Showing papers on "Scintillation published in 1992"

Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: I. Physical characteristics and theoretical considerations

Abstract: A minimally perturbing plastic scintillation detector has been developed for the dosimetry of high-energy beams in radiotherapy. The detector system consists of two identical parallel sets of radiation-resistant optical fibre bundles, each connected to independent photomultiplier tubes (PMTs). One fibre bundle is connected to a miniature water equivalent plastic scintillator and so scintillation as well as Cerenkov light generated in the fibres is detected at its PMT. The other 'background' bundle is not connected to the scintillator and so only Cerenkov light is detected by its PMT. The background signal is subtracted to yield only the signal from the scintillator. The water-equivalence of plastic scintillation detectors is studied for photon and electron beams in the radiotherapy range. Application of Burlin cavity theory shows that the energy dependence of such detectors is expected to be better than the commonly used systems (ionization chambers, LiF thermoluminescent dosimeters, film and Si diodes). It is also shown that they are not affected by temperature variations and exhibit much less radiation damage than either photon or electron diode detectors.

Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: II. Properties and measurements.

Abstract: The properties of a new scintillation detector system for use in dosimetry of high-energy beams in radiotherapy have been measured. The most important properties of these detectors are their hgh spatial resolution and their nearly water-equivalence. Measurements have shown that they have excellent reproducibility and stability, and a linear response versus dose-rate. It is also shown that they have better spatial resolution than ionization chambers and have much less energy or depth dependence in electron fields due to the removal of the influence of the polarization effect. Dose distributions in water, using miniature plastic scintillation detectors, have been measured for different high-energy photon and electron beams.

The Physics of Pulsar Scintillation

Abstract: Scintillation is a well-known phenomenon in astronomy, e.g. twinkling of stars due to scattering in the Earth’s atmosphere, and variability of compact radio sources due to scattering in the ionosphere and the solar wind. These examples correspond to the so-called regime of weak scattering. Radio pulsars scintillate as a result of scattering in the ionized interstellar medium, but in contrast to the previous cases, the physical regime corresponds to strong scattering. Pulsars exhibit two distinct kinds of variability, called diffractive scintillation and refractive scintillation, on timescales of minutes and weeks respectively. The physics of the various regimes of scintillation are reviewed, and some basic theoretical results are summarized. The properties of imaging in the presence of strong scattering are also discussed.

Cerenkov light generated in optical fibres and other light pipes irradiated by electron beams

Abstract: The use of a small plastic scintillator coupled to an optical fibre bundle light pipe as a means for the dosimetry of radiotherapy X-ray or electron beams in a phantom has been studied. Under such conditions, some light is generated by the direct action of the radiation on the optical fibres themselves, and this 'background' signal must be correctly accounted for. In the present study, electron beams were incident on fused silica optical fibres and other light pipes made of polymethylmethacrylate (PMMA), polystyrene and water. The observed light signal generated in all cases was found to depend strongly on the angle between the electron direction and the light pipe axis, and to correlate well with the angular characteristics uniquely associated with Cerenkov radiation. The use of a parallel fibre bundle light pipe, identical to the one that carries light from the scintillator, offers a suitable means of generating a similar background Cerenkov light signal that can be subtracted to obtain the output from the scintillation dosimeter alone.

The measurement of turbulent surface-layer fluxes by use of bichromatic scintillation

Abstract: Scintillation measurements with a HeNe and a CO2 laser were used to derive turbulent fluxes of heat and momentum in the surface layer. This was achieved by the structure constant or dissipation technique, i.e., by relating the measured structure constants and inner scales of refractive index fluctuations to structure constants of temperature fluctuations and dissipation rates of turbulent kinetic energy, respectively, and then assuming Monin-Obukhov similarity.

Review of optical scintillation methods of measuring the refractive-index spectrum, inner scale and surface fluxes

Abstract: The background, recent progress and future trends of remote sensing of three environmental parameters using optical scintillation are reviewed. The parameters are the refractive-index spectrum of turbulence, the inner scale of turbulence, and the surface fluxes of heat, humidity and momentum.

Measuring surface-layer fluxes of heat and momentum using optical scintillation

Abstract: An experiment is described showing that an optical scintillation instrument gives reliable values of heat and momentum fluxes in the surface layer, subject to the usual restrictions of homogeneity and steady state. This instrument measures the turbulence inner scale and refractive-index structure parameter, which are used to obtain the fluxes from Monin-Obukhov similarity relationships. The instrument gives space-averaged values over a propagation path that can range in length from tens to hundreds of meters. The history of the use of optical propagation to estimate fluxes is reviewed.

Measurement of the property of cooled lead molybdate as a scintillator

Abstract: Lead molybdate (PbMoO 4 ) is found to be an efficient scintillator when cooled down to liquid nitrogen temperature. The scintillation light of PbMoO 4 excited by γ-rays amounts to as much as 16% of that of NaI(Tl) in the wavelength range between 180 nm and 650 nm and the decay time is as long as 11 μs if it is cooled below 100 K. Since 100 Mo, whose natural abundance is 9.6%, is a double β decay nuclide, the PbMoO 4 can be useful for the double β decay experiment.

Oriented Scintillation Spectrometer Experiment observations of Co-57 in SN 1987A

Abstract: The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory has observed SN 1987A for two 2 week periods during the first 9 months of the mission. Evidence for gamma-ray line and continuum emission from Co-57 is observed with an intensity of about 10 exp -4 gamma/sq cm/s. This photon flux between 50 and 136 keV is demonstrated by Monte Carlo calculations to be independent of the radial distribution of Co-57 for models of low optical depth, viz., models having photoelectric absorption losses of 122 keV photons no greater than several percent. For such models the observed Co-57 flux indicates that the ratio Ni-57/Ni-56 produced in the explosion was about 1.5 times the solar system ratio of Fe-57/Fe-56. When compared with nearly contemporaneous bolometric estimates of the luminosity for SN 1987A, our observations imply that Co-57 radioactivity does not account for most of the current luminosity of the supernova remnant in low optical depth models. We suggest alternatives, including a large optical depth model that is able to provide the SN 1987A luminosity and is consistent with the OSSE flux. It requires a 57/56 production ratio about twice solar.

The mass dependence of CsI(Tl) scintillation response to heavy ions

Abstract: The response of CsI(Tl) scintillators to heavy ions is investigated as a function of E , A , and Z . In addition to the expected dependence of light output on Z and E , we observe a significant dependence on mass number. A simple parameterization of the quenching in terms of a few physical variables permits characterization of the light output for a variety of nuclear species with a single quenching constant.

Laser Scintillation Measurements of the Temperature Spectrum in the Atmospheric Surface Layer.

Abstract: The locally stationary temperature spectrum in the atmospheric surface layer is estimated using laser scintillation. The fluctuations of the parameters of the turbulence spectrum (the structure constant CT2 and inner scale l0) have a lognormal distribution. The average spectrum is calculated by averaging the locally stationary spectrum over these fluctuations. The average spectrum does not have a universal form. The fluctuations in the turbulence parameters produces a bias in measurements of the Obukhov-Corrsin constant and in estimates of energy dissipation rate ϵ based on average scintillation statistics. The performance of the scintillation technique and the accuracy of scintillation measurements of inner scale and structure constant are estimated using Monte Carlo simulation. One scintillation measurement can provide accurate estimates of the important turbulence parameters and the statistics of the fluctuations of these parameters. The scintillation estimates are true path-averaging estimate...

Luminescence quenching in liquid argon under charged-particle impact: Relative scintillation yield at different linear energy transfers

Abstract: A theoretical model is presented for the linear-energy-transfer (LET) variation of the relative scintillation yield in liquid argon. It is based on energy partition between the core and the penumbra of the charged particle track with little quenching in the penumbra except for fission fragments. Scintillation from the core can be quenched significantly by a biexcitonic mechanism. Some detailed calculations indicate that the electron-ion recombination may occur before exciton self-trapping. Fairly good agreement with experiment has been obtained with respect to the relative variation of the scintillation yield with LET using a diffusion-reaction model of free excitons with a specific reaction rate within acceptable limits. At the same LET different heavy-ion tracks can develop different quenching ratios depending on the density of deposited energy in the core.

Scintillation data collecting apparatus and method

Abstract: A scintillation data collecting apparatus and method are provided for a scintillation camera and a data processor. The apparatus comprises a scintillation detector including an adder and an analyzer for detecting the valid scintillations, a plurality of conversion units each including an analog-to-digital converter for converting intensity signals derived from the valid scintillations into high, accuracy digital values, and controllers for controlling operation of each of the analog-to-digital converters of the conversion units. The method comprises steps of adding the intensity signals generated by the scintillation camera and which are indicative of the scintillations, and analyzing the sum of the intensity signals to determine if the detected scintillations are valid. To select only those values involved in a scintillation event for processing and free up ADCs which are uninvolved to convert future intensity signals, the intensity signals may be compared to a threshold value to allow only those ADCs whose corresponding intensity signal is above the threshold to convert, or a rapid determination of which serial ADCs are involved can be done by allowing all serial ADCs to convert, waiting for a first positive MSB from one of the ADCS, and resetting all ADCs determined to be uninvolved in the event. The converted signals derived from the intensity signals are digital value signals indicative of the valid scintillations computable by the data processor to determine spatial locations of the valid scintillations.

Long-Term Pulsar Flux Monitoring and Refractive Interstellar Scintillation

Abstract: The flux densities of 14 pulsars have been observed for over 1 year at an observing frequency of 610 MHz, using an NRAO 26 m telescope in Green Bank, West Virginia. The observed time scales for long-term flux variation for seven pulsars are in good agreement with refractive scintillation theory. Four pulsars, including the Vela pulsar, have observed time scales that are in poor agreement with theoretical predictions. Pulsars predicted to have extremely long refractive scintillation time scales are observed to have stable fluxes. These pulsars therefore have intrinsically stable luminosities

Liquid xenon ionization and scintillation: Studies for a totally active vector electromagnetic calorimeter

Abstract: In these experiments MeV's to GeV's energies were deposited in a liquid xenon test cell by an electron accelerator with kinetic energy ≤100 keV, intensity ≤10 6 e and pulse width ≤30 ns FWHM. Purification of the liquid xenon is effected by continuous circulation of boil-off gas through Oxisorb. This technique will allow operation of a liquid xenon calorimeter in a high radiation ..environment because the liquid can be continuously purified and replenished. An ionization signal was detected by collecting drift electrons onto an anode mesh, a scintillation signal by collecting photoelectons in a silicon photodiode immersed in the liquid. The energy to create an ionization electron was measured to be W = 9.76±0.70 eV. The corresponding energy to produce a 175 nm scintillation photon was found to be W s = 14.2 eV. The scintillation signal is observed to be fast ( σ t ≤14 ns). Anti-correlation of the ionization and scintillation signals was also seen. Intrinsic energy resolution σ E / E ≈ 0.07%/√( E /GeV) was determined from ionization and σ E / E ≈ 0.2%/√( E /GeV) from scintillation. Doping of xenon with up to 2% methane did not adversely affect the ionization/scintillation yields or resolutions but increased the drift velocity hence, the current signal by about 75%. The intrinsic energy resolution of a totally active xenon scintillation calorimeter will be limited by uniformity of light collection and the dead material of the calorimeter to ( σ E / E ≤ 0.5%). Detection of scintillation with a silicon (or CsI gas) photodiode permits a fast energy measurement which can be used in a first level trigger. The ionization signal will be sampled to determine the vector direction of the photon (or electron) which initiates the shower. Directional resolution σ θ = 5 mrad/√( E /GeV) achievable with this method will allow association of a detected photon to its true vertex point. At LHC ( ≈ 20 interactions per beam crossing) this calorimeter can give a Higgs mass resolution σ M / M H ≈ 0.6% via the 2γ decay mode compared to 3% for calorimeters with equal energy resolution ( σ E / E = 0.5%) but without this vertex capability. Sampling of ionization in the early part of the shower will allow discrimination against π 0 production and detect individual photons from π 0 decays thus enhancing discrimination against these important sources of hadronic background. The essential problem in measuring the shower profiles is the large amount of readout electronics needed. Cheap, current sensitive, digital VLSI electronics has already been developed and produced for the FAST RICH detector project. Studies to adapt this electronics for use in liquid xenon are underway.

Deducing turbulence parameters from transionospheric scintillation measurements

Abstract: The theoretical framework and experimental methodology used to interpret observations of ionospheric scintillations in terms of geophysical processes are reviewed and recent experimental observations of ionospheric scintillations are discussed in this paper. During the past 15 years significant progress has been made in several areas. In particular, significant advances have been made in theoretical understanding of the strong scintillation regime and the effects of short-term temporal variations of the scintillation producing irregularities on observations made with spaced-receiver geometries in both weak and strong scintillations. This improved understanding of the scintillation process has significantly increased the utility of the technique particularly in the equatorial latitudes where geometrical effects are least important.

Absolute number of photons produced by alpha-particles in liquid and gaseous xenon

Abstract: The W s which is defined as an average energy expended per scintillation photon, was found to be 16.3 ± 0.3 eV for alpha-particles in liquid xenon, and 49.6 ± 1.1 eV in gaseous xenon, respectively. These results followed from the number of photoelectrons measured with a VUV sensitive photomultiplier tube, which was used as a photodiode. The number of photoelectrons from the photomultiplier photocathode was measured absolutely with a well calibrated charge sensitive amplifier system as a function of distance between the alpha-source and the photomultiplier photocathode. The detection geometries included both reflective and nonreflective walls. The data were well fitted to corresponding curves obtained with a Monte Carlo simulation, and yielded the total number of scintillation photons.

Rapid two-dimensional dose measurement in brachytherapy using plastic scintillator sheet: linearity, signal-to-noise ratio, and energy response characteristics.

Abstract: Because of the large dose gradients encountered near brachytherapy sources, an efficient, accurate, low-atomic number areal detector, which can record dose at many points simultaneously, is highly desirable. We have developed a prototype of such a system using thin plates of plastic scintillator as detectors. A micro-channel plate (MCP) image intensifier was used to amplify the optical scintillation images produced by radioactive 125 I and 137 Cs sources in water placed 0.5–5.7 cm distance from the detector. A charge-coupled device (CCD) digital camera was used to acquire 2-D light-intensity distributions from the image intensifier output window. For both isotopes, a small area (2 × 3 mm 2 ) PVT detector yields a CCD net count rate that is linear with respect to absorbed dose rate within ± 3% out to 5.7 cm distance. Acquisition times range from 1.5–400 sec with a reproducibility of 0.5–5.5%. If a large-area (6 × 20 em 2 ) PVT detector is used, a four-fold increase in count rate and large deviations from linearity are observed, indicating that neighboring pixels contribute light to the signal through diffusion and scattering in PVT and water. A detailed noise analysis demonstrates that the image intensifier reduces acquisition time 10000-fold, reduces noise relative to signal 200-fold, and reduces amplifier gain noise as well.

Detection of CdS(Te) and ZnSe(Te) scintillation light with silicon photodiodes

Abstract: Some scintillation characteristics of CdS(Te) have been investigated. The scintillation emission spectrum of CdS(Te) single crystals is situated in a band between 560 and 800 nm with, maximum at 640 nm. The light yield of this red emitting crystal on a microsecond time scale is rather large: approximately 1.7*10/sup 4/ photons per MeV of absorbed gamma -ray energy. The scintillation light can be efficiently detected with silicon photodiodes. Results are presented. Main decay time components of 270 ns and 3.0 mu s were measured. The scintillation mechanism and the application of the material are discussed. Some data regarding a similar system, ZnSe(Te), are presented. >

YAlO3: Ce scintillators: application for X- and soft γ-ray detection

Abstract: The possibility of applying YAlO3:Ce scintillation crystals for X-ray and low energy γ-ray detection has been investigated. Examples of scintillation spectra obtained with various irradiation sources are given. The crystal thickness has been optimized.

C and L band transionospheric scintillation experiment: Some results for applications to satellite radio systems

Abstract: Satellite radio systems suffer loss of information in a wide band of frequencies during periods of intense ionospheric scintillation activity when the received signal undergoes rapid and deep fading. In order to assess the problem and to determine a proper fade margin for an Earth-space link, system engineers require information on signal statistics as well as on the morphological aspects of scintillations. Our observations near the northern boundary of the equatorial scintillation belt at (18.9° N geomagnetic) within the Indian zone show that the signal at 4/1.5 Ghz has faded often beyond 10 dB pp, and at times beyond 24 dB pp at 4 Ghz during equinoctial months of high solar activity during the years of 1989–1990. In addition to the morphology at 4 Ghz, information on signal statistics, such as cumulative amplitude distribution function, fade rate distribution, and signal reliability for different message lengths for some events of scintillations, both at C and L band, has been presented. The theoretical Nakagami m distribution has been found to be the best for describing various levels of fade. Autocorrelation and power-spectrum analysis have been used to estimate average fade rates and ground correlation distances. Performance evaluation of satellite Earth terminals using small antennas has been carried out to show the vulnerability of the system in the hostile ionospheric environment notwithstanding the advanced modulation systems being employed.

Study of n-γ discrimination by digital charge comparison method for a large volume liquid scintillator

Abstract: The study of the n-γ discrimination for a large 4 I volume BC501A liquid scintillator coupled to a 130 mm diameter XP4512B photomultiplier was carried out by digital charge comparison method. A very good n-γ discrimination down to 100 keV of recoil electron energy was achieved. The measured relative intensity of the charge integrated at the slow component of the scintillation pulse and the photoelectron yield of the tested counter allow the factor of merit of the n-γ discrimination spectra to be calculated and to be compared with those measured experimentally. This shows that the main limitation of the n-γ discrimination is associated with the statistical fluctuation of the photoelectron number at the slow component. A serious effect of the distortion in the cable used to send the photomultiplier pulse to the electronics for the n-γ discrimination was studied. This suggests that the length of RG58 cable should be limited to about 40 m to preserve a high quality n-γ discrimination.

An efficient single crystal BSE detector in SEM

Abstract: Using the optical modifications described, the signal of the wide-angle BSE detector can be increased 3.7-fold compared with the detector type illustrated in Figure 1. The increase was achieved by more fully exploiting the optical phenomena of diffusion, reflection at the critical angle and antireflection, and especially that of their combined effects. A higher photon energy transfer was obtained by specific optical modifications to the scintillator and the light guide. Based on the current modifications, it can be estimated that about 75% of the generated light reaches the PMT. It is necessary, therefore, to revise all values previously published of the DQE coefficient of the YAG scintillator. A general value of DQE is of very limited use depending as it does on the specific conditions under which light emerges from the YAG scintillator. The DQE, therefore, does not permit sufficiently accurate comparison with other scintillation materials. The DQE can be evaluated only for a completed detector configuration in which the laws of geometrical optics apply, or it can be used for making comparisons of scintillation materials used in the same detector configuration.

Method and apparatus for unpiling pulses generated by piled-up scintillation events

Abstract: A signal containing piled-up pulses from a scintillation camera head is sampled at a high frequency. Samples are summed to provide a time-integration of pulses which are sampled. When pulses pile up, the tail of the first pulse can be determined by extrapolation, permitting the first pulse to be energy-validated. The overlapping tail of the first pulse and the head of the second pulse are also time-integrated, and the area of the tail of the first pulse is subtracted from the time-integration so obtained. It is then possible to validate the second pulse.

Observation of the UV scintillation light from high energy electron showers in liquid xenon

Abstract: The UV scintillation light in liquid xenon has been observed using silicon drift photodiodes. Measurements were carried out in a rectangular light cell of dimentions 7.4 × 7.4 × 60 cm3 using 5 GeV electrons. The photodiodes and the preamplifiers were immersed in the liquid. A high quantum efficiency for 175 nm UV light has been measured. The scintillation light yield is found to be (1.5 ± 0.6) × 104 photons per MeV of energy deposited in liquid xenon. A light attenuation lenght of ≈ 40 cm has been measured.

Development of a thermal scintillating detector for double beta decay of 48Ca

Abstract: Among double beta candidates 48Ca stands out for its 4.271 MeV transition energy, well above most of the contribution of natural γ and β radioactivity, but extremely near to the energy released in the α decay of 238U (4.274 MeV including nucleus recoil). A CaF2(Eu) detector with both thermal pulse and scintillation light readout would give very good discrimination against this very dangerous source of background. We tested CaF2 crystals with 0.01 to 0.07% Eu doping, in the range of temperature between 300 K and 20 mK. The result shows that detection of the scintillation light from alpha particles of 5.4 MeV with a silicon photodiode is possible down to 20 mK with high signal/noise ratio, and that such doping levels do not affect the performance of CaF2 as a thermal detector

X-ray detector including scintillator channel separator capable of improving sensitivity of X-ray detector

Abstract: A novel scintillator channel separator is constructed of a metal thin film and at least two polymer sheets for sandwiching the metal thin film The scintillator channel separator is used in a scintillator type X-ray detector employed in a third-generation, or fourth-generation X-ray CT system This scintillator channel separator optically separates scintillation light emitted from scintillator elements from each other within a plurality of scintillator element channels of an X-ray scintillator array The scintillator channel separator includes at least a first thin film capable of blocking transmission of the scintillation light occurring in the adjoining scintillator element channels; and first and second polymer sheets each having first and second major surfaces, and capable of reflecting the scintillation light The first thin film is sandwiched between the first major surfaces of the first and second polymer sheets, and the second major surfaces of the first and second polymer sheets are stuck to corresponding side surfaces of the respective adjoining scintillator elements, so that the thin film and the first and second polymer sheets constitute the scintillator channel separator

Determination of90Sr in environmental and biological materials with combined HDEHP solvent extraction-low liquid scintillation counting technique

Abstract: Low level90Sr in environmental and biological samples is determined using a combined HDEHP solvent extraction-liquid scintillation procedure. Yttrium-90 is selectively extracted from nitric acid solution into 5% di(2-ethylhexyl) phosphoric acid (HDEHP) in toluene, and90Y in the organic phase is measured directly using an ultra low level liquid scintillation spectrometer. The working program of the Quantulus counter has been optimized. As the counting efficiency using liquid scintillation counting is high and the stripping and precipitation of Yttrium-90 oxalate is omitted, this procedure is simpler and more timesaving than traditional methods. The chemical recoveries of90Y were 85.1% for soil, 75.7% for milk and 65.3% for bone. The detection limit is 8 mBq.

Gain calibration in a scintillation camera

Abstract: The scintillation camera (24) includes a plurality of individually pulsible light emitting devices associated with a number of the photomultiplier tubes. By pulsing the light emitting device using unit (59), certain ones of the photomultiplier tubes of the camera receive substantial amounts of light. Values (38) from the certain photomultiplier tubes are compared to one another in a unit (40) for carrying out an effective gain adjustment of at least one of the certain tubes. The gain calibration is independent of the amount of light emitted from the light emitting devices. There is also provided a method of adjusting the effective gain of the camera by using the readings resulting from a gamma ray induced scintillation instead of the pulsible light emitting devices.

Process and apparatus for the simultaneous selective detection of neutrons and x or gamma photons

Abstract: The simultaneous selective detection of neutrons and X or gamma photons is disclosed, employing two scintillators. One scintillator uses a first doped scintillation detector that is sensitive to neutrons, while the second doped scintillation device is sensitive to X or gamma photons whereby the scintillations occurring at the two devices are in distinct and separate energy bands.