Application of the CIEMAT–NIST method to plastic scintillation microspheres

https://doi.org/10.1016/j.apradiso.2014.12.026Get rights and content

Highlights

  • CN method has been used to the measurement of β emitters with PSm.

  • MICELLE2 has been adapted to simulate particle quenching energy losses in PSm.

  • Activity is dependent on chemical quenching and on geometry used for simulation.

  • Deviations are below 10% for 63Ni and below 3% for 14C, 36Cl and 90Sr/90Y.

Abstract

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 3H 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 63Ni, 14C, 36Cl and 90Sr/90Y 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.

Introduction

The use of Plastic Scintillation microspheres (PSm) is becoming an alternative to Liquid Scintillators (LSs) for the analysis of alpha- and beta-emitting radionuclides. PSm are a solid mixture of fluorescent solutes in a polymer and have diameters of between tens and hundreds of micrometres (Santiago et al., 2013). The preparation of counting samples with PSm is very similar to the procedure followed with LS, and the same counters and vials can be used. When using PSm, detection efficiencies for high-energy beta emitters (i.e. 90Sr/90Y, 36Cl) are very similar to those obtained with LS, whereas relatively small differences are found for medium-energy beta emitters (i.e. 14C) and alpha emitters, and large differences are obtained for low-energy beta emitters (i.e. 3H) (Tarancón et al., 2004). The use of PSm avoids the production of mixed waste when measuring (EPA, 2001; Tahnassian et al., 1991), because the organic solvent is substituted by a polymer that is completely polymerized and cannot degrade by itself. Other advantages come from the solid form of PSm, because PSm can be used in continuous detectors (Grate et al., 2008, Tarancón et al., 2005) and as an extractive scintillation resin (Bagán et al., 2012, Bagán et al., 2011, Grogan and Devol, 2011).

Given the similarities in the composition of plastic scintillators and LS, the mechanism involved in the detection process should be similar. When measuring with LS, three quenching processes (i.e. ionization quenching, chemical quenching and colour quenching) are responsible for almost all energy losses, and the effect caused by these quenching phenomena can be modelled theoretically or experimentally (Birks, 1964; L’Annunziata, 2013). An accurate description of these processes and the detection model, together with some experimental data, is the basis of the tracing methods CIEMAT–NIST (CN) and Triple to Double Coincidence Ratio (TDCR), which can be used as standardization methods for solutions of beta, beta-gamma and electron-capture radionuclides (Broda et al., 2007).

For measuring with PSm, other quenching phenomena in addition to the quenching processes mentioned above may be considered as the cause of the low detection efficiency observed for 3H-like radionuclides. Previous studies using PSm have described two additional quenching phenomena: the loss of energy of the beta or alpha particles into the aqueous solution before the particle reaches the microsphere (i.e. particle quenching) and the inefficient transmission of the scintillating photons in the heterogeneous medium formed by the aqueous solution and polymeric microspheres (i.e. optical quenching) (Santiago et al., 2013).

As already noted, tracing methods can be used not only for radionuclide standardization but also to confirm the mechanism involved in the detection process. In one study, for example, solutions of pure beta emitters were measured using PSm in a TDCR detector (Sanz and Kossert, 2011). The model used to compute the detection efficiency was highly dependent on the description of the geometry and the PSm packing, and consequently an extra tracing radionuclide, 63Ni, was needed to determine the exact PSm packing. Given this approach, the measurement of pure beta emitters was performed with deviation lower than 2.5%, giving validity to the proposed model.

However, use of the TDCR method requires a counter with three photomultipliers, which is not available to all laboratories. Moreover, the use of 63Ni as a tracer makes it impossible to confirm whether the model is valid for low-energy beta emitting radionuclides (like 3H). For these reasons, it would be interesting to evaluate application of the CN tracing method to PSm samples using 3H as a tracing radionuclide in a commercial detector. The CN tracing method is based on computation of the ionization quenching in order to determine the dependence between the detection efficiency and a free parameter. In addition, samples of the tracing radionuclide with increasing amounts of quenching agent are measured in a commercial detector to obtain the correlation between the detection efficiency and a quenching parameter. The measured detection efficiency of the tracing radionuclide is then related to the measured quenching parameter, which is in turn related to a computed efficiency for the sample radionuclide through the free parameter (Grau Malonda, 1999). In the case of PSm, the computation of the detection efficiency must include the estimation of particle quenching. Of the different codes described for application of the CN tracing method, MICELLE2 is the most appropriate because it includes the simulation of the micelle effect in the nanomicelles formed in the liquid scintillation mixture (Grau Carles, 2007, Kossert and Grau Carles, 2010).

The aim of this study is to develop a methodology based on the CN tracing method that would be able to predict the detection efficiencies for beta emitters when measuring with PSm. Such a methodology may be of interest for in-situ calibration of sensors in remote positions and should also improve our understanding of the detection mechanism when using PSm, which could have an impact on PSm applications.

Section snippets

Reagents

Stock solutions used were: 3H solution (3H2O) at a concentration of 3.94(5) kBq/g in deionized water prepared from a standard of 69.8(8) kBq/g, provided by Eckert-Ziegler (Berlin, Germany); a 63Ni solution of 3.868(58) kBq/g prepared from a standard of 397.4(59) kBq/g, from Eckert-Ziegler, in a carrier solution of 30 µg/g of NiCl2 in 0.1 M HCl; a 14C solution (labelled glucose) of 0.1324(17) kBq/g prepared from a standard of 44.70(56) kBq/g, from CERCA/LEA (Pierrelatte, France) in a carrier solution of

Efficiency computation

In the codes used for the CN tracing method, the detection efficiency is obtained by computing the ionization quenching function for beta particles of different energies (defined by the spectrum of the radionuclide) with respect to a kB parameter (which depends on the scintillator) and variable values of the free parameter. This methodology considers that for a given radionuclide in a given scintillation medium, the energy losses are due to ionization quenching, whereas other quenching factors

Activity calculation by the CN tracing method

Once the computation of the detection efficiency by the CN tracing method had been adapted for measurement with PSm, this method was used to perform the activity calculation of 63Ni, 14C, 36Cl and 90Sr/90Y solutions using a 3H standard solution as a tracing radionuclide. The variation in 3H detection efficiency was achieved by using increasing amounts of a chemical quencher (i.e. nitromethane), as is usually done in the field. Fig. 5 shows the variation in detection efficiency relative to the

Conclusions

The CN tracing method has been adapted for determining the activity of beta-emitting radionuclide solutions with PSm. Using 3H as the tracing radionuclide, deviations obtained for high-energy beta emitters are acceptable. However, deviation is higher for low-energy beta emitters, such as 63Ni, probably due to the sensitivity of the model to the geometry and to a non-complete description of the phenomena involved in the detection.

In spite of the deviations observed, the proposed model of

Acknowledgements

The authors wish to thank the Ministerio de Ciencia e Innovación (Spain) (MICINN) and the Agència de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) for financial support under CTM2011-27211 and 2009-SGR-1188, respectively. In addition, the authors are indebted to A.C. Grau for adapting the MICELLE2 code to measurements made with plastic scintillation microspheres, and to K. Kossert for assistance and teaching on free parameter methods.

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