Measurements and simulations of the BLM response to a radiation field inside the CERF target area

The CERN-EU high-energy reference field (CERF) facility is installed in one of the secondary beam lines (H6) of the Super Proton Synchrotron (SPS), in the North Experimental Area at CERN. This facility is used as a reference for testing, inter-comparing and calibrating passive and active instruments. In May 2009, the SPS provided a mixed hadron beam (protons, pions and kaons) during a few days, in order to perform several measurements with different devices such as the Radiation Protection Monitor used for residual dose rates due to Induced Radioactivity in the LHC (PMI), the Secondary Emission Monitor used for high beam losses (SEM), the Radiation Monitor for electronics (RadMon), and the Beam Loss Monitor for the LHC (BLM). This report focuses on the measurements of the BLM response during this year’s operation at CERF. The measurements evaluate the sensitivity of the BLM signal to the particle energy spectrum, with special attention to the contribution coming from thermal neutrons. For this purpose, measurements are performed at various calibrated positions with different fields, as well as by using a Cd layer which is wrapped around the detector. The fields the BLM detector is exposed to are representative for the LHC and range from fields typical to the LHC tunnel, up to radiation fields encountered in shielded areas close to the LHC tunnel. For all configurations, detailed FLUKA simulations were performed to benchmark i n detail the reproducibility of the BLM signal. In addition, to evaluate the BLM response in terms of energy deposition and particles spectra, dedicated calibration calculations were performed to provide energy and particle dependent response functions of the BLM detector. This allowed calculating the contribution of the main particles to the BLM signal and their respective energy range. This successful benchmark shows not only the high accuracy which can be achieved when performing radiation field estimates of LHC like spectra with the FLUKA Monte-Carlo code, but also clarifies the possible issue of low-energy neutrons seen by the BLM detector.