1305.3628 (E. Armengaud et al.)

Background studies for the EDELWEISS dark matter experiment    [PDF]

E. Armengaud, C. Augier, A. Beno\^\it, A. Beno\^\it, L. Bergé, T. Bergmann, J. Blümer, A. Broniatowski, V. Brudanin, B. Censier, M. Chapellier, F. Charlieux, F. Couedo, P. Coulter, G. A. Cox, M. De Jesus, J. Domange, A. -A. Drilien, L. Dumoulin, K. Eitel, D. Filosofov, N. Fourches, J. Gascon, G. Gerbier, M. Gros, S. Henry, S. Hervé, G. Heuermann, N. Holtzer, A. Juillard, M. Kleifges, H. Kluck, V. Kozlov, H. Kraus, V. A. Kudryavtsev, H. Le Sueur, P. Loaiza, S. Marnieros, A. Menshikov, X-F. Navick, C. Nones, E. Olivieri, P. Pari, B. Paul, O. Rigaut, M. Robinson, S. Rozov, V. Sanglard, B. Schmidt, S. Scorza, B. Siebenborn, S. Semikh, D. Tcherniakhovski, A. S. Torrento-Coello, L. Vagneron, R. J. Walker, M. Weber, E. Yakushev, X. Zhang

The EDELWEISS-II collaboration has completed a direct search for WIMP dark matter using cryogenic Ge detectors (400 g each) and 384 kg$\times$days of effective exposure. A cross-section of $4.4 \times 10^{-8}$ pb is excluded at 90% C.L. for a WIMP mass of 85 GeV. The next phase, EDELWEISS-III, aims to probe spin-independent WIMP-nucleon cross-sections down to a few $\times10^{-9}$ pb. We present here the study of gamma and neutron background coming from radioactive decays in the set-up and shielding materials. We have carried out Monte Carlo simulations for the completed EDELWEISS-II setup with GEANT4 and normalised the expected background rates to the measured radioactivity levels (or their upper limits) of all materials and components. The expected gamma-ray event rate in EDELWEISS-II at 20-200 keV agrees with the observed rate of 82 events/kg/day within the uncertainties in the measured concentrations. The calculated neutron rate from radioactivity of 1.0-3.1 events (90% C.L.) at 20-200 keV in the EDELWEISS-II data together with the expected upper limit on the misidentified gamma-ray events ($\le0.9$), surface betas ($\le0.3$), and muon-induced neutrons ($\le0.7$), do not contradict 5 observed events in nuclear recoil band. We have then extended the simulation framework to the EDELWEISS-III configuration with 800 g crystals, better material purity and additional neutron shielding inside the cryostat. The gamma-ray and neutron backgrounds in 24 kg fiducial mass of EDELWEISS-III have been calculated as 14-44 events/kg/day and 0.7-1.4 events per year, respectively. The results of the background studies performed in the present work have helped to select better purity components and improve shielding in EDELWEISS-III to further reduce the expected rate of background events in the next phase of the experiment.

View original: http://arxiv.org/abs/1305.3628