Degradation of polyimide under the irradiation with swift heavy ions

This study concentrates on radiation-induced ageing effects of polymers planned as insulating materials in the rapidly cycling superconducting magnets presently being developed for the FAIR facility. Although beam control and minimization of beam losses will be a major issue, the exposure of insulating parts (e.g. polyimide, polyetherimide, and glassenforced epoxy-compounds) of various accelerator components to x-rays, gammas, neutrons, and scattered light or heavy particles may limit long-term operation. We started to investigate degradation processes of commercial polyimide films (KaptonTM, thickness 12, 25, and 50 μm) by irradiating them with various heavy ions at room temperature. The experiments were performed at the UNILAC with Ti, Mo, and Au of energy up to 11 MeV/u applying fluences between 1×10 and 2×10 ions/cm under a maximum flux of 2×10 ions/cm⋅s. Although Kapton is known to be highly radiation resistant, pronounced material modifications were discovered by means of UV/Vis, infrared, and dielectric spectroscopy [1]. In the spectral range of UV/Vis, the wavelength of the absorption edge of irradiated foils shifts as a function of the applied fluence indicating carbonization and a decrease of the bandgap energy in agreement with studies using ions of lower energies [2]. Analyzing the derivative of the transmission spectra (dT/dλ) reveals that the wavelength at which dT/dλ is maximum (dTmax) depends on the applied dose (Fig. 1a). The effect appears above a critical dose of about 1MGy for Au and Mo ions, and of a somewhat higher valuefor the lighter Ti projectiles (~2.7 MGy). The chemical degradation of several specific polymer groups exhibits a similar dose dependence. Fig. 1b shows as an example the decay of one of the imide bands (725 cm) together with the band of the phenyl group (1169 cm) as a representative for the rather stable aromatic polymer component [3]. Due to track overlapping, the evolution of the infrared and UV/Vis data of the three different ion species follows an exponential law, corresponding to a track radius of about 5, 4, and 2 nm for Au, Mo, and Ti ions, respectively. The insulating property of irradiated Kapton foils was tested by dielectric spectroscopy performed at room temperature and in a frequency range from 1 MHz to 0.1 Hz using a frequency response analysis system (Solartron SI 1260 Impedance/Gain-Phase Analyzer and a Novo-control broadband dielectric converter). The conductivity of the irradiated Kapton foils was significantly increased, depending on the ion species and dose up to three orders of magnitude (Fig. 1c). The threshold behavior and subsequent exponential increase is in agreement with an insulator-conductor transition based on the creation of graphitic clusters along the ion tracks [4]. In contrast to the absorption data, the conductivity change does not directly scale with the dose of the different ions. To better understand these effects, additional measurements are in preparation. 10 10 10 10 10 10 10 10 10