measurement of the neutron flux in the cpl underground laboratory and simulation studies of neutron...

Astroparticle Physics 20 (2004) 549–557

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Measurement of the neutron flux in the CPLunderground laboratory and simulation studies

of neutron shielding for WIMP searches

H.J. Kim a,*, I.S. Hahn b, M.J. Hwang a, R.K. Jain c, U.K. Kang d, S.C. Kim c,S.K. Kim c, T.Y. Kim c, Y.D. Kim d, Y.J. Kwon a, H.S. Lee c, J.H. Lee a,

J.I. Lee d, M.H. Lee e, D.S. Lim d, S.H. Noh f, H. Park c, I.H. Park c,1, E.S. Seo e,E. Won c, M.S. Yang c, I. Yu f

a Physics Department, Yonsei University, ShinChon-Dong, Seoul 120-749, South Koreab Department of Science Education, Ewha Woman’s University, Seoul 120-750, South Korea

c School of Physics, Seoul National University, Seoul 151-742, South Koread Department of Physics, Sejong University, Seoul 143-747, South Korea

e IPST, Department of Physics, University of Maryland, College Park, MD 20742, USAf Physics Department, Seongkywunkwan University, Suwon 440-746, South Korea

Received 9 July 2002; received in revised form 9 September 2003; accepted 26 September 2003

Abstract

Searches for weakly interacting massive particles (WIMPs) can be carried out based on the detection of nuclear

recoil energy in CsI(T‘) crystals. It is crucial to minimize the neutron background as well as to fully understand the

remaining background sources through adequate shielding when using the pulse shape discrimination method for

WIMP detection. We have measured the neutron flux at 350 m minimum depth, where the CheongPyung underground

laboratory (CPL) is located, to be 3.00 ± 0.02 (stat.) ± 0.05 (syst.) · 10�5 neutrons/cm2/s with the neutron energy in the

range 1:5 < En < 6 MeV. Using the GEANT4 simulation, we have demonstrated that the neutron flux can be reduced

sufficiently for dark matter searches with 30 cm of polyethylene passive shield and 20 cm of BC501A liquid scintillator

active shield. The neutron induced background at a few keV energy deposit in CsI crystal is less than 0.001 counts/keV/

kg/day. An active shield not only reduces the neutron background but can also reduce the uncertainty in the neutron

background estimation.

� 2003 Elsevier B.V. All rights reserved.

PACS: 95.35.+d; 29.40.Mc

Keywords: Dark matter; Underground; Neutron background; Active shielding; GEANT4

* Corresponding author. Tel.: +82-2-2123-2601; fax: +82-2-875-4719/392-1592.

E-mail address: [email protected] (H.J. Kim).1 Present address: Department of Physics, Ewha Woman�s University, Seoul 120-750, South Korea.

0927-6505/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.astropartphys.2003.09.001

mail to: [email protected]

550 H.J. Kim et al. / Astroparticle Physics 20 (2004) 549–557

1. Introduction

It is known that dark matter is a major com-

ponent of the universe and WIMPs are a strong

candidate. WIMPs can be detected through elasticscattering with a nucleus in a detector [1]. It is

crucial to minimize background radiation for dark

matter search since the expected interaction rate of

the WIMP-nucleus elastic scattering is small. For

the CsI target with a 2 keV energy threshold and

quenching factor consideration, the event rate ran-

ges from 0.1 to 10�5 counts/kg/day for the WIMP

mass ranges from 20 to 1000 GeV and the spinindependent WIMP cross-section ranges 10�5–10�9

pb. The pulse shape discrimination (PSD) method

can be used to separate the nuclear recoil events

from the c background. NaI(T‘) crystals togetherwith the PSD method have been used for WIMP

searches [2]. It was recently realized that the PSD

capability for CsI(T‘) is even better than that for

NaI(T‘) [3–5]. Neutrons are the most significantbackground source since there is no difference be-

tween neutron and WIMP signals. Misunder-

standing of the neutron background may lead to

false detection of a WIMP signal. Thus it is critical

to eliminate the neutron background effectively

and to fully understand the remaining sources.

Radioactive sources are classified as primordial,

cosmogenic, and man-made. Nucleosynthesis ofall the heavy (A > 7) radioactive nuclei occurs in-

side stars and nuclei with long half-lives such as238U, 232Th, and 40K are the major sources of

background radiation in the underground labora-

tory. There is also significant 222Rn background

radiation in the air. Cosmic-ray muons interacting

in the surrounding rock can generate both c and

neutron backgrounds. Man-made radionuclidesare generated by nuclear bombs and nuclear

reactors. Since 137Cs is a man-made nuclide and is

already known to exist in CsI crystals, it is a major

background source when using CsI crystals for

dark matter searches. It was found that using im-

pure water during CsI powder production is

responsible for the 137Cs background [6]. It is

possible to reduce the cesium radioisotope con-tamination as low as 1 m Bq/kg if highly purified

water is used in throughout the process for making

CsI powder [7].

The KIMS (Korean Invisible Mass Search)

Collaboration [7] has designed an underground

experiment using CsI(T‘) crystals in a low back-

ground facility for a WIMP search. We have

measured the various background sources at the

CheongPyung underground laboratory (CPL) lo-cated in one of several tunnel cavities in Mt. Ho

Myung. They were excavated for a storage water

power plant which is 80 km northeast of Seoul,

Korea. The minimum depth from the ground

surface is about 350 m, which corresponds to

about 1000 m water equivalent (w.e.). The cbackground was measured with an HPGe detector

and a prototype CsI(T‘) crystal detector indepen-dently. It has been demonstrated that less than

1 count/keV/kg/day c background was achieved at

a depth of 1000 m w.e. [8]. Since CPL is at the

same depth, the cosmic-ray induced c background

at CPL is expected to be the same with proper

shielding materials and hence this should not pose

a problem for the WIMP search.

Neutrons at a deep underground site are pro-duced from three major sources: cosmic-ray muon

interactions, spontaneous fission of 238U, and (a; n)reactions. Photonuclear interactions of muons

generate neutrons by fragmenting the target nu-

clei. Neutrons can also be produced by muon

capture, but the rate is much lower than that of

photonuclear interactions at the depth of CPL.

Fission of 238U generates neutrons with an neutronenergy spectrum similar to that of 252Cf neutron

source. Alpha particles from the decay of 238U and232Th can generate neutrons through the (a; n)reaction. The energy and neutron flux of neutrons

from this reaction depends on the cross-section of

the (a; n) reaction as well as the energy and flux of

alpha particles. Thus the rock contents of 238U and232Th play an important role in neutron produc-tion.

Among various organic scintillators used for

fast neutron spectroscopy and time of flight mea-

surements, the BC501A liquid scintillator has the

additional advantage of excellent pulse shape dis-

crimination between neutrons and cs [9]. We used

the BC501A liquid scintillation counter for the

measurement of the fast neutron flux at theunderground laboratory. Our main interest is to

measure the neutron flux precisely enough in order

H.J. Kim et al. / Astroparticle Physics 20 (2004) 549–557 551

to estimate the nuclear recoils in CsI crystals and

to reduce the neutron recoil background below

0.01 counts/keV/kg/day. The KIMS CsI crystals

will be completely surrounded by a proper

shielding system, which consists of a passive shield

of polyethylene, lead, and copper as well as anactive shield of BC501A liquid scintillator. The

recoil signal in CsI crystal induced by the elastic

scattering of neutrons is not separable from the

WIMP signal. Unless the neutron background is

fully understood, it would be impossible to inter-

pret the final results, particularly if a positive sig-

nal is observed. Although the Monte Carlo

simulations can provide some information, theymay not be sufficient to establish a positive WIMP

signal. Active neutron shielding is necessary to

understand and to reduce the neutron back-

ground. If we tag the incoming neutrons efficiently

and the detection efficiency is well known, we can

then estimate the untagged neutron backgrounds

in CsI crystals. In this way, we do not need to rely

on simulations to estimate the neutron back-ground. We can also monitor the PSD perfor-

mance of the CsI crystals by using tagged neutron

events. In addition, the cosmic-ray induced events

can be rejected.

Table 3238U, 232Th and K2O of rock and concrete at CPL

Elements

238U 232Th K2O

Rock (4.8 ± 1.8) ppm (6.0± 2.2) ppm 4.2%

Concrete (10.7 ± 0.30) ppm (2.6± 0.81) ppm 2.1%

2. Background at the underground laboratory

2.1. Cosmic-ray flux

We measured the cosmic-ray muon flux at CPL,

which has a minimum depth of 350 m. The muon

fluxes at the surface level and the underground

laboratory were measured with three 1 cm thick

scintillation counters all in coincidence to be free

Table 1

Major elements concentration of rock at CPL (ppm)

Elements SiO2 Al2O3

Concentration (ppm) 61.35 13.59

Table 2

Minor elements concentration of rock at CPL (ppm)

Elements P Ba Cr Ni Sr C

Concentration (%) 30 445 26 12 106 5

from the huge environmental c background and to

be sure of rare cosmic-ray muon events.

The reduction factor of the muon flux from the

surface to underground is (1.40± 0.15) · 10�4 as

expected at the depth of CPL.

2.2. c ray background

Dominant sources of c ray backgrounds are dueto 238U, 232Th and 40K and their progenitors. The

rock samples at CPL were chemically analysed to

determine elemental compositions. The results are

shown in Tables 1 and 2. The uranium and tho-

rium content of the rock samples was measured byneutron activation analysis. As shown in Table 3,

the rock sample contains (4.8 ± 1.8) ppm of 238U

and (6.0 ± 2.2) ppm of 232Th. Another source of cbackground radiation, 40K, was measured to be

4.0 ppm. Fig. 1 shows the c ray spectrum measured

by an ultra-low background 100% HPGe detector

at CPL. There are several peaks; in particular,

peaks from 40K and 208Tl are clearly shown at thehigh-energy region without any shielding materials

(upper histogram). The background level without

any shields is about 105 counts/keV/kg/day at 200

keV, and with a 10 cm thick lead and 10 cm thick

copper and N2 gas flowing it was reduced by a

factor in excess of 10,000 (lower histogram). The

observed c background from the decay chains of

Fe2O3 MgO K2O Na2O

3.38 0.95 4.19 0.91

u Nb Pb Zn Ga Rb S Cl

2 36 41 67 11 200 20 <400

Fig. 1. c background spectrum at CPL measured with a 100% HPGe detector. Upper histogram is the measurement without shielding,

middle one is with 10 cm Pb shielding and lower one is with 10 cm Pb, 10 cm Cu, and N2 gas flowing.


238U, 232Th, and 40K at CPL is comparable with

results expected from the rock sample analysis.

2.3. Neutron flux

We have constructed 0.5 and 1.0 l BC501A

detectors in cylindrical shapes to measure the high-

energy neutron flux for the primary goal of esti-

mating the neutron recoil background in CsI

crystal detector. The Digital Charge Comparison

method (DCC) was used to separate neutrons

from c rays. The DCC method uses the ratio be-tween the total signal and the tail part of the signal

since the tail of the neutron signal is larger than

that of the c signal. A 2249W LeCroy charge

integration ADC with a CAMAC system was used

to readout the signal from a 2 in. H1161 PMT

attached to the detector. A constant fraction dis-

criminator (CFD) was used for triggering and the

PMT signals were delayed properly to have bothtotal and tail signals. A ROOT [10]-based DAQ

system running on a Linux PC was used to collect

the data.

The energy calibration and resolution was ob-

tained by utilizing the Compton edge of the 662

keV cs from the 137Cs source. The energy calibra-

tion constant, ADC/keV, has been extracted by

comparing the measured Compton edge with

GEANT4 [11] simulation. The energy resolu-

tion was determined to be r=E ðMeVÞ ¼ 0:06=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiE ðMeVÞ

p. The details of the BC501A neutron

detector performance are described in Ref. [12].

We used a 2.4 MeV mono-energetic neutron beam

and the GEANT4 simulations to validate the

nonlinear response of recoiled nucleons. For the

simulation of low-energy neutrons ENDF/B-VI

[13] was used in GEANT4. After converting the

deposited energy to electron-equivalent energy and

folding in the resolution and calibration constant,the Monte Carlo results and the experimental data

are compared in Fig. 2. The beam data was taken

with 300 keV threshold for the clear n=c separa-

tion. The simulated energy spectrum is in good

agreement with the data.

The neutron flux at CPL was measured with

three different shielding conditions: no shield, a 5

cm thick lead shield, and a prototype shield madeof a 15 cm thick lead and a 10 cm thick copper. As a

consequence of major c background events, it is

difficult to measure the neutron flux with no shield.

Since there is not much difference in the fast neu-

tron flux with the 5 cm lead shield and without

shield, the 5 cm lead shield was used for the neutron

flux measurement. The event rate in the energy

Fig. 2. Comparison of electron-equivalent energy distribution

between GEANT4 MC (histogram) and test-beam data (filled

circles) taken with a 2.4 MeV mono-energetic neutron beam.


range from 0.3 to 3.0 MeV was 0.6 Hz with the 0.5 l

detector, where the c background is dominant. We

Fig. 3. Energy deposit in BC501A by neutron scattering at the un

(b) Measured electron-equivalent energy distribution with 300 keV th

obtained data for the neutron detector inside the

prototype shield for two weeks. A comparison of

the total and tail energy deposits at the neutron

detector using the DCC method is shown in Fig.

3a. The measured electron-equivalent energy spec-

trum induced by neutrons is shown in Fig. 3b. Theneutron counting rate between 0.3 and 3 MeV

electron-equivalent energy was 280 events per day

with a 0.5 l neutron detector.

Since what we measured is not the neutron

energy but the electron-equivalent energy of re-

coiled proton, we need to use an unfolding pro-

cedure to determine the neutron energy spectrum.

The GEANT4 program was used for the neutronrecoil simulation. We assumed that the neu-

tron enters the detector from a random direction,

and that the neutron energy was generated uni-

formly from 0 to 20 MeV. The nonlinear re-

sponse to the recoiled proton and carbon by a

neutron in a liquid scintillator can be written as

derground. (a) Total versus tail energy deposit distribution.

reshold.


Eee¼0:83Ep�2:82½1�expð�0:25E0:93p Þ� [14], where

Eee is the electron-equivalent energy and Ep is

the recoiled proton energy which was taken into

account in the simulation. Also the detector en-

ergy resolution was folded in, and the 300 keV

threshold was applied. Using the above infor-mation, the Bayesian unfolding method [15] was

used to obtain the neutron energy spectrum and

flux. A simple unfolding method, such as matrix

inversion, is difficult to use in this case because of

the broad correlation between the nucleon recoil

energy and the incident energy. Also the detector

resolution and threshold cut make the unfolding

complicated. The Bayesian method offers a nat-ural way to unfold experimental distributions in

order to derive the best estimation of the true

distribution. Its iterative technique and smooth-

ing procedure provide reliable and stable results

with respect to variations of the initial probabil-

Fig. 4. Neutron MC data; (a) generated neutron energy spectrum (line

between generated and reconstructed energy.

ities. We took the uniform distribution as an in-

put and obtained reliable results for both the

Monte Carlo and the real data. The unfolded

simulation data obtained by the Bayesian method

is shown in Fig. 4a in comparison with the input

energy spectrum given in a line. As a result of thethreshold effect and the poor resolution, it is

difficult to obtain reliable results when the neu-

tron energy is below 1.5 MeV, as shown in the

correlation plot of the generated and simulated

neutron spectra in Fig. 4b. Since there is no

neutron above 3 MeV electron-equivalent energy,

this unfolding is valid up to approximately 6

MeV neutron energy. With exactly the samemethod, the measured electron-equivalent energy

by neutron scattering as shown in Fig. 3b was

unfolded. The neutron flux was measured to be

3.00 ± 0.02 (stat.) ± 0.05 (syst.) · 10�5 neutrons/

cm2/s with the neutron energy in the range

) and unfolded simulation (filled circles). (b) Correlation matrix

Fig. 5. Unfolded neutron energy spectrum at CPL in the energy range from 1.5 to 6 MeV.


1.5 MeV < En < 6 MeV and the energy spectrum

is shown in Fig. 5. The systematic uncertaintiesassociated with the n=c separation and unfolding

procedure were taken into account. We set a 1.5

MeV threshold on the unfolded neutron flux since

the neutron flux distribution below the threshold

is not reliable as demonstrated by our simulation

studies.

Fig. 6. Conceptual design of shieldi

The shape of the neutron energy spectrum is

similar to the simulated neutron spectrum atModane where the rock composition is different

from CPL [16]. However, the flux at CPL is about

10 times higher than that at Modane at 1708 m

underground, but is similar to that at Hoken Hill,

Australia at 1230 m underground, where the rock

composition is similar to the CPL [17].

ng for the KIMS experiment.

Fig. 7. Fraction of penetrating 3.0 MeV neutrons through

BC501A as a function of its thickness.


3. Monte Carlo simulation for the neutron shielding

Using the GEANT4 simulation, we studied the

shielding optimization for the KIMS experiment

based on the measured c and neutron background.The conceptual design of this shielding system is

shown in Fig. 6. The outermost layer is a poly-

ethylene shield (PS) of 30 cm thickness for the

neutron shield. Just inside the polyethylene, a 15

cm thickness low background Boliden lead (LS) is

situated to reduce the c background. An active

shield composed of 20 cm BC501A liquid scintil-

lation counter (LSC) is placed inside of the LS.The LSC is located inside of the LS for the fol-

lowing reasons. It would be difficult to separate

neutrons from cs because of the overwhelming

number of c events if it were located outside of the

LS. Also neutrons produced by cosmic muon

interactions with the lead may not be tagged by the

LSC. A 1 mm thick cadmium sheet will be located

inside the LSC to absorb thermal neutrons. Theinnermost shielding layer is a 10 cm thick oxygen-

free highly conductive (OFHC) copper layer to

reduce surviving c rays.

We estimated the required thickness of liquid

scintillator for the neutron shield. Fig. 7 shows the

inefficiency curve with respect to the scintillator

thickness for 3 MeV neutrons with the electron-

equivalent energy threshold of 300 keV. We aim to

Fig. 8. Expected low-energy spectrum in CsI crystal p

achieve a rate of neutrons hitting the CsI(T‘)crystal detector of below 0.01 counts/keV/kg/day,

assuming that every neutron hitting the CsI crystal

generates a sizable signal around a few keV. We

simulated neutron propagation with the baseline

detector and shield. The measured neutron flux and

energy distribution as shown in Fig. 5 was used forthe flux of neutrons incident on the polyethylene

shield. A reduction factor of 800 was achieved with

30 cm of polyethylene. Additional reduction by

other shielding materials including the liquid scin-

tillator was about a factor of 60. Moreover, we can

roduced by neutron scattering in the shielding.


eliminate the neutron background by the liquid

scintillator signals. It was found that neutron tag-

ging efficiencies are 84% and 73% with the 0.3 and

0.6 MeV n=c separation thresholds, respectively.

When we apply this information, we can achieve a

neutron background as low as 4.5 · 10�4 and7.6 · 10�4 counts/keV/kg/day with 0.3 and 0.6 MeV

energy thresholds, respectively, at 2–3 keV energy

deposit in CsI crystal as shown in Fig. 8. The

neutron background rate with baseline shielding

system reaches a value which is much lower than

our design goal for the total background rate. We

also used a GEANT4 simulation to study the

neutron background from muon interactions in theshielding material. The measured muon flux was

used for this study. The background from this

source in the CsI crystal in a few keV energy range

is less than 10�4 counts/keV/kg/day level which is

smaller than that due to other neutron background

sources.

4. Conclusion

We have measured the neutron flux at 350 m

underground at CheongPyung to be 3.00 ± 0.02

(stat.) ± 0.05 (syst.) · 10�5 neutrons/cm2/s with the

neutron energy in the range 1.5 MeV < En < 6

MeV. We studied the effects of various arrange-

ments of shielding materials and their geometriesfor minimizing the neutron flux. One such shield-

ing arrangement consists of a 10 cm thick copper

shield for the innermost section, a 20 cm thick

BC501A liquid scintillator for an active shielding,

a 15 cm thick lead shield, and a 30 cm thick

polyethylene shield for passive shielding at the

outermost section. Using GEANT4 simulation, we

demonstrated that the neutron flux can be suffi-ciently reduced to enable a dark matter search with

a 30 cm thick polyethylene passive shielding at the

outermost section and a 20 cm thick BC501A li-

quid scintillator active shielding between a 10 cm

thick copper and a 15 cm thick lead shielding at

the inner section.

Acknowledgements

This work is supported by the Korean Ministry

of Science and Technology under a Creative Sci-

ence Research Initiative program. Y.J. Kwonwishes to acknowledge the financial support of the

atomic research and development project of the

year of 1999. Y.D. Kim also acknowledges the

support by Korea Research Foundation Grant

(KRF-99-015-DP0078). We are grateful to J.P.

Wellisch for assistance with the GEANT4 neutron

simulation.

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