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)