KIMS Collaboration Korea Invisible Mass Search experiment since 2000

Status of WIMP search in KIMS experiment

Kwak, Jungwon ( KIMS Collaboration )The dark Side of the Universe

KIAS-APCTP-DMRC Workshop in KIAS May 26th 2005

2005-05-26 KIAS-APCTP-DMRC Workshop 2

KIMS Collaboration Korea Invisible Mass Search experiment since 2000

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. LeeSejong University

H.J.Kim Kyungpook National University

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

I.S.Hahn, I.H.Park Ewha Womans University

M.H.Lee, E.S.Seo Univ. of Maryland

J.LiInstitute of High Energy Physics

J.J.Zhu, D. He, Q.Yue, X. LeeTsinghua University


Korea Middleland Power Co.Yangyang Pumped Storage Power Plant

Yangyang Underground Laboratory, Y2L

Minimum depth : 700 m / Access to the lab by car (~2km)Completion of the power plant(2006)

Construction of the laboratory buildings done (2003)


Environment Parameters in Y2L

Depth Minimum 700 m

Temperature 20 ~ 25 oC

Humidity 35 ~ 60 %

Rock contents 238U less than 0.5 ppm 232Th 5.6 2.6 ppm 40K 270 5% ppm

Muon flux 2.7 10-7 /cm2/s ( 1 10-2 /cm2/s )

Neutron flux 8 10-7 /cm2/s ( 1.4 10-3 /cm2/s )

222Rn in air 1~2 pCi/liter ( 4 pCi/liter )

Blue,Green : Gneiss (2 Gyr ) Red : Igneous Rock ( 200 Myr ) have more radioactivity

238U lifetime = 4.4 Gyr 232Th lifetime = 14.05 Gyr40K lifetime = 1.277 Gyr


KIMS Main shield in Y2L

Mineral oil 30cm Pb 15cm : 30t

OFHC Cu 10cm : 3tPE 5cm

HPGe detector measurement


External backgrounds

1. 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 PSD N2 flowing to remove air contaminated by Rn222

2. Neutron background Undistinguishable from WIMP signal (Nuclear recoil)

• (, 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 density Veto system using Muon detector Underground experimental facility is necessary


Muon detector (MUD)

2 x 2” PMT for each channel muon modules 28 signal channels ( 6 + 4 x 4 + 3 x 2 )

Liquid Scintillator 5 % • PC 1 liter + PPO 4 g + POPOP 15 mg

Mineral Oil 95 % - Neutron moderator 10-5 times of ground Muon rate at 700 m deep underground


Muon detection efficiencies

Trigger : Use Plastic scintillator and Bottom modules

Muon trigger - Trigger in MUD8 is issued or not… - Muon trigger efficiency of single module : 97.0 % - Muon veto efficiency : 99.9 %

Muon Tagging efficiency : 94.0 % - Required coincidence triggers in different modules

Plastic scintillatorMUD8

Bottom modules


Muon flux measurement

Tagging efficiency : 94.0 % Area of top module = 70220 cm2

Measured Muon flux : 2.7 x 10-7 /cm2/s

DAQ rate of Muon detector = 1 ~ 3 Hz

Real Muon rate of KIMS Muon detector = 4.5 x 10-2 Hz


• 1liter BC501A liquid scintillator - Teflon container - Quartz window - Low background 3” PMT • n/ separation using PSD method - 500MHz sampling FADC

Neutron Monitoring Detector (NMD)


Alpha background study – 238U and 232Th chain214Bi -decay 214Po -decay

222Rn -decay 218Po -decay

Coincidence time [m]

Coincidence time [ms]

220Rn -decay 216Po -decay

Coincidence time [m]


Inside main shield < 1.8 neutrons/day/liter @90%CL, E threshold = 300 keVTag events using and coincidences in 238U & 232Th chain. All neutron candidates are consistent with alphas from internal sources

Outside main shield = 8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV ) Subtract energy spectrum inside shield to reject internal background Expected 10-3 times of neutron flux outside shield 5 x 10-3 cpd level of background contribution on CsI(Tl) detector

Neutron flux measurement in Y2L


Log10(t)

t = min(Abs(t Muon event – t Neutron event))Require t < 1 s for coincidence events tmean = 130 ns delay cable for muon and more electronics

t = 32 ns 16ns clock pulse used

High energy events of neutron detector are mostly from muons. E mean ~ 230 GeV of muons at 2000m w.e. underground

3.5 muons /day in neutron detector ~ 2.7 x 10–7 /cm2/s muon flux

Energy [MeV]

Muon induced neutron


2 events of Muon induced neutron during 67.4 days DAQ run ~ 0.03 counts/day/liter Expected muon induced neutron rate = 0.06 counts/days/liter 50% neutron detection efficiency - Lower efficiency for Higher Energy of neutron Neutron yield for muon = 2 x 10-4 ( g/cm2)-1 for 15cm-thick lead Muon flux = 2.7x10-7 /cm2/s Muon veto efficiency – 99.9% Expected non-vetoed muon = 2.7x10-10 /cm2/s non-vetoed Neutron rates = 1.2 x 10-4 counts/day/liter

Muon induced neutron (cont’d)

Energy [MeV]


• 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 detector


NaI(Tl)

CsI(Tl)

Advantage High light yield ~60,000/MeV Pulse shape discrimination Easy fabrication and handling High mass number(both Cs and I) Relatively easy to get large mass with an affordable cost

Disadvantages Emission spectra does not match with normal bi-alkali PMT

=> Effectively reduce light yield 137Cs(1/2 ~30y) ,134Cs(1/2 ~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

Why CsI(Tl) Crystal ?


Best available Crystal at MarketBest available Crystal at Market

Powder SelectionPowder Selection

Cs137 Reduction Using Pure waterCs137 Reduction Using Pure water

Rb87 Reduction by Re-crystallizationRb87 Reduction by Re-crystallization

Internal background of CsI(Tl) crystal detector

• 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 (8.7 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

Geant SimulatioGeant Simulationn


Further reduction of internal background

2mBq/kg 0.7 cpd internal background

New CsI powder produced with ultra pure water


WIMP search using CsI(Tl)

CsI(Tl) Crystal 8x8x30 cm3 (8.7 kg) 3” PMT (9269QA) Quartz window, RbCs photo cathode 5 Photo-electron/keV DAQ 500MHz Home Made FADC 5 photo-electron within 2μsec trigger condition total 32μsec window

Now on DAQ run with 3 modules – 24 kg - 6.3 cpd background level

Typical FeTypical Fe5555 SignalSignal

Crystal

Data # of p.e.

Size

0406 237 kg days

5.5 8x8x23 cm3 6.7 kg

0501A

Running - 8x8x30 cm3 8.7 kg

0501B

Running - 8x8x30 cm3 8.7 kg


Data analysis for WIMP Limit

Single photon clustering

Mean time definition i = cluster number ti = time of i th cluster

Ai = charge of i th cluster Pulse Shape Discrimination

i

ii

A

tAt


Neutron Calibration facility in SNU In-house Am/Be source

9Be ( , n ) 12C ~ 50% 9Be ( , n ) 12C* , 12C* 12C + (4.4 MeV) ~ 50% Activity = 300 mCi = 1.1 x 1010 Bq - neutron rate = 7 x 105 neutrons /sec - a few 100 neutrons/s hit 3X3x3 cm3 CsI(Tl) crystal

46o

CsI

BC501a BC

501

a

BC501a LSC

90o

17.4

o

n

Am/Be

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

BC501A Neutron detectors - Tag the scattered neutron from CsI at various angles - Calculate the scattering angle between nuclear and neutron Measure time of flight between LSC and CsI(Tl) detector - Energy of Incident neutron 3x3x3 cm3 CsI(Tl) crystal - Nuclear recoil energy


Neutron Calibration facility in SNU

@Energy = 4 ~ 5 keV a) Comparison between mean time distibutions of neutron data and compton data for test crystal b) Comparison between mean time distributions of compton data for test crystal and full sized crystal

Energy of neutrons from Am/Be source [MeV]


Data for WIMP search 430 kg days for 8x8x30 cm3 crystal of 14 cpd background level 2004 spring 237 kg days for 8x8x23 cm3 crystal of 6.3 cpd background level

MC data using Geant4 simulation Calibration data

• Neutron data from Neutron calibration facility Reference distribution for nuclear recoil events• 57Co ( 122.06 keV ), 241Am (59.54 keV) and 55Fe(5.899 keV) Compton data Check cut efficiency, Single photon calibration • 137Cs ( 661.657 keV ) Compton data Reference distribution for electron recoil events

Summary CsI(Tl) Data taking


Analysis Cut Efficiency

Analysis Cuts To remove PMT noise and surface events PMT noise - Originate from PMT structure ( spark in dinode) - Cluster due to PMT noise is abnormally big Qc/Nc cut – mean charge of single cluster

Efficiency curve for PMT Noise cut Compton data neutron reference data

The difference of efficiencies is taken into account as systematic error.


3~4 keV 4 ~5 keV

10~11 keV9~10 keV

8~9 keV7~8 keV6~7 keV

5~6 keV

Log Mean Time distribution of Neutron & 137Cs Compton & Data


WIMP candidates Log 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 correctedOpen 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


• WIMP Nucleus Scattering

• Spin Independent

• Spin Dependent

ρρχχ=galatic halo density=galatic halo density

vvEE=earth velocity in galatic frame=earth velocity in galatic frame

vv00=sun velocity in galatic frame=sun velocity in galatic frame

WIMP Proton Cross Section


WIMP Limit curve ( spin independent )

3879 kg days exposure of NAIAD - NaI(Tl) 237 kg days exposure of KIMS - CsI(Tl) 4123 kg days exposure of DAMA - NaI(Tl) Solid line for DAMA region by annual modulation

Although the amount of data is less than 1/10 times of NaI(Tl) experiments, lower limit thanks to the better PSD power of CsI(Tl) crystal and lower recoil energy threshold.

W-A combined WIMP-neucleon X-section

W-A (EK) WIMP-neucleon X-section for E k energy bin


WIMP Limit curve ( spin dependent )

WIMP – ProtonWIMP – Proton

<S<Spp>>CsCs = 0.370 = 0.370

<S<Spp>>II = 0.309 = 0.309

WIMP – NeutronWIMP – Neutron

<S<Snn>>CsCs = 0.003 = 0.003

<S<Snn>>II = 0.075 = 0.075

For I , SM(Bonn A), M.T.Ressell, D. J. Dean Phys. Rev. C58 (1997) 535For Cs, IBFM, F. Iachello, L.M. Krauss, G. Maino Phys. Lett. B254 (1991) 220


Summary 1. External background

- Gamma background ~ less than 10-4 reduction rate by shield - Neutron background Neutron from environmental radioactive sources ~ neutron moderator = 30cm of Mineral oil and 5cm of PE ~ expected rate inside shield 10-9 /cm2/s ~ 5 x 10-3 cpd on CsI(Tl) Neutron induced by cosmic muon ~ 700m deep underground lab. in Y2L ~ muon flux in 700m underground 2.7 x 10-7 /cm2/s ~ 99.9% of veto efficiency ~ expected rate inside shield 10-8 /cm2/s for 15cm thick lead layer

2. Internal background

- Achieve 6.3 cpd level internal background level ~ 24 kg CsI(Tl) crystal detector is running - Successful powder R&D ~ Purification of process water and re-crystallization - PMT noise reduction with pulse shape analysis of signal photon cluster


Prospects

Reduction of internal background of CsI successful Expect ~ 1cpd crystal

600 kg powder production was finished 1cpd level CsI(Tl) crystal one prototype crystal ready to run Current shield structure can house 250kg of CsI(Tl) crystal detector

237 kg days data of 5.5 cpd background level

DAMA result by annual modulation

Current best limit curve by CDMS experiment

1 year data taking with 250 kg CsI(Tl) crystals

of 2cpd background level


Prospects

WIMP - ProtonWIMP - Proton WIMP - NeutronWIMP - Neutron

2005-05-26 KIAS-APCTP-DMRC Workshop 33Limited by threshold

ULE HPGe detector

Collaboration with China and Taiwan

Low mass WIMP search in KIMS


Status of ULE HPGe detector setup

CsI(Tl) crystal Compton veto Built by TU Delivered to SNU

5g 1 cpd level detectorTested at Academia Cinica, TaiwanTo be delivered to SNU in Dec.

If successful upgrade to 1kg mass


Thanks !!!

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KIMS Collaboration Korea Invisible Mass Search experiment since 2000