Coincidence System Research Articles

Development of plastic scintillator based primary standard for activity measurements and its performance evaluation

4πβ-γ coincidence technique is a powerful tool and widely recognised method to determine the absolute activity concentration of radioactive solutions. A new plastic scintillator based coincidence system has been developed and established as a primary standard for radioactivity measurements. The performance of the system was evaluated by the standardisation of 60Co radioactive solution due to its simple decay scheme. The activity concentration results obtained by the new system were compared with the existing proportional counter and liquid scintillation based 4πβ-γ coincidence systems. This paper discusses the design details of the new system and its performance evaluation.

Primary standardization and determination of gamma ray emission intensities of Ho-166

The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN-CNEN/SP, in São Paulo, for the primary standardization of 166Ho is described. The activity of 166Ho was determined by the efficiency extrapolation technique applied to a 4πβ(PC)-γ coincidence system using a gas flow proportional counter in 4π geometry coupled to a 76 × 76 mm NaI(Tl) crystal. The results for the γ-rays intensities at 80.57 and 1379.45 keV were 0.0651(11) and 0.00904(11), respectively.

PALS probing of photopolymerization shrinkage in densely packed acrylate-type dental restorative composites.

Using positron annihilation lifetime spectroscopy (PALS), microstructural changes in commercial dental restorative composites under light-curing polymerization were identified as a modification in mixed positron/Ps trapping, where the decay of positronium (Ps; the bound state of positrons and electrons) is caused by free-volume holes mainly in the polymer matrix, and positron trapping is defined by interfacial free-volume holes in a mixed filler-polymer environment. In loosely packed composites with a filler content of <70-75%, this process was related to the conversion of Ps-to-positron trapping. To disclose such peculiarities in densely packed composites using the example of he commercially available acrylate-based composite ESTA-3® (ESTA Ltd., Kiev, Ukraine), which boasts a polymerization volumetric shrinkage of only 1.5%. ESTA‑3® was used as a commercially available acrylate-based dental restorative composite. A fast-fast coincidence system of 230‑ps resolution based on 2 photomultiplier tubes coupled to a BaF2 detector and ORTEC® electronics was used to register lifetime spectra in normal-measurement statistics. The raw PAL spectra were treated using x3-x2-CDA (coupling decomposition algorithm). The annihilation process in the densely packed dental restorative composites (DRCs), as exemplified by the commercially available acrylate-based composite ESTA‑3®, is identified as mixed positron/ Ps trapping, where o-Ps decay is caused by free-volume holes in the polymer matrix and interfacial filler-polymer regions, and free positron annihilation is defined by free-volume holes between filler particles. The most adequate model-independent estimation of the polymerization volumetric shrinkage can be done using averaged positron annihilation lifetime. A meaningful description of the transformations in Psand positron-trapping sites under light curing can be developed on the basis of a semiempirical model exploring x3‑x2‑CDA. There is a strong monolithization of agglomerated filler nanoparticles in these composites, caused by the photo-induced disappearing of positron traps at the cost of Ps-decaying holes. Governing the polymerization void-evolution process in densely packed DRC ESTA‑3® occurs mainly in the filler sub-system as positron-to-Ps trapping conversion, which is the reason for the low corresponding volumetric shrinkage.

Search for variations of [formula omitted] decay constant

Exploration of possible anomalies in the decay of different radionuclides is an active research area in the fields of astrophysics and nuclear physics. The most significant changes in decay rates can be expected for β−, β+ or electron capture decay nuclides, due to the proposed influence of the solar neutrino flux by some research groups. In this work, the nuclear decay rate of β+ decay of N22a was investigated during the time interval of ~33 days. For this purpose, a coincidence system based on a planar high purity germanium detector and a plastic scintillation detector was designed and used for the acquisition of experimental N22a coincidence spectrum. The corresponding experimental conditions: temperature, atmospheric pressure, and relative humidity, which may affect the measurement system, were monitored. In addition, Monte-Carlo simulations of this system were performed in order to obtain the N22a simulated coincidence spectrum. The time-dependent coincidence spectra in 0.5 h bins were collected by the multiparameter MPA-3 system. The fluctuations in the decay rate (i.e. possible variations of the decay constant) were analyzed by Fourier and Lomb-Scargle algorithms. No significant evidence for periodic behavior in decay rate or aperiodic anomalies was found during the acquisition period.

Guidelines and Limits for the Quantification of Ra Isotopes and Related Radionuclides With the Radium Delayed Coincidence Counter (RaDeCC)

AbstractThe Radium Delayed Coincidence Counter (RaDeCC) is one of the most extensively used equipment for measuring 223Ra and 224Ra activities in water and sediment samples. Samples are placed in a closed He‐circulation system that carries the Rn produced by the decay of Ra to a scintillation cell. Each alpha decay recorded in the cell is routed to an electronic delayed coincidence system which enables the discrimination of 223Ra and 224Ra. In this study, the measurement and quantification methods using the RaDeCC system are assessed through analyses of registered data in different RaDeCC systems worldwide and a set of simulations. Results of this work indicate that the equations used to correct for 223Ra and 224Ra cross‐talk interferences are only valid for a given range of activities and ratios between isotopes. Above certain limits that are specified in this study, these corrections may significantly overestimate the quantification of 223Ra and 224Ra activities (up to ~40% and 30%, respectively), as well as the quantification of their parents 227Ac and 228Th. High activities of 226Ra may also produce an overestimation of 224Ra activities due to the buildup of 222Rn, especially when long measurements with low activities of 224Ra are performed. An improved method to quantify 226Ra activities from the buildup of 222Rn with the RaDeCC system is also developed in this study. Wethus provide a new set of guidelines for the appropriate quantification of 223Ra, 224Ra, 227Ac, 228Th, and 226Ra with the RaDeCC system.

A new coincidence module using pulse-mixing method applied in the 4π(LS)-γ coincidence system with TDCR detector

Application of a new coincidence module using the pulse-mixing method in the 4π(LS)-γ coincidence system with TDCR detector in the beta channel (TDCRG) is presented. The pulse-mixing method extended for the TDCR system with gamma channel was compared with the classical coincidence method where the coincidence resolving time is introduced. The characteristics of the resolving and dead time of the above two methods were investigated. The level of accidental coincidences in the system was determined. The new developed coincidence module was validated by activity measurement of a set of 51Cr, 54Mn, 60Co, 65Zn, 85Sr and 134Cs standards.

Data analysis software package for radionuclide standardization with a digital coincidence counting system

The Nuclear Metrology Laboratory (LMN) – IPEN, São Paulo, Brazil – developed a Digital Coincidence System (DCS), based on the Coincidence Counting Methodology, in order to improve its capabilities in radionuclide primary-standardization. Digital process is implemented in two steps: data-acquisition (a set of measurements) and offline software data-analysis and calculation. The present work shows the basics of the data-acquisition unit (Software Coincidence System – SCS), describes the DCS’ data-analysis process and the initial approaches chosen for the implementation of the software package (Coincidence Analyzing Task – CAT). 152Eu standardization, performed for DCS testing, software expansion and validation, is briefly discussed.

Locating events in a segment of a segmented HPGe detector at KRISS

Abstract A specific segment of the 24 fold segmented HPGe detector was studied for improving the interaction position resolution from several cm to mm. The segmented HPGe detector was modeled by CAD program with their electrodes which was isolated by the Boolean function. The Shockley-Ramo theorem changed the Gauss law to Laplace equation problem for calculating the induced charge on each electrode. Finite Element Method solved the Poisson and Laplace equation with their boundary conditions for Electric field and potential. The drift velocities of charge carriers were compared between both along the 100 > and one between 100 > and 110 > Ge crystal axis at a specific segment and used to simulate the pulse shapes with 1 mm position resolution. Coincidence system between a specific segment and 3 × 3 HPGe detector verified the simulated pulse shapes by comparing these with the experimental ones which are generated at the specific target region with a fixed scattering angle. The experimental and simulated ones were χ 2 fitted to find the least χ 2 value and locate the interaction positions. The interaction positions of experimental pulse shapes were analyzed statistically after the rise time correction along 100 > and 110 > Ge crystal axis. The position resolution as FWHM on x, y, and z axis is the 3.12, 4.38 and 8.69 mm with the collimator size 3, 6, and 6 mm, respectively.

SiPM Smart Sensor based portable coincidence system for distributed field applications

A novel, low-cost, light-weight, portable coincidence system based on autonomous smart sensors using off the shelf Silicon photomultipliers (SiPMs) and microcontrollers has been developed, which can detect radiation events, in particular charged particles such as muons. Innovative software techniques allow real-time self compensation and synchronisation of multiple smart sensors with an external GPS or a self-referencing clock, making the system suitable for surface and subterranean coincidence-based measurements. The modular and rugged design supports real-time coincidence as well as offline analysis, and permits easy scalability to multiple detectors. It is a viable option for a range of applications such as education and outreach, globally distributed cosmic ray coincidence systems, and background radiation detection in homes, industry and academia.

Standardisation of 85Sr with digital anticoincidence counting and half-life determination of the 514 keV level of 85Rb

The activity of a 85Sr solution was determined by means of fully digital 4πβ(LS)-γ anticoincidence counting. The measurements were carried out in a custom-built, combined TDCR / 4πβ(LS)-γ coincidence system, utilising a commercially available CAEN N6751C digitizer. The analysis of the experimental data, collected in list-mode format, was performed off-line by using the SoftKAM computer code developed at PTB. The data were also used to determine the half-life of the 514 keV level of 85Rb which was found to be (1020.2 ± 6.0) ns.

Simple coincidence technique for cosmic-ray intensity exploration via low-energy photon detection

Changes in cosmic-ray intensity can significantly influence the search for rare events or processes in nuclear and astroparticle physics through corresponding variations in detector background count rate. In this work, we present an approach to explore cosmic-ray intensity and corresponding cascade production of secondary particles in the detector vicinity using low-energy photon background spectra induced by cosmic rays at the earth’s surface. The coincidence system based on a plastic scintillator and an extended range HPGe detector, including a multiparameter device, was used for the acquisition of low-energy photon spectra. This system was also simulated by the GEANT4 toolkit, and the simulated and experimental spectra were compared. Single aperiodic events, as well as possible periodic behavior of low-energy photon emission were searched for.

Light-cured dimethacrylate dental restorative composites under a prism of annihilating positrons.

Breakthrough resolutions in current biopolymer engineering rely on reliable diagnostics of atomic-deficient spaces over the finest sub-nanometer length scales. One such diagnostic is positron annihilation lifetime spectroscopy, which probes space-time continuum relationships for the interaction between electrons and their antiparticle (positrons) in structural entities like free-volume defects, vacancies, vacancy-like clusters, interfacial voids and pores, etc. This paper is intended to highlight the possibilities of positron annihilation lifetime spectroscopy as an informative instrumentation tool to parameterize free-volume evolution in light-cured dimethacrylate dental restorative composites exemplified by Charisma® (Heraeus Kulzer GmbH, Hanau, Germany) and Dipol® (Oksomat-AN Ltd, Kyiv, Ukraine). The subjects of the study were the commercially available dimethacrylate-type dental restorative composites Charisma® and Dipol®. The analysis used a fast-fast coincidence system of 230 ps resolution based on 2 photomultiplier tubes coupled to BaF2 scintillator detectors and ORTEC® (ORTEC, Oak Ridge, USA) electronics to register lifetime spectra in normal-measurement statistics evolving ~1 million coincidences. The annihilation process in both composites is identified as mixed positron-Ps (positronium) trapping, where ortho-Ps decaying is caused entirely by free-volume holes in the polymer matrix, and the 2nd component is defined mainly by interfacial free-volume holes between filler nanoparticles and the surrounding polymer. The most appropriate model-independent estimation of photopolymerization volumetric shrinkage in dental restorative composites can be done using averaged positron annihilation lifetime. Partiallyconstrained x4-term analysis of lifetime spectra is less efficient, giving greater scatter of variance with an additional artifact of fixed shortest lifetime allowing unresolved mixing in the 2nd component. A meaningful phenomenological description of transformations in Ps and positron-trapping sites under light curing, which occurs more efficiently in Charisma® than in Dipol® nanocomposites, can be developed at the basis of a semi-empirical model exploring a x3-x2-coupling decomposition algorithm. A deep understanding of void-evolution processes in dimethacrylate dental composites employing positron annihilation lifetime spectroscopy makes it possible to diagnose, characterize and engineer novel biomaterials for advanced use in medical practice.

An automated LS(β)- NaI(Tl)(γ) coincidence system as absolute standard for radioactivity measurements

4πβ-γ coincidence method is a powerful and widely used method to determine the absolute activity concentration of radioactive solutions. A new automated liquid scintillator based coincidence system has been designed, developed, tested and established as absolute standard for radioactivity measurements. The automation is achieved using PLC (programmable logic controller) and SCADA (supervisory control and data acquisition). Radioactive solution of 60Co was standardized to compare the performance of the automated system with proportional counter based absolute standard maintained in the laboratory. The activity concentrations determined using these two systems were in very good agreement; the new automated system can be used for absolute measurement of activity concentration of radioactive solutions.

Primary Standardization of 152Eu by 4πβ(LS) – γ (Nal) coincidence counting and CIEMAT-NIST method

The 4πβ-γ coincidence counting and CIEMAT/NIST liquid scintillation method were used in the standardization of a solution of 152Eu. In CIEMAT/NIST method, measurements were performed in a Liquid Scintillation Counter model Wallac 1414. In the 4πβ-γ coincidence counting, the solution was standardized using a coincidence method with ‘‘beta-efficiency extrapolation”. A simple 4πβ-γ coincidence system was used, with acrylic scintillation cell coupled to two coincident photomultipliers at 180° each other and NaI(Tl) detector. The activity concentrations obtained were 156.934 ± 0.722 and 157.403 ± 0.113 kBq/g, respectively, for CIEMAT/NIST and 4πβ-γ coincidence counting measurement methods.

Calibration of HPGe–HPGe coincidence spectrometer through performing standardisation of 125I activity by X-ray-gamma coincidence spectrometry using two HPGe detectors

An X-ray-gamma coincidence measurement method for efficiency calibration of a HPGe–HPGe system, using the methodology for activity standardisation of 125I, has been developed. By taking one list-mode time-stamped measurement of the 125I source, six spectra were generated in post-processing: total spectra, coincidence spectra and energy gated coincidence spectra for each of the two detectors. The method provides enough observables for source activity to be determined without a prior knowledge of the detector efficiencies. In addition, once the source is calibrated in this way the same spectra can also be used to perform efficiency calibration of the individual detectors in the low energy range. This new methodology for source activity determination is an alternative to the already established X-ray-(X-ray, gamma) coincidence counting method; with two NaI(Tl) detectors and the sum-peak method using a single HPGe detector. When compared to the coincidence counting method using two NaI(Tl) detectors, the newly developed method displays improved energy resolution of HPGe detectors combined with measurement of only full peak areas, without the need for total efficiency determination. This enables activity determination even in presence of other gamma emitters in the sample. Standard coincidence counting with NaI(Tl) detectors provides lower uncertainties. The method has been used for calibration of a coincidence HPGe spectrometer in the low energy range of 125I and fine adjustments of a Monte Carlo model of the coincidence system.

Disintegration rate and gamma-ray emission probability per decay measurement of Cu-64

The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN for the standardization of activity of 64Cu is described. The measurement was carried out in a 4π (PC) β-γ coincidence system. The activity per unit mass of the solution was determined by the extrapolation technique. The events were registered using a Time to Amplitude Converter (TAC) associated with a Multi-channel Analyzer. The gamma-ray emission probability per decay of the 1345.7keV transition of 64Cu determined with a calibrated REGe gamma-ray spectrometer was (0.472 ± 0.010) %, in agreement with the literature.

On neutron activation analysis with γγ coincidence spectrometry.

A new γγ coincidence system has been set up at NIST. It is operated with a digital data finder supported by new software developed at NIST. The system is used to explore possible enhancements in instrumental neutron activation analysis (INAA) and study applicability to neutron capture prompt gamma activation analysis (PGAA). The performance of the system is tested with certified reference materials for efficiency calibration and quantitative performance. Comparisons of INAA results based on conventional gamma-ray spectrometry data with INAA results based on coincidence data obtained from the same samples show improvements in the counting uncertainties and demonstrates the quantitative accuracy of the new system.

Technical Basis for the Use of a Correlated Neutron Source in the Uranium Neutron Coincidence Collar

Active neutron coincidence systems are commonly used by international inspectorates to verify a material balance across the various stages of the nuclear fuel cycle. The Uranium Neutron Coincidence Collar (UNCL) is one such instrument; it is used to measure the linear density of 235U (g 235U/cm of active length in assembly) in fresh light water reactor fuel in nuclear fuel fabrication facilities. The UNCL and other active neutron interrogation detectors have historically relied on americium lithium (241AmLi) sources to induce fission within the sample in question. Californium-252 is under consideration as a possible alternative to the traditional 241AmLi source. This work relied upon a combination of experiments and Monte Carlo simulations to demonstrate the technical basis for the replacement of 241AmLi sources with 252Cf sources by evaluating the statistical uncertainty in the measurements incurred by each source and assessing the penetrability of neutrons from each source for the UNCL.

Analysis of radioxenon and Krypton-85 at the BfS noble gas laboratory

Proportional counters and a β-γ coincidence system and their performance characteristics are described, with emphasis on internal and external quality controls. Typical limits of detection (LD) are 8mBqm−3 for 85Kr and 4mBqm−3 for 133Xe for the proportional counters, while for similar xenon volumes the LD for 133Xe in the β-γ system is approximately 4 times lower. Results of comparative analyses agree above the limit of quantification. Xenon-133 levels in southern Germany are presently at or below the LD of the proportional counters, but above the LD of the β-γ system, which is now routinely used for analysis of environmental samples.

Development and evaluation of a practical method to measure the Depth of Interaction function for a single side readout PET detector

In small animal and organ dedicated PET scanners, the knowledge of depth of interaction (DOI) of the gamma ray along the main axis of the scintillator is a fundamental information in order to avoid parallax error and to achieve high performances in terms of spatial resolution. Recently we developed a new method to obtain the DOI function for a single side readout PET module, recirculating the scintillation light in the matrix by means of a mirror placed on top of the module. In a complete PET scanner, periodical DOI calibrations have to be performed to prevent time dependent miscalibrations and performance degradations. The current DOI calibration relies on a coincidence system between the module and an external scintillator to provide a priori the DOI information and it is clearly not feasible in a real system without unpractical disassemblies of the scanner. In this paper we develop instead a fast and precise calibration method based on uniform irradiation of the scintillators. Three irradiation modalities are presented, in particular one where the source is placed on top of the module, one with the source placed on one side of the module and one that exploits the internal radioactivity of the scintillator. The three different procedures are evaluated and the calibration method is validated by comparing the information provided by the coincidence setup.