Neutron Flux Measurement at TAPIRO Fast Reactor for APD's Irradiation Fluence Evaluation

Abstract

The Avalanche Photodiodes ( APD) were chosen as photon sensors for the region of the CMS electromagnetic calorimeter. The LHC will be a hard environment for what concerns the radiation levels in the detectors. The most relevant damage on APDs is caused by neutrons that produce an increase in the dark current of these devices. In the CMS-ECAL collaboration a big effort was indeed done to understand this damage, but the evaluation of the absolute effect was limited by the knowledge of the neutron flux calibration of the various irradiation facilities. This investigation describes the calibration of the neutron flux of the Tapiro reactor in Rome and the calculation of the Non-Ionizing-Energy-Loss on Silicon for this reactor. The damage parameter alpha for the APDs is evaluated to be about 10-11*10^-17 A/cm/neutron at 18C and 2 days after the irradiation. Some cross-checks with other irradiation facilities are also presented