Neutron flux Measurement at Tapiro fast reactor for APD’s Irradiation fluence evaluation

The Avalanche Photodiodes (APD) were chosen as photon sensors for the barrel 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 for the APDs is evaluated to be about 10 11 10 17 A/cm/neutron at 18oC and 2 days after the irradiation. Some cross-checks with other irradiation facilities are also presented. Introduction The Avalanche Photodiodes (APD) have been chosen as photon sensors for the scintillating crystals of the CMS electromagnetic calorimeter in the barrel region [1]. The prototypes which are nowadays available have been optimized for radiation hardness in order to survive the neutron fluence expected in ten years of operation of the detector in LHC. In order to study the damage on the APDs, several prototypes have been irradiated in the various irradiation facilities available for the CMS-ECAL collaboration. To compare the results a good absolute calibration of the dose is mandatory. Furthermore, as the neutron damage on silicon devices depends on the neutron energy, a detailed knowledge of the neutron spectrum of each source is fundamental for this type of measurements. The neutron source used by our group is the TAPIRO reactor at ENEA-Casaccia near Rome. This note is divided in two parts. The first part describes the TAPIRO reactor calibration performed with two different methods. The second part shortly reviews the neutron damage on silicon devices and exploits the calculation of the Non-Ionizing-Energy-Loss relative to 1 MeV neutrons for silicon in different irradiation positions in the TAPIRO. Then the measurements done on the APDs with this reactor are described. In particular the results obtained during an APD irradiation performed simultaneously to the reactor calibration are discussed. Finally some cross-checks done with other sources are described. All this work is finalized to a precise and reliable evaluation of the damage parameter of neutrons on APDs.