Studies of the muon-induced neutron background in LSM: detector concept and status of the installation

📝 Abstract

A good particle candidate for Cold Dark Matter (CDM) is the supersymmetric neutralino or more generally a weakly interacting massive particle (WIMP). The expected interaction rate of WIMPs with the detector medium in the direct detection experiments is below 0.01 events/kg/day. This makes a good knowledge of the background conditions highly important, especially with ever increasing sensitivity of the detectors. One of the background components is related to cosmic muons and in particular to muon-induced neutrons. Detailed studies carried out by the Edelweiss collaboration in this respect are presented. This activity includes GEANT4 simulations with full event topology as well as a dedicated measurement with a new neutron counter installed in the fall of 2008 in LSM (Laboratoire Souterrain de Modane, France). This counter is incorporated into the existing muon veto system thus allowing to monitor neutrons in coincidence with the incoming muons.

💡 Analysis

A good particle candidate for Cold Dark Matter (CDM) is the supersymmetric neutralino or more generally a weakly interacting massive particle (WIMP). The expected interaction rate of WIMPs with the detector medium in the direct detection experiments is below 0.01 events/kg/day. This makes a good knowledge of the background conditions highly important, especially with ever increasing sensitivity of the detectors. One of the background components is related to cosmic muons and in particular to muon-induced neutrons. Detailed studies carried out by the Edelweiss collaboration in this respect are presented. This activity includes GEANT4 simulations with full event topology as well as a dedicated measurement with a new neutron counter installed in the fall of 2008 in LSM (Laboratoire Souterrain de Modane, France). This counter is incorporated into the existing muon veto system thus allowing to monitor neutrons in coincidence with the incoming muons.

📄 Content

Studies of the muon-induced neutron background in LSM: detector concept and status of the installation V. Yu. Kozlov∗(for the Edelweiss collaboration) Forschungszentrum Karlsruhe, Institut für Kernphysik, Postfach 3640, 76021 Karlsruhe, Germany E-mail: Valentin.Kozlov@ik.fzk.de A good particle candidate for Cold Dark Matter (CDM) is the supersymmetric neutralino or more generally a weakly interacting massive particle (WIMP). The expected interaction rate of WIMPs with the detector medium in the direct detection experiments is below 0.01 events/kg/day. This makes a good knowledge of the background conditions highly important, especially with ever increasing sensitivity of the detectors. One of the background components is related to cosmic muons and in particular to muon-induced neutrons. Detailed studies carried out by the Edelweiss collaboration in this respect are presented. This activity includes GEANT4 simulations with full event topology as well as a dedicated measurement with a new neutron counter installed in the fall of 2008 in LSM (Laboratoire Souterrain de Modane, France). This counter is incorporated into the ex- isting muon veto system thus allowing to monitor neutrons in coincidence with the incoming muons. PACS: 29.30.Hs; 95.35.+d; 95.55.Vj Keywords: Neutron background; Muon-induced neutrons; Dark matter; Underground physics Identification of dark matter 2008 August 18-22, 2008 Stockholm, Sweden ∗Speaker. c ⃝Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ arXiv:0902.4858v1 [astro-ph.IM] 27 Feb 2009 Studies of the muon-induced neutrons in LSM V. Yu. Kozlov

1. Introduction The Edelweiss experiment searches for the WIMP candidates of the Dark Matter. The set-up is located in LSM in the French Alps which provides a shielding factor of ∼4800 m.w.e. The detection principle is based on measuring energy of the recoil nucleus originating from the WIMP elastic scat- tering. Bolometers of pure natural Ge are used both as the detectors and the target material. These detectors cooled down to about 20 mK allow to measure simultaneously heat and ionization signals. Due to the quenching of the ionization signal present for nuclear recoils one achieves a very high dis- crimination of β and γ background from the recoil candidates [1]. However, neutrons coming from the natural radioactivity or induced by the remaining muons can still mimic the nuclear recoil signal of WIMP events and thus can not be discriminated in the same way as β’s and γ’s. Therefore, this type of background requires special careful investigation. The knowledge of it also becomes highly important in view of large 1-tonne scale experiments like EURECA [2].
2. Neutron background For kinematic reasons, not every neutron can mimic the WIMP nuclear recoil event but only those who have an energy of 0.5-10 MeV when they reach the Ge bolometers. Such neutrons appearing due to natural radioactivity in the surrounding (e.g. U/Th contamination) can be avoided by using a passive hydrogen-rich moderator (50 cm polyethylene shield in case of Edelweiss) and in addition by radiopurity selection of materials to be used. Monitoring of the ambient neutron flux in proximity of the Edelweiss experimental set-up is performed with the help of 3He gas detectors. This measurement yields a flux of about 2·10−6 n/cm2/day [3] which is in good agreement with the previously measured value [4]. Another part of the neutron background is caused by muon interactions in the rock and in fact everywhere in the set-up (especially in high-Z materials such as the gamma shield based on lead). High energy neutrons (well above 10 MeV) created in such deep inelastic scattering (DIS) processes further lead to the production of secondary neutrons with energies below 10 MeV. The effect of this µ−induced neutron component is commonly reduced by tagging the original muons. In Edelweiss experiment the plastic scintillator modules covering the full bolometer set-up act as the muon veto [5]. Full simulations of the Edelweiss set-up including the muon veto were performed in GEANT4 in order to estimate the influence of muons for the Dark Matter search. These simulations involve muon generation to reproduce the muon flux specific for LSM and allow to get complete event topology [6]. It was shown then that muons which miss the veto can still induce some neutrons reaching the bolometers and giving rise to WIMP-like events not vetoed by the muon system. To verify these simulations one has to normalize them to the experimental data, i.e. one needs explicit µ−induced neutron measurements. One way to achieve this is to check the rate of events which are in coinci- dence between muon veto and the bolometers. This rate currently measured in Edelweiss is about 0.03 events/kg/day and it is reasonably well reproduced in the simulations. However, the rareness of these coincidence events makes it hard to get enough statis

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