Transcript Muon and Neutron Background

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
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
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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)
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Environment Parameters in Y2L
Depth
Minimum 700 m
Temperature
20 ~ 25 oC
Humidity
35 ~ 60 %
Rock contents
238U
less than 0.5 ppm
5.6 A 2.6 ppm
40K
270 A 5% ppm
232Th
Muon flux
Neutron flux
222Rn
in air
2.7 x 10-7 /cm2/s
( 1 x 10-2 /cm2/s )
8 x 10-7 /cm2/s ( 1.4 x 10-3 /cm2/s )
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
40K
lifetime = 14.05 Gyr
lifetime = 1.277 Gyr
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KIMS Main shield in Y2L
HPGe detector measurement
Mineral oil 30cm
PE 5cm
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Pb 15cm : 30t
OFHC Cu 10cm : 3t
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External backgrounds
1. External gamma
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Isotopes in surrounding materials (Rock)
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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)
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•
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
(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
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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
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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
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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
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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
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Alpha background study – 238U and 232Th chain
214Bi
-decay 
214Po
a-decay
Coincidence time
222Rn
a-decay 
[ms]
218Po
a-decay
Coincidence time
220Rn
a-decay 
216Po
[m]
a-decay
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Coincidence time
[m]
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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 -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 10-3 times of neutron flux outside shield
 5 x 10-3 cpd level of background contribution on CsI(Tl) detector
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Muon induced neutron
Log10(Dt)
Energy
[MeV]
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
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Muon induced neutron
Energy
(cont’d)
[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
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Radon Monitoring detector
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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
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Why CsI(Tl) Crystal ?
Advantage
High light yield ~60,000/MeV
Pulse shape discrimination
NaI(Tl)
Easy fabrication and handling
High mass number(both Cs and I)
CsI(Tl)
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(t ~30y) ,134Cs(t ~2y) may be problematic
1/2
1/2
CsI(Tl)
Photons/MeV
~60,000
Density(g/cm3)
4.53
Decay Time(ns)
~1050
Peak emission(nm) 550
Hygroscopicity
slight
NaI(Tl)
~40,000
3.67
~230
415
strong
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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
: 10 mBq/kg
: 20 mBq/kg
87Rb : 10 ppb
134Cs
137Cs
87Rb
Geant Simulation
137Cs
134Cs
keV
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
137Cs
134Cs
1.07 cpd/1ppb
0.35 cpd/1mBq/kg
0.07 cpd/1mBq/kg
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Further reduction of internal background
New CsI powder produced with ultra pure water
2mBq/kg  0.7 cpd internal background
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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 cm3 6.7 kg
0501A
Running
-
8x8x30 cm3 8.7 kg
0501B
Running
-
8x8x30 cm3 8.7 kg
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Data analysis for WIMP Limit
 Single photon clustering
 Mean time definition
t

 At
A
i
i
i
i = cluster number
ti = time of i th cluster
Ai = charge of i
th
cluster
 Pulse Shape Discrimination
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Neutron Calibration facility in SNU
 In-house Am/Be source
( 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

9Be
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
CsI
BC501a
90o
LSC
n
 Measure time of flight between LSC and
CsI(Tl) detector
- Energy of Incident neutron
Am/Be
 3x3x3 cm3 CsI(Tl) crystal
- Nuclear recoil energy
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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
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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 ( 122.06 keV g ), 241Am (59.54 keV) and 55Fe(5.899 keV)
Compton data
Check cut efficiency, Single photon calibration
• 137Cs ( 661.657 keV g ) Compton data
Reference distribution for electron recoil events
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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.
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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
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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
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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
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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
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WIMP Limit curve ( spin dependent )
WIMP – Proton
WIMP – Neutron
<Sp>Cs = 0.370
<Sn>Cs = 0.003
<Sp>I = 0.309
<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
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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
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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
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Prospects
WIMP - Proton
WIMP - Neutron
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Low mass WIMP search in KIMS
Collaboration with China and Taiwan
ULE HPGe detector
Limited by threshold
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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
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Thanks !!!
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