Internal background of CsI(Tl) crystal detectors for dark matter search

Tae Yeon Kim

Seoul National University

For the KIMS Collaboration

Seoul National Univ. : J.M.Choi, R.K.Jain, S.C.Kim, S.K.Kim, S. Y. Kim, T.Y.Kim*, H.S.Lee, H.S.Park, I.H.Park, E.I.Won, H.S.Won, H.Y.Yang, M.S.YangSejong Univ. : W.K.Kang, J.I. Lee, D.S.Lim, Y.D.Kim Yonsei Univ. : J.Hwang, H.J.Kim, J.H.Lee, Y.J.Kwon Iwha Womans Univ. : I.S.Han, E.K.Lee SeongKyunKwan Univ. : I.S.Cho, D.H.Choi, S.H.Noh, I.T.Yu Chonbuk National Univ. : S.Y.Choi KAIST : P.Ko Univ. of Maryland : M.H.Lee, E.S.Seo National Taiwan Univ., :H.B.Li, C.H.Tang, M.Z.Wang Academia Cinica :W.P.Lai, H.T. Wong Inst. Of High Energy Physics : J.Li, Y.Liu, Q.Yue Inst. Of Atomic Energy : B.Xin, Z.Y.Zhou Tsing Hua University, China : J.J.Zhu

(Korean Invisible Mass Search)

- Location

• Underground Pumped Storage Power Plant of Korea South Electric Company

• 80 km northeast from Seoul • 2 hours from Seoul by car

- Depth

• Minimum : 350 m • Access tunnel : 1.4 km

• Temperature : 11 - 16 oC• Relative humidity : more than 90 %

- Experimental hall

1. Underground laboratory

2. External background- Cosmic rays

Measured with 3 plastic scintillation(BC408) counters of 1 cm thickness in coincidence.

A B

A : spectrum at sea level for 1 dayB : spectrum at underground lab. for 2 weeks

Count rate B / count rate A = (1.4±0.15)x10-4

- NeutronMeasured with 0.5 liter BC501A liquid scintillator for 13 days

3675 counts/13days

Fast neutron flux = 4x10-5 /cm2/sec in the energy range between 1.5 MeV and 6.0 MeV.

- Gamma ray

The rock composition (ICP-Mass analysis) :238U = 9 ppm 232Th = 3 ppm can be controlled < 0.005 cpd40K = 4 ppm in the above shielding.

GEANT4 simulation :The shieilding of (Pb 15 cm + Cu 10 cm) can reducegamma ray background of 1.5 MeV by a factor of 106 .

210Pb 210Bi

210Po 206Pb (stable)

Beta-decay

Beta-decay

Alpha-decay

Gamma ray : 46.5 keVBeta ray(Q-value) : 16.5, 63.0, 1161.4 KeV

In lead shield

  CsI crystal

A B C D

Dimension

Front surface 7070 mm2 5555 mm2 5555 mm2 6060 mm2

Rear surface 7070 mm2 6565 mm2 6262 mm2 6060 mm2

length 300 mm 300 mm 230 mm 300 mm

Mass(kg) 6.63 4.87 3.56 4.87

Exposure(kgday) 13.0 8.9 22.5 27.4

3. CsI(Tl) crystal detector-Crystal ; Commercially available one and stored at underground laboratory for a few month.

- Shielding

• Lead 15 cm• OFHC copper 10 cm

• N2 gas flow : 1 liter/min.• Temperature : 26-29 oC• Humidity : 30-40 %

- Photoelectron yield ( 7x7x30 cm3)

55Fe source(5.9 KeV X-ray)

(4.5±0.9) photoelectorns /keV

- Energy resolution - Position dependence (60Co)

Relative standard deviation = 1.7%0021.0)(

58.0

)(

keVEkeVE

- Background spectra

IsotopeCsI crystal

A B C D

134Cs 38.5 2.8 34.9 2.9 537.2 10.2 51.4 3.2

137Cs 74.4 4.8 177.2 3.8 192.5 9.8 161.1 3.2

64.7±5.1 Counts/keV/kg/day- GEANT4 MC simulation

Internal 662 keV gamma-ray ;Photo-peak efficiency ~ 38 %Total detection efficiency ~ 58%Internal beta-ray ; ~ 100 %

- Contamination of cesium radioisotopes

4. Measurement Results

CsI crystal

A B C D

Li <1.7 <1.7 <1.7 10.6

Na 1646 <120 <120 21290

Mg 33.5 28.3 27.4 661.6

Al 50.5 78.6 139.8 503.5

K <327 <327 <327 -

Cr 0.8 2.0 2.5 121.3

Fe 218.3 488.0 253.4 705.8

Rb 816.0 3.2 205.1 202.8

Sr <0.08 <0.08 <0.08 298.9

Ba 1052.7 1062.4 1042.7 654.4

La 18.7 18.8 18.6 12.2

Sm 2.2 2.3 2.5 1.3

Lu 0.015 0.0097 0.0076 2.5

Tl 269500

554600

216400

265500

Th <0.02 <0.02 <0.02 0.5

U 0.05 0.0086 0.0066 <0.002

- ICP-Mass analysis (Unit in ppb) - Alpha-ray spectrum

1 ppb Rb = 0.6 cpd at a few keV regionCrystal B is closed to this value.

238U = 0.39 ± 0.09 ppt232Th = 4.5 ± 0.5 ppt

-HPGe measurement

- An expected spectrum (GEANT4)

134Cs = 1mBq/kg137Cs = 1mBq/kgRb = 1ppb238U = 0.39 ± 0.09 ppt132Th = 4.5 ± 0.5 ppt

Pollucite < 5.8 mBq/kg

63.0 ±3.7 mBq/kg

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