gamma-ray spectroscopy.

Increased Light Extraction From Inorganic Scintillators With Laser-Etched Microstructures

A technique of laser-etching repeating micrometer-scale structures into the exit surface of a thin, YAP:Ce scintillator to increase light extraction has been performed. Different etching techniques were performed on a single, monolithic crystal using varying laser power to study how light extraction is influenced by these different treatments. For the best case, a 90.4% increase in light extraction was measured without optical coupling, which compares to photonic crystal techniques. This technique finds application in associated particle imaging (API) where no optical coupling can be used for the alpha detector's scintillator, inside the Deuterium-Tritium (D-T) neutron generator vacuum. The surface characteristics of each treatment are quantified, and changes in light collection relative to a normal, polished surface are presented. Measurement of the number of detected photons is given for the case where no optical coupling is used between the scintillator and photosensor. Additionally, implications of performance improvements in YAP:Ce based alpha transducers within D-T generators, due to the measured increases in light collection, are presented.

Assessment of photon detectors for a handheld gamma-ray and neutron spectrometer using Cs2LiYCl6:Ce (CLYC) scintillator

The coupling of Cs2LiYCl6:Ce (CLYC) scintillator to silicon photon converters has been evaluated with the goal of investigating replacements for the traditional photomultiplier tube (PMT) in small handheld spectrometers. Energy spectra produced under irradiation by a range of gamma-ray and neutron sources were collected with CLYC mounted to several avalanche photodiodes, PIN photodiodes, and silicon photomultipliers. The performance for both gamma rays and neutrons was compared to that obtained by coupling CLYC to PMTs. None of the silicon devices evaluated provide comparable performance to that of a PMT with CLYC. This is attributed to the photon-detection efficiency of the silicon detectors over the wavelength range of CLYC emissions, as well as the noise characteristics of the devices.

Linearity of the energy scale of a detector based on a LaBr3(Ce) scintillator

The linearity of a LaBr3(Ce) detector response in the energy range of 0.6–11.0 MeV has been investigated using various PMT voltage divider circuits. A tapered voltage divider circuit is proposed to reduce the nonlinearity.