1. 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

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.Lee
H.S.Lee, S.E.Lee, J. Lee, S.S.Myung, H.Y.Yang
Seoul National University
Y.D.Kim, J.I. Lee
Sejong University
H.J.Kim
Kyungpook National University
M.J.Hwang, Y.J.Kwon
Yonsei University
I.S.Hahn, I.H.Park
Ewha Womans University
M.H.Lee, E.S.Seo
Univ. of Maryland
J.Li
Institute of High Energy Physics
J.J.Zhu, D. He, Q.Yue, X. Lee
Tsinghua University
KIAS-APCTP-DMRC Workshop

3. Yangyang Underground Laboratory, Y2L
Korea Middleland Power Co.
Yangyang Pumped Storage Power Plant
Minimum depth : 700 m / Access to the lab by car (~2km)
Completion of the power plant(2006)
Construction of the laboratory buildings done (2003)
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 A 2.6 ppm
40K A 5% ppm
Muon flux
2.7 x 10-7 /cm2/s ( 1 x 10-2 /cm2/s )
Neutron flux
8 x 10-7 /cm2/s ( 1.4 x 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 = Gyr
40K lifetime = Gyr
KIAS-APCTP-DMRC Workshop

5. 2005-05-26 KIAS-APCTP-DMRC Workshop
KIMS Main shield in Y2L
Mineral oil 30cm
Pb 15cm : 30t
OFHC Cu 10cm : 3t
PE 5cm
HPGe detector measurement
KIAS-APCTP-DMRC Workshop

6. 2005-05-26 KIAS-APCTP-DMRC Workshop
External backgrounds
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
Neutron background
Undistinguishable from WIMP signal (Nuclear recoil)
(a, 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
KIAS-APCTP-DMRC Workshop

7. 2005-05-26 KIAS-APCTP-DMRC Workshop
Muon detector (MUD)
2 x 2” PMT for each channel muon modules
28 signal channels ( x 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
KIAS-APCTP-DMRC Workshop

8. Muon detection efficiencies
Plastic scintillator
MUD8
Bottom modules
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
KIAS-APCTP-DMRC Workshop

9. 2005-05-26 KIAS-APCTP-DMRC Workshop
Muon flux measurement
Tagging efficiency : 94.0 %
Area of top module = 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
KIAS-APCTP-DMRC Workshop

10. Neutron Monitoring Detector (NMD)
1liter BC501A liquid scintillator
- Teflon container
- Quartz window
- Low background 3” PMT
n/g separation using PSD method
- 500MHz sampling FADC
KIAS-APCTP-DMRC Workshop

11. Alpha background study – 238U and 232Th chain
214Bi -decay  214Po -decay
Coincidence time [ms]
222Rn -decay  218Po -decay
Coincidence time [m]
220Rn -decay  216Po -decay
KIAS-APCTP-DMRC Workshop
Coincidence time [m]

12. Neutron flux measurement in Y2L
Inside main shield < 1.8 neutrons/day/liter @90%CL, E threshold = 300 keV
Tag a events using a-a and b-a 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 times of neutron flux outside shield
5 x 10-3 cpd level of background contribution on CsI(Tl) detector
KIAS-APCTP-DMRC Workshop

13. 2005-05-26 KIAS-APCTP-DMRC Workshop
Muon induced neutron
Energy [MeV]
Log10(Dt)
Dt = min(Abs(t Muon event – t Neutron event))
Require Dt < 1 ms for coincidence events
Dtmean = 130 ns delay cable for muon and more electronics
sDt = 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
KIAS-APCTP-DMRC Workshop

14. Muon induced neutron (cont’d)
Energy [MeV]
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 (m 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
KIAS-APCTP-DMRC Workshop

15. Radon Monitoring detector
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
KIAS-APCTP-DMRC Workshop

16. 2005-05-26 KIAS-APCTP-DMRC Workshop
Why CsI(Tl) Crystal ?
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
NaI(Tl)
CsI(Tl)
Disadvantages
Emission spectra does not match with normal bi-alkali PMT
=> Effectively reduce light yield
137Cs(t1/2 ~30y) ,134Cs(t1/2 ~2y) may be problematic
CsI(Tl) NaI(Tl)
Photons/MeV ~60, ~40,000
Density(g/cm3)
Decay Time(ns) ~ ~230
Peak emission(nm)
Hygroscopicity slight strong
KIAS-APCTP-DMRC Workshop

17. Internal background of CsI(Tl) crystal detector
137Cs : 10 mBq/kg
134Cs : 20 mBq/kg
87Rb : 10 ppb
87Rb
137Cs
134Cs
keV
Geant Simulation
137Cs (artificial)
serious background at low energy
134Cs (artificial+133Cs(n,gamma))
87Rb (natural)
Hard to reject
 reduction technique in material is known
Best available Crystal at Market
Powder Selection
Cs137 Reduction Using Pure water
Rb87 Reduction by Re-crystallization
Single Crystal (8.7 kg) background
@ ~10keV
87Rb cpd/1ppb
137Cs cpd/1mBq/kg
134Cs cpd/1mBq/kg
KIAS-APCTP-DMRC Workshop

18. Further reduction of internal background
New CsI powder produced with ultra pure water
2mBq/kg  0.7 cpd internal background
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 Fe55 Signal
Crystal
Data
# of p.e.
Size
0406
237 kg days
5.5
8x8x23 cm kg
0501A
Running -
8x8x30 cm kg
0501B
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
KIAS-APCTP-DMRC Workshop

21. Neutron Calibration facility in SNU
In-house Am/Be source
9Be ( a , n ) 12C ~ 50%
9Be ( a , n ) 12C* , 12C*  12C + g (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
LSC
90o
17.4o 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
KIAS-APCTP-DMRC Workshop

22. Neutron Calibration facility in SNU
Energy of neutrons from Am/Be source [MeV]
@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
KIAS-APCTP-DMRC Workshop

23. Summary CsI(Tl) Data taking
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 ( keV g ), 241Am (59.54 keV) and 55Fe(5.899 keV) Compton data
Check cut efficiency, Single photon calibration
137Cs ( keV g ) Compton data
Reference distribution for electron recoil events
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.
KIAS-APCTP-DMRC Workshop

25. Log Mean Time distribution of Neutron & 137Cs Compton & Data
3~4 keV
4 ~5 keV
5~6 keV
6~7 keV
7~8 keV
8~9 keV
9~10 keV
10~11 keV
KIAS-APCTP-DMRC Workshop

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

27. WIMP Proton Cross Section
WIMP Nucleus Scattering
ρχ=galatic halo density
vE=earth velocity in galatic frame
v0=sun velocity in galatic frame
Spin Independent
Spin Dependent
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.
sW-A combined WIMP-neucleon X-section
sW-A (EK) WIMP-neucleon X-section for E k energy bin
KIAS-APCTP-DMRC Workshop

29. WIMP Limit curve ( spin dependent )
WIMP – Proton
<Sp>Cs = 0.370
<Sp>I = 0.309
WIMP – Neutron
<Sn>Cs = 0.003
<Sn>I = 0.075
For I , SM(Bonn A), M.T.Ressell, D. J. Dean Phys. Rev. C58 (1997) 535
For Cs, IBFM, F. Iachello, L.M. Krauss, G. Maino Phys. Lett. B254 (1991) 220
KIAS-APCTP-DMRC Workshop

30. 2005-05-26 KIAS-APCTP-DMRC Workshop
Summary
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
KIAS-APCTP-DMRC Workshop

31. 2005-05-26 KIAS-APCTP-DMRC Workshop
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
KIAS-APCTP-DMRC Workshop

32. 2005-05-26 KIAS-APCTP-DMRC Workshop
Prospects
WIMP - Proton
WIMP - Neutron
KIAS-APCTP-DMRC Workshop

33. Low mass WIMP search in KIMS
Collaboration with China and Taiwan
ULE HPGe detector
Limited by threshold
KIAS-APCTP-DMRC Workshop

34. 2005-05-26 KIAS-APCTP-DMRC Workshop
Status of ULE HPGe detector setup
5g 1 cpd level detector
Tested at Academia Cinica, Taiwan
To be delivered to SNU in Dec.
If successful  upgrade to 1kg mass
CsI(Tl) crystal
Compton veto
 Built by TU
 Delivered to SNU
KIAS-APCTP-DMRC Workshop

35. 2005-05-26 KIAS-APCTP-DMRC Workshop
Thanks !!!
KIAS-APCTP-DMRC Workshop