Enhancement of the polydimethylsiloxane (PDMS) luminescence to develop a proton scintillator

Abstract

The polydimethylsiloxane (PDMS) luminescence, induced by MeV proton beams, has been investigated in the pure and doped polymer. Gold nanoparticles (AuNPs) and graphene oxide microparticles (GO$\mathrm{\mu }$Ps) have been employed at low concentration (0.1 wt%) to modify the PDMS properties. Measurements have demonstrated that AuNPs enhance the PDMS luminescence, while GO$\mathrm{\mu }$Ps quench the polymer luminescence. The first ones, embedded into PDMS, produce a visible luminescence whose intensity is proportional to the absorbed proton dose. A linearity between the luminescence intensity and the proton absorbed dose is observed up to about 2.5 kGy, while at higher doses a luminescence saturation region shows up. The polymer can be employed to monitor in vacuum the proton beam spot size and shape and as a plastic scintillator dosimeter with peculiar elastic properties and high biocompatibility, as it will be presented and discussed.

Introduction

Polydimethylsiloxane (PDMS) and its compounds are polymers which can be used as scintillator for ionizing radiation detection, as recent literature are reporting [1], [2].

Polydimethylsiloxane (PDMS) is a polymer belonging to the group of the silicone compounds [3]. Its chemical stoichiometry is CH₃ [Si(CH₃)₂O]${}_n$Si(CH₃)₃, where n is the number of repeating [SiO(CH₃)₂] monomer units. It has a density lower than water, of 0.965 g/cm³, high elastic properties and a high transparence to the visible radiation. It is chemically inert and has good gas (CO₂, He, ${\mathrm{O}}_2$, ${\mathrm{H}}_2$, ${\mathrm{H}}_2$O, ${\mathrm{N}}_2$,…) permeability [4]. It can be produced starting from liquid solutions, such as the Sylgard produced by Dow Corning [5]. It has high tensile modulus (1.8 MPa), high toughness (4.77 MPa), and high elongation (160%) [6]; it is resistant to oxidation,  can be used in combination with water and alcohol solvents without material deformation, solid PDMS will not allow aqueous solvents to infiltrate and swell the material. However, some solvents, such as chloroform, can diffuse into the material and cause swelling [7].

PDMS molecules have flexible polymer chains due to their siloxane linkages, which confer high level of viscoelasticity to the structure [8]. PDMS has a low elastic modulus which enables it to be easily deformed also by low stress, behaving like a rubber. It is hydrophobic, and thus its surface is liquid repellent [9]. Organic solvents can diffuse in PDMS generating swell [10].

The PDMS properties strongly depend on its method of preparation. It has two-part chemical (Part A as a base and Part B as a curing agent) containing the silicone base and a curing agent. The preparation depends on the ratio between the part A and B, mechanical stirrer velocity, used temperature and optimal timing [11].

The PDMS synthesis is carried using the Sylgard™ 184 Silicone Elastomer Kit, an oven to heat at approximately 100 °C and a vacuum system to eliminate the gas that develops by mixing the elastomer with the curing agent. Generally, the elastomer and the curing agent are mixed at the 10 parts to 1-part ratio (10:1) [5] for a few minutes and then left to degas in a vacuum oven for 15–60 min or more, until almost all bubbles disappear. PDMS is usually prepared by replica molding: the liquid prepolymer is poured into a container, which has a given shape and may also have raised structures on its surface, in which the hot solution is allowed to cool.

The PDMS applications concern many fields ranging from biomedicine to microelectronic, to realize prostheses [12], sensors and transducers [13], optical devices and lens [14].

PDMS changes its physical and chemical properties when: (i) it is mixed with other substances, (ii) it is covered by thin films, (iii) nanoparticles are embedded into it. The color, the optical properties, the elasticity, the hardness and wettability, the permeability and other parameters are strong dependent on the concentration of nanoparticles embedded into PDMS [15], [16], [17].

The ionizing radiation energy deposition in PDMS produces electron excitation and photon emission, giving it the characteristics of a plastic scintillator medium, which can be used for the radiation detection, as reported in the literature [18]. When an ionizing radiation hits an inorganic scintillator, the absorbed energy promotes the electrons across the gap into the conduction band, leaving the holes (i.e. positive charges) into the valence band. This creates electron–hole pairs which, when recombined, produce photon emission. The impurities help to enhance the visible light emission by introducing intermediate energy states within the energy gap. Upon recombination, the electrons will transit to these intermediate levels and then, falling back to the valence band, emit photons. The additive impurities permit to shift the emission wavelength to the desired value. Generally, the photon yield is amplified by using a photo-multiplier stage, which gives very high sensitivity to the scintillator detection system [19].

In this paper, we propose a method to prepare scintillation detectors for ionizing radiations. Preliminary results obtained for the proton beams detection exploiting the visible fluorescence of PDMS with added gold nanoparticles (AuNPs) or/and graphene oxide micrometric sheets (GO$\mathrm{\mu }$Ps) will be presented and discussed.