Measurement of the neutron flux in the CPL underground laboratory and simulation studies of neutron shielding for WIMP searches

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1 Astroparticle Physics 20 (2004) Measurement of the neutron flux in the CPL underground 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 , South Korea b Department of Science Education, Ewha Woman s University, Seoul , South Korea c School of Physics, Seoul National University, Seoul , South Korea d Department of Physics, Sejong University, Seoul , South Korea e IPST, Department of Physics, University of Maryland, College Park, MD 20742, USA f Physics Department, Seongkywunkwan University, Suwon , 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/cm 2 /s with the neutron energy in the range 1:5 < E n < 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 counts/kev/ kg/day. An active shield not only reduces the neutron background but can also reduce the uncertainty in the neutron background estimation. Ó 2003 Elsevier B.V. All rights reserved. PACS: d; Mc Keywords: Dark matter; Underground; Neutron background; Active shielding; GEANT4 * Corresponding author. Tel.: ; fax: / address: hongjoo@phya.yonsei.ac.kr (H.J. Kim). 1 Present address: Department of Physics, Ewha WomanÕs University, Seoul , South Korea /$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi: /j.astropartphys

2 550 H.J. Kim et al. / Astroparticle Physics 20 (2004) Introduction It is known that dark matter is a major component of the universe and WIMPs are a strong candidate. WIMPs can be detected through elastic scattering 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 ranges from 0.1 to 10 5 counts/kg/day for the WIMP mass ranges from 20 to 1000 GeV and the spin independent WIMP cross-section ranges pb. The pulse shape discrimination (PSD) method can be used to separate the nuclear recoil events from the c background. NaI(T ) crystals together with 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 significant background source since there is no difference between neutron and WIMP signals. Misunderstanding 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 of all the heavy (A > 7) radioactive nuclei occurs inside stars and nuclei with long half-lives such as 238 U, 232 Th, and 40 K are the major sources of background radiation in the underground laboratory. There is also significant 222 Rn background radiation in the air. Cosmic-ray muons interacting in the surrounding rock can generate both c and neutron backgrounds. Man-made radionuclides are generated by nuclear bombs and nuclear reactors. Since 137 Cs 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 impure water during CsI powder production is responsible for the 137 Cs background [6]. It is possible to reduce the cesium radioisotope contamination 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 background facility for a WIMP search. We have measured the various background sources at the CheongPyung underground laboratory (CPL) located 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 c background was measured with an HPGe detector and a prototype CsI(T ) crystal detector independently. 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 produced from three major sources: cosmic-ray muon interactions, spontaneous fission of 238 U, and (a; n) reactions. Photonuclear interactions of muons generate neutrons by fragmenting the target nuclei. 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 238 U generates neutrons with an neutron energy spectrum similar to that of 252 Cf neutron source. Alpha particles from the decay of 238 U and 232 Th 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 238 U and 232 Th play an important role in neutron production. Among various organic scintillators used for fast neutron spectroscopy and time of flight measurements, the BC501A liquid scintillator has the additional advantage of excellent pulse shape discrimination between neutrons and cs [9]. We used the BC501A liquid scintillation counter for the measurement of the fast neutron flux at the underground laboratory. Our main interest is to measure the neutron flux precisely enough in order

3 H.J. Kim et al. / Astroparticle Physics 20 (2004) 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 an active 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 interpret the final results, particularly if a positive signal is observed. Although the Monte Carlo simulations can provide some information, they may not be sufficient to establish a positive WIMP signal. Active neutron shielding is necessary to understand and to reduce the neutron background. 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 background. We can also monitor the PSD performance of the CsI crystals by using tagged neutron events. In addition, the cosmic-ray induced events can be rejected. 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 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 c ray background Dominant sources of c ray backgrounds are due to 238 U, 232 Th and 40 K 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 thorium content of the rock samples was measured by neutron activation analysis. As shown in Table 3, the rock sample contains (4.8 ± 1.8) ppm of 238 U and (6.0 ± 2.2) ppm of 232 Th. Another source of c background radiation, 40 K, 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 40 K and 208 Tl are clearly shown at the high-energy region without any shielding materials (upper histogram). The background level without any shields is about 10 5 counts/kev/kg/day at 200 kev, and with a 10 cm thick lead and 10 cm thick copper and N 2 gas flowing it was reduced by a factor in excess of 10,000 (lower histogram). The observed c background from the decay chains of Table U, 232 Th and K 2 O of rock and concrete at CPL Elements 238 U 232 Th K 2 O 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% Table 1 Major elements concentration of rock at CPL (ppm) Elements SiO 2 Al 2 O 3 Fe 2 O 3 MgO K 2 O Na 2 O Concentration (ppm) Table 2 Minor elements concentration of rock at CPL (ppm) Elements P Ba Cr Ni Sr Cu Nb Pb Zn Ga Rb S Cl Concentration (%) <400

4 552 H.J. Kim et al. / Astroparticle Physics 20 (2004) 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 N 2 gas flowing. 238 U, 232 Th, and 40 K at CPL is comparable with results expected from the rock sample analysis Neutron flux We have constructed 0.5 and 1.0 l BC501A detectors in cylindrical shapes to measure the highenergy neutron flux for the primary goal of estimating 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 between 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 discriminator (CFD) was used for triggering and the PMT signals were delayed properly to have both total 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 obtained by utilizing the Compton edge of the 662 kev cs from the 137 Cs source. The energy calibration constant, ADC/keV, has been extracted by comparing the measured Compton edge with GEANT4 [11] simulation. The energy resolution pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi was determined to be r=e ðmevþ ¼0:06= E ðmevþ. 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 separation. 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 made of 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 neutron 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

5 H.J. Kim et al. / Astroparticle Physics 20 (2004) 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 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 spectrum induced by neutrons is shown in Fig. 3b. The neutron 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 recoiled proton, we need to use an unfolding procedure to determine the neutron energy spectrum. The GEANT4 program was used for the neutron recoil simulation. We assumed that the neutron enters the detector from a random direction, and that the neutron energy was generated uniformly from 0 to 20 MeV. The nonlinear response to the recoiled proton and carbon by a neutron in a liquid scintillator can be written as Fig. 3. Energy deposit in BC501A by neutron scattering at the underground. (a) Total versus tail energy deposit distribution. (b) Measured electron-equivalent energy distribution with 300 kev threshold.

6 554 H.J. Kim et al. / Astroparticle Physics 20 (2004) E ee ¼ 0:83E p 2:82½1 expð 0:25Ep 0:93 ÞŠ [14], where E ee is the electron-equivalent energy and E p is the recoiled proton energy which was taken into account in the simulation. Also the detector energy resolution was folded in, and the 300 kev threshold was applied. Using the above information, 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 natural way to unfold experimental distributions in order to derive the best estimation of the true distribution. Its iterative technique and smoothing procedure provide reliable and stable results with respect to variations of the initial probabilities. We took the uniform distribution as an input 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 the threshold effect and the poor resolution, it is difficult to obtain reliable results when the neutron 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 same method, 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/ cm 2 /s with the neutron energy in the range Fig. 4. Neutron MC data; (a) generated neutron energy spectrum (line) and unfolded simulation (filled circles). (b) Correlation matrix between generated and reconstructed energy.

7 H.J. Kim et al. / Astroparticle Physics 20 (2004) Fig. 5. Unfolded neutron energy spectrum at CPL in the energy range from 1.5 to 6 MeV. 1.5 MeV < E n < 6 MeV and the energy spectrum is shown in Fig. 5. The systematic uncertainties associated 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. The shape of the neutron energy spectrum is similar to the simulated neutron spectrum at Modane 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]. Fig. 6. Conceptual design of shielding for the KIMS experiment.

8 556 H.J. Kim et al. / Astroparticle Physics 20 (2004) 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 polyethylene 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 scintillation counter (LSC) is placed inside of the LS. The LSC is located inside of the LS for the following 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. The innermost shielding layer is a 10 cm thick oxygenfree 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 electronequivalent energy threshold of 300 kev. We aim to Fig. 7. Fraction of penetrating 3.0 MeV neutrons through BC501A as a function of its thickness. 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 for the 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 scintillator was about a factor of 60. Moreover, we can Fig. 8. Expected low-energy spectrum in CsI crystal produced by neutron scattering in the shielding.

9 H.J. Kim et al. / Astroparticle Physics 20 (2004) eliminate the neutron background by the liquid scintillator signals. It was found that neutron tagging 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 and 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 the shielding 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/cm 2 /s with the neutron energy in the range 1.5 MeV < E n < 6 MeV. We studied the effects of various arrangements of shielding materials and their geometries for minimizing the neutron flux. One such shielding 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 sufficiently 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 liquid 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 Science Research Initiative program. Y.J. Kwon wishes to acknowledge the financial support of the atomic research and development project of the year of Y.D. Kim also acknowledges the support by Korea Research Foundation Grant (KRF DP0078). We are grateful to J.P. Wellisch for assistance with the GEANT4 neutron simulation. References [1] M.W. Goodman, E. Witten, Phys. Rev. D 31 (1985) [2] P.F. Smith et al., Phys. Lett. B 379 (1996) 299; R. Bernabei et al., Phys. Lett. B 389 (1996) 757. [3] H.J. Kim, S.K. Kim, E. Won, H.J. Ahn, Y.D. Kim, in: Proceeding of the 29th International Conference on High Energy Physics, Vancouver, p. 1543; H.J. Kim et al., Nucl. Instrum. Methods A 457 (2001) 471. [4] H. Park et al., Nucl. Instrum. Methods A 491 (2002) 460. [5] S. Pecourt et al., Astropart. Phys. 11 (1999) 457; V.A. Kudryavtsev et al., Nucl. Instrum. Methods A 456 (2001) 272. [6] Y.D. Kim et al., J. Korean Phys. Soc. 40 (2002) 520; T.Y. Kim et al., Nucl. Instrum. Methods A 500 (2003) 337. [7] S.K. Kim et al., Nucl. Phys. B 124 (2003) 217 (Proc. Suppl.); H.J. Ahn et al., KIMS Collaboration, Technical Design Report, [8] D. Abriola et al., Astropart. Phys. 6 (1996) 63. [9] J.B. Birks, Theory and Practice of Scintillation Counter, Pergamon press, Oxford, [10] R. Brun, F. Rademakers, Nucl. Instrum. Methods A 389 (1997) 81. [11] The GEANT4 Collaboration, geant4/geant4.html. [12] M.J. Hwang et al., Nucl. Instrum. Methods, in press. [13] Cross Section Evaluation Working Group, ENDF/B-VI Summary Document, Report BNL-NCS (ENDF- 201), [14] D.M. Drake et al., Nucl. Instrum. Methods A 274 (1986) 576. [15] G. DÕagostini, Nucl. Instrum. Methods A 362 (1995) 487. [16] V. Chazal et al., Nucl. Instrum. Methods A 9 (1998) 163. [17] S.R. Hashemi-Nezhad et al., Nucl. Instrum. Methods A 357 (1995) 52.