ELSEVIER Nuclear Physics B (Proc. Suppl.) 124 (2003) 217-220 www.elscvier.com/locatelnpe

Development of CsI crystals for W IMP search S.K.Kima* I.S.Cho D.H.Choi J.M.Choia I.S.Hahnb M.J.Hwang R.K.Jai+’ W.K.Kang H.J.KimC J.H.Kima S.C.Kima T.Y.Kima Y.D.Kimd Y.J.KwonC H.S.Leea M.H.Leee S.E.Leea S.H.Noh H.Park” I.H.Parka E.S.Seo” E.Wona H.S.Wona H.Y.Yan$ M.S.Yang” I.Yuf

aDMRC and School of Physics, Seoul National University, Seoul 151-742, Korea

bDepartment of Science Education, Ewha Woman’s University, Seoul 120-750, Korea

‘Department of Physics, Yonsei University, Seoul 120-749, Korea

dDepartment of Phy sits, Sejong University, Seoul 143-747, Korea

eDepartment of Physics, University of Maryland, College Park,MD 20742, USA

‘Department of Physics, Seongkyunkwan University, Suwon 440-746, Korea

An experimental search for WIMP using CsI(T1) crystals(KIMS) is being prepared at the underground labo- ratory, Cheongpyung, Korea. Characteristics and internal background of CsI(T1) crystal have been investigated in detail to develop highly sensitive detector for WIMP search. The background at the underground laboratory is measured and shielding design has been done with an optimization with a MC study. The result of detector development and prospect of KIMS experiment is reported.

1. Introduction

Scintillation detectors have been widely used for nuclear and high energy physics experiments. Some crystals with high light yield can measure low energy radiation for WIMP search. NaI(T1) crystals have been adopted by several experi- ments[l],[2] for WIMP search. However, due to high hygroscopicity, encapsulation of the crystal is necessary and this could cause unwanted sur- face contamination that could not be removed easily. Recently UKDMC group suggested that excessive signal with abnormally short decay time might be caused by the surface contamination [3]. The CsI(T1) crystal is one of good detector can- didates for WIMP search as studied by [4], [5] and [6]. Because of its good pulse shape discrim- ination(PSD) capability and the much less hy- groscopicity, CsI(TI) can be better crystal for the WIMP search than NaI(T1). In addition, both Cs and I have large mass numbers to give somewhat higher sensitivity than NaI(T1) crystal for spin in-

*e-mail : skkimQhepl.snu.ac.kr

dependent WIMP cross section. Korea Invisible Mass Search(KIMS) group has been developing the CsI(TI) crystal detector for WIMP search.

2. Light yield and pulse shape discrimina- tion

Light yield of CsI(T1) crystal is comparable with that of NaI(T1) crystal. However because of the spectral mismatch between the normal bi- alkali photocathode and the emission spectrum of CsI(T1) crystal, the effective light yield is often quoted as 40% of NaI(T1) crystal. Using pho- tomultiplier tubes with green enhanced Rb-Cs photo-cathode, we measured the photoelectron yield for a 7 x 7 x 30 cm2 crystal. From the sig- nals of 5.9 keV x-ray from 55Fe source, about four photoelectrons per keV is obtained, with which threshold of 2 keV can be easily achieved. In or- der to study the response of the crystal to the WIMP like scattering, we performed a beam test using the mono-energetic neutron beam of 2.62 MeV produced by the bombardment of a 3.2 MeV

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218 S.K. Kim et ~11. /Nuclear Physics B (Proc. Suppl.) 124 (2003) 217-220

0 20 40 60 80 loo 120 140

JLni, OceV)

Mean time

Figure 2. Mean time distribution of signals from neutrons and gammas.

Figure 1. Quenching factor for CsI(T1) crystal with various Tl doping concetnration.

proton beam to a tritium target with a Tandem accelerator located in KIGAM[7]. Six neutron de- tectors made of BC501A liquid scintillator were used to tag the scattered neutrons. By tagging the angle of scattered nuetrons, we estimated the recoil energy of nucleus. Both quenching factors and pulse shape discrimination power are mea- sured. Quenching factor is calculated by the ra- tio between the measured electron equivalent en- ergy and the nuclear recoil energy. The measured quenching factors are shown in Figure 1 for var- ious doping concentration. No significant dop- ing dependence is observed. In Figure 2 we show mean time distributions for neutrons and gam- mas at energies between 3keV to 10 keV. In order to quantify the pulse shape discrimination power we adopt the quality factor given by [8] that is used for sensitivity estimation as a function of PSD. The quality factors for CsI(T1) crystals with different doping concentrations by our measure- ments as well as the quality factors for CsI(T1) and NaI(T1) measured by other groups are shown in Figure 3.

3. Internal background of CsI crystal

Contamination of 13’Cs and 134Cs in CsI crys- tal may cause a serious problem in terms of inter-

nal background. Both 134Cs and 137Cs emit beta rays as they decay to excited states of 134Ba and 13’Ba respectively. The excited stated of 134Ba goes to its ground state by emitting gamma rays immediately. As both gamma ray and electron can be measured in the crystal, the total energy deposit is large and it does not contribute to the background in the WIMP signal region. On the other hand, 137Ba emits 662 keV gamma ray with life time of about two minutes to go to the ground state. Because of the time delay, beta rays from 13’Cs can be serious background in WIMP sig- nal region. *‘Rb is identifed an additional source of internal background, which emits beta rays but without any accompanying gamma rays. All beta and gamma rays from 134Cs, 13’Cs and *‘Rb are generated and fed into the GEANT4 simula- tion program to find out background contribution from each source. The result in comparison with measured background spectrum from an existing crystal is shown in Figure 4. Fairly good agree- ment between the data and simulation is made. From this result we estimated that contamina- tion levels are 155mBq/kg, 35mBq/kg and 3.9 ppb for 13’Cs, 134Cs and Rb, which correspoonds to roughly 60 counts/keV/kg/day(cpd) below 20 keV.

Both 13’Cs and 134Cs do not exist in nature and created by nuclear tests and nuclear reactors after 1940s. Therefore, they must have entered into the

S.K. Kim et ul. /Nuclccn Physics B (Proc. Suppl.) 124 (2003) 217-220 219

Figure 3. Measured quality factors for CsI(T1) crystals with various doping concentrations are shown with previous measurements and NaI(T1).

CsI powder during the chemical processing for the extraction of Cs from the ore. We have measured Cs and Rb contamination levels in various sam- ples of CsI crystals, powders and some intermedi- ate Cs compounds from several vendors[9]. 137Cs and 13*Cs in the crystal are directly measured from the crystal data taking inside the shielding at the underground laboratory. Their contamina- tion in powders are measured by HPGe detector with a proper shielding. Rb concentration is mea- sured by ICP-MASS method. We also measured a pollucite sample, which is the ore of the Cs, and did not observe the contamination of 137Cs confirming the 137Cs was introduced during the extraction process. About 70 liters of water is used to produce lkg of CsI powder and small con- tamination of 137Cs in water can pollute the CsI powder. A sample of processing water was mea- sured to have 137Cs contamination in the order of 10 mBq/liter. Extraction of certain Cs com- pound with highly purified water demonstrated that water is highly suspected to be one of major sources for the contamination.

4. Underground laboratory

The underground laboratory is located in the tunnel about 400 m underground facility of Cheongpyung Stored Water Power Plant which

Figure 4. Measured background spectrum of a CsI(T1) crystal and simulated spectrum from var- ious internal background sources.

is about 80 km away from Seoul. Cosmic ray flux is measured to be l/10000 relative to the ground level that is consistent with the expectation. The elemental composition of the rocks in the tunnel was analyzed by the ICP-MASS method. 238U and 232Th are found to be 9 ppm and 3 ppm respectively. The neutron flux in the tunnel was also measured using a BC501A liquid scintillation counter. After unfolding the measured spectrum, we obtained the neutron energy spectrum and the estimated neutron flux to be 4 x low5 /cm2/s for the energy range of 1.5 N 6 MeV. Details of the measurement of neutron bckground is given else- where [lo].

5. Design of the detector and the back- ground shielding structure

CsI(T1) crystal has the size of 9cm x 9cm x 30 cm that corresponds to 11 kg and two 3 inch RbCs photo-multiplier tubes are mounted at both ends. Four side surfaces are wrapped with thin Teflon tape. The whole detector is an array of 5 x 5 crys- tals. Also neutron detector array made of liquid scintillation counter(LSC) will be located inside the lead shielding to be used as a monitor of rem- nant neutron background inside the shielding as well as the additional neutron shielding. With 20 cm of LSC, about 75% of neutron tagging effi-

220 S. K. Kim et al. /Nuclear Physics B (Pmt. Suppl.) 124 (2003) 217-220

ciency is expected at the threshold of 600 keV. Just outside of the crystal array is 10 cm OFHC copper surrounded by 20 cm thick liquid scintil- lator modules made of BC501A. Then, 15 cm of lead shield made of Boliden lead is located. The outermost layer is 30 cm polyethylene or equiva- lent shielding for slowing down the neutrons. The size of whole shielding structure is 2.2 m x 2.2 m x 3 m and the weight 38 tons. The external background measured in the CPL was used as an input to the GEANT4 MC simulation pack- age to model the shielding structure. Remaining background estimated by the GEANT4 simula- tion is two orders below 1 cpd for both gamma and neutron backgrounds. To verify this simu- lation result measurement of background inside the prototype shield is underway. An option of replacing the polyethylene shielding by the min- eral oil with small amount of scintillation mate- rial that can also play the role of muon tagging is under investigation.

6. Sensitivity and schedule

In order to etimate the sensitivity of the KIMS experiment, our measured quality factors and 2 keV threshold is assumed. WIMP density of 300MeV/cm3, average velocity of 23Okm/s and escape velocity of 650km/s are used. Using 100 kg of crystals with the internal background level of 1 cpd, we can improve the current limit on the cross section by one order of magnitude in one year of data taking as shown in Figure 5. Even with 10 cpd level background, there is enough mar- gin to cover the whole signal region claimed by the DAMA experiment with 90% confidence level. The reduction of 137Cs and Rb is still in progress. However, it is feasible to have crystals with 10 cpd level background rate with the CsI powder that we currently have in hand. The further im- provement in reducing the internal background is expected. The search experiment will start im- mediately after receiving 10 cpd level crystals by the summer of 2002.

Figure 5. Sensity of KIMS experiment estimated for 100 kg year data taking for different back- ground levels.

7. Acknowledgement

This work was supported by the Creative Re- search Initiative program of Ministry of Science and Technology of Korea. We would like to thank the Korea South Electric Co. and the Cheong- pyung Storage Power Plant for allowing us to use their underground facility.

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