Evaluation of the neutron background in an HPGe target for WIMP direct detection when using a reactor neutrino detector

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Evaluation of the Neutron Background in an HPGe Target for WIMP Direct Detection When Using a Reactor Neutrino Detector as a Neutron Veto System1 Xiangpan Jia, Ye Xua,b*, Junsong Lina, Yulong Fenga, and Haolin Lia a

bDepartment

School of Physics, Nankai University, Tianjin, 300071 China of Mathematics and Physics, Fujian University of Technology, Fuzhou, 350108 China *email: [email protected], [email protected] Received April 23, 2013

Abstract—A direct WIMP (weakly interacting massive particle) detector with a neutron veto system is designed to better reject neutrons. The experimental configuration is studied in this paper involves 984 Ge modules placed inside a reactorneutrino detector. The neutrino detector is used as a neutron veto device. The neutron background for the experimental design is estimated using the Geant4 simulation. The results show that the neutron background can decrease to O(0.01) events per year per tonne of highpurity germa nium and it can be ignored in comparison with electron recoils. DOI: 10.1134/S106377611313013X 1

1. INTRODUCTION

In direct searches for WIMPs (Weakly interacting massive particle), there are three different methods used to detect the nuclear recoils: collecting ioniza tion, scintillation, and heat signatures induced by them. The background of this detection is made up of electron recoils produced by γ and β scattering on electrons, and nuclear recoils produced by neutrons scattering elastically on target nuclei. Nuclear recoils can be efficiently discriminated from electron recoils with pulse shape discrimination, hybrid measure ments, and so on. The rejection power of these tech niques can even reach 106 [1, 2]. For example, the CDMSII [1] and EDELWEISSII [3] experiments measure both ionization and heat signatures using cryogenic germanium detectors in order to discrimi nate between nuclear and electron recoils, and the XENON100 [4] and ZEPLINIII [5] experiments measure both ionization and scintillation signatures using twophase xenon detectors. However, it is very difficult to discriminate between nuclear recoils induced by WIMPs and by neutrons. This discrimina tion and reduction of neutron backgrounds are the most important tasks in direct dark matter searches. The cross sections of neutronnucleus interactions are much larger than the WIMP nucleus ones, and therefore the multiinteractions between neutrons and detector components are used to tag neutrons and thus separate WIMPs from neutrons. In the ZEPLINIII experiment, the 0.5% gadolinium (Gd) doped polypropylene is used as the neutron veto device, and 1 The article is published in the original.

its maximum tagging efficiency for neutrons reaches about 80% [6]. In [7], the 2% Gddoped water is used as the neutron veto, and its neutron background can be reduced to 2.2 (1) events per year per tonne of liquid xenon (liquid argon). In our previous work [8], the reactor neutrino detector with 1% Gddoped liquid scintillator (GdLS) is used as the neutron veto sys tem, and its