Study of the degradation process of polyimide induced by high energetic ion irradiation

The dissertation focuses on the radiation hardness of Kapton under extreme radiation environment conditions and is motivated by the application of this polyimide as insulator in superconducting magnets for the new Facility for Antiproton and Ion Research (FAIR) planned at the Gesellschaft für Schwerionenforschnung (GSI). The new FAIR accelerators are expected to deliver protons and heavy ions of extreme energies (10 GeV/u) and unprecedented intensities (10 ions/pulse). Reliable data of the radiation hardness of polymers concerning mechanical, electrical, and outgassing properties are of great interest also for other facilities such as the Large Hadron Collider (LHC) of CERN. To study ion-beam induced modifications, Kapton foils were irradiated at the GSI linear accelerator UNILAC using several projectiles (e.g. Ti, Mo, Au, and U) within a large fluence regime (1x10 – 5x10 ions/cm). The irradiated Kapton foils were analysed by means of infrared and UV/Vis spectroscopy, tensile strength measurement, mass loss analysis, and dielectric relaxation spectroscopy. For testing the radiation stability of Kapton at the cryogenic operation temperature (5-10 K) of the superconducting magnets, additional irradiation experiments were performed at the Grand Accelerateur National d’ Ions Lourds (GANIL, France) focusing on the online analysis of the outgassing process of small volatile degradation fragments. Results obtained by optical spectroscopy, tensile strength measurement and mass loss analysis show similar trends and can be scaled by the irradiation dose given by the product of fluence and energy deposited along the ion trajectory. Critical material degradation appears above a dose of 1 MGy. The investigations of the electrical properties analysed by dielectric relaxation spectroscopy exhibit a different trend: high fluence irradiations with light ions (e.g. Ti) lead to a slight increase of the conductivity, whereas heavy ions (e.g. Sm, Au) cause a drastic change already in the fluence regime of nonoverlapping tracks (5x10 ions/cm). Online analysis of the outgassing process during irradiation at cryogenic temperatures shows the release of a variety of small gaseous molecules (e.g. CO, CO2, and short hydro carbons). Also a small amount of large polymer II _____________________________________________________________________________________________________________________________ fragments is identified and confirms the degradation mechanism of Kapton proposed in earlier studies. Simultaneous in-situ infrared spectroscopy gives evidence of accumulation of these small molecules inside the bulk polymer at cryogenic temperatures. During heat-up cycles, these fragments outgas in specific temperature zones. The results obtained by the different analytical techniques allow the following conclusions which are of special interest for the application of Kapton as insulating material in a high-energetic particle radiation environment. a) The material degradation measured with the optical spectroscopy and tensile strength tests are scalable with the dose deposited by the ions. The high correlation of the results allows the prediction of the mechanical degradation with the simple and non-destructive infrared spectroscopy. The degradation curve points to a critical material degradation which has to be expected above a dose of 1 MGy. b) The dielectric relaxation spectroscopy indicates a dramatic increase in the conductivity induced by irradiation with heavy ions which pass a threshold of mass and deposited energy (dE/dx). The phenomenon indicates that only a few hits (fluences of 10 ion/cm) of a heavy high energetic ion leads to a significant increase of conductivity. c) The degradation induced formation of small molecules and their outgassing even at cryogenic temperature cause a gas release during irradiation. At temperatures below 20 K, an additional accumulation of these molecules in the bulk material occurs and leads to a critical gas evolution during heat-up cycles. III _____________________________________________________________________________________________________________________________