Scintillation Detector System Research Articles

Extended Algorithm for Simulation of Light Transport in Single Crystal Scintillation Detectors for S(T)EM

The new extended Monte Carlo (MC) simulation method for photon transport in S(T)EM back scattered electron (BSE) scintillation detection systems of various shapes is presented in this paper. The method makes use of the random generation of photon emission from a scintillator luminescent centre and describes the trajectory of photons and the efficiency of their transport toward the photocathode of the photomultiplier tube. The paper explains a new algorithm for determining the position of interaction of the photon with the surface of the single crystal scintillator or of the light guide with nearly arbitrary shapes. Some examples of the utilization of the simulation method are also included, and conclusions for very simple edge-guided signal (EGS) scintillation detection systems made. The computer optimized design of the BSE scintillation detector for the S 4000 Hitachi SEM was chosen to demonstrate the capability of this MC simulation method.

Determination of effective atomic numbers and effective electron densities for Cu/Zn alloy

The mass attenuation coefficients, total interaction cross-sections, effective atomic numbers, effective electron densities and photon mean free paths of the Cu/Zn alloy were determined on the basis of the mixture rule at 356, 511, 662, 835 and 1275 keV gamma-ray energies. The gamma-rays were detected by using an ordinary NaI(Tl) scintillation detection system with a resolution of 10.2% at 662 keV.It was observed that the mixture rule is a suitable method for determination of these parameters. The effective atomic numbers and effective electron densities tend to be almost constant as a function of energy. There is good agreement between experiment and theory, calculated by WinXCom.

Determination of potassium in several plants and study of potassium transfer to different beverages, including tequila, by measurement of 40K

Measurement of 40K was used for determination of potassium concentrations in leaves of agave and maguey cactus leaves, and coffee beans of various origins. The procedure was also used to study potassium transfer to tequila (alcoholic drink made of agave cactus), and the cactus and coffee infusions using 40K as a natural radioactive tracer. Counting of 40K in Marinelli containers with the aid of a low background NaI(Ti) scintillation detection system for 12–24 hours was employed. The method appeared to be simple and suitable for determination of potassium concentrations in large samples, which eliminates homogeneity problems.

Gaseous scintillation detection and amplification in variable pressure scanning electron microscopy

This work investigates the generation and detection of gaseous scintillation signals produced in variable pressure scanning electron microscopy through electron-gas molecule excitation reactions. Here a gaseous scintillation detection (GSD) system is developed to efficiently detect photons produced via excitation reactions in electron cascades. Images acquired using GSD are compared to those obtained using conventional gaseous secondary electron detection (GSED) and demonstrate that images rich in secondary electron (SE) contrast can be achieved using the gaseous scintillation signal. A theoretical model, based on existing Townsend theories, is developed. It describes the production and amplification of photon signals generated by cascading SEs, high energy backscattered electrons, and primary beam electrons. Photon amplification (the total number of photons produced per sample emissive electron) is then investigated and compared to conventional electronic amplification over a wide range of microscope operating parameters, scintillating imaging gases, and photon collection geometries. These studies revealed that argon gas exhibited the largest GSD gain, followed by nitrogen then water vapor, exactly opposite to the trend observed for electron amplification data. It was also found that detected scintillation signals exhibit larger SE signal-to-background levels compared to those of conventional electronic signals detected via GSED. Finally, dragging the electron cascade towards the light pipe assemblage of GSD systems, or electrostatic focusing, dramatically increases the collection efficiency of photons.

Stabilizing scintillation detector systems with pulsed LEDs: a method to derive the LED temperature from pulse height spectra

Photomultiplier tubes (PMTs) can be stabilized with light emitting diodes (LEDs) used as reference light sources. The LED is supplied with short voltage pulses that produce well-defined light portions if the LED temperature is kept constant. However, if the PMT must be operated in a wide temperature range, the LED light output is no longer a constant but becomes a function of the junction temperature. This problem can be solved at low expense by means of a new method. The LED is operated in two alternating pulse modes distinguished by the pulse voltage. The LED light output depends on the pulse voltage and, therefore, the PMT pulse height spectrum shows two distinct peaks. However, not only is the total amount of light L emitted per pulse but also the shape of the temperature dependence L(T) varies with the pulse voltage. The numerical ratio R of the peak positions corresponding to the two different LED modes is therefore a function of the LED temperature as well. Since it is a ratio, R is independent of the actual PMT gain but characterizes the LED junction temperature. Thus, the pulse height ratio of the different LED signals can serve as an LED thermometer. Moreover, measuring L and R at several temperature points covering the full operational range yields a calibration function L(R). With this knowledge, commercial off-the-shelf LED components can be used as precise reference light sources in a wide range of ambient temperatures

Stabilizing scintillation detector systems by exploiting the temperature dependence of the light pulse decay time

Scintillation detectors must tolerate a wide range of ambient temperatures and strong temperature slopes when used in outdoor applications. Such demanding conditions are typical for all homeland security applications. An effective and efficient detector stabilization compensating for temperature dependent gain shifts is essential to maintain energy calibration and resolution. Reliable, well-established solutions are based on radioactive reference sources; however, alternatives are much asked for. The gain shift correction for the temperature dependence of the scintillation light output requires elaborate hardware and software means without a reference source. Strong and rapid temperature changes further complicate the situation as there is no thermal equilibrium in the detector but rather a temperature field. This paper presents a new technique of gain stabilization which considers the effective scintillator temperature by analyzing the average pulse shape of detector signals. The pulse shape is correlated with the scintillation light decay time. This parameter can be extracted online from digitized detector signals. The decay time data are used to eliminate all the temperature determined system gain shifts without radioactive reference source. This technique has been verified in extensive climate chamber measurements. The results are discussed.

<Superscript>64</Superscript>Cu, <Superscript>66</Superscript>Cu and <Superscript>99m</Superscript>Tc for studies related to copper-associated hepatitis in dobermann dogs

An extractive scintillating (ES) resin was evaluated for its performance as an on-line monitor of uranium in water. The TRU-ES resin is comprized of an inert macroporous polystyrene core impregnated with the organic fluors [diphenyloxazole (PPO) and 1,4-bis-(4-methyl-5-phenyl-2-oxazolyl)benzene (DM-POPOP)) and the selective extract (octyl(phenyl)-N, N-diisobutyl-carbamoylmethylphosphine oxide (CMPO) in tri-butyl phosphate (TBP)]. The TRU-ES resin, packed into translucent FEP Teflon tubing, was placed into a flow-cell scintillation detection system. Acidified aqueous solutions, 233 U spiked synthetic ground water and EPA natural uranium QA samples, were pumped through the flow-cell while gross count rate and pulse-height spectra were collected. The increase in count rate is attributed to the uranium being extracted from the aqueous medium and retained by the TRU-ES resin with simultaneous detection of the resultant scintillation photons. The TRU-ES loading efficiency was nearly quantitative out of 2M HNO 3 with a flow rate of 0.5 ml . min -1 . The detection efficiency was measured to be 51p for 233 U. The detection limit was determined to be ~2 Bq . l -1 for 233 U based on a resin free column volume of 0.2 ml, and 50 ml of 2M HNO 3 acidified groundwater.

Neutron Detector Systems for the Insruments at JSNS

Neutron detector systems are key to the success of high power pulsed spallation neutron sources like JSNS and SNS. We organized many small meetings to discuss about neutron detector systems for the instruments at JSNS, especially about requirements and candidates. Detector developers summarized specifications of detector systems that are available now or under development, and discussed about the requirements and candidates with neutron instrument designers. At the first stage, we will employ improved versions of conventional-detector systems such as He-3 gas linear position sensitive detector and improved version of the ZnS scintillation detector systems developed at ISIS. We will continue R&D's to develop high performance detector systems, such as MSGC/MPGC, MSGC in a gas tube and various scintillation based detector systems. Our plan is to add such high performance detector systems at a later time to the instruments.

Development of a scintillator detector system for γ-ray scan measurements of industrial distillation columns

A CsI(Tl) scintillator detector set was developed for γ-ray scan measurements aiming to identify process failures in distillation columns of an industrial petrochemical plant. A system with a 60Co sealed source as a γ-ray emitter and the developed CsI(Tl) detector were applied to evaluate the performances in a simulated model of a tray-type distillation column.

Performance measurements of a new large-area neutron scintillation detector system

During the upgrade of the small-angle neutron scattering instrument KWS-1 at the research reactor FRJ-2, the 15-yr-old detector system has been replaced. While the Anger camera-based concept of the detector remained unchanged, the signal and data processing branch has been completely revised. Because of higher count rate requirements, a highly configurable parallel readout electronics has been developed aiming at counting rates of several hundred kilohertz. The whole detector system has been recently installed at KWS-1 and measurements to study the system performance have been carried out. The linearity and spatial resolution of the new detector have been determined by an analysis of an acquired image taken with a hole diaphragm in front of the scintillator. The overall system dead time has been measured by comparing the detector count rates at several scattering intensities to the count rates achieved with a fission chamber. Finally, scattering patterns of well-known samples have been taken in order to demonstrate the quality of the acquired images.

Search for neutrino-less double beta decay of 48Ca by CaF 2 scintillator

A CaF 2 scintillation detector system (ELEGANT VI) has been operating at Oto Cosmo Observatory to study double beta decays of 48Ca. No events were observed around the Q-value energy region after the analysis of 4.23 kg yr data. To derive the lower limit for the half-life of the neutrino-less double beta decay of 48Ca, the expected number of background events in that energy region was estimated by a Monte Carlo simulation using the measured activities of 214Bi and 220Rn inside CaF 2 crystals. A new lower limit is obtained to be 1.4×10 22 yr at the 90% C.L. An experimental sensitivity is 5.9×10 21 yr at the 90% C.L.

Double beta decay study of 48Ca by CaF 2 scintillator

A CaF 2 scintillation detector system (ELEGANT VI) is developed to search for neutrinoless double beta decay (0νDBD) of 48Ca. No events were observed around the Q-value energy region after the analysis of 4.23 kg yr data. To derive the lower limit for the half-life, the expected number of background events was estimated by a Monte Carlo simulation using the measured activities of 214Bi and 220Rn inside CaF 2 crystals. A new lower limit is obtained to be 1.8 × 10 22 yr at the 90% C.L.

Simultaneous separation and detection of actinides in acidic solutions using an extractive scintillating resin.

An extractive scintillating resin was evaluated for the simultaneous separation and detection of actinides in acidic solutions. The transuranic extractive scintillating (TRU-ES) resin is composed of an inert macroporous polystyrene core impregnated with organic fluors (diphenyloxazole and 1,4-bis-(4-methyl-5-phenyl-2-oxazolyl)benzene) and an extractant (octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide in tributyl phosphate). The TRU-ES resin was packed into FEP Teflon tubing to produce a flow cell (0.2-mL free column volume), which is placed into a scintillation detection system to obtain pulse height spectra and time series data during loading and elution of actinides onto/from the resin. The alpha-particle absolute detection efficiencies ranged from 77% to 96.5%, depending on the alpha energy and quench. In addition to the on-line analyses, off-line analyses of the effluent can be conducted using conventional detection methods. The TRU-ES resin was applied to the quantification of a mixed radionuclide solution and two actual waste samples. The on-line characterization of the mixed radionuclide solution was within 10% of the reported activities whereas the agreement with the waste samples was not as good due to sorption onto the sample container walls and the oxidation state of plutonium. Agreement between the on-line and off-line analyses was within 35% of one another for both waste samples.

A novel detection system for laser cooling of ortho-positronium

We report on the development of a position-sensitive scintillation detector system for viewing laser-cooled ortho-positronium (oPs) annihilation points. Ortho-positronium can be cooled down to a recoil limit of 0.1 K by an ultraviolet laser in spite of its relatively short lifetime. In order to perform this experiment the wavelength of Cr:LiSAF laser is considered to be 243 nm which corresponds to the transition energy of 1S–2P states of oPs. Thermal positronium plays an important role in laser cooling of oPs. We investigated various kinds and sizes of inorganic scintillators . Position resolutions of YAP:Ce and CsI(Tl), respectively, were determined to be 3.00 and 3.8 mm. Extensive Monte Carlo simulation studies were performed to view the decay positions of oPs using the time-of-flight method. Related issues of position resolution measurements and oPs decaying points will be discussed.

The ALTA global positioning satellite based timing system

The Alberta Large-area Time-coincidence Array (ALTA) experiment uses a number of scintillation detector systems to form a sparse very large area cosmic air-shower detection array. An important scientific goal of the ALTA collaboration is to search for coincidences in the ALTA array due to large area cosmic ray phenomena. A local cosmic ray event, determined by a coincidence of the triplet of cosmic ray detectors forming a local detector system, is time stamped with a temporal coordinate obtained from a GPS receiver. The readout of the data, the local coincidence and the GPS time stamp are all performed in the local readout crate. This time stamp, along with the local shower direction is used to search for coincidences within the large area array. Using two GPS receivers and duplicate sets of ALTA electronics the timing resolution of the GPS time difference between sites was estimated to be 16 ns .

A temporal method of avoiding the Cerenkov radiation generated in organic scintillator dosimeters by pulsed mega-voltage electron and photon beams

The output signal of an organic scintillator probe consists of a scintillation signal and Cerenkov and fluorescence radiation (CFR) signal when the probe is exposed to a mega-voltage photon or electron beam. The CFR signal is usually unwanted because it comes from both the scintillator and light guide and so it is not proportional to the absorbed dose in the scintillator. A new organic scintillator detector system has been constructed for absorbed dose measurement in pulsed mega-voltage electron and photon beams that are commonly used in radiotherapy treatment, eliminating most of the CFR signal. The new detector system uses a long decay constant BC-444G (Bicron, Newbury, OH, USA) scintillator which gives a signal that can be time resolved from the prompt CFR signal so that the measured contribution of prompt signal is negligible.The response of the new scintillator detector system was compared with the measurements from a plastic scintillator detector that were corrected for the signal contribution from the CFR, and to appropriately corrected ion chamber measurements showing agreement in the 16 MeV electron beam used.

An alternative approach to tritium-in-water monitoring

A paradigm shift in the approach to tritium-in-water monitoring is considered and its viability demonstrated through a series of proof-of-principle experiments. The fundamental difference in the proposed detection system consists of changing the state of the tritiated water from the liquid phase to the gas phase and subsequently using any one of a variety of detection methods to detect tritium in the water vapour. Proof-of-principle experiments using an un-optimized heated ionization chamber illustrate the viability of the proposed method. Operation of the experiment as a continuous in-line detection system shows a detection limit of ∼20 μCi/l, a detection time constant of 10 s, and simplicity and effectiveness of the overall system. It is projected that an optimized heated ionization chamber system using state-of-the-art current measuring circuitry, could yield a detection limit of ∼0.2 μCi/l. Furthermore, application of a more sensitive detection system (for example, the gas electron multiplier) is expected to result in additional gain in sensitivity. The advantages of the proposed tritium-in-water monitoring system are: high sensitivity with a detection limit beginning to approach current day batch liquid scintillation detection systems; a truly continuous, on-line tritium-in-water monitor with response time of less than tens of seconds; a detection system potentially applicable for multiple stream monitoring; a system that does not generate any waste; and, a system that overcomes the fouling issues associated with current day on-line LSC systems.

COSMIC RAY MUON MEASUREMENT BY L3 SPECTROMETER AT CERN

L3 + C is a branch experiment on L3 magnet spectrometer, which is located on the ring of LEP accelerator at CERN. To take the advantage of L3 muon chambers in its low threshold, wide dynamic range and high resolution, the momentum of cosmic ray muons in the range of 15–2000 GeV/c at a shallow depth of 30 m of molasse can be measured precisely. Since 1998, a scintillator detector system, a new fast trigger and DAQ system, and a small air shower array had been established for study the CR muon events independently. Up to August 2000, 8 billion muons and 25 million air shower events had been recorded. The first results for CR muon spectrum and the charge ratio etc. had been obtained.

A new two-dimensional scintillation detector system for small-angle neutron scattering experiments

A new two-dimensional position-sensitive detector system for small-angle neutron scattering experiments is under development. It is based on a 60/spl times/60 cm/sup 2/ large /sup 6/Li-glass scintillator for the neutron capture and dispersion of the emitted light on an array of photomultiplier tubes for the derivation of the event position. For subthermal neutrons of 8 /spl Aring/, an efficiency of 96% and a spatial resolution of 8 mm are to be achieved. Major improvements have been made in the signal- and data-processing branch, which aims at an electronic dead time of 1 /spl mu/s. A fast digitalization method using free-running analog-to-digital converters for each photomultiplier combined with field-programmable gate arrays for a parallel event detection in all channels has been applied. A subsequent farm of up to 16 floating-point digital signal processors is used to provide enough computing power for an improved reconstruction of the event position according to a maximum likelihood method.

A miniature "scintillator-fiberoptic-PMT" detector system for the dosimetry of small fields in stereotactic radiosurgery

Linear accelerator-based stereotactic radiosurgery is an important modality in the management of localized intracranial abnormalities and often is the only option for treatment of certain lesions. The quantification of the absorbed radiation doses delivered for this modality has been a challenge due to the small size of the treatment fields (typically 5-30 mm collimated fields) and the type of detectors presently available. This paper presents comparative dose measurements performed using a miniature (1.6/spl times/10/sup -3/ cm/sup 3/) plastic scintillator detector to the commonly used detectors for the dosimetry of small photon fields used for stereotactic radiosurgery. The spatial resolution of the plastic scintillator detector was compared to that of a p-type Si diode and a 0.1-cm/sup 3/ ionization chamber. Small field dosimetry parameters (beam profiles, percent depth doses, and dose output factors) using the scintillation detector, the 0.1-cm/sup 3/ ionization chamber, the p-type Si diode, and radiographic film are presented. This comparative study shows that designing a miniature scintillator detector system is feasible and when compared to other benchmark detectors is an appropriate detector for the dosimetry of small stereotactic radiosurgery photon fields.