Analysis of the Radiation Field in ATLAS Using 2008 2011 Data from the ATLAS-MPX Network

The ATLAS-MPX group has installed a network of 16 Medipix2 pixel detector-based, single-quantum-sensitive devices before the LHC start-up in 2008 at various positions in the ATLAS experimental and technical caverns. The aim of the network is to perform real-time measurements of spectral characteristics and composition of the radiation environment inside the ATLAS detector during its operation and in particular already in the early stages with low luminosity. The detectors are generally sensitive to all radiation species capable to deposit energy of at least 8 keV per single pixel. With the devices, in two different modes of operation, a large dynamic range of particle flux can be covered, of at least 9 orders of magnitude, which corresponds to the highest luminosity, while also background measurement can be made. An important goal is the determination of the neutron component of the mixed radiation field. To identify different types of neutrons, the 300 μm thick silicon sensor area of each ATLAS-MPX device is divided into several regions, covered by different neutron converter materials. A 6 LiF layer is used for thermal neutron detection and a polyethylene foil for fast neutron detection. The calibration of the detection efficiency for several energies in different regions of all devices has been performed with various known neutron sources: 252 Cf, 241 AmBe, thermal neutrons and 14 MeV neutrons from Van de Graaff accelerator. Based on these calibrations the spectral composition of the unknown neutron field can be estimated by comparing the responses from different regions. ATLAS-MPX devices have been operated almost continuously starting from early 2008 (background and cosmic radiation measurements and the first LHC beam appearances in 2008 and 2009) through the stable LHC operation during the 2010 and 2011 run periods up to nowadays. Data are acquired instantaneously by each detector station, with adaptable exposure times (currently 0.1 ms – 600 s) according to local particle flux. Rates of incidence of different particle types normalized per area and per luminosity during regular LHC running are evaluated online. This report highlights a selection of the results. Measured thermal neutron fluxes are found to be largely in agreement with the original simulations, mostly within a factor of two. Significant deviations are observed in the low radiation regions of ATLAS cavern, where measured thermal neutron fluxes are found to be lower than predicted by Monte Carlo simulations. High sensitivity of ATLAS-MPX devices also permits determination of LHC induced radioactivity. Starting from luminosity of about 10 30 cm -2 . s -1 the induced radioactivity of short lived radionuclides has become observable. Presently also radioactive decays with longer half-lives are measurable. Online responses of the ATLAS-MPX devices follow reliably the course of the luminosity curve. Devices close to the beam demonstrated * Corresponding author. enough sensitivity to visualize van der Meer luminosity scans and provide measurement of the effective overlapping beam sizes and maximum collision rate.