Effect of UVC radiation on the optical properties of thermally treated CR-39 polymer films: A new approach for the use of CR-39 as an optical dosimeter

https://doi.org/10.1016/j.radphyschem.2020.109253Get rights and content

Highlights

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    UVC and thermal treatments combined as a new UV detection approach for CR-39 polymer films.

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    The UVC and thermal treatments linearly increase the absorptivity for the UVC compared to the other treatments.

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    A great enhancement in the values of carbonaceous cluster M and N and a continuous steadily decrease in the Egd and Egind values.

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    The absorptivity for the short-wavelength UV radiation does alter with the order of treatments.

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    This new approach can potentially enable the estimates of UV dose equivalent.

Abstract

A new approach combining UV and thermal treatments has been studied for improved UV dosimetry of CR-39 polymeric detector films. Prior to thermal annealing, the detector films were exposed to radiation from a UV lamp (250–263 nm) at a distance of 1 cm for different durations of 10, 20, 40, 60, 80, and 100 h. Reversing the sequence, other detector films were first thermally treated in a controlled oven at 160 °C for 1 h and then subjected to UV exposure as above. The qualitative impacts of these treatments have been analyzed by means of ultraviolet/visible (UV/Vis) spectrophotometry in the range 190–450 nm, considering optical properties such as absorbance, band-gap energy, and Urbach energy. As controls, CR-39 detector films subjected only to UV treatment and a pristine film were also examined. The obtained results revealed that the methods combining UV and thermal treatments enhanced the absorptivity for short-wavelength UV radiation, depending on the order of the treatments. The pre-treated films (UV exposure followed by thermal annealing) permitted excellent estimation of UV exposure times, as a proxy of UV dose, over the range 10–100 h.

Introduction

Polyallyl diglycol carbonate (PADC), commonly known by the brand name CR-39, has been proven to be the most highly sensitive polymeric solid-state nuclear track detector (PSSNTD) (Cartwright et al., 1978). PADC is transparent in the visible spectral region, and has the chemical composition C12H18O7 (density 1.30 g/cm3). PADC can register tracks from charged particles with high resolution. Thus, protons and heavy ions produce latent damage trails, which can be visualized after chemical treatment with a suitable etchant solution. PADC can also register tracks from recoil protons under impact of neutrons or alpha particle emissions from naturally occurring radioactive materials (NORM). It can thus be used in neutron spectrometry and dosimetry (Castillo et al., 2013; Sahoo et al., 2014; Vasconcelos et al., 2019; Saad et al., 2020) and radon dosimetry (Mozzo et al., 1996; Tommasino, 2001; Saad et al., 2013, 2014a, 2018). The properties of polymers can be altered in various ways, such as annealing (Nidhi Gupta et al., 2009; Saad et al., 2012, Saad et al., 2014b); exposure to IR radiation (Prasher and Singh, 2003; Singh and Prasher, 2004; Saad et al., 2019), UV radiation (Saad et al., 2015; Jaleh et al., 2017), gamma-rays (Malek and Chong, 2002; Singh and Prasher, 2004; Saad et al., 2005; Abdul-Kader et al., 2014), alpha particles or heavy ions (Saad et al., 2001; Ishikawa et al., 2008; El Ghazaly and Hassan, 2014); chemical doping (Miyoshi et al., 2015, 2016); or ion implantation (Butt et al., 2014; Chawla et al., 2019).

We have applied ultraviolet/visible (UV/Vis) spectrophotometry to study radiation damage trails for the detection of gamma-rays in CR-39 nuclear track detectors (Saad et al., 2005). Recently, we have used this detector as a gamma dosimeter (Saad et al., 2018, Saad et al., 2020), addressing the question as to whether a CR-39 nuclear track detector might also be used as an optical dosimeter in the UV region following thermal treatment (Khalil, 2019). It has been shown that thermal treatment of CR-39 polymeric detector material, heating it above its glass transition temperature (Tg), can modify its morphology and improve its performance (Nidhi et al., 2009; Gendya, 2019.). Thus, modifications within CR-39 annealed polymeric detectors can have a beneficial effect on their registration and structural properties, and especially their optical properties (Nidhi et al., 2009; Saad et al., 2012, Saad et al., 2014a, Saad et al., 2014b, Saad et al., 2020, Saad et al., 2020). A CR-39 detector consists of repeated units of PADC. PADC consists of two adjacent carbonate ester groups separated by an ether chain. The ether chain can be considered as a radiation-sensitive section, whereas the carbonate ester groups are less radiation-sensitive (Yamauchi et al., 2005a, 2005b, 2008; Ogawara et al., 2020). The structure of PADC is a three-dimensional network consisting of polyallyl chains (Stejny, 1987). Two ethylene units are interposed between the ether and carbonate esters, which can also be regarded as radiation-sensitive. Our UV/Vis studies have indicated that complex compounds are generated as a result of cleavage of the main chain of the polymer material. Clearly, chain rupture increases with increasing gamma-ray dose level for CR-39. A shift in the absorption edge towards higher wavelength indicates a decrease in the band-gap energy for irradiated detectors (Saad et al., 2005, 2018). Our UV/Vis observations suggested that a CR-39 polymer detector could enable determination of a gamma-radiation dose (Saad et al., 2018). Further measurements by FTIR spectroscopy indicated that characteristic functional groups with vibrational bands, such as carbonate ester and ether, in CR-39 polymeric detector material are significantly damaged by different gamma-ray doses ranging from 500 to 3000 kGy (Nwara, 2018). In that study, we delineated distinct mechanisms for the destruction of ether and carbonate groups by gamma-radiation, with ether groups being much more sensitive. Recently, we carried out measurements on thermally treated CR-39 exposed to relatively low gamma-ray doses of up to 1000 kGy, applying UV/Vis spectroscopy (Khalil, 2019; Saad et al., 2020). This study yielded a new approach, introducing a thermal treatment (gamma exposure post-annealing) to obtain a unique dosimeter for gamma-radiation doses ranging from 200 to 1000 kGy.

On the other hand, when PADC polymer (a form of CR-39) is exposed to UVC radiation of sufficient energy (111.28 kcal mol−1) as an ionizing radiation, in the presence of oxygen, a free radical chain is formed as a result of chemical bond cleavage (Adams and Garton, 1993). A mechanism of photo-oxidative degradation (initiation, propagation, termination, and chain branching) ensues. Chemical changes can be attributed to scission of the polymer chains and formation of cross-linkages. Photo-oxidation of CR-39 detectors mainly involves chain scission (Tse et al., 2006; Saad et al., 2015). From the point of view of polymer degradation as a result of UV exposure, we would answer the above question by stating that extension of this approach to UV radiation detection is indeed promising. The aim of this work was to investigate the effects of UV irradiation for different durations on a CR-39 polymeric detector material, with and without thermal treatment, using a UV/Vis spectrophotometer. Finally, we sought to identify the optimum conditions for using CR-39 as a UV radiation-recording polymer with unique resolution.

Section snippets

Materials and methods

CR-39 polymer sheets with a thickness of 750 μm were purchased from Track Analysis Systems Ltd. (TASL), Bristol, UK. The sheets were cut with a laser beam into identical pieces of dimensions 1.5 cm × 2 cm.

Three sets of detectors were prepared for this study. The first set of CR-39 detectors was only exposed to UV radiation from a lamp. The second set of detectors was first annealed in a digital oven and then exposed to UV radiation, i.e. pre-UV thermal annealing (thermal annealing + UV

Measurement of UV/Vis spectra of non-thermally treated and thermally treated UV-radiation exposed CR-39 detector films

Optical absorptions were measured to assess the effect of UV irradiation in non-thermally treated and thermally treated (pre-/post-UV thermal annealing) CR-39 after different exposure times. Fig. 1(a–c) show the UV/Vis spectra of CR-39 polymer films exposed to UV radiation, pre-UV thermal annealing, and post-UV thermal annealing, respectively. In these figures, a shift towards higher wavelengths (red or bathochromic shift) in the absorption spectra of CR-39 polymer films is observed, which

Conclusion

We have introduced a new approach of combined UV and thermal treatments to obtain a unique dosimeter for UV radiation determination over exposure periods of 10–100 h. The obtained results suggest that the extent of UV-induced thermal damage is dose-dependent. Our results imply that this new technique enhances CR-39 detector sensitivity to UVC radiation, allowing high-precision estimation of UV radiation doses. Post-UV thermal annealing treatment has proven to be the most efficient process in

Credit author statement

A.F. Saad: Conceptualization, Methodology, Writing - Reviewing, Editing; Ebrahim M. Sedqy: Data analysis, Data curate, Investigation, Writing - Original draft preparation; Rania M. Ahmed: Investigation, Writing - Original draft preparation.

Declaration of competing interest

No conflict.

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