Elsevier

Journal of Luminescence

Volume 199, July 2018, Pages 225-231
Journal of Luminescence

Hydrothermal synthesis of CdWO4 for scintillator-polymer composite films development

https://doi.org/10.1016/j.jlumin.2018.03.056Get rights and content

Abstract

Composite films of CdWO4 embedded in polystyrene have been prepared and characterized through microtomography, photoluminescence and X-ray excited optical luminescence. CdWO4 scintillator powder was successfully produced via microwave-assisted hydrothermal method using oxide precursors. With the control of synthesis pH, CdWO4 wolframite structure was obtained at temperatures as low as 100 °C for reaction time of 1 h. Surface modification of CdWO4 particles by stearic acid promoted good dispersion of scintillator nanoparticles within the polymeric matrix. The composites have been prepared with 220 µm or 440 µm thickness and CdWO4/PS mass ratios of 10% and 20%. Optical measurements show the characteristic CdWO4 emission in the blue region as well as PS luminescence due to PPO, POPOP and PTO organic impurities. Under X-ray excitation, the light output was proportional to the increase of scintillator content and sample thickness. The emission intensity varied for each film face irradiated, being the most homogeneous condition achieved for the composite of 440 µm and CdWO4/PS = 10%.

Introduction

Scintillator compounds are continuously investigated for improving their potential applications which include medical image detectors, dosimetry and industrial inspection. Besides the features of efficiency, decay time and chemical stability, it is also important to achieve low costs of production especially when large areas need to be covered [1], [2]. In this sense, systems based on inorganic particles embedded in polymeric matrix are shown to be promising since they can combine the mechanical features of polymers with the optical properties of scintillators. These organic/inorganic composites have thus aroused great interest in recent years as inexpensive and efficient detectors for ionizing radiation [3], [4], [5], [6].

The use of a material with high effective atomic number Z will increase the stopping power, that is, the absorption of high energy radiation by the composite in comparison with the polymer without such particles [7], [8]. In many cases, energy transfer between these elements take place, with the luminescent properties of the composite determined by these processes. On the other hand, one of the main issues that need further attention is the control of the particles dispersion within the polymeric matrix. Particle clusters can act as scattering centres, which compromise the spatial resolution of the detectors, whereas low amounts of scintillator result in decrease of the luminescence efficiency [8], [9].

Cadmium tungstate (CdWO4) is a well-known scintillator due to its useful features such as high density (7.9 g/cm3), large Z (61.2) and efficient scintillation output (~40% of NaI:Tl). Owing to the difficulties to obtain CdWO4 as single crystals, many other synthesis routes have been developed to produce polycrystalline samples including sol-gel, sonochemical and hydrothermal [10], [11], [12], [13]. Up to our knowledge, however, few reports have explored the ability to prepare composites loaded with this compound [14]. The search for new scintillating films is relevant since the convenience of using as the primary radiation sensor in many detection systems, including synchrotron radiation facilities [15], [16].

In this work, scintillator composites have been prepared using CdWO4 and polystyrene (PS). The advantages of using PS consist on its low cost, good resistance to thermal and light deterioration, flexibility and high transparency over a large spectra range [1], [17]. The synthesis of CdWO4 powder employed a simple microwave-assisted hydrothermal method, which allowed the formation of single crystalline phase at temperatures as low as 100 °C. Our approach involves the surface modification of the particles in order to improve their compatibility with the polymeric matrix and achieve a higher homogeneity of the composite films [18].

Section snippets

Material and methods

For the synthesis of CdWO4 powder, a microwave-assisted hydrothermal method was developed. In a typical procedure, stoichiometric amounts of CdO (Vetec, 99.5%) and WO3 (Fluka, 99.9%) were homogenized in an agate mortar and placed into a Teflon autoclave filled with 100 ml of distilled water. Then, the autoclave was sealed and placed in the microwave system (Brazilian patent 2008-PI0801233-4) with frequency of 2.45 GHz and maximum power of 800 W. Reactions were conducted at 100 °C and 120 °C

Results and discussion

Fig. 1 shows the powder XRD of the samples prepared using pH 7.0, 8.0 and 9.0, at the temperature of 120 °C. One can observe that the oxide precursors reacted under hydrothermal conditions and CdWO4 was obtained at significantly lower temperature and faster than in the solid state synthesis [21], [22]. The pattern of the sample obtained at pH 7.0 presented a small amount of WO3 as secondary phase, due to Cd losses during the synthesis. It could be compensated using CdO excess to the

Conclusions

The single crystalline phase of CdWO4 obtained using oxide precursors under microwave-assisted hydrothermal conditions has been demonstrated for the first time. This low-cost method involves low temperatures and short synthesis times, and thus it was suitable to produce the scintillator powder employed as load of composite films. The surface modification of CdWO4 particles by stearic acid was efficient to improve the dispersion within the polystyrene matrix. Microtomography scans have shown

Acknowledgments

This work has been supported by the Brazilian funding agencies CNPq, CAPES, FINEP and FAPITEC. The authors are also grateful to the Brazilian Synchrotron Light Laboratory (LNLS) and the beamlines staff for helping with proposals TGM#20170460 and IMX#20160751.

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