Muon and Neutron Backgrounds at Yangyang underground lab.
2005. 2. 18. Muju Workshop
Kwak, Jungwon Seoul National University
1. External Backgrounds 2. Muon flux in Yangyang underground lab. 3. Neutron background study4. Conclusion
2005-02-18 MuJu workshop 2
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 Improve PSD power of CsI crystal detector N2 flowing to remove air contaminated by Rn222
2. Neutron Undistinguishable with WIMP (Nuclear recoil)
• (n,) reaction • Nuclear fission • Induced by cosmic muon (~ 200GeV)
In shield - possible to veto In Rock - impossible to veto
Neutron shield of High Hydrogen density material Veto using Muon detector Underground experimental lab
2005-02-18 MuJu workshop 3
KIMS Main shield in Y2L
CSI
Pb shield (15cm) PE shield (5cm)
Copper shield (10cm)
Mineral Oil shield (30cm)
NMD
MUDMineral oil 30cm Pb 15cm : 30t
OFHC Cu 10cm : 3tPE 5cm
HPGe detector measurement
2005-02-18 MuJu workshop 4
Muon flux in Yangyang underground lab. (1)
MUD
MUD 2
MUD 3
MUD 1
MUD 7
MU
D
4
MU
D
6
MUD 5
MUD 8
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 shield 10-5 times of ground Muon rate at 700 m deep underground
2005-02-18 MuJu workshop 5
Muon flux in Yangyang underground lab. (2)
Attenuation length Fast components ~ 50 cm ~ Distance between PMTs Slow components ~ 250 cm ~ Size of Muon detector
Muon Efficiency = 98.1 % Cosmic Muon rate at Ground level ~ 10-2 cm-2s-1
Coincidence of two trigger detectors
2005-02-18 MuJu workshop 6
Muon flux in Yangyang underground lab. (3)
Muon flux [cm-2 m-1]
Muo
n flu
x
[cm
-2 m
-1]
Time [days]
Net DAQ time = 154.5 daysTotal 403227 muon events
Muon flux = 4.30 x 10-7 cm-2 s-1
Energy [MeV]
2005-02-18 MuJu workshop 7
Muon flux in Yangyang underground lab. (4)
mud5
mud4
mud7
mud6
Lab is here
2005-02-18 MuJu workshop 8
Neutron background study (1-1)
1. Use 1 liter BC501A Liquid Scintillator Teflon container ( 10 x 18 cm ) Quartz window and low background 3“ PMT
2. Backgrounds of Neutron detector Very small neutron flux measurement Gamma
External radioactive source and internal radioactive impurity • Development new PSD method
Alpha 238U and 232Th impurities in Liquid Scintillator Coincidence study of decay chains
2005-02-18 MuJu workshop 9
Pi and Pn i Normalized distribution of accumulated pulse shape
Pni - P i Bin by bin difference of probability function
Neutron background study (1-2)
Bin number i ( 1 bin = 2 ns ) Bin number i ( 1 bin = 2 ns )
FO
M
Old New
2005-02-18 MuJu workshop 10
Neutron background study (2)
Use the data of 67.4 days DAQ period
Energy Threshold 300 keV
Stable PSD power
2005-02-18 MuJu workshop 11
Energy [MeV]
Inside of shield for 67.41 days
Suspect the most of the events in neutron region are backgrounds
Check the coincidence of 238U and 232Th decay chains
Energy [MeV]
Neutron background study (3)
2005-02-18 MuJu workshop 12
238U decay chain
0.064
1.163
5.407
Beta
Beta
Alpha
22.4 y
5.013 d
138.376 d
210 Pb 210 Bi 210 Po
210Pb
4.871
5.590
6.115
1.024
3.272
7.833
Alpha
Alpha
Alpha
Beta
Beta
Alpha
1600 y
3.824 d
3.10 m
26.8 m
19.9 m
164.3 us
226 Ra 222 Rn 218 Po 214 Pb 214 Bi 214 Po
226 Ra
4.770Alpha7.538 E+4 y230 Th230 Th
4.859Alpha2.455 E+5 y234 U 234 U
4.270
0.274
2.117 (0.08)
2.197
Alpha
Beta
Beta (IT)
Beta
4.468 E+9 y
24.10 d
1.18 m
6.70 h
238 U234 Th234 Pa*
234 Pa
238 U
Q-value (MeV)
DecayLifetimeIsotope Family in 238 U
2005-02-18 MuJu workshop 13
232Th decay chain
2.254
6.207
8.954
5.001
Beta (64.06%)
Alpha (35.94%)
Alpha
Beta
60.55 m
299 ns
3.057 m
212 Bi
212 Po208 Tl
212 Bi
5.520
5.789
6.405
6.906
0.574
Alpha
Alpha
Alpha
Alpha
Beta
1.9116 y
3.66 d
55.6 s
0.145 s
10.64 h
228 Th 224 Ra 220 Rn 216 Po 212 Pb
228 Th
4.083
0.046
2.127
Alpha
Beta
Beta
1.405 E+10 y
5.75 y
6.15 y
232 Th228 Ra228 Ac
232 Th
Q-value (MeV)
DecayLifetimeIsotope Family in 232 Th
2005-02-18 MuJu workshop 14
Neutron background study (4) – 238U and 232Th chain214Bi -decay 214Po -decay
222Rn -decay 218Po -decay
Coincidence time [m]
Coincidence time [ms]
220Rn -decay 216Po -decay
Coincidence time [m]
2005-02-18 MuJu workshop 15
Neutron background study (5)
• - coincidence • 553 events of 222Rn -decay 218Po -decay - 553 +- 44 events ( 1341 events of background ) • 26 events of 220Rn -decay 216Po -decay - 26 +- 5 events
• -coincidence • 471 events of 214Bi -decay 214Po -decay Efficiency correction 534=471/0.976/0.903 - 534 +- 25 events
• 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
• 2722(all events) – 2851(expected alpha) = -121 +- 114 - 1.5 +- 1.4 neutron cnts/liter/day
• less than 1.8 cnts /day/liter (90 %CL) of neutron background inside of shield.
2005-02-18 MuJu workshop 16
Neutron background study (6)
Real Alpha Energy [MeV]Gam
ma
equi
vale
nt A
lpha
Ene
rgy
[MeV
]
Equilibrium from 238U Equilibrium from 226Ra
Quenching factor using tagged Alpha events
Decay chain in equilibrium from 226Ra
2005-02-18 MuJu workshop 17
Neutron rate outside of shield
8 x 10 –7 /cm2/s ( 1.5 < E neutron < 6 MeV )
Subtract energy spectrum inside of shield to reject internal background from real neutron
Energy [MeV]
Neutron background study (7)
2005-02-18 MuJu workshop 18
Conclusion
1. Muon Flux measurement 4.3 x 10-3 m-2 s-1
Stable flux - no modulation
2. Neutron Flux measurement Inside shield < 1.8 cnts/liter/day (90% CL) Outside shield = 8 x 10-7 cm-2 s-1 (1.5 ~ 6 MeV ) Alpha (226Ra decay chain) dominant internal background
Kims(Korea)