CsI crystal for WIMP Search
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Transcript of CsI crystal for WIMP Search
CsI crystal for WIMP SearchSun Kee Kim
Seoul National University
For the KIMS Collaboration
Seoul National Univ. : J.M.Choi, R.K.Jain, S.C.Kim, S.K.Kim*, T.Y.Kim, H.S.Lee, H.Park I.H.Park, E.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.Hahn, E.K.Lee SeongKyunKwan Univ. : I.S.Cho, D.H.Choi, S.H.Noh, I.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 : J.J.Zhu
Korea Invisible Mass Search
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Brief History of KIMS
• 1997 Summer : First discussion on WIMP search in Korea
• 1997 Fall : Started R&D on CsI(Tl) for WIMP search
• 1998 Summer : Demonstrated that CsI(Tl) can be used
as a WIMP detector - presented at ICHEP98
• 1999 Spring : Started background measurement at Cheongphyung
• 2000 Summer : Project funded, KIMS collaboration formed
• 2001 August : TDR, Project Review Committee
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
CsI(Tl) Crystal
CsI(Tl) NaI(Tl)Density(g/cm3) 4.53 3.67Decay Time(ns) ~1050 ~230Peak emission(nm) 550 415Hygroscopicity slight strong
Advantages High light yield ~50,000/MeV Pulse shape discrimination Easy fabrication and handling High mass number(both Cs and I)
Disadvantages Emission spectra does not match with normal bialkali PMT => effectively reduce light yield 137Cs(1/2 ~30y) ,134Cs(1/2 ~2y) may be problematic
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Photoelectron yield
Full size crystal (7x7x30) : ~ 4 p.e./keVSmall crystal(3x3x3) : 5~7 p.e./keV
RbCs(green extended) PMT(D726UK)70% larger p.e. than the normal bialkali PMT
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Neutron Beam Test
KIGAM(Korea Institute of Geology and Material)3.2 MeV p : 3H(p,n)3He 2.62 MeV n
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Quenching factor
QF = Emeasured / Erecoiln n
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Pulse Shape
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Mean Time
i
ii
A
tAt
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Quality factor
2)(
)1(
KS
B
S B
cut
Ideal detector ~ 1, ~ 0
K << 1
MT
BKS
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
CsI(Na)
1st measurement of CsI(Na)
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
~ 2 hours from SNU
Power plant
Homyung Mt.( 虎鳴山 )
Reservoir
Laboratory
Access tunnel(1.4km)350mPukhan River( 北漢江 )
Underground Lab. at CheongpyungStorage Water Power Plant
~1000 mwe
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
External background
Cosmic rays : ~ 10-4 relative to the sea level Expected at ~400 m underground The rock composition : 238U ~ 4.8 ppm, 232Th ~ 6 ppm 40K ~ 4 ppm (ICP-Mass)
Gamma background With a shielding of 15cm Pb + 10cm Cu(OFHC) Can be controlled < 0.005 cpd (counts/keV/kg/day)
Neutron background Photonuclear interaction of cosmic ray muon Fission of 238U, (alpha,n) reaction With 30cm polyethylene + 20 cm Liquid scintillator Can be controlled <0.001 cpd
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
W/o shielding
10cm Pb + 10 cm Cu(16 days data taking)
100% HPGe installed in CPL
Gamma Background
222Rn_up(226Ra)/222Rn_down(214PB,214Bi)=0.22+-0.06
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Neutron Background
~ 4x10-5 /cm2/sec
Input to the GEANT4 simulation
Measured with 0.5 liter BC501A liquid scintillator
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Internal background
137Cs : 1/2= 30.07 year decay to 137Ba* (Q = 1175.6 keV) 2 min life time, emitting 661.6 keV gamma Hard to reject => serious background at low energy
134Cs : 1/2 = 2.065 year decsy to 134Ba* (Q=2058.7 keV) immediate gamma emission Can be rejected easily => not a serious problem
Radioisotopes in the crystal - most important background after the proper shielding
87Rb : 1/2 half life = 4.75 x 1010 year (27.8% natural abundance) Beta deacy to 87Sr (Q=282.3 keV) no gamma emission Hard to reject => potentially a serious problem => reduction technique in material is known
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Internal background
In a crystal currently available
rays andrays of 87Rb, 134Cs, and 137Cs
+ GEANT
60 cpd
137Cs : 13.3mBq/kg, 134Cs : 54.2mBq/kg, 87Rb :~203ppbAnother crystal
137Cs : ~155mBq/kg, 134Cs : ~35mBq/kg, 87Rb : 3.9 ppb
0.63 cpd/1ppb 87Rb0.3 cpd/1mBq 137Cs0.005 cpd/1mBq 134Cs
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
238U, 232Th in the crystal
10~20 alphas/kg/day ~3 ppt (U+Th)
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Measurement of Internal Background
Crystal 137Cs, 134Cs : direct measurement (fit to the MC prediction) Rb : direct measurement + chemical analysis U, Th : direct measurement of alphas ~ 15 ppt
Powder 137Cs, 134Cs : 100% HPGe detector Rb : chemical analysis (ICP Mass)
CsOH, CsNO3, Pollucite, Water 137Cs, 134Cs : 100% HPGe detector
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0 20 40 60 80 100 120 140 160 180 200137Cs(mBq/kg)
134Cs(mBq/kg)
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Background measurement of powders
0 50 100 150
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CsMnO4
~ 3mBq/kg
CsI Powders
137Cs ~14mBq/kgRb ~ 21 ppb
CsOHCsNO3
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
137Cs in Pollucite ?
In CsI powder : 10~100 mBq/kg In Pollucite : < 5.8 mBq/kgProcessing water : 10.1 4.8 mBq/literExtraction of CsMnO4 from Pollucite with pure water 2.05 3.16 mBq/kg
< 5.8 mBq/kg
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
CsI (30-50 keV) : 14.3 kg daysNaI(Tl) (35-40 keV) : 330 kg days
Surface effect ?
V.A. Kudryavtsev et al., hep-ex/0109013
62 kg days exposure
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Shielding Design
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Detector Design
CsI(Tl) 9cm x 9cm x 30cm ~ 11 kg
PMT (3 inch RbCs)
5x5 = 25 crystals ~ 275 kg
Quartz
20cm BC501A tagging efficiency > 75%
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Sensitivity
After 1 year data takingwith 100 kg CsI(Tl) 2 keV threshold
Dark Matter 2002, Marina del Rey, CA, Fev. 22, 2002 Sun Kee Kim(Seoul National Univ.)
Summary & Prospect
Underground Lab. at CheongPyung is established Environmental background : low enough Comfortable place for long term experiment
Extensive R&D on CsI(Tl) crystal has been carried out Low energy threshold ~ 2keV Pulse shape discrimination of -rays : promising Intrinsic background : almost understood
Shielding design/Simulation/Electronics/DAQ Design almost at final stage
Prospect ~100 kg CsI(Tl) crystal within this year 1 year data taking will cover DAMA region