Samuel Henry

Abstract:

The EDELWEISS-II collaboration has completed a direct search for WIMP dark matter using cryogenic Ge detectors (400 g each) and 384 kg × days of effective exposure. A cross-section of 4.4×10-8 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 ×10 -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 (≤0.9), surface betas (≤0.3), and muon-induced neutrons (≤0.7), do not contradict five 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. © 2013 Elsevier B.V. All rights reserved.

Abstract:

A dedicated analysis of the muon-induced background in the EDELWEISS dark matter search has been performed on a data set acquired in 2009 and 2010. The total muon flux underground in the Laboratoire Souterrain de Modane (LSM) was measured to be Φμ=(5.4±0.2-0.9+0.5) muons/m 2/d. The modular design of the μ-veto system allows the reconstruction of the muon trajectory and hence the determination of the angular dependent muon flux in LSM. The results are in good agreement with both MC simulations and earlier measurements. Synchronization of the μ-veto system with the phonon and ionization signals of the Ge detector array allowed identification of muon-induced events. Rates for all muon-induced events Γμ=(0.172±0.012)evts/(kgd) and of WIMP-like events Γμ-n=0.008-0.004+0.005evts/(kgd) were extracted. After vetoing, the remaining rate of accepted muon-induced neutrons in the EDELWEISS-II dark matter search was determined to be Γirredμ- n<6·10-4evts/(kgd) at 90% C.L. Based on these results, the muon-induced background expectation for an anticipated exposure of 3000 kg d for EDELWEISS-III is N3000kgdμ-n<0.6 events. © 2013 Elsevier B.V. All rights reserved.

Abstract:

The luminescence and scintillation properties of ZnS-Ag/6LiF where studied in the 7-295 K temperature range to evaluate the suitability of the scintillator for neutron detection at very low temperature (<1 K). It is shown that decrease of temperature has little effect upon principal luminescence and scintillation characteristics of ZnS-Ag: the changes of emission intensity are small for photoexcitation and negligible for excitation with α-particles. The recombination kinetics of the scintillation decay exhibits modest shortening of the fast decay time constant, from 4.52 to 3.35 μs with cooling to 10 K. It is concluded that ZnS-Ag/6LiF is a promising scintillator for cryogenic application. © 2012 Elsevier B.V. All rights reserved.