Investigation of induced radioactivity in the CERN Large Electron Positron collider for its decommissioning

Abstract

The future installation of the Large Hadron Collider in the tunnel formerly housing the Large Electron Positron collider (LEP) required the dismantling of the latter after 11-year operation. As required by the French legislation, an extensive theoretical study was conducted before decommissioning to establish the possible activation paths both in the accelerator and in the four experiments (L3, ALEPH, OPAL and DELPHI) installed around the ring. The aim was to define which areas may contain activated material and which ones would be completely free of activation. The four major sources of activation in LEP, i.e., distributed and localized beam losses, synchrotron radiation and the super-conducting RF cavities, were investigated. Conversion coefficients from unit lost beam power to induced specific activity were established for a number of materials. A similar study was conducted for the four experiments, evaluating the four potential sources of induced radioactivity, namely e +e − annihilation events, two-photon events, e +e − Bhabha scattering events and beam-related radiation, i.e., synchrotron radiation and off-momentum beam particles. Decommissioning started early in 2001 and was completed by February 2002. Every single piece removed from the machine tunnel and experimental areas was double-checked for induced radioactivity, with complementary gamma-spectrometry measurements performed on samples of the various materials. The measurements have to a large extent confirmed the predictions. The overall intent of this paper is to provide data and computational guidelines for assessing induced radioactivity in high-energy electron accelerators, which could be of use in future decommissioning.