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
2005-05-26 KIAS-APCTP-DMRC Workshop 3
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)
2005-05-26 KIAS-APCTP-DMRC Workshop 4
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
2005-05-26 KIAS-APCTP-DMRC Workshop 5
KIMS Main shield in Y2L
Mineral oil 30cm Pb 15cm : 30t
OFHC Cu 10cm : 3tPE 5cm
HPGe detector measurement
2005-05-26 KIAS-APCTP-DMRC Workshop 6
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
2005-05-26 KIAS-APCTP-DMRC Workshop 7
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
2005-05-26 KIAS-APCTP-DMRC Workshop 8
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
2005-05-26 KIAS-APCTP-DMRC Workshop 9
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
2005-05-26 KIAS-APCTP-DMRC Workshop 10
• 1liter BC501A liquid scintillator - Teflon container - Quartz window - Low background 3” PMT • n/ separation using PSD method - 500MHz sampling FADC
Neutron Monitoring Detector (NMD)
2005-05-26 KIAS-APCTP-DMRC Workshop 11
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]
2005-05-26 KIAS-APCTP-DMRC Workshop 12
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
2005-05-26 KIAS-APCTP-DMRC Workshop 13
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
2005-05-26 KIAS-APCTP-DMRC Workshop 14
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]
2005-05-26 KIAS-APCTP-DMRC Workshop 15
• 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
2005-05-26 KIAS-APCTP-DMRC Workshop 16
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 ?
2005-05-26 KIAS-APCTP-DMRC Workshop 17
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
2005-05-26 KIAS-APCTP-DMRC Workshop 18
Further reduction of internal background
2mBq/kg 0.7 cpd internal background
New CsI powder produced with ultra pure water
2005-05-26 KIAS-APCTP-DMRC Workshop 19
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
2005-05-26 KIAS-APCTP-DMRC Workshop 20
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
2005-05-26 KIAS-APCTP-DMRC Workshop 21
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
2005-05-26 KIAS-APCTP-DMRC Workshop 22
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]
2005-05-26 KIAS-APCTP-DMRC Workshop 23
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
2005-05-26 KIAS-APCTP-DMRC Workshop 24
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.
2005-05-26 KIAS-APCTP-DMRC Workshop 25
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
2005-05-26 KIAS-APCTP-DMRC Workshop 26
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
2005-05-26 KIAS-APCTP-DMRC Workshop 27
• 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
2005-05-26 KIAS-APCTP-DMRC Workshop 28
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
2005-05-26 KIAS-APCTP-DMRC Workshop 29
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
2005-05-26 KIAS-APCTP-DMRC Workshop 30
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
2005-05-26 KIAS-APCTP-DMRC Workshop 31
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
2005-05-26 KIAS-APCTP-DMRC Workshop 32
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
2005-05-26 KIAS-APCTP-DMRC Workshop 34
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
2005-05-26 KIAS-APCTP-DMRC Workshop 35
Thanks !!!