1. Muon and Neutron detector of KIMS experiment
Jungwon Kwak and JingJun Zhu DMRC

2. Backgrounds in KIMS experiment
External background Sources
Cosmic rays – muon
Reduce in Deep underground
( 10-5 level of ground in 700 m underground )
Radioactive source in Environmental material – Gamma, Neutron
Lead, Mineral Oil, Copper shield ( mainly related with U238, Th232, K40)
Rn in air – Alpha
N2 flowing
Internal background sources
CSI crystal – Cs137,134 Rb
R&D for ultra low background CSI crystal ( currently 10 cpd level)
PMT background – glass window, noise signal
Quartz window PMT with low background
Main shielding - Stainless steel, lead, copper
Pure copper, low background lead
MUD
CSI
NMD

3. Diagram of KIMS detector
Active Detector
CSI
Cs(Tl) Crystal Detector
MUD
NMD
Neutron Monitor Detector
PE shield (5cm)
Pb shield (15cm)
MUD
Muon Detector - LS 5%
Copper shield (10cm)
CSI
Passive shield
NMD
Copper : 10 cm
PE : 5 cm
Lead : 15 cm
Mineral Oil : 30 cm
Mineral Oil shield (30cm)

4. NMD Neutron background
To understand neutron background well is important in dark matter search experiment because neutron signals mimic the WIMP signals.
Neutron background source:
cosmic ray induced neutron
spontaneous fission of U238
(α,n) reaction of U & Th in the rock
Neutron flux at underground laboratory is very small, so we need a large detector and a long term measurement.
MUD
CSI

5. NMD Neutron Monitor Detector
Liquid Scintillator
BC501A 20 liter ( 16 cm diameter x 100 cm )
10 liter ( 16 cm diameter x 50 cm )
2 x 5” PMT
10-3 times of Neutron rate inside main shield
MUD
CSI
inside Pb 5 cm shield
inside main shield

6. NMD DCC method MUD CSI 20liter BC501A Neutron Detector
DCC(Digital Charge Comparison) method:
tail of signal / total signal for BC501A scintillator
neutron and gamma signal separation using pulse shape
MUD
CSI

7. NMD Neutron background inside main shield

8. NMD time variation
Fitting function = lifetime (3.8 days for Rn) + constant
Constant flux : 27 counts / day /liter
Data taken just after windows are sealed

9. NMD FADC DAQ system 10liter BC501A Neutron Detector used
1G Hz data sampling using digital oscilloscope
Data taken two month later window sealing
Scatter plot of part/full for each PMT
Energy spectrum of neutron candidates (MeV)
Flux for Neutron candidate = 29 counts / day / liter

10. MUD MUon Detector/Neutron Shield
CSI
2x2” PMT for each channel
8 muon modules , 28 signal channels
Liquid Scintillator 5 %
PC 1 liter + PPO 4 g + POPOP 15 mg
Mineral Oil 95 %
10-5 times of ground Muon rate at Y2L

11. MUD Muon detection Efficiency
CSI
Trigger Muon using two other scintillator detectors in the Ground lab
Use one(MUD2) of muon modules

12. MUD Attenuation length
CSI
Fitting function : two exponential decay function
Fitting results : fast term - 50 cm , slow term – need more length

13. MUD Muons in Y2L and CPL MUD CSI
Ground level : ~ 8.64 x 106 muons / day.m2 = Fluxg
CPL(-350m) : ~ 1.45 x 103 muons /day.m2 = 1.6 x 10-4 Fluxg
Y2L(-700m) : ~ 2.20 x 102 muons /day.m2 = 2.5 x 10-5 Fluxg

14. MUD Hit position reconstruction
CSI
MUD8 module used
Trigger Muon using Plastic scintillator and MUD2
Divide 64 x 64 cells and calculate c2 values for each cell
choose one cell with minimum c2
Use only ADC information ( in progress for TDC )

15. Conclusion MUD CSI NMD (Neutron Monitoring Detector)
Upper limit of Neutron flux ~ 30 counts/day/liter
Few ppt level of U, Th can cause comparable Alpha rate
Need Checking U, Th level of NMD
MUD (Muon Detector)
Muon efficiency ~ 98 %
Muon flux in Y2L ~ 2.5 x 10-5 of ground level
Muon track reconstruction in progress
MUD
CSI

16. NMD Neutron background outside main sheild