Scintillator Elements Research Articles

Large-scale production of PWO scintillation elements for CMS ECAL

JSC Bogoroditsk Technical Chemical Plant, BTCP, has produced up to date more than 20,000 lead tungstate scintillation elements for the electromagnetic calorimeter of CMS Collaboration. Here we report on the status of the crystal production and results of the quality insurance program, which is performed by the Collaboration in cooperation with BTCP to keep crystal properties within specifications.

Characterization of pilot batch of LuYAP crystals to be used in prototypes of small animal positron emission tomograph ClearPET

Small animal positron emission tomograph ClearPET® is under development by Crystal Clear Collaboration, CERN. ClearPET® detector with Depth Of Interaction (DOI) determination capability is based on LSO and (Lu-Y)AlO3:Ce (LuYAP) scintillation crystals. Pilot batch of LuYAP crystals was grown at Bogoroditsk Techno Chemical Plant, and more than 9000 scintillation elements (pixels) with dimensions 2×2×8mm3 and 2×2×10mm3 were manufactured for small animal PET prototyping. During this effort, complete characterization of scintillation properties of grown crystal and selective control of scintillation pixels was performed. The aim of this work is to evaluate repeatability of LuYAP production technology and uniformity of scintillation pixel parameters. Methods and main results of characterization are presented.

Production of the high-quality CdWO4 single crystals for application in CT and radiometric monitoring

Effects of the intrinsic and dopant-induced defects in cadmium tungstate (CWO) crystals upon their luminescence and functional characteristics have been studied. It has been shown that oxygen vacancies and complexes with trivalent dopants lead to formation of deep charge carrier traps that are responsible for afterglow in the millisecond range. These defects also give rise to color centers that cause absorption in the intrinsic emission band, thus worsening scintillation parameters of the crystals. On the basis of the results obtained, an improved technological process has been developed, ensuring stable technological yield of high-quality crystals. This allowed development of scintillation elements for computer tomography with afterglow of less than 100ppm after 20ms and light yield of not less than 19500 photon/MeV. Ways and advantages of the use of high-quality CWO crystals in scintillation detectors and spectrometric detection blocks are discussed.

The in-flight background of IBIS/PICsIT

The PICsIT instrument is the high-energy imaging detector of IBIS on board INTEGRAL and comprises 4096 individual scintillator elements each working in the energy range from about 175 keV to 10 MeV. Images of the sky are created using the coded aperture technique wherein each point in the detection plane contributes to the final image. For this reason the sensitivity at any point in the image is highly dependent on both the total background and its spatial and temporal uniformity. Herein we describe the PICsIT in-flight background, for the principal modes of operation, from the point of view of total count rate and spectrum, and discuss both the spatial and temporal stability.

“Scintillator-photodiode” detectors for scanning introscopy with high-spatial resolution

The improvement of spatial resolution, together with reducing the dose load upon the tested object and making the scanning time shorter, is an essential aim in designing new radiographic systems. In this paper, we consider a possibility to use “scintillator-photodiode” detectors in radiographic systems in order to obtain resolution better than 1 line pair per mm (detector array pitch less than 1mm). A possibility is considered to use various types of scintillators (CsI(Tl), CdWO4, ZnSe(Te)), various kinds of scintillation elements (separate scintillation elements, scintillation monoplates, plates made of a small-crystalline scintillator and a binding substance) with differently treated surfaces, different binders, reflective coatings, photoconverters (photodiode or photodiode+amplifier+commutator in one housing). It has been shown that ZnSe(Te) scintillators are preferable for detectors of X-ray and low-energy gamma-radiation (20–100keV) with array pitch as small as 0.2mm. CsI(Tl) should be used for 70–200keV energies (0.4mm pitch), CdWO4 crystals—for 0.3–10MeV energies (pitch 0.4mm).

The TOFOR spectrometer for 2.5 MeV neutron measurements at JET

Neutron emission spectroscopy has been demonstrated to be a powerful plasma diagnostic at tokamaks. This was shown with the magnetic proton recoil spectrometer developed for measurement of the 14 MeV neutron emission from deuterium–tritium (DT) plasmas at the Joint European Torus (JET). For diagnosis of D plasmas, a 2.5 MeV spectrometer is needed with a factor of 100 higher efficiency to maintain the count rate because of the lower neutron emission rate. The time-of-flight (TOF) technique has the right attributes for these measurements. However, previous instruments have not achieved the full potential of the technique, especially, with respect to count rate capability. A TOF spectrometer for optimized rate (TOFOR) has been conceptually defined and is now under design and development for construction and use at JET. The TOFOR design goal is a count rate capability of about 300 kHz which should be attainable in high power D discharges at JET. The TOFOR project is now in a research and development stage where the instrument is being characterized through neutron transport calculations and test experiments. The tests aim to determine the response of the plastic scintillator elements from which TOFOR will be constructed. This contribution will present the TOFOR design principles, the results of the simulations and test measurements, and the steps taken to reach the optimized design.

Evaluation of Hamamatsu R5900 series PMTs for readout of high-resolution scintillator arrays

Two types of Hamamatsu R5900 series photomultiplier tubes (PMTs) have been evaluated as position-sensitive photon detectors for reading out arrays of small scintillator elements. Both PMTs have metal-channel dynode structures and compact dimensions of 26×26×20 mm 3 . Model R5900-C12 is a position sensitive PMT (PS-PMT) with photoelectrons shared among 12 plate anodes arranged in a crossed pattern such that there are six anodes each in the X and Y directions. Model R5900-M64 is a multi-channel PMT (MC-PMT) with 64 individual channels and low inter-channel crosstalk. Scintillator arrays can be coupled to these PMTs either directly or by using optical fibers. In this paper, we present measurements of single photoelectron spectra, pulse linearity, positioning response, inter-channel gain variation and pixel-to-pixel crosstalk for these PMTs. We have also measured their positioning response in reading out high-resolution lutetium oxyorthosilicate (LSO) scintillator arrays. Across a sample of 30 R5900-M64 MC-PMTs which were manufactured recently, the maximum gain variation over all channels is less than 3:1, while the difference between any two adjacent channels is less than 2:1. With a tapered voltage divider, the output of the anode signal at a gain of 10 6 remains linear up to 200 photoelectrons for the R5900-M64. For the R5900-C12, the summed anode signal remains linear up to 400 photoelectrons when irradiated through a 2 mm diameter aperture. Images obtained with a Na-22 source irradiating an array of 1×1×10 mm 3 LSO crystals coupled by an optical fiber bundle to the R5900-C12 or the R5900-M64 show excellent crystal identification.

Proton detection with large-acceptance scintillator detection systems in electron-scattering environments

Two highly segmented plastic-scintillator arrays have been developed for proton detection in electron scattering experiments. The detectors subtend solid angles of 225 and 550 msr and cover energy ranges of 50–225 and 25–165 MeV, respectively. The charge and arrival time of each photomultiplier signal are digitized by flash ADCs and temporarily stored in a dual-port memory. The readout parameters are computer controlled, tuned, and monitored. These detectors have been employed in (e, e′p) and (e, e′pp) experiments for proton emission angles greater than 30° and for luminosities up to 10 36 nucleons cm −2 s −1. The singles counting rates in the scintillator elements of the first layers were about 0.5×10 6 particles s −1 and the trigger rate 1 MHz. The measured resolution in the excitation energy and timing spectra are 2.7% and 0.7 ns, respectively.

Novel X-ray detectors for medical imaging

A number of different imaging systems are in use in X-ray medical diagnostics (e.g. digital radiography or computer tomography). The design goal of these imaging systems is to optimally use the information contained in X-ray quanta that have passed through the patient. The best image quality, as well as the minimisation of the X-ray dose applied to the patient are of prime importance. We report about innovations for novel detectors which reduce the X-ray dose and improve the image quality simultaneously. Advances in thin film electronics have permitted the development of large a-Si:H imaging arrays to design fat panel solid state detectors (short FD, up to 45 × 45 cm 2) for both digital radiography and fluoroscopy. The proposed detector consists of a CsI:T1 needle shaped scintillation crystal layer (thickness: 450 μm, needle diameter ∼ 10 μm) in front of an a-Si:H-panel. The Detective Quantum Efficiency (DQE) is about 65% (at spatial frequency zero) and spatial resolution is 2.8 lp/mm at 20% of the MTF and 6.2 lp/mm at 4%. In computed tomography (CT), a new geenration of linear detector array consists of Gadolinium Oxysulfid (GOS) ceramic scintillator elements, glued onto photodiodes. Important criteria for the selection of the detector material are good absorption of the incident X-rays (α > 95%) and high efficiency of conversion of the absorbed radiation energy to an electrical signal. A very short decay time to extremely low levels of afterglow is an advantage for the very short scanning times in CT. One gets a DQE (at 0 mm −1) about 80%. The next step toward dose reduction could be implemented by the application of monochromatic instead of polychromatic X-rays. This would additionally improve the DQE and thus enhance image quality. In addition, with the application of monochromatic X-rays, scattered radiation could be suppressed to a large extent by energy-selective single photon measurement, without loss of unscattered photons. At present, large area detectors in particular suffer from image quality losses, if no scattered radiation (multiline) grid is used.

Suppression of the radiation damage in lead tungstate scintillation crystal

Methods to suppress damage centres in lead tungstate (PWO) crystals under irradiation are discussed. It is shown that in large size scintillation elements (23 cm in length) the minimization of both electron and hole centers created under irradiation by recharge of structural defects is achieved by proper stoichiometry tuning and additional simultaneous doping of crystals by Y and Nb. The different aspects of the suppression of the recharge process in PWO scintillation crystals are also discussed.

Studies of ways to reduce defects in CdWO 4 single crystals

Studies are reported of the nature and formation conditions of structural defects affecting principal working characteristics of cadmium tungstate single crystals (macro- and microinclusions, scattering centers, block structure, twinning, violations of stoichiometry). It is shown that macro- and microinclusions are morphologically related to the crystal growth from the supercooled melt and are caused by the capture of crystal-forming components as intrinsic impurities by the growing crystal. Macro- and microinclusions are shown to form layered structures parallel to the (1 0 0) planes, which are also twinning planes. Ways are proposed to improve structural perfection of the crystals, providing for their high functional characteristics. Accounting for the studied features of intrinsic defect formation, optimum crystallographic directions were suggested for the crystal growth and subsequent cutting into scintillation elements with high and reproducible functional characteristics.

Optical and physical characteristics of HBLAN fluoride glasses containing cerium

The optical and physical characteristics of HBLAN fluoride glasses containing cerium have been studied using a variety of techniques. These stable glass compositions have been considered as candidates for electromagnetic particle detectors due to their high density (5.77 g cm −3), high average atomic number and scintillation properties. Optical fluorescence measurements on glass samples containing different concentrations (5% and 8%) of cerium show the characteristic emission spectrum of cerium in a fluoride environment. Replacing the hafnium with zirconium quenched the fluorescence almost completely. The lowest exciton level ( E 0) for the HBCeAN glass samples was measured to be 4.39 ± 0.01 eV. XPS scans of these glass samples showed that the strong lines from hafnium, and oxygen present as an impurity, obscure the valence band energy levels of cerium. The refractive index, as a function of wavelength was measured in the visible region and fitted with a Sellmeier formula. Abbe numbers of 68 and 56 were calculated for HBLAN glasses containing cerium and lanthanum respectively. These glasses are promising candidates for fast, radiation-resistant scintillator elements in electromagnetic calorimeters used at high energies.

A study of inter-crystal scatter in small scintillator arrays designed for high resolution PET imaging

Inter-crystal scatter causes mispositioning of scintillation events, which is of particular concern in imaging detectors based on small discrete scintillator elements. Because it is difficult to measure the scatter and its effects on detector intrinsic spatial resolution, a Monte Carlo simulation has been used to study inter-crystal scatter effects for evaluating and optimizing the design of a high resolution animal PET detector based on an array of small scintillator crystals. In this simulation, the authors quantitatively assess the mispositioning of events due to inter-crystal scatter as a function of parameters such as different scintillator materials, crystal geometry, /spl gamma/-ray incident angle and applied energy threshold. In analyzing the tradeoff between the detector efficiency and the position detection accuracy, the authors found that the mispositioning is not sensitive to the energy threshold, however it does change rapidly with the crystal length and the gap between crystals. The authors also compared four different crystal positioning algorithms to provide a theoretical estimate of positioning accuracy and to determine the best algorithm to use. To study how inter-crystal scatter affects detector spatial resolution, the authors analyzed the coincidence line spread function with and without inter-crystal scatter and found that the inter-crystal scatter had very little effect on the FWHM and FWTM of the coincidence line spread function.

A three layer circular scintillator hodoscope

The rotationally symmetric kinematics of hadronic reactions around the beam direction call for a detector system with an appropriate symmetry. A hodoscope has been invented which optimally meets such conditions. A combination of both contrarotating and straight scintillator elements form a three layer circular hodoscope. The overlap of individual counters defines pixels with a structure reminiscent of a sunflower pattern. Thus, a rather large, granulated and flat detector arrangement has been achieved economically. Such hodoscopes are in operation for measurements at LEAR/CERN and CELSIUS/Uppsala. A third enlarged version is presently being tested and will be used in the Time Of Flight spectrometer (TOF) at COSY/Jülich. The performance of individual detector elements and of the complete system was investigated with various particle beams and cosmic rays. Comparisons with Monte Carlo calculations confirm the operation of the detector.

The optimisation of small CsI(Tl) gamma-ray detectors

As part of the INTEGRAL (International Gamma-Ray Astrophysical Laboratory) project, CsI(Tl) scintillation elements with a cross-sectional area of 1 cm/sup 2/ and between 1 and 6 cm in length have been developed to form the basis of the imager detector plane. The crystal preparation and wrapping, crystal geometry, optical coupling, and matching to the photodiode have all been optimized in order to maximize the light output from the crystal, and hence produce the lowest energy threshold and best spectral resolution for any given readout electronics. Energy resolutions of 22% of 122 keV and 7% at 662 keV have been obtained using a 1 cm/sup 3/ crystal on a 10*10 mm crystal. With standard laboratory electronics, a low energy threshold of approximately 40 keV has been obtained. >

Position sensitivity by light splitting in scintillator arrays

A novel detector design of overlapping plastic scintillator elements in cylindrical geometry has been developed for detection of low multiplicity events of fast protons and other light charged particles: each particle traversing the detector from the axis outwards will produce light in several elements. The relative amounts of energy deposited in those elements allow one to interpolate on the particle trajectory beyond the resolution given by the granularity. The detector covers the angular range 10° ≤ Θ lab ≤ 72° and 0° ≤ ϕ ≤ 360° with an inner layer of scintillator bars of triangular cross section and an outer layer of rings. The material is BC408. Tests with minimum ionizing electron beams show that spatial resolutions of ΔΘ lab ≈ 1.5° and Δϕ 12 ≈ 1.5° (FWHM) can be obtained for electrons or proton pairs with energies in the GeV range. In the EDDA experiment the ultimate spatial resolution is then determined by the size of the interaction area rather than by the intrinsic pulse height resolution of the detector.

Study on Optical Properties of Ceramics Scintillator for X-Ray CT Equipment (Part 2)

The scintillator elements in X-ray CT equipment of the third generation type require high uniformity of distribution of light output as well as quality response characteristic. Parameters responsible for the distribution of light output were studied using the Monte Carlo method. Although the scattering coefficient was neglected, close agreement was obtained between calculated and experimental results. Calculated results indicated that relative output drop at both ends decreased with increasing optical absorption coefficient and was constant for X-ray absorption factor.

Study on Optical Properties of Ceramics Scintillator for X-Ray CT Equipment (Part 1)

Demand for the improvement of the quality of X-ray CT detectors is increasing. In order to realize the requirement, Gd2O2S: Pr (Ce, F) scintillating powder was sintered using a HIP technique. The highest light output from a 1mm thick specimen combined with a silicon photodiode, when excited by 120kV X-rays, has reached 1.9 times of CdWO4 single crystals. The CT equipment of the third generation type requires high uniformity among many scintillator elements. Particularly, quality response characteristic and distribution of light output is very important. In this report, parameters which have a great influence on the quality response characteristic were studied using the Kubelka-Munk theory. Agreement between calculated and experimental results was obtained, and calculated results indicated that quality response characteristic increased with increasing optical absorption coefficient, scattering coefficient and X-ray absorption factor.

The light response of plastic scintillator and the calibration of large arrays

The light response function for both fast and slow plastic scintillator (BC-412 and BC-444) has been studied in a series of experiments with phoswich (fast/slow plastic scintillator) detectors. The light response of the slow plastic element was determined as a function of the charge (Z), the mass number (A) and the energy(E) deposited by incident particles that were stopped within the scintillator material. The light response of the fast scintillation element was determined as a function of Z, A and E for both stopping and for transmitted particles. The response function was found to be independent of particle species for fragments that were transmitted through the scintillator. The response functions are independent of experimental configuration and are not specific to individual detectors. A method for the analysis of phoswich spectra which employs these calibration functions is developed. This method is suitable for calibrating large numbers of detectors.

Design characteristics of a low-energy gamma-ray imager for future space missions

The performance characteristics and detailed design of a position sensitive γ-ray detector, constructed from an array of CsI(Tl) scintillation elements viewed by individual photodiodes, is discussed on the basis of a series of laboratory tests. The imager matrix is configured as a two layer arrangement of position sensitive detectors and has a number of modes of operation including single site events in the upper layer and Compton scatter events between the two layers. The spectral range of operation, detection efficiency and energy resolution are assessed in terms of the dimensions of the matrix elements, their optical properties, and the detailed design of the front-end electronics.