Stretchable scintillator composites for indirect X-ray detectors

Highlights

• Polymer-based scintillator materials have been developed based on Gd₂O₃:Eu³⁺ nanoparticles.

• The films exhibit high levels of stretchability with suitable performance.

• The composites can be used for the fabrication of large area stretchable X-ray detectors.

Abstract

Flexible and stretchable materials are increasingly being investigated for future technological platforms, polymer based materials being the most suitable candidates for those emerging technologies. This work reports on polymer based scintillator composites based on the thermoplastic elastomer Styrene-Ethylene/Butadiene-Styrene (SEBS) and Gd₂O₃:Eu³⁺ scintillator nanoparticles, to form a polymer-based flexible and stretchable material for X-ray indirect detectors. Further, visible light yield under X-ray irradiation was improved by the inclusion of 2,5 dipheniloxazol (PPO) and (1,4-bis (2-(5-phenioxazolil))-benzol (POPOP) within the polymer matrix. Together with high levels of stretchability, with deformations up to 100%, the films exhibit suitable performance with low mechanical hysteresis (less than 1.5 MJ/m³ for cycles up to 100% of strain) and reproducibly such as a scintillator material for the conversion of X-ray radiation into visible radiation. The decrease of just ∼13% of the X-ray radiation into visible light upon stretching up to 100% is attributed to a reduction of the effective filler concentration and proves the suitability of the developed materials for large area and stretchable X-ray radiation detectors.

Introduction

Flexible electronics is an increasingly growing technology as it allows the development of innovative products, opening new application possibilities [1]. One step further in this concept and a new paradigm for electronics is to achieve stretchable materials that are essential to create stretchable organic electronics, sensors and novel devices [2], [3].

In this context, characteristics such as moldable, flexible, printable and stretchable [4], [5] emerge as one of the most relevant technological research fields nowadays, aiming to improve the applicability and integration of electronic systems [4], [6]. Thus, stretchable electronics will allow applications in close contact to the human body and improve installation on curved interfaces [7], [8], among others. In the particular case of radiation detectors, stretchability will reduce the inconvenience caused by the rigid panels of conventional detectors and will allow the integration of sensors for medical monitoring and control on three dimensional architectures/configurations [9]. Stretchable radiation detectors will allow applications in the areas of security equipment and in large strain deformation sensors for medical imaging [10], [11].

Indirect X-ray detectors based on scintillators are a key engineering tool on industrial environment as they are widely used in many business and consumer products with ever increasing frequency in areas such as medicine and security, among others [12], [13], [14]. Traditional indirect X-ray detectors are limited by the scintillators critical properties such as efficiency, response time, light yield, surface roughness and for large area applications, low flexibility and cost [15], [16], [17]. Flexible nanocomposites have been prepared for the development of indirect radiation detectors by dispersing scintillating nanoparticles within a polymer matrix [18], [19]. The polymer matrix allows short scintillation time and low manufacturing costs when compared to current available scintillators [13]. These limitations have led to the search for novel materials [20], [21]. Among them, X-ray detectors from polymer-based scintillator composites have sparked large attention due to their scintillation decay time, thermal stability, flexibility, low cost and an easy fabrication in large areas [13], [22].

Thus, in order to develop flexible and stretchable materials able to convert X-ray radiation into visible radiation, with appropriate mechanical properties, showing thermally stability, chemical and radiation resistance and high energy resolution [22], this work reports on a novel composite based on gadolinium oxide doped with europium (Gd₂O₃:Eu³⁺) and fluorescence molecules 2,5-dipheniloxazol (PPO) and 1,4-bis-(2-(5-phenioxazolil))-benzol (POPOP) able to efficiently convert X-ray radiation into visible light [23], [24], dispersed into a thermoplastic elastomer polymer. Due to their atomic number, wide band gap (∼5.2 eV), high density (∼7.4 g cm⁻³), proper light yield (∼2 × 10⁴ photons. M eV⁻¹) and radioluminescence [25], Gd₂O₃:Eu³⁺ nanoparticles were selected as scintillators. Further, the overall visible light output in the X-ray to electrical conversion process was improved by incorporating the fluorescence molecules PPO and POPOP [26].

The polymer matrix was selected from the elastomer styrene-butadiene-styrene (SBS) family, as these materials show a chemical resistance with highly repeatable deformation [27]. However, applications of SBS in the biomedical area is not recommended due to the low biostability, associated to the double bonds of butadiene present in the elastomeric block [28], [29]. The SBS family are thermoplastic elastomers (TPE), show properties of both rubbers and thermoplastics. They have proven suitability for the development of composite materials for strain sensors applications due to their characteristic large deformation, high flexibility and good optical resistance [30], [31], [32]. Form this family of TPE, the co-polymer Styrene-Ethylene/Butylene-Styrene (SEBS) due to their lower butadiene ratio arises as an efficient and stable binder with high biostability, allowing the biomedical application [28]. Besides that, SEBS present an excellent radiation and temperature resistance, good mechanical properties and good optical properties [33], [34].

In this way, the main objective of this work is the development of an innovative generation of flexible and stretchable materials for large area and stretchable indirect X-ray detectors with applicability in areas such as security and healthcare.