Triple-to-double Coincidence Ratio System Research Articles

Primary standardization and half-life determination of [formula omitted]Ac at NRC

A solution of 225Ac was standardized by NRC using the triple-to-double coincidence ratio (TDCR) method. The counting efficiencies were calculated assuming a counting efficiency of 100% for alpha decays and those calculated using the MICELLE2 Monte Carlo code for beta decays and was approximately 500% for the NRC TDCR system. The relative uncertainty for the activity concentration was determined to be 0.25%. This agreed with measurements performed using gamma spectroscopy and a predicted calibration factor for the Vinten 671 ionization chamber as calculated using an EGSnrc model, implementing radioactive decay. Finally, the half-life of 225Ac was determined from long-term measurements using ionization chambers and liquid scintillation counting. The NRC measured half-life for 225Ac was found to be 9.914(4) days and is consistent within an expanded uncertainty coverage of k = 2 with the most recent (Kossert et al., 2020; Pommé et al., 2012) measurements of this decay parameter.

Development of FPGA-based standalone, portable TDCR system

A standalone, lightweight, portable, FPGA-based triple to double coincidence ratio (TDCR) system is developed. The optical chamber of the system is fabricated in a dual, coaxial cylindrical geometry, with the inner cylinder made up of Teflon and the outer one of aluminium. Three square PMTs, each having an active cathode area of 2.3 cm × 2.3 cm, are housed in the optical chamber. The variation in detection efficiency to identify the optimum kB (ionization quenching parameter) value is achieved by changing the vial position vertically with respect to the centre line of the PMTs. The coincidence analyzer implemented in a field programmable gate array (FPGA) transfers the pulse counts and associated parameters in real time to a Raspberry Pi single board computer (SBC). The TDCR algorithm to compute the efficiency is implemented in the SBC. In addition, the SBC is interfaced with a local 7” touchscreen to provide an intuitive graphical user interface (GUI) for operating the standalone instrument. The system performance is validated with 3H and 14C radionuclide standards.

Development of FPGA-based coincidence module for TDCR counting system

A new coincidence module for the Triple-to-Double Coincidence Ratio (TDCR) counting system has been developed at KRISS. The module is developed based on the Field-Programmable Gate Array (FPGA) to replace the MAC3, a well-known coincidence module developed by the LNHB for a TDCR system. A cRIO controller equipped with a Zynq-7020 FPGA from the National Instrument is used to develop the FPGA code that emulates the functionality of the MAC3. The performance of the new module was tested by comparing the output count rates of the new module with the output from the MAC3 module. A comparison in activity measurement of 3H and 14C was also performed between the two modules. A good agreement was observed with a difference of less than 1.0% between the new module and the MAC3 module, confirming the ability of the new module to replace the MAC3.

Preliminary measurements using a Triple to Double Coincidence Ratio (TDCR) Liquid Scintillator Counter System

The preliminary measurements using a Triple to Double Coincidence Ratio (TDCR) Liquid Scintillator Counter System developed by the Nuclear Metrology Laboratory (LMN) at IPEN, is presented. The TDCR system makes use of three photomultipliers positioned at 120° relative angle, operating in coincidence. For this preliminarymeasurement, 14C was selected to be standardized; 14C was chosen due to be a pure beta emitter with low end-point energy of 156 keV, which is suitable to be measured by the TDCR system. This solution was previously calibrated by the efficiency tracing technique using a 4p(PC)b-g coincidence system, employing 60Co as a tracer. The activity of the 14C solution by efficiency tracing technique was determined by using the extrapolation technique, changing the beta efficiency by pulse height discrimination. In order to determine the final activity, a Monte Carlo simulation was used to generate the extrapolation curve. The Software Coincidence System (SCS) developed by the LMN was used for both systems to register the events. MICELLE 2 code was used to calculate the theoretical TDCR efficiency. Measurements using HIDEX, a commercial liquid scintillator system, were also carried out and the results from the three methods were compared, showing a good agreement.

Set-up of a new TDCR counter at IRA-METAS

A triple-to-double coincidence ratio (TDCR) counter was recently constructed at IRA-METAS for liquid scintillation based primary activity standardisations. A description of its optical chamber, efficiency change tools, photomultiplier tubes (PMTs) and electronics is given. This TDCR system was validated by measuring several standard solutions of beta emitters including 45Ca, 14C, 63Ni and 3H. The activity concentrations, obtained from these measurements and efficiencies computed with a FORTRAN code we developed for symmetric and asymmetric PMTs, agree with the certified values within uncertainties.

A prototype of a portable TDCR system at ENEA

A prototype of a portable liquid scintillation counting system based on the Triple-to-Double Coincidence Ratio (TDCR) technique was developed at ENEA-INMRI in the framework of the European Metrofission project. The new device equipped with the CAEN digitizers was tested for the activity measurements of pure β-emitters (99Tc and 63Ni). The list-mode data recorded by the digitizers were analyzed by software implemented in the CERN ROOT environment, which allows the application of pulse shape discrimination using the new device.

Activity determination of 59Fe

Iron-59 was measured in three commercial and two custom-built liquid scintillation counters. The counting efficiencies were determined using CIEMAT/NIST efficiency tracing and the triple-to-double coincidence ratio (TDCR) method, respectively.The efficiency computation for the TDCR method was realized by means of the MICELLE2 program, applying a stochastic model for the computation of electron emission spectra. The program was extended to make calculations of spectra originating from complex decay schemes possible. In addition, a new parameterization of electron stopping powers for 10 commercial liquid scintillation cocktails was included in the software.The activities determined with the two methods were in very good agreement; the relative standard uncertainty of the combined result was found to be 0.16%. It was used to calibrate a 4π ionization chamber at PTB for future calibrations of this isotope which is used for investigations of iron metabolism.A standardized solution was submitted to the Bureau International des Poids et Mesures (BIPM) to be measured in the ionization chambers of the International Reference System (SIR) for comparison purposes.The liquid scintillation samples were also measured in a new portable TDCR system with three channel photomultipliers. Although this system has a much lower counting efficiency, the activity was in satisfactory agreement with the conventional TDCR system. The usage of the portable TDCR system, thus, provides an important test of the free parameter model.

Activity determination of 229Th by means of liquid scintillation counting

Liquid scintillation measurements of 229Th in radioactive equilibrium with its progenies were carried out. The counting efficiency was determined by means of a free parameter model. The measurements were made in a custom-built triple-to-double coincidence ratio (TDCR) system. In addition, a new portable mini-TDCR system with three channel photomultipliers and two commercial counters were tested. The decay of the short-lived 213Po requires great care, since it often occurs during the dead time of the counter systems. Also the rather short-lived 217At may decay during the dead time caused by 221Fr decay events. The overall counting efficiency of the TDCR system of PTB was found to be about 700% (depending on the degree of chemical quenching) and the relative standard uncertainty of the activity concentration was found to be about 0.23%. The determined activity concentration was compared with the outcome of alpha spectrometry under defined solid angle and excellent agreement was found. The TDCR efficiency calculations can be easily adapted to activity determinations of 225Ra or 225Ac in equilibrium with their progenies.

A miniature TDCR system dedicated to in-situ activity assay

In the framework of the European Metrology Research Programme (EMRP), the Joint Research Project MetroFission has a dedicated work package for the development of a portable Triple-to-Double-Coincidence-Ratio (TDCR) system dedicated to in-situ activity measurements of low-energy beta emitters arising from the operation of the next generation of nuclear power plants. In the design phase of the NPL version of the mini-TDCR, a wide range of metrological aspects and detector types was considered. This paper summarizes these aspects of design, in the light of previous experience with the primary TDCR system at NPL. For example, in this miniature version of the TDCR, the optical chamber was simplified and cylindrical geometry was deemed sufficient. The reflectivity of the surface was increased by painted layers of Spectraflect®, a specially formulated barium sulphate coating with high reflectivity across a wide range of wave lengths including UV. This option was chosen rather than the high performing and more expensive Spectralon® material used for the primary NPL TDCR. The miniature TDCR system is intended for on-site monitoring and will not require as high a performance as the primary system. Other factors that were considered included sample changing, light tightness, type of photo detector, method for varying the detection efficiency, shielding and the possible addition of an internal gamma-ray source for determination of the quench parameter of the source. In this version, the sample changing is performed using a piston and an automatic shutter. Significant design effort has been applied to ensure minimal ingress of light from the piston. Efficiency variation is accomplished by increasing the vertical displacement of the vial. Provision has been made to automate this at a later stage. Maximum light transmission to the photo-multiplier tubes is obtained at the “zero” reference height. Validation measurements were successfully performed using four different radionuclides: 3H, 241Pu, 63Ni, and 99Tc.

Activity standardisation of 45Ca and 204Tl using the new TDCR system at CMI

A new triple-to-double coincidence ratio (TDCR) system has been established at CMI as an additional technique for primary activity standardisation. Details of the detectors and the electronics are given. Construction of the optical chamber was validated using measurement of a 3H standard—the efficiency achieved with Ultima Gold was 58%. Several methods of efficiency variation (coloured bands painted on vial, positioning and photomultipliers defocussing) are compared. Activity concentrations of 204Tl and 45Ca were determined with the TDCR method and with the efficiency tracer technique. Advantages of the TDCR method compared to the efficiency tracing method and agreement between results are shown.

Measurements of some radionuclides using a new TDCR system and an ultra low-level conventional LSC counter in CPST, Lithuania

Beta-emitters 36Cl and 63Ni, and the electron capture radionuclide 55Fe were used to calibrate a secondary standardization instrument Quantulus-1220TM. An external standard technique was applied to assess the quenching level of samples. Nitromethane as an artificial quencher was used for sample preparation at different quenching levels. The dependence of the counting efficiency on the quenching level was determined. Samples of beta-emitters were re-measured with the triple-to-double coincidence ratio (TDCR) primary standardization instrument. The specific activities of beta-emitters were determined with the help of the software TDCRB-02 and compared with known values. It was found that for 36Cl and 63Ni the discrepancy was 0.9% and 2.7%, respectively, within the specified uncertainty limits. A new sample with 90Sr was prepared and measured with the TDCR device. In this case, the divergence from the expected value was 1.7%. Results suggest the suitability of the TDCR techniques for low-level counting.

Application of a free parameter model to plastic scintillation samples

In liquid scintillation (LS) counting, the CIEMAT/NIST efficiency tracing method and the triple-to-double coincidence ratio (TDCR) method have proved their worth for reliable activity measurements of a number of radionuclides. In this paper, an extended approach to apply a free-parameter model to samples containing a mixture of solid plastic scintillation microspheres and radioactive aqueous solutions is presented.Several beta-emitting radionuclides were measured in a TDCR system at PTB. For the application of the free parameter model, the energy loss in the aqueous phase must be taken into account, since this portion of the particle energy does not contribute to the creation of scintillation light. The energy deposit in the aqueous phase is determined by means of Monte Carlo calculations applying the PENELOPE software package. To this end, great efforts were made to model the geometry of the samples. Finally, a new geometry parameter was defined, which was determined by means of a tracer radionuclide with known activity. This makes the analysis of experimental TDCR data of other radionuclides possible. The deviations between the determined activity concentrations and reference values were found to be lower than 3%.The outcome of this research work is also important for a better understanding of liquid scintillation counting. In particular the influence of (inverse) micelles, i.e. the aqueous spaces embedded in the organic scintillation cocktail, can be investigated. The new approach makes clear that it is important to take the energy loss in the aqueous phase into account. In particular for radionuclides emitting low-energy electrons (e.g. M-Auger electrons from 125I), this effect can be very important.

Validation of a new TDCR system at NPL

A new triple to double coincidence ratio (TDCR) system has been established at NPL. The system incorporates a spherical optical chamber, a manual sample changing facility and an integral light-tight housing. A 6 in diameter NaI(Tl) detector has been incorporated to allow 4pibeta-gamma coincidence measurements to be performed in parallel. Details of the detectors, the electronics and the overall TDCR system are given. Validation measurements using suitable low-energy beta and electron capture nuclides, i.e. (3)H and (55)Fe have been performed. The highest efficiency achieved with a (3)H solution in Ultima Gold AB in a glass vial was 53% and in a high-efficiency LS cocktail 65%. This indicates that the optical chamber is performing well. After varying the efficiency by de-focusing the PM tubes, the activity of the sources was determined. The activity concentration determined with TDCR agreed with certified values within the range of uncertainties. Further results from validation measurements and the corresponding uncertainty budgets are presented.

Activity standardization of 3H with the new TDCR system at PTB

A triple-to-double coincidence ratio (TDCR) liquid scintillation counting system (Pochwalski et al., 1988; Grau Malonda and Coursey, 1988) was set up at the Physikalisch-Technische Bundesanstalt (PTB) to be used as an additional technique for primary activity standardization. The optical chamber of the system was designed in our laboratory, expending great effort to achieve maximum light transmission to the photomultiplier tubes (PMT) and, consequently, a maximum counting efficiency and low uncertainties. A programme to compute the counting efficiencies and to analyse the measurement data was developed and tested. The software can also be used to allow for PMT asymmetries. The system and recently developed analysis procedures were used for the activity standardization of a tritiated water solution within the scope of an international comparison under the auspices of the Bureau International des Poids et Mesures (BIPM).

Construction and implementation of a TDCR system at ANSTO

A triple-to-double coincidence ratio (TDCR) liquid scintillation counting system has been recently constructed at Australian Nuclear Science and Technology Organization (ANSTO). A description of the system and measured activities for sources such as 3H, 14C, and 241Am are presented.