Observation of Radiation Degradation of Electrical Insulators in the CERN Particle Accelerators

For the selection of polymer-based materials to be used in radiation environments, radiation tests have been performed at the European Organization for Particle Physics Research (CERN) for several decades. According to the recommendations of the IEC Standard 544, mechanical tests are carried out, and the radiation degradation is measured after accelerated irradiations. It is well known that during long-term exposures, oxygen and moisture are allowed to diffuse in the materials and hence to induce more severe degradation; this phenomenon is known as the `dose-rate effect'. During machine shut-downs, samples of rigid and flexible polymeric insulators (magnet-coil resins and cable insulations) have been taken out and tested after several years of exposure in the Super Proton Synchrotron (SPS) and in the Large Electron-Position Collider (LEP). The mechanical test results are compared to the ones after the accelerated qualification tests, and to the ones of a study conducted in 1991 to estimate the lifetime of cables in the radiation environment of LEP 200. They confirm that thermoplastics are more sensitive to long-term irradiations than the thermosetting resins and the composites, but that the dose-rate effect cannot be neglected in the latter.