Gamma irradiation induced chemical and structural modifications in PM-355 polymeric nuclear track detector film

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Abstract

This experimental study investigated the modification of chemical and structural properties of PM-355 films by irradiation of 1.25 MeV 60Co γ-radiations at doses ranging from 0–675 kGy. The induced modifications were followed by micro-Raman and X-ray diffraction (XRD) spectroscopy. Further, the induced modifications were confirmed by UV–Visible spectroscopy. Raman spectra show that the films are highly disordered at the highest gamma dose. The XRD pattern of PM-355 shows the decreasing intensity of peak positions with an increase in the gamma dose, which suggests the loss of crystallinity of the films due to irradiation. Observed results indicate the formation of a disordered system in the irradiated films. Furthermore, the crystallite size for pristine and bombarded sample has been calculated. Moreover, interchain distance, micro strain, interplanar distance, dislocation density and distortion parameters were calculated. The analysis revealed a significant increase in micro strain, dislocation density and distortion parameters with an increase of gamma dose, which is in line with the Raman analysis. With increasing γ-dose, the value of the direct and indirect band gap found to decrease. To the best of our knowledge, this work is the first to show the simultaneous existence of direct as well as the indirect band gap in PM-355 polymer.

Introduction

Solid state nuclear track detectors (SSNTDs) have been extensively used for the detection of ions. Nowadays, ion track membranes, also known as nuclear track filters, have emerged as the main offshoot from SSNTDs. Many applications of SSNTDs have been developed, including biological filters, detection of light ions, dosimetry to use in ion track etching, magnetic nano wires as magneto resistive sensors and much more [1], [2], [3], [4].

As advanced technology keeps on developing every day, the demand for polymers having improved properties is continuously on the rise due to their use of various, scientific and technological applications [5]. Irradiation plays a prominent role in modification of properties of polymers significantly. Ionizing radiation passing through matter, deposit energy in the material and cause irreversible changes in the macromolecular structure of the target material. The most prominent effects of radiation involve a change of phase in the absorbing material due to the scissions or cross linking formation. At the same time, it is also likely possible to observe the creation of chemical bonds between different molecules (intermolecular cross linking) in the main chain.

Most of the efforts have been directed towards the effects of radiations on the track etch and bulk etch rates of PM-355 polymeric detectors [6], [7], [8]. In fact; many authors have used such detectors basing only on approximate calibration data, which presented a dependence of track diameters on energy of selected particles and etching time for the detectors [9], [10], [11], [12], [13]. The PM-355 detectors have used for the detection of light as well as heavy ions, including protons, deuterons, He, C, N, O, S, etc. [14], [15], [16]. Recently; PM-355 detectors have been used for the assessment of solar ultra-violet radiation dosimetry [17]. The polymer PM-355 has the same chemical composition as the solid state nuclear track detector CR-39 or PADC that find diverse applications [5], [18].

From the above discussion, it was concluded that there is a lacuna in the systematic study on the optical, chemical, structural and thermal properties of PM-355 after irradiation. Very few attempts have been carried out in the study of optical, thermal, chemical and structural properties of PM-355 SSNTDs as a function of radiation doses [19], [20]. Moreover, to be best of our knowledge a few reports have been carried out on the use of gamma radiation for the modifications of optical and structural properties of PM-355 SSNTDs [21]. Interestingly, as for many other polymeric nuclear track detectors (viz. CR-39, Makrofol KG, PET, etc.), the optical, chemical, structural, and thermal properties may be greatly affected by irradiation [22], [23], [24], [25]. Because of the potential usefulness of PM-355, a study was carried out on the chemical and structural modifications induced by gamma irradiation.

The aim of this paper is to investigate the change in chemical and structural properties of PM-355 film after gamma irradiation. An attempt has been made to correlate the results with the earlier reported data on the effects of irradiation on different polymeric materials in order to make the present investigation more informative.

Section snippets

Experimental aspects

Small pieces of size (1 × 1 cm2) and thickness 250 μm were cut from the commercially available sheet of the PM-355 detector having density of 0.9 gm/cm3. This target assembly was then taken for gamma irradiation. Detectors were irradiated using 1.25 MeV gamma radiation (the average energy of the two 60Co photons) source of 60Co in the radiation chamber with dose rate 7.328 kGy/h at IUAC, New Delhi. Doses from 150 to 675 kGy were used in the present study.

The nature of changes induced by gamma

Raman spectra

Raman spectra of pristine and highest irradiated PM-355 films have been shown in Fig. 1 against Raman shift in cm−1. The relative changes in the bonds have been estimated from the relative increase or decrease in Raman intensity of the peaks associated with the functional groups present the polymer. It is clear from the figure that the Raman intensity of most of the peaks increased, and some peaks disappeared, suggesting that some structural changes occur in the polymer film after gamma

Conclusion

Based on the present results, the chemical and structural properties of the PM-355 polymer are affected by the gamma irradiation. Raman spectra show the decrease in the peak intensity. UV–Visible spectral studies of pristine and gamma irradiated PM-355 polymer films reveals the simultaneous existence of the direct and indirect band gap; an observation which is being reported for the first time to the best of our knowledge. The values of an indirect band gap found to be lower than the

Acknowledgments

Authors are highly thankful to Dr. S P Lochab, IUAC New Delhi, India, for providing gamma irradiation facility. Special thanks are due to Prof. Ravi Kumar, Department of Material Science and Engineering, National Institute of Technology, Hamirpur, India, for providing XRD, Raman and UV–Visible facilities.

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