2003-04-25 79th KPS meeting

1

WIMP Search with CsI(Tl) Crystals – Status and Future

The Future of Dark Matter Detection Y.D. Kim ( KIMS collaboration )

KIMS CollaborationKorea Invisible Mass Search experiment H.C.Bhang, S.C.Kim, S.K.Kim*, S.Y.Kim, J.W.Kwak, H.S.Lee

J. Lee, S.S.MyungSeoul National UniversityY.D.Kim, J.I. Lee, D.S.Lim

Sejong UniversityH.J.Kim

Kyungpook National UniversityM.J.Hwang, Y.J.Kwon

Yonsei UniversityI.S.Hahn, I.H.Park

Ewha Womans UniversityM.H.Lee, E.S.Seo

Univ. of Maryland J.Li

Institute of High Energy Physics J.J.Zhu, D.He

Tsinghua University

Seoul

Yangyang

Laboratory

3.5 hours by car

YangYang Underground Laboratory(Y2L)

Yangyang

airport

• Located in mountain area of east Korea

• 3.5 hours by car from Seoul• Only one underground lab in Korea

[1] Location

Run byDMRC

(Dark Matter Research Center)@ Seoul National University

http://dmrc.snu.ac.kr

[2] Profile of YangYang Underground Laboratory(Y2L)• located besides of Pumped Storage Power Plant• 700 m minimum depth, 2km Access tunnel by car

700mentranceAccess Tunnel(2km)

Top Lake

KIMS

Neutron shield / Muon det.(30cm)

Lead shield(15cm)

Polyethylene(5cm)

Copper shield(10cm)

CsI(Tl) crystal

Neutron flux are measured in realtime as actual data

Environment ParametersDepth Minimum 700 m

Temperature 20 ~ 25 oC

Humidity 35 ~ 60 %

Rock contents 238U less than 0.5 ppm 232Th 5.6 +/- 2.6 ppm

K2O 4.1 %

Muon flux 4.4 x 10-7 /cm2/s

Neutron flux 8 x 10-7 /cm2/s

222Rn in air 1 ~ 4 pCi/liter

Temperature stability of CsI(Tl) crystal detector

21.7 +/- 0.2 oC

N2 flowing rate = 4 liter / minFor Rn Reduction and low humidity

Muon Detector

• 4 coverage muon detector : 28 channels • Liquid Scintillator(5%) + Mineral Oil (95%) = 7 ton • Measured Muon flux = 4.4 x 10 –7 /cm2/s • Position resolution : x, ~ 8 cm • Reconstructed muon tracks with hit information

Neutron Monitoring Detector • 1liter BC501A liquid scintillator • n/ separation using PSD• Measured Neutron flux

Inside of main shield < 3 neutrons/day/liter @90%CL, E threshold = 300 keVTag events using and coincidences in 238U & 232Th chain. 0 consistent result of Neutron rate Expect 10-3 of neutron flux outside of main shield No problem with current CsI background level

outside of main shield = 8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV ) Subtract energy spectrum inside of main shield to reject internal background

Neutrons induced by muons

Log10(t)

Energy [MeV]Neutron region

Energy [MeV]

• Two strong neutron candidates induced by muon 2 events for 67.41 days 0.025 +- 0.017 cnts/liter/day

• Needs more statistics

• It seems low enough compared other background

Coincidence between muon and neutron detector

Pulse Shape Discrimination : Reference spectra of Recoil

i

ii

A

tAt

Mean TimeMean Time

6<E<7 keVE=2.62MeV

Quenching Quenching FactorFactor

Previous test with mono-energetic neutron beam

i

ii

A

tAt

recoilEE

Motivation Neutron beam is usually limited by beam time.Moving detector to accelerator place is sometimes difficult.

IdeaUse Intense Am-Be source for both PSD and Quenching

factor 9Be(α,n)12C (~50%) neutron only (higher energy) 9Be(α,n)12C* (~ 50%) lower energy neutron + γ(4.43MeV)

Have to use TOF to get the En

New Facility for PSD measurement

300 mCi(1.1x1010 Bq) Am/Be source emits 7 x 105n/sec (70 neutrons/106 α’s) ~a few 100 neutrons/sec hit 3cmX3cm crystal

Neutron Calibration Setup at SNU

•Am-Be Source + •20cm Liquid Scintillation Counter(LSC)•5cm Pb shielding +•10cm Borated Paraffin

)(

recoilE

5MeV

1MeV

10MeV

TOF En

neutron detectors

BC

501A

BC501A

LSC

90o

n

Am/Be

55o neutron detectors

4.4MeVCsIθ

Reference Recoil Spectrum : Recoil vs. Cs137

2-4keV 4-6keV 6-8keV

10-12keV8-10keV

ToF functioning for quenching factor

60Co

γ (4.43MeV)neutron

2γ’s(1.173 & 1.333 MeV)

σToF = 1.9ns2

2

)1)((2

n

nn

ttlc

cmE

Qmax

Qtot

nγ

n

γ

Quenching Factor

Neutron Energy

90º

55º

E CsI(k

eV)

En(MeV)

Background data and limits

CsI(Tl) Crystal 8x8x23 cm3 (6.6 kg)

3” PMT (9269QA) Quartz window, RbCs photo cathode

DAQ 500MHz FADC

cp

d

full size crystal ~ 5 p.e./keV

Trigger Threshold less than 2 keV

( 5 p.e. within 2 s )

137Cs & Rb87 137Cs & Rb87 ReductionReduction

6cpd6cpd

Contributions

1. Cs137 ~ 3.0 cpd

2. Cs134 ~ 1.8 cpd

3. Rb87 ~ 1.0 cpd

Total ~ 6 cpd @ 10keV

237 kg days data

Coincidences

T1/2 = 0.138+/-0.012 sec

(T1/2 = 0.145 sec)

Th : 0.411 +/- 0.078 ppt of Th232 if it is equilibrium

Th Chain : 216Po212Pb(145 msec)

T1/2=4.23+-1.10 min

(T1/2 = 3.10 min)

After subtraction of Th232

294.8 +/- 49.4 events

U : 1.16 +/- 0.19 ppt

Accidentals

U Chain : 218Po214Pb(145 msec)

CsI Data Taking

Data for WIMP search 237 kg days for 8x8x23 cm3 crystal of 6 cpd background level

MC data using Geant4 simulation Calibration data

• Neutron data from Neutron calibration facility Reference distribution for recoil events - equivalent to 1175 kg days of underground data • PMT background data Determine the cut values • 57Co ( 122.06 keV ) and 55Fe (5.9 keV) Compton data Check Surface events, Single photon calibration • 137Cs ( 661.657 keV ) Compton data Reference distribution for events - equivalent to 350 kg days of underground data

Cut efficiency calculation

PMT related noises

UKDMC NaI(Tl) crystal

Raw Data

Underground dataPMT noise

The low energy events are PMT related.Most likely, radioactivities(K-40,U,Th) in glassScintillates weakly, propagated through crystal.

Analysis cuts for WIMP search Cut conditions for reduction of PMT background

• Numbers of photoelectron red marker : PMT noise black marker : 137Cs Compton # of p.e. > 4 for both channels

• Charge & # of p.e. ratio Energy threshold > 3 keV

Preliminary Preliminary

Cut efficiency correction

Before Cut After Cut

Efficiency curve After efficiency correction

Underground dataPMT noise

Preliminary Preliminary

Reference PSD distribution of Neutron & 137Cs Compton & Data

3~4 keV 4 ~5 keV

11~12 keV10~11 keV9~10 keV

8~9 keV7~8 keV6~7 keV

5~6 keV

Preliminary Preliminary

Log Mean Time fitting using Distribution of Neutron & 137Cs

ComptonEnergy [k

eV]Recoil events [cpd]

3~4 0.44 +/- 1.23

4~5 0.08 +/- 0.75

5~6 0.46 +/- 0.58

6~7 -0.42 +/- 0.35

7~8 -0.37 +/- 0.30

8~9 -0.16 +/- 0.24

9~10 0.11 +/- 0.18

10~11 0.21 +/- 0.18

Background level

WIMP mass 20 GeV/c2

60 GeV/c2

130 GeV/c2

250 GeV/c2

Preliminary Preliminary

Preliminary Limit curve 1 - SI Dark matter density at the solar system D = 0.3 GeV c-2 cm -3

Use annual average parameters V0 = 220 km s-1, VE = 232 km s-1, VEsc = 650 km s-1

Preliminary results Recoil energy threshold > 20 keV Comparable limit with NAIAD with ten times more data

Preliminary Limit curve 2 - SD

I

Cs

All

spin values of I-127 & SD form factor

M. T. Ressel and D. J. Dean ,

Phys. Rev. C 56 535 (1997)

Spin values of Cs-133

Iachello et al., PLB 254(1991)

Method by Tovey et al, used.

0.01

0.1

1

10

100

103

104

10 100 1000

plim(A

) (pb)

M (GeV/c2)

SIMPLE (F)

SIMPLE ( 35Cl)

Tokyo (F)

Tokyo (Na)

NAIAD (Na)

NAIAD (I)

SIMPLE ( 37Cl)

CRESST (Al)

DAMA (129Xe)

DAMA (Na)

DAMA (I)

PICASSO (F)

an = 0

Plan - Projected Limit

1.7 tons of CsI powder with 2mBq/kg of 137Cs produced (Chemetall, Germany) and crystallization starts. 137Cs contribution < 1 cpd is assured.

250 kg(25 crystals) may start in 2005 with < 2 cpd background level. DAMA data can be tested with similar crystal detector containing Iodine.

should be helpful to identify current discrepancy

Current powder still contains a little 137Cs.Further reduction anticipated.

Other exp. in Yangyang: low mass WIMP

Limited by threshold

ULE HPGe detector(5g)

Collaboration with China and Taiwan : HPGe installation in Dec.

0.1 keV threshold

Summary

Established an underground laboratory (700 m deep)

Examined Environment parameters in underground laboratory Detailed MC study is in progress. CsI R&D

• 5 p.e./keV , Eth = 2 keV, E recoil > 20 keV

• PSD technique works at low energy

• successful reduction of internal background ( 6 cpd level )

Shielding structure and other detectors in operation

Preliminary SI and SD limits obtained with 1 crystal ( 6.6 kg ) of 6 cpd background.

250 kg detector with < 2 cpd will start 2005.