научная статья по теме EVALUATION OF THE NEUTRON BACKGROUND IN AN HPGE TARGET FOR WIMP DIRECT DETECTION WHEN USING A REACTOR NEUTRINO DETECTOR AS A NEUTRON VETO SYSTEM Физика

Текст научной статьи на тему «EVALUATION OF THE NEUTRON BACKGROUND IN AN HPGE TARGET FOR WIMP DIRECT DETECTION WHEN USING A REACTOR NEUTRINO DETECTOR AS A NEUTRON VETO SYSTEM»

EVALUATION OF THE NEUTRON BACKGROUND IN AN HPGe TARGET FOR WIMP DIRECT DETECTION WHEN USING A REACTOR NEUTRINO DETECTOR AS A NEUTRON VETO SYSTEM

Xiangpan Ji, Ye Xu* Junsong Lin, Yulong Feng, Haolin Li

School of Physics, Nankai University 300071, Tianjin, China

Received April 23, 2013

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 reactor-neutrino 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 0(0.01) events per year per tonne of high-purity germanium and it can be ignored in comparison with electron recoils.

DOI: 10.7868/S0044451013110035

1. INTRODUCTION

In direct searches for WIMPs (Weakly interacting massive particle), there are three different methods used to detect the nuclear recoils: collecting ionization, scintillation, and heat signatures induced by them. The background of this detection is made up of electron recoils produced by 7 and 3 scattering 011 electrons, and nuclear recoils produced by neutrons scattering elasti-cally 011 target nuclei. Nuclear recoils can be efficiently discriminated from electron recoils with pulse shape discrimination, hybrid measurements, and so 011. The rejection power of these techniques can even reach 10® fl, 21. For example, the CDMS-II |!| and EDELWEISS-II [3] experiments measure both ionization and heat signatures using cryogenic germanium detectors in order to discriminate between nuclear and electron recoils, and the XENONIOO 11| and ZEPLIN-III |5| experiments measure both ionization and scintillation signatures using two-phase xenon detectors. However, it is very difficult to discriminate between nuclear recoils induced by WIMPs and by neutrons. This discrimination and reduction of neutron backgrounds are the most important tasks in direct dark matter searches.

E-mail: xuye76(fflnankai.edu.cn

The cross sections of neutron nucleus interactions are much larger than the WIMP nucleus ones, and therefore the multi-interact ions between neutrons and detector components are used to tag neutrons and thus separate WIMPs from neutrons. I11 the ZEPLIN-III experiment, the 0.5% gadolinium (Gd) doped polypropylene is used as the neutron veto device, and its maximum tagging efficiency for neutrons reaches about 80% [6]. I11 Ref. [7], the 2% Gd-doped 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). I11 our previous work [8], the reactor neutrino detector with 1 % Gd-doped liquid scintillator (Gd-LS) is used as the neutron veto system, and its neutron background can be reduced to about 0.3 per year per tonne of liquid xenon. These neutron background events are mainly from the spontaneous fission and (a.n) reactions due to 238U and 232Th in the photomultiplicr tubes (PMTs) in the liquid xenon.

Because of its advantages of the low background rate, energy resolutions, and low energy threshold, high-purity germanium (HPGe) is widely used in dark matter and ncutrinolcss double beta decay experiments [9, 10]. I11 our work, ,6Ge is used as a WIMP target material and WIMPs are detected by only the ionization channel (like the CDEX and CoGeNT experiments). This makes its neutron background much

loss than in the ease of a xenon target without PMTs in HPGo detectors. The CDEX and CoGoNT experiments are respectively located in an underground laboratory with a depth of 7000 meter water equivalent (iii.w.o.) and 2100 iii.w.o. Neutrons can only be shielded but not tagged in these two experiments. A detector configuration that can shield and tag neutrons rejects neutron background better in dark matter experiments. The feasibility of direct WIMP detection with the neutron veto based on the neutrino detector was validated in our previous work [8]. In this paper, a neutrino detector with Gd-LS (1 % Gd-doped) is still used as a neutron-tagged device and WIMP detectors with HPGo targets (called Go modules) are placed inside the Gd-LS. Hero, wo designed an experimental configuration of 984 Go modules individually placed inside four reactor-neutrino detector modules used as a neutron veto system. The experimental hall of the configuration is assumed to be located in an underground laboratory with a depth of 910 m.w.e., which is similar to the far hall in the Daya Bay reactor-neutrino experiment [11]. Collecting ionization signals is considered the only method of WIMP detection in our work. The neutron background for this design is estimated using the Geant4 [12] simulation.

The basic detector layout is described in Sec. 2. Some features of the simulation in our work are described in Sec. 3. The neutron background of the experimental configuration is estimated in Sec. 4. The contamination due to reactor neutrinos is discussed in Sec. 5. We conclude in Sec. 6.

2. DETECTOR DESCRIPTION

Four identical WIMP detectors with HPGo targets are individually placed inside four identical neutrino detector modules. The experimental hall of this experimental configuration is assumed to be located in an underground laboratory with a depth of 910 m.w.e., which is similar to the far hall in the Daya Bay reactor neutrino experiment. The detector is located in a 20 x 20 x 20 ni3 cavern. The four identical cylindrical neutrino modules (each 413.C cm in height and 393.C cm in diameter) are immersed into a 13 x 13x8 in3 water pool at a depth of 2.5 m from the top of the pool and at a distance of 2.5 m from each vertical surface of the pool. The detector configuration is shown in Fig. 1.

Each neutrino module is partitioned into three enclosed zones. The innermost zone is filled with the 1 % Gd doped liquid scintillator [8] (2.C m in height, 2.4 m in diameter), which is surrounded by a zone filled with

unloaded liquid scintillator (LS) (35 cm thick). The outermost zone is filled with transparent mineral oil (40 cm thick) [13]. 3CC 8-inch PMTs are mounted in the mineral oil. These PMTs are arranged into 8 rings of 30 PMTs on the lateral surface of the oil region, and 5 rings of 24, 18, 12, 6, 3 on the top and bottom caps.

Each WIMP detector consists of an outer copper vessel (144.6 cm in height, 82.8 cm in diameter, and 0.8 cm in thickness), which is surrounded by an aluminum (Al) reflector (0.2 cm thick) and an inner copper vessel (116.G cm in height, 54.8 cm in diameter and 0.5 cm in thickness). There is a vacuum zone between the outer and inner copper vessels (about 13 cm thick). The part inside the inner copper vessel is made up of two components: the upper component is a cooling system with liquid nitrogen of a very high purity (32 cm in height, 54.8 cm in diameter) and the lower one is an active target of 246 Go modules arranged into 6 rows (each row includes 4 rings of 20, 14, 6, 1). Each Go modulo is made of a copper vessel and an HPGo target: there is an HPGo target (6.2 cm in height, 6.2 cm in diameter, ~ 1 kg) in a 0.1 cm thick copper vessel (12.6 cm in height, 6.4 cm in diameter).

3. SOME FEATURES OF THE SIMULATION

The Geant4 (version 8.2) package [12] was used in our simulations. The physics list in the simulations includes transportation processes, decay processes, low-energy processes, electromagnetic interactions (multiple scattering processes, ionization processes, scintillation processes, optical processes, Cherenkov processes, Bremsstrahlung processes, etc.), and hadron interactions (leptoii nuclear processes, fission processes, elastic scattering processes, inelastic scattering processes, capture processes, etc.). The respective cuts for the productions of gamma quanta, electrons and positrons are 1 mm, 100 //in and 100 //in. The quenching factor is defined as the ratio of the detector response to nuclear and electron recoils. The Birks factor for protons in the Gd-LS is set to 0.01 g-cni_2-MoV_1, corresponding to the quenching factor 0.17 at 1 MeV, in our simulation.

4. NEUTRON BACKGROUND ESTIMATION

Figure 2 shows the recoil spectra for WIMP interactions with l6Ge nucleus in the case of the WIMP mass of 100 GeV (the tool from Ref. [14] has been used). To reject neutrino background, the recoil energy was set in a range from 10 koV to 100 koV in this work. Multi-scattered in the detector, neutrons can be tagged by

06.4

06.2

Ge target

6.2 12.4 12.6

Ge module

WIMP detector d

I

--Mineral oil

LS

Gd-LS

Neutrino detector veto

WIMP

detector

Water

Neutrino

detector

veto

WIMP

detector

Fig. 1. a) A Go module with the HPGe material (length unit, cm), b) a WIMP detector with 246 Go modules (length unit, cm), c) a neutrino detector where a WIMP detector is placed, d) four WIMP detectors individually placed inside four

neutrino detectors in a water shield

the Go modules. But there is energy deposited in only-one Ge module for a WIMP interaction. Proton recoils induced by neutrons and neutron-captured signals are used to tag neutrons that reach the Gd-LS. The energy deposition produced by proton recoils is close to a uniform distribution. Neutrons captured on Gd and H respectively lead to a release of about 8 MoV and 2.2 MoV of 7 particles. Due to the instrumental limitations of the Gd-LS, we assume neutrons to be tagged if their energy deposition in the Gd-LS is more than 1 MoV, corresponding to 0.17 MoVee (electron equivalent energy). In the Gd-LS, it is difficult to distinguish signals induced by neutrons from electron recoils, which

are caused by radioactivity in the detector components and the surrounding rocks. But this radioactivity can be controlled to less than ~ 50 Hz according to the Daya Bay experiment [11]. If we assume a value of 100 //s for the neutron tagging time window, the indistinguishable signals due to radioactivity resu

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