Showing papers on "Scintillation published in 1997"

The low-energy concentrator spectrometer on-board the BeppoSAX X-ray astronomy satellite

Abstract: The payload of the Italian/Dutch spacecraft BeppoSAX includes a set of four X-ray concentrators each of geometric area 124 cm 2 together with imaging gas scintillation proportional counter detectors located at the focal planes. One of these detectors, the Low-Energy Concentrator Spectrometer (LECS), is sensitive to X-rays in the energy range 0:1 10 keV, while the other three cover a narrower energy range of 1:3 10 keV. In order to achieve the extended low-energy response of the LECS a novel type of gas scintillation proportional counter has been developed which dispenses with the separate drift and scintillation regions of conventional instruments. The design and performance of the instrument together with its calibration and data analysis system are described here.

Physical processes in inorganic scintillators

Abstract: BASIC PRINCIPLES AND PROCESSES Physical Mechanism of Scintillation Creation of Electron Hole Pairs Excitation and Emission of Luminescence Centers Scintillation Materials Halides Oxides and Oxide Systems Chalcogenides Glasses Interaction of Ionizing Radiation with Scintillators High Energy Photons Charged Particles Neutral Particles General Characteristics of Inorganic Scintillators Light Yield Duration of Scintillation Pulse Afterglow Temperature Response Optical Properties Radiation Hardness Density Emission Spectra Mechanical and Chemical Properties Physical Parameters Cost Consideration Scintillator Requirements in Various Applications High Energy Physics Intermediate Energy Physics Positron Emission Tomography Gamma Spectroscopy Energy Resolution Intrinsic Scintillator Resolution Nonproportional Response Time Resolution Low Energy Quanta and Electrons CONVERSION OF ELECTRONIC EXCITATIONS IN SOLIDS Charge Carrier Behaviors Delta Rays Secondaries Excitation of Luminescence Centers Effect of Ionization Density Energy Losses Simple Phenomenological Model Plasmon Model Polaron Model Scintillation Yield Spectra Vacuum Ultraviolet Region Ultrasoft X-Rays X-Rays Gamma Rays Heavy Ionizing Particles INTRINSIC LUMINESCENCE OF INORGANIC SCINTILLATORS Excitonic Luminescence Alkali Halide Crystals Alkaline-Earth Fluorides Ternary Halide Compounds Excitonic-Like Luminescence Cesium Iodide Tungstate and Molybdate Phosphors Core-to-Valence Transitions First Evidence for Radiative Core-to-Valence Transitions Excitation Spectra Emission Spectra Experiment Theory Luminescence Kinetics Experimental Theoretical Investigations Temperature Dependence of Luminescence Parameters Conditions of Detection Prospects for Research EXTRINSIC LUMINESCENCE OF INORGANIC SCINTILLATORS Thallium-Activated Halide Scintillators Crystals with NaCl-Type Structure Crystals with CsCl-Type Structure Other Thallium-Based and Thallium-Doped Crystals Crystals Containing Other ns2 Ions Bismuth Germanate (BGO) Sodium-Activated Cesium Iodide Rare-Earth-Activated Crystals General Considerations Europium-Activated Crystals Cerium-Activated Compounds Preliminary Comments LaF3-CeF3 Systems Cerium Chloride Barium Fluoride Gadolinium-Containing Crystals Lu- and Y-Containing Crystals Nd- and Pr-Activated Crystals DEFECT FORMATION BY IONIZING RADIATION Effect on Scintillator Characteristics Mechanisms of Defect Formation Efficiency of Defect Production Geometrical Factor Separation between F and H Centers Temperature Dependence of Production Efficiency Role of Halogen Ion Impurities Effect of Cation Impurities Formation Time of F-H Pairs Closing Comments References Index

Equatorial scintillation and systems support

Abstract: The need to nowcast and forecast scintillation for the support of operational systems has been recently identified by the interagency National Space Weather Program. This issue is addressed in the present paper in the context of nighttime irregularities in the equatorial ionosphere that cause intense amplitude and phase scintillations of satellite signals in the VHF/UHF range of frequencies and impact satellite communication, Global Positioning System navigation, and radar systems. Multistation and multifrequency satellite scintillation observations have been used to show that even though equatorial scintillations vary in accordance with the solar cycle, the extreme day-to-day variability of unknown origin modulates the scintillation occurrence during all phases of the solar cycle. It is shown that although equatorial scintillation events often show correlation with magnetic activity, the major component of scintillation is observed during magnetically quiet periods. In view of the day-to-day variability of the occurrence and intensity of scintillating regions, their latitude extent, and their zonal motion, a regional specification and short-term forecast system based on real-time measurements has been developed. This system, named the Scintillation Network Decision Aid, consists of two latitudinally dispersed stations, each of which uses spaced antenna scintillation receiving systems to monitor 250-MHz transmissions from two longitudinally separated geostationary satellites. The scintillation index and zonal irregularity drift are processed on-line and are retrieved by a remote operator on the Internet. At the operator terminal the data are combined with an empirical plasma bubble model to generate three-dimensional maps of irregularity structures and two-dimensional outage maps for the region.

The high pressure gas scintillation proportional counter on-board the BeppoSAX X-ray astronomy satellite

Abstract: In this paper we describe the High Pressure Gas Scintillation Proportion Counter, one of the Narrow Field Instrument of the X–ray astronomy satellite BeppoSAX. Sensitive in the 4-120 keV band, the HPGSPC will study all astrophysical sources emitting in the poorly studied hard X–ray band. The design and performances of the instrument are reported together with results of the on–ground calibration data analysis.

Scintillation reduction using multiple transmitters

Abstract: Uplink scintillation is a serious issue for laser communication between a satellite and the ground. Fluctuations on the uplinked beacon and communications laser can be minimized by transmitting multiple independent lasers from separate apertures which then sum incoherently at the satellite. The objective of the experiment described here was to determine the number and spacing required for separate transmitters to reduce fluctuations in the received power due to atmospheric scintillation to acceptable levels. Received power vs. time data were collected for horizontal laser links established between a laser transmitting platform and a receive telescope assembly separated by distances of 1.2 and 10.4 km to mimic the expected atmospheric effects of an uplink slant path to a satellite. Reduction in signal fluctuations was observed as the number of laser transmitters was increased from 1 to 16. A ground terminal design with 16 lasers on an 18 inch diameter circle with a 7 dB fade margin should be adequate to compensate form sot scintillation fades, while the remaining deep fades may be corrected by using forward error correction techniques.

Whole atmospheric-turbulence profiling with generalized scidar

Abstract: Statistical analysis of stellar scintillation on the pupil of a telescope, known as the scidar (scintillation, detection, and ranging) technique, is sensitive only to atmospheric turbulence at altitudes higher than a few kilometers. With the generalized scidar technique, recently proposed and tested under laboratory conditions, one can overcome this limitation by analyzing the scintillation on a plane away from the pupil. We report the first experimental implementation of this technique, to our knowledge, under real atmospheric conditions as a vertical profiler of the refractive-index structure constant CN2(h). The instrument was adapted to the Nordic Optical Telescope and the William Hershel Telescope at La Palma, Canary Islands. We measure the spatial autocorrelation function of double-star scintillation for different positions of the analysis plane, finding good agreement with theoretical expectations.

Probability distribution of irradiance for the onset of strong scintillation

Abstract: We calculated the probability distribution function (PDF) from simulations for an initially spherical wave propagated through homogeneous atmospheric turbulence. The onset of strong scintillation was calculated. By onset of strong scintillation, we mean conditions of weak scintillation changing to the condition of strong focusing. In addition, one case in the saturation regime was calculated. The simulations’ PDF’s are compared with several heuristic models of the PDF and are seen to progress from close to log normal for the cases of weakest scintillation to close to the log normally modulated exponential PDF (LNME PDF) for the cases of strong scintillation. The simulations’ PDF’s are not in agreement with the K PDF for any of the calculated cases. The best agreement was obtained in comparison with Beckmann’s PDF [P. Beckmann, Probability in Communication Engineering (Harcourt, Brace, & World, 1967)]. Beckmann’s PDF varies from being the log-normal PDF for weak scintillation to being the LNME PDF for strong scintillation and progresses further to the theoretically expected exponential PDF in the limit of saturated scintillation. We recommend that simulation be used to predict the irradiance PDF for plane and diverged waves in homogeneous turbulence in preference to using heuristic models. More complicated propagation cases remain in the domain of heuristic argumentation.

Channel characterization and modeling for Ka-band very small aperture terminals

Abstract: An increasing number of commercial applications are being promoted for future Ka-band satellite communication systems. Many of these systems will involve low-margin very small aperture terminals (VSATs). These systems are subject to important atmospheric propagation degradations that affect the quality of transmission and the link availability. The objective of this paper is to characterize the Ka-band channel and evaluate the performance degradation in VSATs resulting from atmospheric propagation, impairments. In particular microwave propagation through a turbulent atmosphere is discussed, and the statistical characterization and modeling of tropospheric scintillation is reviewed. Moreover, the paper extends the method proposed by Filip and Vilar (1990) for the long-term characterization and modeling of the combined effect of rain impairments and scintillation. Specifically, the increase in noise temperature during rain events is added to the Filip-Vilar model. This leads to a five-parameter global fading distribution that is used to predict typical Ka-band satellite link outage time, the mathematical formalism is illustrated by applying the method to the selected case of the Advanced Communications Technology Satellite (ACTS)-Georgia Tech experimental downlink. Numerical results confirm that both rain impairments and scintillation are important factors in the design of Ka-band VSAT systems.

Properties of the new LuAP:Ce scintillator

Abstract: The scintillation properties of LuAP (lutetium aluminum perovskite, LuAlO 3 ) have been investigated at three different levels of Ce doping: ≥ 0, 0.035 and 0.105 mol%. The light yield, in photoelectrons per MeV, was measured as 122±20, 1300±100 and 2850±200, respectively. The light pulse shapes were largely exponential, with a decay constant of 16.5±1 ns for all the samples studied. In all cases, however, an additional slow component, amounting to about 10±3% of the total light, was also found, characterized by a time constant of 74±7 ns. The sample doped with 0.105 mol% Ce showed an energy resolution of 9.3% for the 662 keV full energy peak from a 137 Cs source. The high detection efficiency of the material for γ-rays (because of its high density of 8.4 g/cm 3 ) is confirmed by a photofraction of about 13% for a specimen with a volume of only 0.05 cm 3 . The time resolution for 60 Co γ-rays at a 1 MeV threshold was measured as 160 ps, somewhat poorer than expected. Nevertheless, the high light yield, fast light pulse, high detection efficiency for γ-rays and excellent time resolution make this material a very attractive scintillator, particularly in positron emission tomography.

Luminescence properties and scintillation mechanisms of cerium- and praseodymium-doped lutetium orthoaluminate

Abstract: Absorption, reflection as well as luminescence emission, excitation, and decay curves for single crystals of and grown by the Bridgman technique have been measured at various temperatures. The fluorescence spectra photo-excited over a wide energy domain ranging from the UV to the x-ray region, and the kinetics are typical of the cerium and praseodymium ions. These experimental results show that the exciton transfer to the dopant occurs at around 8 eV, and the energy transfer via sequential hole and electron trapping is dominant at higher energy. This process must be considered as the main scintillation mechanism in this crystal. The high efficiency of this mechanism is explained by the small energy difference between the 4f level of the dopant and the top of the valence band, estimated from XPS measurements.

Diffuse Cosmic Gamma Radiation Measured by HEAO 1

Abstract: The High Energy Astronomical Observatory 1 (HEAO 1) contained as one of its instruments the UCSD/MIT hard X-ray and gamma-ray instrument, a complex of actively shielded scintillation counters that operated over the nominal 10 keV to 10 MeV range. The two medium-energy detectors (MEDs) employed in this investigation covered the range 80 keV-2 MeV, had a geometrical area of 43 cm2, and were collimated to a circular field of view of 17? FWHM. During a period of operation starting in 1978 April, these detectors were operated in a manner designed to provide a precise measure of the diffuse cosmic background gamma-ray flux. Previous measurements of the spectrum in this range were not of sufficient precision to distinguish between various models explaining the hard X-ray background either as an ensemble of discrete sources or due to a global effect at cosmological distances. The detectors could be alternately blocked or unblocked with a moveable shutter of CsI, which could be operated in an active anticoincidence mode or as a passive shield. Data taken in these various modes and analyzed with respect to the varying radiation environment of the 23? inclination 400 km circular orbit permitted separation of various background effects that have plagued previous measurements of this radiation using scintillation spectrometers. Over the ~80-400 keV band, systematic errors were small and correctable, with a resulting spectrum fitted by a power law of dN/dE = (2.62 ? 0.05) ? 10-3(E/100 keV)-(2.75?0.08 photons cm-2 s-1 keV-1 sr-1. This fit, in general agreement with previous results in this energy range, joins smoothly with measurements at lower energies, and when extrapolated to higher energies, it agrees with the recently reported COMPTEL results at energies above 2 MeV obtained on the Compton Gamma Ray Observatory. The accompanying demonstration of the experimental difficulties associated with scintillation spectrometer measurements of the diffuse cosmic background provides significant clarification of the observational status of this important measurement. When account is taken of measurements with possible local background estimation uncertainties, the spectrum between ~200 keV and ~50 MeV appears more simplified than previous estimates.

Atmospheric intensity scintillation of stars, i. statistical distributions and temporal properties

Abstract: Stellar intensity scintillation in the optical was extensively studies at the astronomical observatory on La Palma (Canary Islands). Photon-counting detectors and digital signal processors recorded temporal auto-and cross-correlation functions, power spectra, and probability distributions. This first paper of a series treats the temporal properties of scintillation, ranging from microseconds to seasons of year. Previous studies, and the mechanisms producing scintillation are reviewed. Atmospheric turbulence causes 'flying shadows' on the ground, and intensity fluctuations occur both because this pattern is carried by winds, and is intrinsically changing. On very short timescales, a break in the correlation functions around 300 mus may be a signature of an inner scale (approx. 3 mm in the shadow pattern at windspeeds of ms -1). On millisecond timescales, the autocorrelation decreases for smaller telescope apertures until approx. 5 cm, when the 'flying shadows' become resolved. During any night, timescales and amplitudes evolve on scales of tens of minutes. In good summer conditions, the flying-shadow patterns are sufficiently regular and long-lived to show anti-correlation dips in autocorrelation functions, which in winter are smeared out by apparent wind shear. Recordings of intensity variance together with stellar speckle images suggest some correlation between good [angular] seeing and large scintillation. Near zenith, the temporal statistics (with up to 12:th order moments measured)is best fitted by a Beta distribution of the second kind (F-distribution), although it is well approximated by log-normal functions, evolving with time.

High-latitude upgrade to the Wideband ionospheric scintillation model

Abstract: The high-latitude sections of the Wideband ionospheric scintillation model (WBMOD) have been upgraded extensively, based on analysis of scintillation data from the Defense Nuclear Agency Wideband, HiLat, and Polar BEAR satellite-beacon experiments. Data collected at Sondre Stromfjord, Greenland; Tromso, Norway; Fort Churchill, Canada; and Bellevue, Washington (United States) over a 4-year period were analyzed, and the results of these analyses were used to construct a completely new model for the behavior of the height-integrated irregularity-strength parameter (CkL) at high latitudes. The new high-latitude CkL model includes variations with sunspot number, geomagnetic activity (Kp), latitude, local time, longitude, and season. The new WBMOD CkL models (equatorial and high-latitude) have been implemented in a more versatile code, denoted SCINTMOD, which has the capability to generate a wide range of user-controlled maps of scintillation effects over large spatial areas. Examples of the types of graphical output that SCINTMOD can generate are presented.

Algorithms for Obtaining Atmospheric Surface-Layer Fluxes from Scintillation Measurements

Abstract: Algorithms are derived for obtaining atmospheric surface-layer fluxes of heat, humidity, and momentum from scintillation measurements. Both the three-wavelength and two-wavelength methods are examined, as is combining the two-wavelength method with other micrometeorological measurements. Corrections for the effects of strong scintillation are discussed. Deriving turbulence inner-scale and refractive structure parameters from scintillation variances is discussed, as is using these quantities to obtain the energy dissipation rate and structure parameters of temperature and humidity. Given these latter quantities, Monin–Obukhov similarity theory is applied to specific humidity and temperature (equivalently, potential temperature) to obtain the fluxes.

A high pressure xenon self-triggered scintillation drift chamber with 3d sensitivity in the range of 20-140 kev deposited energy

Abstract: A high pressure gas self-triggered scintillation drift chamber has been built with a working volume of 30 cm diameter and 4 cm depth, viewed through glass windows by an array of 19 PMTs. The detector tested at 9 bar xenon filling showed 2.7% FWHM energy resolution at 122 keV over a 22 cm field of view and three-dimensional sensitivity with X , Y resolution of 3.7 mm FWHM and Z resolution of 0.6 mm FWHM for 60 keV photoabsorbed γ-rays. The readout system of the detector is able to select multi-vertex events occurring in the sensitive volume due to γ-ray absorption and discriminate fluorescent photons, Compton scattering and photoabsorption vertices. The energy resolution in “fluorescent photon gated mode” has been measured as 2.2% FWHM at 122 keV γ-ray. Some possible applications of the detector are discussed.

A scintillation detector signal processing technique with active pileup prevention for extending scintillation count rates

Abstract: A new method for processing signals from scintillation detectors is proposed for very high count-rate situations where multiple-event pileups are the norm. This method is designed to sort out and recover every impinging event from multiple-event pileups while maximizing the collection of scintillation signal for every event to achieve optimal accuracy in determining the energy of the event. For every detected event, this method cancels the remnant signals from previous events, and excludes the pileup of signals from following events. With this technique, pileup events can be recovered and the energy of every recovered event can be optimally measured despite multiple pileups. A prototype circuit demonstrated that the maximum count rates have been increased by more than 10 times, comparing to the standard pulse-shaping method, while the energy resolution is as good as that of the pulse shaping (or the fixed integration) method at normal count rates. At 2/spl times/10/sup 6/ events/sec for NaI(Tl), the true counts acquired are 3 times more than the delay-line clipping method (commonly used in fast processing designs) due to events recovered from pileups. Pulse-height spectra up to 3.5/spl times/10/sup 6/ events/sec have been studied.

Ka-band scintillations: measurements and model predictions

Abstract: New propagation data from a 30/20-GHz propagation experiment at several US sites, including Fairbanks, AK, and Norman, OK, are presented to examine existing models for scintillations. Beacon measurements were collected at one sample per second continuously and at 20 samples per second for selected intervals. The widely separated measurement frequencies and the wide range of measurement elevation angles of about 8-52/spl deg/ provide a solid set of measurements to test existing models and formulate new ones. It is found that current scintillation prediction models overestimate the magnitude of scintillation. The Karasawa model was modified to fit the Fairbanks data, with this modification valid for the climate zone containing Fairbanks. The model of the radiocommunication sector of the International Telecommunication Union was examined for several sites, and an elevation angle modification was attempted to bring it into agreement with the data sets. Measurements indicate a frequency dependence of f/sup 7/12/.

Atmospheric intensity scintillation of stars. ii. dependence on optical wavelength

Abstract: Atmospheric intensity scintillation of stars on milli- and microsecond time scales was extensively measured at the astronomical observatory on La Palma (Canary Island). Scintillation statistics and temporal changes were discussed in Paper I, while this paper shows how scintillation depends on optical wavelength. Such effects originate from the changing refractive index of air, and from wavelength-dependent diffraction in atmospheric inhomogeneities. The intensity variance σ2/I increases for shorter wavelengths, at small zenith distances approximately consistent with a theoretical λ -7/6 slope, but with a tendency for a somewhat weaker dependence. Scintillation in the blue is more rapid than in the red. The increase with wavelength of autocorrelation time scales (roughly proportional to sqrt{λ} is most pronounced in very small apertures, but was measured up to \o 20 cm. Scintillation at different wavelengths is not simultaneous: atmospheric chromatic dispersion stretches the atmospherically induced 'flying shadows' into 'flying spectra' on the ground. As the 'shadows' fly past the telescope aperture, a time delay appears between fluctuations at different wavelengths whenever the turbulence-carrying winds have components parallel to the direction of dispersion. These effects increase with zenith distance (reaching \approx 100 ms cross-correlation delay between blue and red at Z = 60°), and also with increased wavelength difference. This time delay between scintillation in different colors is a property of the atmospheric flying shadows, and thus a property that remains unchanged even in very large telescopes. However, the wavelength dependence of scintillation amplitude and time scale is 'fully' developed only in small telescope apertures (less than about 5 cm), the scales where the 'flying shadows' on the Earth's surface become resolved. Although these dependences rapidly vanish after averaging in larger apertures, an understanding of chromatic effects may still be needed for the most accurate photometric measurements. These will probably require a sampling of the [stellar] signal with full spatial, temporal and chromatic resolution to segregate the scintillation signatures from those of astrophysical variability.

Evaluation of statistical models for clear-air scintillation prediction using olympus satellite measurements

Abstract: The objective of this study is to evaluate and to compare some of the statistical models for the monthly prediction of clear-air scintillation variance and amplitude from ground meteorological measurements. Two new statistical methods, namely the direct and the modelled physical-statistical prediction models, are also introduced and discussed. They are both based on simulated data of received scintillation power derived from a large historical radiosounding set, acquired in a mid-latitudue site. The long-term predictions derived from each model are compared with measurements from the Olympus satellite beacons at the Louvain-la-Neuve site at 12·5 and 29·7 GHz and at the Milan site at 19·77 GHz during 1992. The model intercomparison is carried out by checking the assumed best-fitting probability density function for the variance and log-amplitude fluctuations and analysing the proposed relationships between scintillation parameters and ground meteorological measurements. Results are discussed in order to understand the potentials and the limits of each prediction model within this case study. The agreement with Olympus measurements is found to be mainly dependent on the proper parametrization of prediction models to the radiometeorological variables along the earth–satellite path. ©1997 by John Wiley & Sons, Ltd.

Degradation of navigational accuracy with Global Positioning System during periods of scintillation at equatorial latitudes

Abstract: The effect of ionospheric scintillation on navigational accuracy with the GPS (global positioning system) in the equatorial region is presented. The accuracy of position fixing with the GPS as indicated by the PDOP (position dilution of precision) factor is degraded when the raypath from the satellite shows deep fading. It is understood that navigation, particularly using a moderately sophisticated GPS receiver, in the equatorial zone will be severely affected during maximum sunspot number years.

Radiation detection device

Abstract: The invention relates to a device for detecting input radiation such as X-rays, η-rays, ionising radiation or fluorescent and low-level light. The device has at least one detection element comprising a sensor component (scintillator) for converting the input radiation into photons (scintillation light) in the UV, visible or IR part of the electromagnetic spectrum and an optical amplifier component which receives the light converted by the sensor component, forwards it for further processing and amplifies it at the same time. According to the invention, the amplifier component comprises at least one optical waveguide (1, 2, 2a, 2b, 20) whose material is optically pumped to amplify the scintillation light (3, 24, 25).

Scintillation properties of RbGd2Br7 : Ce3+ crystals; fast, efficient, and high density scintillators☆

Abstract: The scintillation properties of RbGd 2 Br 7 crystals ( ϱ ≈ 4.7 g/cm 3 ) doped with Ce 3+ concentrations of 1 and 5 mol% studied under X-ray and γ-quanta excitation are presented and compared with the properties of NaI(Tl) and other scintillators. For a sample doped with 5 mol% Ce, a light yield of about 55 000 photons/MeV with a main decay time of 60 ns was measured. With a photomultiplier tube, an energy resolution of 3.8% (FWHM) for the 662 keV full absorption peak has been observed.

Depth dose measuring device

Abstract: In a depth dose measuring device, a light is outputted from an end surface of a block-type detector (202) formed by tying a plurality of scintillation fibers in a bundle, a spectroscope (204) is used to disperse the light so as to measure an amount of light having a wavelength corresponding to an emission spectrum of the scintillation fibers, a picture measuring device (205) and a picture processing device (206) calculate an emission distribution of only a scintillation light depending upon an emission distribution in which the scintillation light and a Cerenkov light are mixed, and a result is displayed on a display device (207).

Scintillation fiber type radiation detector

Abstract: A radiation detector obtains radiation information by detecting a light pulse occurred in response to a radiation in a scintillation fiber at one end or opposite ends of the scintillation fiber. The scintillation fiber is surrounded by a scattering member which emits an electron by interaction with the radiation.

Surface polishing of GSO scintillator using chemical process

Abstract: A Ce doped Gd/sub 2/SiO/sub 5/, (GSO) single crystal is an excellent scintillator featuring a large light output, a short decay constant and a high absorption coefficient. It was shown in our previous studies that the surfaces of the scintillator should be polished to get scintillation light output efficiently especially for long and narrow scintillators. Therefore, we newly developed a chemical process to polish the GSO crystal. The surface of the GSO crystal was found to become smooth when it was etched using anhydrous orthophosphoric acid at 150-300/spl deg/C. The process was also proved to be applicable to large size crystal such as 20/spl times/20/spl times/200 mm/sup 3/ preventing thermal shock crack. An improvement in the scintillator efficiency was confirmed using the chemical polishing process.