A. Tarancón

Mixed waste reduction in radioactivity determination by using plastic scintillators

Abstract In this work, we tested whether plastic scintillation (PS) is a suitable alternative to liquid scintillation (LS) and Cerenkov techniques for beta emitter detection. The main advantage of this alternative is the reduction of mixed waste produced as consequence of the measurement process. In addition, the quality parameters obtained with PS are as reliable as those obtained with LS and allow determination, with a relative error 90 Sr / 90 Y in low level activity aqueous samples. Gamma and alpha emitters were also measured with plastic scintillators using linear and logarithmic amplified scintillation detectors.

Radiostrontium separation and measurement in a single step using plastic scintillators plus selective extractants. Application to aqueous sample analysis.

This study describes a new protocol for (90)Sr determination in water samples based on the use of a selective extractant (DtBuCH18C6) and plastic scintillator microspheres. The proposed procedure unifies chemical separation and sample measurement preparation in a single step to reduce the effort, time and reagents required for analysis. In addition, the final measurement does not produce mixed waste. The minimum activity detectable for 10 mL of sample solution is 0.46 Bq L(-1). Relative errors for the determination of (90)Sr activity in drinking, sea and river waters are less than 4%.

Plastic vs. liquid scintillation for 14C radiotracers determination in high salt matrices.

The use of radiotracers is a common procedure for better understanding of the dynamics of oil reservoirs. Several molecules and radionuclides are implemented for this purpose; one of these is (14)C labelled thiocyanate. Samples taken from fluids in production wells require a pre-treatment step in order to purify and concentrate the activity of the radiotracer before measurement. Pre-treatment is based on ionic exchange and the solution eluted includes a high concentration of salt that can be a serious drawback for the development of a stable emulsion when the liquid sample and the scintillation cocktail are mixed for Liquid Scintillation (LS). The objective of this study is to evaluate the capability of Plastic Scintillation (PS) to determine the activity of radiotracers in salt matrices. For this purpose, an initial comparative study of the influence of salt and coloured matrices on the detection efficiency for PS and LS was performed. Results indicate that in both techniques colour quenching produces the same effects: efficiency decrease and spectra shift, whereas salt quenching produces different effects for PS and LS, ionization quenching for PS and chemical quenching for LS. As a result of this divergence, the calibration required for each counting method is different: two steps for PS and a single step for LS. After system study and procedure optimization, radiotracer activities in real samples taken from oil reservoirs have been determined. The results indicated that PS shows similar capability to the classical LS to determine the activity in these salt matrix samples with the additional advantages that no separation of the phase can appear and no mixed waste is produced after measurement.

Determination of oil reservoir radiotracer (S14CN-) in a single step using a plastic scintillator extractive resin.

The analysis of radiotracers is important in the study of oil reservoir dynamics. One of the most widely used radiotracer is S(14)CN(-). Prior to activity measurements by Liquid Scintillation (LS), routine determinations require the pretreatment steps of purification and concentration of the samples using anion exchange columns. The final elution media produces samples with high salt concentration that may lead to problems with phase separation during the LS measurement. Plastic Scintillation (PS) is an alternative technique that provides a solid surface that can be used as a platform for the immobilisation of selective extractants to obtain a PS resin. The proposed procedure unifies chemical separation and sample measurement preparation in a single step, serving to reduce the number of reagents needed and manpower required for the analysis while also avoiding mixed waste production by LS. The objective of this study is to develop a PS resin for the determination of (14)C-labelled thiocyanate radiotracer in water samples. For this purpose, the immobilisation procedure was optimised, including optimisation of the proportion of PS microspheres:extractant and the use of a control blank to monitor the PS resin immobilisation process. The breakthrough volume was studied and the detection and quantification limits for 100 mL of sample were determined to be 0.08 Bq L(-1) and 0.31 Bq L(-1), respectively. The established procedure was applied to active samples from oil reservoirs and errors lower than 5% in the sample determinations were obtained.

Alpha/beta pulse shape discrimination in plastic scintillation using commercial scintillation detectors.

Activity determination in different types of samples is a current need in many different fields. Simultaneously analysing alpha and beta emitters is now a routine option when using liquid scintillation (LS) and pulse shape discrimination. However, LS has an important drawback, the generation of mixed waste. Recently, several studies have shown the capability of plastic scintillation (PS) as an alternative to LS, but no research has been carried out to determine its capability for alpha/beta discrimination. The objective of this study was to evaluate the capability of PS to discriminate alpha/beta emitters on the basis of pulse shape analysis (PSA). The results obtained show that PS pulses had lower energy than LS pulses. As a consequence, a lower detection efficiency, a shift to lower energies and a better discrimination of beta and a worst discrimination of alpha disintegrations was observed for PS. Colour quenching also produced a decrease in the energy of the particles, as well as the effects described above. It is clear that in PS, the discrimination capability was correlated with the energy of the particles detected. Taking into account the discrimination capabilities of PS, a protocol for the measurement and the calculation of alpha and beta activities in mixtures using PS and commercial scintillation detectors has been proposed. The new protocol was applied to the quantification of spiked river water samples containing a pair of radionuclides ((3)H-(241)Am or (90)Sr/(90)Y-(241)Am) in different activity proportions. The relative errors in all determinations were lower than 7%. These results demonstrate the capability of PS to discriminate alpha/beta emitters on the basis of pulse shape and to quantify mixtures without generating mixed waste.

Synthesis of plastic scintillation microspheres: alpha/beta discrimination.

Plastic scintillation microspheres (PSm) have been developed as an alternative for liquid scintillation cocktails due to their ability to avoid the mixed waste, besides other strengths in which the possibility for alpha/beta discrimination is included. The aim of this work was to evaluate the capability of PSm containing two combinations of fluorescence solutes (PPO/POPOP and pT/Bis-MSB) and variable amounts of a second organic solvent (naphthalene) to enhance the alpha/beta discrimination. Two commercial detectors with different Pulse Shape Discrimination performances (Quantulus and Triathler) were used to evaluate the alpha/beta discrimination. An optimal discrimination of alpha/beta particles was reached, with very low misclassification values (2% for beta particles and 0.5% for alpha particles), when PSm containing PPO/POPOP and between 0.6 and 2.0 g of naphthalene were evaluated using Triathler and the appropriate programme for data processing.

Crosslinked plastic scintillators: a new detection system for radioactivity measurement in organic and aggressive media.

The measurement of radioactive solutions containing organic or aggressive media may cause stability problems in liquid and plastic scintillation (PS) techniques. In the case of PS, this can be overcome by adding a crosslinker to the polymer structure. The objectives of this study are to synthesise a suitable crosslinked plastic scintillator (C-PS) for radioactivity determination in organic and aggressive media. The results indicated that an increase in the crosslinker content reduces the detection efficiency and a more flexible crosslinker yields higher detection efficiency. For the polymer composition studied, 2,5-diphenyloxazole (PPO) is the most adequate fluorescent solute and an increase in its concentration causes little change in the detection efficiency. The inclusion of a secondary fluorescent solute 1,4-bis-2-(5-phenyloxazolyl) benzene (POPOP) improves the C-PS radiometrical characteristics. For the final composition chosen, the synthesis of the C-PS exhibits good reproducibility with elevated yield. The obtained C-PS also displays high stability in different organic (toluene, hydrotreated vegetable oil (HVO) and methanol) and aggressive media (hydrochloric acid, nitric acid and hydrogen peroxide). Finally, the C-PS exhibits high detection efficiency both in water and in aggressive media and can also be applied in organic media showing similar or even higher detection efficiency values.

Pilot Study of the Application of Plastic Scintillation Microspheres to Rn-222 Detection and Measurement

This work demonstrates that when plastic scintillation microspheres (PSms) synthesized from polystyrene are exposed to 222Rn in air or water, they concentrate 222Rn in their volume. A theoretical model, based on the solution of the diffusion equation is developed, in order to describe the 222Rn sorption and desorption processes in PSms. A macroscopic quantity termed “sampling efficiency” is used to quantify the absorption properties of the PSms. The sampling efficiency is estimated by exposure of PSms to 222Rn in air and water. Estimates of the counting efficiency for measurement of the exposed PSms by commercial scintillation counters are presented. The results show that polystyrene PSms are simultaneously good samplers and detectors of 222Rn. These properties indicate that sensitive approaches for the measurement of 222Rn based on absorption and scintillation counting of PSms can be developed.

Mixture quantification using PLS in plastic scintillation measurements.

This article reports the capability of plastic scintillation (PS) combined with multivariate calibration (Partial least squares; PLS) to detect and quantify alpha and beta emitters in mixtures. While several attempts have been made with this purpose in mind using liquid scintillation (LS), no attempt was done using PS that has the great advantage of not producing mixed waste after the measurements are performed. Following this objective, ternary mixtures of alpha and beta emitters ((241)Am, (137)Cs and (90)Sr/(90)Y) have been quantified. Procedure optimisation has evaluated the use of the net spectra or the sample spectra, the inclusion of different spectra obtained at different values of the Pulse Shape Analysis parameter and the application of the PLS1 or PLS2 algorithms. The conclusions show that the use of PS+PLS2 applied to the sample spectra, without the use of any pulse shape discrimination, allows quantification of the activities with relative errors less than 10% in most of the cases. This procedure not only allows quantification of mixtures but also reduces measurement time (no blanks are required) and the application of this procedure does not require detectors that include the pulse shape analysis parameter.

Application of the CIEMAT-NIST method to plastic scintillation microspheres.

An adaptation of the MICELLE2 code was used to apply the CIEMAT-NIST tracing method to the activity calculation for radioactive solutions of pure beta emitters of different energies using plastic scintillation microspheres (PSm) and (3)H as a tracing radionuclide. Particle quenching, very important in measurements with PSm, was computed with PENELOPE using geometries formed by a heterogeneous mixture of polystyrene microspheres and water. The results obtained with PENELOPE were adapted to be included in MICELLE2, which is capable of including the energy losses due to particle quenching in the computation of the detection efficiency. The activity calculation of (63)Ni, (14)C, (36)Cl and (90)Sr/(90)Y solutions was performed with deviations of 8.8%, 1.9%, 1.4% and 2.1%, respectively. Of the different parameters evaluated, those with the greatest impact on the activity calculation are, in order of importance, the energy of the radionuclide, the degree of quenching of the sample and the packing fraction of the geometry used in the computation.