5 th Australasian Congress on Applied Mechanics, ACAM 27 1-12 December 27, Brisbane, Australia Radiation damage to polymer optical fibres C. Yan 1, S.H. Law 2, N. Suchowerska 3,4 and S.H. Hong 5 1 School of Engineering Systems, Queensland University of Technology, Brisbane, QLD 41, Australia 2 Optical Fibre Technology Centre, The University of Sydney, 26 National Innovation Centre, ATP, Eveleigh, NSW 143, Australia 3 Department of Radiation Oncology, Royal Prince Alfred Hospital, Missenden Road, Camperdown NSW 4 School of Physics, The University of Sydney NSW 26, Australia 5 School of Aerospace, Mechanical and Mechatronic Engineering, J7, The University of Sydney, NSW 26, Australia Abstract: Recently, polymer optical fibre dosimeters have been proposed for use in radiation oncology. Prior to clinical acceptance the effects of radiation on the materials involved must be assessed. In this work, the effects of radiation on mechanical strength and power transmission of polymer optical fibres were investigated. The fibres were subject to high energy electrons and photons generated by a linear accelerator for a range of doses. The mechanical properties and melting point of the fibres after the radiation were evaluated using tensile and differential scanning calorimetry (DSC) tests. The power transmission and absorption spectrum to visible light and their dependence on radiation conditions were also estimated. The overall performance of these polymer optical fibres as a part of the dosimeter subjected to radiation is discussed. Keywords: mechanical property, polymer optical fibre, radiation, transmission. 1 Introduction Polymer optical fibres (POF) are continuously finding applications in data and information transfer, especially in the automobile, aerospace and medical industries, due to their low cost, light weight, higher flexibility and durability and ease of termination [1-4]. A typical example is polymer optical fibre dosimeters in radiation oncology [5, 6]. However it is known that high energy radiation may cause deterioration in the mechanical and optical properties of POF [7-9]. To meet the needs for reliable operation, the mechanical behaviour and possible failure under radiation, remain the key materials issues. In this work, the effects of radiation on mechanical strength and power transmission of polymer optical fibres were investigated. The fibres were subject to electron and photon radiation generated by a linear accelerator over a range of doses. The mechanical properties and melting point of the fibres after exposure to radiation were evaluated using tensile and differential scanning calorimetry (DSC) tests, respectively. The power transmission and its dependence on radiation conditions were also estimated. 2 Experimental procedure To conduct the radiation test, polymethylmethacryate (PMMA) polymer optical fibres with a core diameter of 1. mm were arranged into tight spirals on 15 mm thick Perspex sheets. Additional sheets of Perspex were placed on the fibre to create full radiation scatter conditions and conditions of electronic equilibrium. The fibres were irradiated using a Varian EX linear accelerator with 2 types of radiation and 5 different radiation doses, i.e., 6 MV photons and 12 MeV electrons for doses of 1, 3, Fig. 1 The polymer optical fibre
Stress (MPa) Stress (MPa) 5, 1 and 1, Gy. Fig. 1 shows the POF mounted on the Perspex sheets. The mechanical properties of the POF were measured both before and after the irradiation using tensile tests in accordance with the American Standard for Test and Measurement (ASTM) D 3379-75. An Instron 5567 was used with a crosshead speed of 5 mm/min. The gauge length of the specimen was 1 cm. Changes in the composition of the polymer were evaluated by differential scanning calorimetry using a TA Instruments Model 292. The power transmission test was carried out using a Kingfisher KI76 power meter and a red laser pointer as the light source. The POF end surfaces were carefully polished before the test. A cut-back method was used, in which the original 396 cm POF was cut 44 cm shorter each time for 9 times after recording the power attenuation, as schematically shown in Fig. 2. The absorption spectrum of white light was estimated using an Ocean Optics 2 spectrometer. Fig. 2 Power transmission test. 3 Results and discussion Fig. 3 shows the stress-strain curves of the POF after 6MV photon radiation after exposure to doses of 1 and 1, Gy. For comparison, the result of unexposed POF is also included in Fig. 3. The curves can be divided into two parts, i.e., the initial elastic regime and the plastic deformation until the final failure. It is evident that there is little effect of radiation on the Young s modulus. The yield point is also not sensitive to the radiation used. However, the elongation at failure decreases with the radiation dose. The same trend can be observed in the tensile strength, i.e., the tensile strength decreasing with the radiation dose. In summary, the tensile strength and ductility (elongation), of the POF deteriorated after irradiation although the initial elastic behaviour was almost identical before and after irradiation. For many polymers, chain scission is likely after exposure to radiation. Kudoh et al. found that the molecular weight of polymers (PMMA and PTFE) was reduced after irradiation [1]. Since lower 35 3 Unexposed 1 Gy 1 Gy 35 3 Unexposed 1 Gy 1 Gy 25 25 2 15 1 5 2 15 1 5 5 1 15 2 25 3 Strain (%) 5 1 Strain (%) (a) (b) Fig. 3 Stress-strain curve of the POF after irradiation with 6MV photons, (a) full curves, and (b) Elastic region
Power Output (mw) Heat Flow (mw) molecular weight corresponds to a lower tensile strength in polymers, this can potentially affect the strength of the polymer. The effect of radiation on melting temperatures of the POF was investigated using differential scanning calorimetry, as shown in Fig.4. It can be seen that the melting temperatures of the POF (PMMA) is slightly increased after the 6MV photon radiation. However, there is no relationship between the radiation dose and the melting temperatures. -1-2 Unirradiated 1 Gy 3 Gy 5 Gy 1 Gy 1 Gy -3-4 4 6 8 1 12 14 16 Temperature ( C) Fig. 4 DSC analysis of the POF before and after radiation. The 1 Gy line is overlying the 1 Gy line. 2.E-5 1.8E-5 1.6E-5 1.4E-5 1.2E-5 1.E-5 8.E-6 6.E-6 4.E-6 2.E-6.E+ 1 2 3 4 5 POF length (cm) Unirradiated POF 6MV Photons 1Gy 6MV Photons 1Gy 12MeV Electrons 1Gy 12MeV Electrons 1Gy Fig. 5 Power attenuation after different irradiations Fig. 5 shows the effect of irradiation on the power transmission in the POFs. In Fig. 5, the highest power output is observed in the unexposed POF. Significant power attenuation can be found in the POF subject to irradiation. At a given fibre length, the lowest power transmission is associated with the POF exposed to 6 MV photon radiation. In other words, the transparency of the PMMA is more sensitive to 6 MV photons than 12 MeV electrons with all the damage occurring by the time the dose has reached 1 Gy. 12 MeV electrons have less effect on the transparency of the PMMA. The
Wavelength (nm) attenuation of the material at 1 Gy is only slightly less than that for the unexposed fibre. Further exposure (to 1 Gy) significantly increases the attenuation of the core material. The effect of radiation on the spectral absorption of white light was studied. As indicated in Fig. 6, the unexposed POF has the highest and second highest peaks at the wavelengths of approximate 584 nm and 645 nm, which correspond to yellow and red light, respectively. Fig. 7 shows the variation of wavelength at the absorption peaks with the dose after 6 MV photon irradiation. Fig. 6 Spectral absorption of white light in unexposed POF 8 75 7 65 6 55 5 15 3 5 1 1 Dose (Gy) 1st Peak Min Turn Pt 2nd Peak 3rd Peak It can be seen in Fig. 7 that there is slight shift of the first absorption peak to the shorter wavelength at a very high radiation dose. Overall, it seems the dose level employed here does not have a great effect on the absorption spectrum of white light. 4 Conclusions Fig. 7 Variation of wavelength at the absorption peaks with radiation dose with linear trend lines. In this work, the effects of electron and photon irradiation on the mechanical behaviour and light transmission of POF were investigated. The results showed that the tensile strength and elongation to the failure points (ductility) were reduced after irradiation. The higher the radiation dose imposed, the lower the tensile strength and ductility. However, the elastic modulus and yield strength were not sensitive to irradiation. The proposed reason is chain scission in the polymer fibres caused by the radiation. There was a slight increase of melting temperature after irradiation, as indicated by the DSC
test. Irradiation, especially photon irradiation caused increased attenuation in the transmission of light. On the other hand, the absorption spectrum of visible light was not sensitive to the radiation. Based on all testing results, the POFs are mechanically suitable for application in radiation dosimeters if only elastic strain is expected. Further work will be extended to reducing the power attenuation. Acknowledgements This work was supported by the Australian Research Council, the New South Wales Cancer Council, the Bandwidth Foundry Pty Ltd, The Sydney Cancer Institute and CMS Alphatech Pty Ltd. The Bandwidth Foundry is a Major National Research Facility supported by the Australian and New South Wales Governments. The irradiation of the fibre was performed at Royal Prince Alfred Hospital, Sydney. References [1] Ji, P.N. and Wang, T. (24) Development of special polymer optical fibres and devices, in Proceedings of the SPIE, v 5595, 38-52. [2] Fan, H. et al. (1998) Analogue and digital transmission using polymer optical fibre, Electronics letters, 34, 1999-2. [3] Law, S.H., Eijkelenborg, M.A., Barton, J and Yan, C., Lwin, R., and Gan, J. (26) Cleaved endface quality of microstructured polymer optical fibres, Optics communications, 265, 513-52. [4] Krauser, J., Zamzow, P.E., Daum, W. and Zieman, O. (22) Polymer optical fibers for data communication, Springer-Verlag. [5] Lambert, J., McKenzie, D.R., Law, S., Elsey, J. and Suchowerska, N. (26) A plastic scintillation dosimeter for high dose rate brachytherapy, Physics in Medicine and Biology, 51, 556-5516. [6] Lambert, J., Nakano, T., Law, S., Elsey, J. McKenzie, D.R. and Suchowerska, N. (27) In vivo dosimeters for HDR brachytherapy: A comparison of a diamond detector, MOSFET, TLD and scintillation detector, Medical Physics, 34, 1759-1764. [7] Clough, R. L and Shalaby, S.W. (1991) Radiation effects on polymers, American Chemical Society, Washington. [8] Coffey, T. et al. (22) Characterization of the effects of soft X-ray irradiation on polymers, Journal of electron spectroscopy and related phenomena, 122, 65-78. [9] Sasuga, T. et al. (1991) Effects of ion irradiation on the mechanical properties of several polymers, Radiation physics and chemistry, 37, 135-14. [1] Okudaira, K.K. et al. (1998) Radiation damage of poly(methylmethacrylate) thin films, Journal of electron spectroscopy and related phenomena, 88-91, 913-917.