KIMS : Dark Matter Search Experiment in Korea

Jik LeeSeoul National University

For the KIMS Collaboration

DSU2006, Madrid

KIMS

H.C.Bhang, J.H.Choi, S.C.Kim, S.K.Kim, S.Y.Kim, J.W.Kwak, J.H.LeeH.S.Lee, S.E.Lee, J. Lee, S.S.Myung, H.Y.Yang

Seoul National University

Y.D.Kim, J.I. Lee, U.G. KangSejong University

H.J.Kim, S.C. Yang, J.H. SoKyungpook National University

M.J.Hwang, Y.J.KwonYonsei University

I.S.Hahn, I.H.ParkEwha Womans University

M.H.Lee, E.S.SeoUniv. of Maryland

J.LiInstitute of High Energy Physics

J.J.Zhu, D. He, Q.YueTsinghua University

Korea Invisible Mass Search

WIMP

SUSY model with R-parity conservation Neutralino : stable LSPweak interactions scale annihilation cross sectionproper relic density for dark matter

Neutralino:super-partner of neutral gauge and Higgs bosons

0214

01131211

01

~~~~χ~ HNHNWNBN +++=

⇒ Excellent CDM candidate

Direct WIMP SearchSignature : WIMP-nucleus elastic scattering

Signal by recoiled nucleus

scalar interaction

5 5( ) ( ) ( ) ( ) ....q qL f qq d q qμμχχ χγ γ χ γ γ= ⋅ + ⋅ +

spin-dep. interaction

2 2 232 ( 1)spin F rG m J Jσπ

= Λ +

1 ( )p p n na S a SJ

Λ ≡ < > + < >

224 ( )scalar r p nm Zf A Z fσπ

⎡ ⎤= + −⎣ ⎦

2A∝

SNU

Seoul

3.5hours by car

Yangyang

Korea Middleland Power Co.Yangyang Pumped Storage Power Plant

Yangyang Underground Laboratory

Minimum depth : 700 m / Access to the lab by car (~2km)

(Upper Dam)

(Lower Dam)

(Power Plant)

AdvantageHigh light yield ~60,000/MeV Pulse shape discrimination

Moderate background rejectionEasy fabrication and handlingEasy to get large mass with anaffordable cost

Good for AM study

DisadvantagesEmission spectra does not match with normal bi-alkali PMT

=> Effectively reduce light yield137Cs(τ1/2 ~30y) ,134Cs(τ1/2 ~2y) may be problematic

CsI(Tl) NaI(Tl)Photons/MeV ~60,000 ~40,000Density(g/cm3) 4.53 3.67Decay Time(ns) ~1050 ~230Peak emission(nm) 550 415Hygroscopicity slight strong

CsI(Tl) Crystal

∑∑=

i

ii

AtA

t

NaI(Tl)

CsI(Tl)

Pulse shape discriminationof gamma background

External backgrounds1. External gamma

Isotopes in surrounding materials (Rock)• Decay chain of U238 and Th232

• Isotopes (K40, …) • Rn222 in air Shielding structure made of pure and high Z materials Partially or fully distinguishable from WIMP signal by PSDN2 flowing to remove air contaminated by Rn222

2. Neutron backgroundIndistinguishable from WIMP signal (Nuclear recoil)• (α, n) reaction • Nuclear fission • Induced by cosmic muon ( E mean ~ 230 GeV )

- possible to veto with muon detector Neutron moderator made of material with High Hydrogen densityVeto system using Muon detector

KIMS

Neutron shield / Muon det.

Lead shield

Polyethylene

Copper shield

CsI(Tl) crystal

Mineral oil 30cm Pb 15cm : 30t

OFHC Cu 10cm : 3tPE 5cm

Muon Detector • 4π coverage muon detector : 28 channels • Liquid Scintillator(5%) + Mineral Oil (95%) = 7 ton • Measured Muon flux = 2.7 x 10 –7 /cm2/s• Position resolution : σx,y ~ 8 cm • Reconstructed muon tracks with hit information• Muon veto efficiency ~99.9%

1liter BC501A liquid scintillatorn/g separation using PSDE_vis > 300 keV

Neutron Monitoring Detector

neutronscandidates

Log10(Δt) Energy [MeV]

Muon induced neutrons

2 events of Muon induced neutron during 67.4 days ~ 0.03 counts/day/liter

neutronscandidates

Neutron Monitoring Detector214Bi β-decay 214Po α-decay

222Rn α-decay 218Po α-decay

Neutron Monitoring Detector(cont’d)

230Ra dominant contamination in 238U chain - 6.66 +- 0.27 cnts/liter/day – 1.5 x10-6 ppt of 230Ra level

232Th dominant contamination in 232Th chain - 0.32 +- 0.06 cnts/liter/day - 0.63 ppt of 232Th level

• All neutron events : alphas from internal sourcesNeutron rate < 1.8 cpd (90 % C.L. ) inside the shield

• Neutron rate outside the shield8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV )

After subtracting internal background estimated from the data inside the shield

• Electrostatic alpha spectroscopy : 70 liter stainless container

• Use Si(Li) photodiode : 30 x 30 mm • Estimate 222Rn amount

with energy spectrum of a from 218Po & 214Po.• Photodiode calibration : 210Po, 241Am • 222Rn in air = 1 ~ 2 pCi/liter• Absolute efficiency calibration done with 226Ra

Radon Monitoring

Internal background of CsI Crystal

• 137Cs (artificial)– serious background at low energy

• 134Cs (artificial+133Cs(n,gamma))• 87Rb (natural)

– Hard to reject reduction technique in material is known

Single Crystal (~10 kg) background @ ~10keV

87Rb 1.07 cpd/1ppb 137Cs 0.35 cpd/1mBq/kg 134Cs 0.07 cpd/1mBq/kg

137Cs : 10 mBq/kg134Cs : 20 mBq/kg87Rb : 10 ppb

87Rb

137Cs

134Cs

keVkeV

CP

DC

PD

GeantGeantSimulationSimulation

From simulation

Reduction of Internal Background

• CsI solubility in water is very high.• Recrystallization reduces Rb87.• 10 ppb powder ~ 1 ppb (< 1.1cpd)

Rb87 ReductionRb87 Reduction

Cs137 ReductionCs137 Reduction• Water is main

source of Cs137• It was reduced by

using purified water

Further reduction of internal background

2mBq/kg 0.7 cpd internal background

New CsI powder produced with ultra pure water

Best available Crystal at MarketBest available Crystal at Market70cpd70cpd

Powder SelectionPowder Selection20cpd20cpd

Cs137 Reduction Using Pure waterCs137 Reduction Using Pure water14cpd14cpd

Rb87 Reduction by ReRb87 Reduction by Re--crystallizationcrystallization6cpd6cpd

Ultra Pure Water UsedUltra Pure Water Used4cpd4cpd

Reduction of Internal Background cont’ed

CsI(Tl) Crystal 8x8x30 cm3 (8.7 kg) 3” PMT (9269QA) Quartz window, RbCs photo cathode

4~6 Photo-electron/keVDAQ 500MHz Home Made FADC

5 photo-electron within 2μsec trigger conditiontotal 32μsec window

Name B.G. Weight Company

0406 6.7 kg

8.7 kg

8.7 kg

8.7 kg

8.7 kg

8.7 kg

0512 ~4CPD 8.7 kg beijing

8.7 kg

0605A ~4CPD 8.7 kg beijing

8.7 kg

0501A

shanghai

shanghai

shanghai

beijing

beijing

beijing

beijing

0501B

0510A

0510B ~4CPD

0511 ~4CPD

beijing

0601 ~4CPD

0605B ~4CPD

6CPD

6CPD

6CPD

~4CPD

Data taking with CsI(Tl)

Neutron calibration facility in SNU 300 mCi Am/Be source

neutron rate 7 x 105 neutrons /sec a few 100 neutrons/sec hit 3cmX3cm crystalQuenching factor of Recoil Energy Take Neutron calibration data PSD check – Quality factor

46o

CsIBC501a BC

501aBC501a LSC

90o17.4

o

n

Am/Be

Tag γ(4.4MeV)to measure TOF and energy of neutrons

@Energy = 4-5 keV137 Cs Compton Neutron Recoil

PSD discriminationMean time distribution

3~4 keV 4 ~5 keV

10~11 keV9~10 keV

8~9 keV7~8 keV6~7 keV

5~6 keV

Extraction of nuclear recoil events

)log(

)()(

><=

+

tx

xfNxfN iiiNR

iERERNR

Rate of nuclear recoil eventsLog mean time distribution of background events are fitted to distributions of Compton events and neutron events

Filled circle : w/o PMT noise cut

Open square : with PMT noise cut - efficiency corrected

Open circle : fitted the number of nuclear recoil events

55Fe energy distribution solid line for MC and dotted points for data

Simulated energy distributions of WIMP for different masses

20 GeV,50 GeV,100 GeV,1 TeV

Local WIMP density ~ 0.3GeV/cm3

Maxwellian velocity distribution with v ~ 270km/sLocal flux of WIMP ~ 100 GeV/mχ x 105/cm2/s

ρρχχ==galaticgalatic halo densityhalo density

vvEE=earth velocity in =earth velocity in galaticgalatic frameframe

vv00=sun velocity in =sun velocity in galaticgalatic frameframe

22

2(max) 2( )recoil x N

N

mE v m

m mχ

χ

=+

Recoil energy < 100 keVMeasured energy < 10 keV due to quenching

Interaction rate of WIMP

Dark matter density at the solar systemρD = 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

Spin Independent Limit

3879 kg days NAIAD - NaI(Tl) 237 kg days KIMS - CsI(Tl)4123 kg days DAMA - NaI(Tl)

KIMS First WIMP Limit

PLB 633(2006) 201In Feb. 2006

Spin dependent WIMP limit only with I

Pure proton casePure proton case Pure neutron casePure neutron case

Form factor and spin expectation value for Form factor and spin expectation value for ““II”” are obtained from are obtained from

““M.T.ResselM.T.Ressel and and D.J.DeanD.J.Dean PRC 56(1997) 535PRC 56(1997) 535

Spin independent

Spin dependent (pure proton)

• First physics result was published

• Various R&D run was done – About 4000 kg day data acquisited– With and without quartz block (5cm

thick) – 0 degree and Room temperature

operation– Analysis is ongoing

• Successfully reduce internal backgrounds of CsI(Tl) crystals

– 70 kg full size crystals(8x8x30cm3) are being prepared

– Mass production stage • 2 Crystal/Month growing is

possible

• With more than 100 kg crystals, we start to probe

– Annual modulation – SD+SI interaction search

WIMP search with CsI(Tl) crystalSummary & Prospects

ULE HPGe detector setup

CsI(Tl) crystal Compton veto

Built by TUDelivered to SNU

5g 1 cpd level detector Tested at Academia Cinica, Taiwanand delivered to SNU in Dec, 2004.

If successful upgrade to 1kg mass

Search for Low Mass WIMP

Collaboration with China and Taiwan

Double beta decay

(A,Z)(A,Z+1)

(A,Z+2)

(A,Z) (A,Z) --> (A,Z+2) +> (A,Z+2) + 22ββ +2+2νν

22

202/1/1

i

i

iv

iei emUm

mT

αββ

ββν

∑>=<

>∝<

Experimental search for Experimental search for DBDDBD

♦ Two approaches:

+ event shape reconstruction- low energy resolution- low efficiency

e-

e-

source

detector

detector

Source ≠ Detector

e-

e-

Source ≡ Detector(calorimetric technique)+ high energy resolution+100% efficiency- no event topology

sum electron energy / Q

Signature: shape of the two electron sum energy spectrum

1

10-2

10-6

R = 5%two neutrino DBD

continuum with maximum at ~1/3 Q

neutrinoless DBDpeak enlarged only by the detector energy resolution

low background- underground operation

- shielding- low radioactivityof materials

low energy resolution

2ν events canmask 0ν ones

♦ If you use the calorimetric approach

1. TMSN50% + STD Liquid scintillator124Sn 124Te + 2β−decay

2287keV

2. CaMoO4 Crystal 1.8x1.8x3.5 cm3

100Mo 100Ru + 2β−decay3034keV

T1/2 > 3.41x1019 yr at 90% C.L

Double beta decay R&D

10kg Mo10kg Mo--100 CaMoO100 CaMoO44

5-year runningT1/2 > 1x1025 yr at 68% C.L.

Present Present Best LimitBest Limit >>4.6x104.6x102323 yr @ 90% C.L.yr @ 90% C.L.

Other activitiesSummary and prospect

ULE HPGe detector for low mass WIMP searchCompton veto detector deliveredPrototype detector and shield is being soon

Cryogenic detector for low energy detectionFabrication and test of TES sensor on-going Need fast cycling of sample test for the TES optimization

Metal loaded liquid scintillator for 0νββ decayPilot experiment with 1 liter 50% Sn loaded LS encouraging resultStudy on internal background in progressLoading other element in progress

CaMoO4Test with a 50g crystal has been done encouraging resutR&D for growing optimization and background reduction in progress10kg crystal can be easily installed in the current WIMP search setup