Effects of long-term low-level exposure to radiation as observed in acrylic scintillator. Cause and prevention of radiation disease in uranium calorimeters

The acrylic scintillator which was used for several years as the active medium in the Axial Field Spectrometer uranium calorimeter at the CERN Intersecting Storage Rings was found to be severely damaged. The observed light output has decreased on the average by a factor of 2, and the attenuation length by a factor of 3, causing a severe degradation of the energy resolution of the system for the detection of both electromagnetic and hadronic showers. It could be shown that the effects are due to the radiation induced by uranium decay. The fact that the integrated dose obtained from this process (50 Gy) is 2 orders of magnitude smaller than what was currently believed to be a safe level, strongly suggests that the dose rate is a dominating factor in these radiation damage problems. It could be shown that the radiation resistivity is improved by at least an order of magnitude if contact with oxygen is avoided.