Estimation of the Optical Aging Rate for 1.00 mm Plastic Scintillating Fibre of Type SCSF-78M

Natural aging, in absence of ionizing radiation, leads to a very slow decrease of the optical transparency in plastic scintillating fibres. This aging phenomenon is attributed to chemical reactions of the fibre core material polystyrene, which can be a photo-reaction by natural light or an oxidation reaction with oxygen diffused in from the surrounding atmosphere. This note treats the natural aging of a plastic scintillating fibre as an oxidation reaction of the polystyrene core and oxygen diffused from the surrounding air atmosphere at room temperature and in the dark. The study was motivated by the observation of a systematic degradation of the optical attenuation length of scintillating fibres in the context of quality assurance measurements for the CERN LHCb SciFi project for which 12’000 km of scintillating fibres with 0.25 mm diameter were produced and tested. In order to estimate the slow aging speed of optical properties, we made accelerated aging tests at elevated temperatures, by measuring transmission loss and attenuation length of test scintillating fibers over time. Compared to the oxygen diffusion into the fibre core from air atmosphere, the oxidation reaction is slow such that the polystyrene core is kept always saturated in equilibrium atmosphere. Under these conditions, the oxidation reaction is a “0-th order reaction”, and increases the color center concentration linearly with time. The ultra-violet (UV) light absorption of color centers also increases the transmission loss in the short visible wavelength region, such as 450nm or 500nm, also in a linear manner. Therefore, the rates of these transmission loss increases correspond to the oxidation reaction rate, and should obey Arrhenius-type equations as a function of the absolute temperature. We used SCSF-78M 1.00mm (KURARAY’s standard plastic scintillating fiber, multi-cladding and 1mm diameter) as test samples and performed accelerated tests at different temperatures. We described the test results by an Arrhenius equation, and extrapolated the UV loss increase rate due to oxidation to room temperature. Finally, we concluded that the speed of attenuation length decrease at 22 degrees C in air atmosphere can be estimated as 0.8% per year initially, and as 7% accumulated over 10 years. The degradation of the attenuation length is found to be sublinear in time, i.e. gradually slowing down.