Geant4 Monte Carlo radioxenon beta-gamma coincidence efficiency simulation for a phoswich detector

In this work, a Monte Carlo (MC) simulation model is established to accurately characterize a phoswich beta-gamma coincidence detector system. This model can be easily used to predict the beta-gamma coincidence efficiencies of xenon radioisotopes at various stable xenon concentrations in the counting cell. The results demonstrate that there is a significant inverse correlation between beta-gamma coincidence efficiency and stable xenon concentration. The influence of stable xenon concentration on beta-gamma coincidence counting efficiency has been investigated for each individual xenon radioisotope. The results indicate that the effect of stable xenon concentration on beta-gamma coincidence efficiency depends on the xenon radioisotope and its decay modes. The coincidence efficiency of ¹³³Xe with 31.0-keV X-ray decay mode is the most affected one; and then followed by ^131mXe, ¹³³Xe with 81.0-keV gamma-ray decay mode, ^133mXe and finally ¹³⁵Xe. The study also indicates that the gamma absorption by xenon gas plays more of a role in the decrease of beta-gamma coincidence efficiency for ¹³³Xe and ¹³⁵Xe, and that the conversion electron spectrum shifting and broadening plays more of a role in the reduction of beta-gamma coincidence efficiency for the metastable radioxenon of ^131mXe and ^133mXe.     

Comparison of phoswich and ARSA-type detectors for radioxenon measurements

The monitoring of atmospheric radioxenon to ensure compliance with the Comprehensive Nuclear Test Ban Treaty (CTBT) has driven the development of improved detectors for measuring xenon, including the development of a phoswich detector. This detector uses only one PMT to detect β–γ coincidence, thus greatly reducing the bulk and electronics of the detector in comparison to the ARSA-type detector. In this experiment, ¹³⁵Xe was produced through neutron activation and a phoswich detector was used to attain spectra from the gas. These results were compared to similar results from an ARSA-type β–γ coincidence spectrum. The spectral characteristics and resolution were compared for the coincidence and beta spectra. Using these metrics, the overall performance of the phoswich detector for β–γ coincidence of radioxenon was evaluated.     

Determining detection limits and minimum detectable concentrations for noble gas detectors utilizing beta-gamma coincidence systems

Beta-gamma coincidence counting is one of two acceptable noble gas monitoring measurement modes for Comprehensive Nuclear-Test-Ban-Treaty (CTBT) verification purposes defined in CTBT/PC/II/WG.B/1. Rigorous derivations of detection limits and minimum detectable activity concentrations for - coincidence data are derived in this paper. Different sampling methodologies are modeled to show how the MDC is affected by different sample collection times, spectral collection times, background radon levels, and other factors.     

Multi-sample beta-gamma coincidence spectrometry in an underground laboratory

The installation of a multi-sample beta-gamma coincidence spectrometer in an underground laboratory with a rock covering of 47 m yielded further background reduction by factors of between 10 (at 2 MeV) and about 4 (below 1 MeV). In silicon samples Fe can be measured by neutron activation analysis with a detection limit of 40 pg.     

Utilization of phoswich detectors for simultaneous,multiple radiation detection

Summary A phoswich radiation detector is comprised of a phosphor sandwich in which several different phosphors are viewed by a common photomultiplier. By selecting the appropriate phosphors, this system can be used to simultaneously measure multiple radiation types (alpha, beta, gamma and/or neutron) with a single detector. Differentiation between the signals from the different phosphors is accomplished using digital pulse shape discrimination techniques. This method has been shown to result in accurate discrimination with highly reliable and versatile digital systems. This system also requires minimal component count (i.e., only the detector and a computer for signal processing). A variety of detectors of this type have been built and tested including: (1) a triple phoswich system for alpha/beta/gamma swipe counting, (2) two well-type detectors for measuring low levels of low energy photons in the presence of a high energy background, (3) a large area detector for measuring beta contamination in the presence of a photon background, and (4) another large area detector for measuring low energy photons from radioactive elements such as uranium in the presence of a photon background. An annular geometry, triple phoswich system optimized for measuring alpha/beta/gamma radiation in liquid waste processing streams is currently being designed.     

Automatic analysis algorithm for radionuclide pulse-height data from beta-gamma coincidence systems

Since the first magnetic analyses of archaeological materials were carried out over a century ago, archaeomagnetic reference curves are now available covering the last few millennia. It would seem to be an appropriate time to examine the archaeomagnetic record to see how it can be improved. For directional studies the disturbing factors include magnetic refraction, mechanical deformation, local magnetic field anomalies, and magnetic anisotropy. In the complex field of archaeointensity determination there is a real need for faster and more reliable methods. The use of sediments on Palaeolithic sites will be increasingly important for the dating of early hominids.     

Application of multi-sample beta-gamma coincidence spectroscopy for high sensitivity activation analysis

The advantage of , -coincidence spectroscopy for the measurement of radiochemically separated nuclides of low radioactivity is demonstrated. The background is reduced by about three orders of magnitude, and its main component, the natural radioactivity of the laboratory walls, is reduced by about four orders of magnitude. The contribution of cosmic rays becomes important at higher energies and within large shields.     

A Geant4 Monte Carlo method for synthesizing radioxenon beta-gamma coincidence spectra

This paper describes a Monte Carlo (MC) simulation method for calculating radioxenon beta-gamma coincidence spectral information. These spectral components include detector response simulations by Geant4 modeling, detector energy and resolution calibrations using the histograms of detector response, beta-gamma coincidence efficiency values and spectral interference ratios. The work presented in this paper demonstrates the feasibility of using the spectral information to create beta-gamma coincidence spectra at various radioxenon activity concentrations. The analysis of these synthetic spectra by XECON software shows an excellent correlation between the analysed radioxenon activity concentration and number of MC samplings.     

Deconvolution of three-dimensional beta-gamma coincidence spectra from xenon sampling and measurement units

Summary In view of the terrorist threats to the United States, the country needs to consider new vectors and weapons related to nuclear and radiological threats against our homeland. The traditional threat vectors, missiles and bombers, have expanded to include threats arriving through the flow of commerce. The new commerce-related vectors include: sea cargo, truck cargo, rail cargo, air cargo, and passenger transport. The types of weapons have also expanded beyond nuclear warheads to include radiation dispersal devices (RDD) or “dirty' bombs. The consequences of these nuclear and radiological threats are both economic and life threatening. The defense against undesirable materials entering our borders involves extensive radiation monitoring at ports of entry. The radiation and other signatures of potential nuclear and radiological threats are examined along with potential sensors to discover undesirable items in the flow of commerce. Techniques to improve radiation detection are considered. A strategy of primary and secondary screening is proposed to rapidly clear most cargo and carefully examine suspect cargo.     

Measurement of coincidence timing resolution of scintillation detectors compared to semiconductor detectors to image three-photon positron annihilation

The coincidence timing resolution is a critical parameter for triple coincidence to measure the three photon positron annihilation yield. The yield is inversely proportional to the concentration of oxygen which acts as a quenching agent within the tissue and, therefore, provides important information about the state of the cancerous tumor. A comparison between scintillation and semiconductor detectors with respect to their timing properties for triple coincidence measurements and imaging was carried out. The coincidence timing resolution for example for HPGe detectors was found to vary between 35.4±0.02 and 42.8±0.02 ns. A comparison of all detector characteristics with respect to HPGe is presented.     

Comparison of resolving time values of different scintillation detectors in the coincidence experiments

The resolving time values of the coincidence spectrometers composed by NaI(Tl) inorganic, BC408 type and BC418 type organic scintillation detectors were obtained in this work. For this purpose, fast–fast coincidence measurements were carried out. Best timing performance was obtained by BC418 type plastic scintillator for various source-detector distances.     

Quantitative analysis in field-flow fractionation using ultraviolet-visible detectors: an experimental design for absolute measurements

In previous works, it has been shown that a standard ultraviolet-visible detection system can be used for quantitative analysis of heterogeneous systems (dispersed supermicron particles) in field-flow fractionation (FFF) by single peak area measurements. Such an analysis method was shown to require either experimental measurements (standardless analysis) or an accurate model (absolute analysis) to determine the extinction efficiency of the particulate samples. In this work, an experimental design to assess absolute analysis in FFF through prediction of particles' optical extinction is presented. Prediction derives from the semiempirical approach by van de Hulst and Walstra. Special emphasis is given to the restriction of the experimental domain of instrumental conditions within which absolute analysis is allowed. Validation by statistical analysis and a practical application to real sample recovery studies are also given.     

Comparison of portable detectors for uranium enrichment measurements

Uranium enrichment and holdup measurements require a detector capable of accurately obtaining the 186-keV peak area. NaI detectors have been widely used for these tasks. However, for recycled uranium, the interference of the 239-keV peak from the ²³²U decay chain challenges the capabilities of the NaI detectors to accurately extract the area of the 186-keV peak. Using CZT detectors, which have much better resolution than the NaI detectors, has temporarily solved this interference problem. However, the CZT detectors have setbacks in that they are generally small and have low efficiencies, which require long acquisition times for reasonable statistics. Recently, two new types of scintillator detectors have become available commercially, LaCl₃(Ce) and LaBr₃(Ce). These cerium-doped lanthanum halide detectors, with comparable resolution but better efficiency than the CZT detectors, appear to permanently solve the interference problem for recycled uranium measurements. In this report, we compare the uranium enrichment measurement performances of a portable NaI detector, a large coplanar-grid CZT detector, and a LaBr₃ detector.     

Three photon annihilations of positrons and positronium in solids with two detectors in coincidence

The two-detector coincidence positron annihilation system at Washington State University was used to observe three-photon annihilation events of positrons. The decay of ortho-positronium (o-Ps) can be distinguished for para-Ps decay and positron two photon annihilations by the unique difference energy distribution. The apparatus was used to detect rare three-photon annihilations of positrons in metals.     

Auger photoelectron coincidence spectroscopy using synchrontron radiation

The technique of Auger-photoelectron coincidence spectroscopy (APECS) is described and illustrated with a case study of the Cu(100) 3p and M₂₃VV spectra. APECS offers many advantages over the conventional singles spectroscopy such as isolating overlapping spectral features, reducing secondary electron background, and revealing new decay modes. In the coincidence Cu Auger spectra discussed here, the multiplet structure of the quasi-atomic 3d⁸ Auger final state is clearly observed, as well as different intensities for the multiplet components for the p_1/2 and p_3/2 transitions. Furthermore, the spectra reveal evidence for a Coster-Kronig decay channel for 3p_1/2 core holes, and illustrate that the sum of the Auger electron and photoelectron kinetic energies is conserved. Possible technical improvements that can increase the counting efficiency are also discussed.     

Study of silicon detectors for high resolution radioxenon measurements

Measurement of radioactive xenon in the atmosphere is one of several techniques to detect nuclear weapons testing, typically using either scintillator based coincidence beta/gamma detectors or germanium based gamma only detectors. Silicon detectors have a number of potential advantages over these detectors (high resolution, low background, sensitive to photons and electrons) and are explored in this work as a possible alternative. Using energy resolutions from measurements and detection efficiencies from simulations of characteristic electron and photon energies, the minimum detectable concentration for Xe isotopes was estimated for several possible detector geometries. Test coincidence spectra were acquired with a prototype detector.     

True coincidence correction for k0-method in INAA of plant materials with well-type detectors

Summary The k₀-method in INAA has been used at CENA/USP Brazil for analyzing geological and biological materials. With the acquisition of a well-type detector, it was realized that the true-coincidence effects, originally corrected by semi-empirical procedures with the in-house k₀-software (Quantu-INAA), would affect the accuracy of results. Thus, new coincidence correction algorithms were developed, based on a theoretical equation for obtaining the correction factors. Validation of the algorithms was performed by analysis of plant certified reference materials.     

Pressure control of a pressurized proportional counter for 4 pi beta-gamma coincidence counting--effects of pressure variation and study of a non-flow type system

Effects of the pressure variation of counting gas on the efficiency function were examined using a 4 pi beta-gamma coincidence counting system with a pressurized proportional counter. The results showed that the pressure variation of about ten kPa could not give large effects on the efficiency function and that a non-flow type of pressurized proportional counter with a simple gas control system could work well in the case of the pressure higher than 1 MPa, an ordinary pressure for the counting.     

Investigation of passivated silicon detectors for direct measurements of actinides

Although much progress was made by using high purity gemanium detectors for direct measurement of²³⁹Pu in man, the improvement of lung spectroscopy sensitivity remains a challenge. Previous work showed that passivated implanted planar silicon detectors could provide an alternative by combining high-energy resolution at room temperature and planar geometry with large detection arrays. This paper reports on the status of this research based on the characterisation of 3×3 cm² and 6×3 cm² strip silicon detectors. Furthemore, the possibility to apply such detectors towards other monitoring problems for accidental situations such as the direct measurement of small contaminated surfaces was investigated.     

Environmental dosimetry using high-sensitivity TL detectors

Two highly sensitive thermoluminescent (TL) materials (Al₂O₃:C and CaSO₄:Tm) were investigated for environmental monitoring. We compared their response to low and to high linear energy transfer (LET) radiations in order to develop a two-element TL system.The relative TL efficiency for α-particle irradiation as compared to γ-exposure was about 0.04 and 0.5 for Al₂O₃:C and CaSO₄:Tm, respectively. Measurements of external environmental radiation doses in an international balloon experiment were performed with our highly-sensitive TL system having various sensitivities to low and to high LET radiations.