1. Status of Neutron flux Analysis in KIMS experiment
Jungwon Kwak
Seoul National University

2. Korea Invisible Mass Search experiment
International collaboration - 5 nations, 8 institutes
Dark matter , missing mass in Universe
Universe = missing ( ~ 95%) + Visible Mass ( ~ 5 %)
Dark Matter means matter whose existence has been inferred only through its gravitation effects … Particle Data Group
WIMP (Weakly Interacting Massive Particle)
Excellent CDM candidate
Super partner of neutral gauge particle and higgs
Underground Laboratory
To avoid neutrons induced by cosmic ray
- Neutron signal is identical to the WIMP signal in CsI(Tl) crystal
YangYang underground Laboratory
In YangYang Water pumping power plant
700 m underground : 4.4 x 10-7 /cm2/s cosmic rate
Main-shield : 30cm Mineral Oil, 15cm Lead, 5cm PE, 10cm Cu
- Gamma (10-4 ) and Neutron background(10-3) rate of outside
CsI(Tl) crystal detector, 4p full coverage Muon detector, Neutron monitoring detector and Rn monitoring detector
KPS meeting

3. Backgrounds of Neutron Detector
Neutron monitoring detector
1.2 liter BC501A liquid scintillator
Quartz window and Teflon (CF2) container (10f x 16 cm3 )
D “ PMT – ultra low background
g Background
External radioactive source and internal radioactive impurity
PSD make enable to reject out gamma background
- Develop new PSD method using probability function for g and neutron
a Background - Mimic neutron signal in neutron detector
Source : 238U and 232Th impurities in Liquid Scintillater
Tagging a – a coincidence events
- Lifetime Limit < 4.3 m
Tagging b – a coincidence events
Lifetime Limit < 0.1 s
KPS meeting

4. Probability function for g and neutron pulses
Pg i and Pn i
Normalized distribution of accumulated pulse shape
Pn i - Pg i
Bin by bin difference of probability function
Bin number i ( 1 bin = 2 ns )
Bin number i ( 1 bin = 2 ns )
During DATA run period, never changed DAQ and Detector system to keep the same probability functions of pulse shapes.
Blue and Pink line indicate the start points of partial pulses for simple DCC method. (Blue 20ns , Pink 60ns delayed points from peak point of pulse)
KPS meeting

5. Simple DCC and Weighted DCC method
Simple DCC patial (60ns) / Full ratio
Weighted DCC A+/A- ratio
Energy [MeV]
Energy [MeV]
if (Pn i - Pg i > 0 ) A+ = Sum( (Pn i - Pg i ) x Pulse i )
if (Pn i - Pg i < 0 ) A- = - Sum( (Pn i - Pg i ) x Pulse i )
Better Pulse Shape Discrimination
Applied the more Weighting factor to bins in which the more difference between n and g probability functions
KPS meeting

6. Comparison between two DCC methods
FOM
Energy [MeV]
FOM = ( Peak n – Peak g ) /sqrt( sn2 + sg 2 )
Worse PSD of 20 ns DCC at low energy
more contamination of short component term
Worse PSD of 60 ns DCC at high energy
smaller statistics of long component term
Weighted DCC shows better PSD than simple DCC method
KPS meeting

7. Data analysis cuts A+/A- Use the data of 67.4 days DAQ period
Energy [MeV]
Data taking time [Day]
Use the data of 67.4 days DAQ period
Energy Threshold 300 keV
Ratio A+/A- > for Neutron region
Require 390 ns < peak of pulse < 430 ns
KPS meeting

8. Energy spectrum Inside of shield for 67.41 days
Rate: 0.1 counts/liter/s
Energy [MeV]
Energy [MeV]
Inside of shield for days
Suspect the most of the events in neutron region are a backgrounds
Check the coincidence of 238U and 232Th decay chains
Lifetime limit of a – a coincidence < 4.3 m
- 1 a event per every 42.8 m ( cnts/liter/day )
Lifetime limit of b – a coincidence < 1 s
- 1 g event per every 10 s ( cnts/liter/s)
KPS meeting

9. 238U decay chain Family in 238 U Isotope Lifetime Decay Q-value (MeV)
234 Th
234 Pa*
234 Pa
4.468 E+9 y
24.10 d
1.18 m
6.70 h
Alpha
Beta
Beta (IT)
4.270
0.274
2.117 (0.08)
2.197
234 U
2.455 E+5 y
4.859
230 Th
7.538 E+4 y
4.770
226 Ra
222 Rn
218 Po
214 Pb
214 Bi
214 Po
1600 y
3.824 d
3.10 m
26.8 m
19.9 m
164.3 ms
4.871
5.590
6.115
1.024
3.272
7.833
210Pb
210 Pb
210 Bi
210 Po
22.4 y
5.013 d d
0.064
1.163
5.407
KPS meeting

10. 232Th decay chain Family in 232 Th Isotope Lifetime Decay
Q-value (MeV)
232 Th
228 Ra
228 Ac
1.405 E+10 y
5.75 y
6.15 y
Alpha
Beta
4.083
0.046
2.127
228 Th
224 Ra
220 Rn
216 Po
212 Pb y
3.66 d
55.6 s
0.145 s
10.64 h
5.520
5.789
6.405
6.906
0.574
212 Bi
212 Po
208 Tl
60.55 m
299 ns
3.057 m
Beta (64.06%)
Alpha (35.94%)
2.254
6.207
8.954
5.001
KPS meeting

11. b – a coincidence candidates
238U chain
214Bi : MeV b-decay g 214Po : MeV a-decay
Lifetime of 214Po = ms … 4 ms dead time 97.6% efficiency
100 keV Energy threshold … 90.3 % efficiency of b-spectrum
232Th Chain
212Bi : MeV b-decay g 212Po : MeV a-decay
Lifetime of 212Po = 299 ns … 4 ms dead time 2.3 E-4 % efficiency
Require two pluses within 2 ms window … 99.9 % efficiency
About 50 keV Hardware threshold … more than 90% efficiency
KPS meeting

12. 214Bi b-decay g 214Po a-decay Coincidence time [ms] 2018-12-26
KPS meeting

13. a – a coincidence candidates
238U chain
222Rn : MeV a-decay g 218Po : MeV a-decay
Lifetime of 218Po = 3.10 m
232Th Chain
224Ra : MeV a-decay g 220Rn : MeV a-decay
Lifetime of 220Rn = 55.6 s
220Rn : MeV a-decay g 216Po : MeV a-decay
Lifetime of 216Po = s
KPS meeting

14. 222Rn a-decay g 218Po a-decay Coincidence time [m] 2018-12-26
KPS meeting

15. 220Rn a-decay g 216Po a-decay Coincidence time [m] 2018-12-26
KPS meeting

16. Summary of Internal background
a-a coincidence
553 events of 222Rn a-decay g 218Po a-decay
events ( 1341 events of background )
26 events of 220Rn a-decay g 216Po a-decay
events
b-a coincidence
471 events of 214Bi b-decay g 214Po a-decay
Efficiency correction 534=471/0.976/0.903
events
230Ra dominant contamination in 238U chain
cnts/liter/day – 1.5 x10-6 ppt of 230Ra level
232Th dominant contamination in 232Th chain
cnts/liter/day ppt of 232Th level
2722(all events) – 2851(expected alpha) =
neutron cnts/liter/day
less than 1.8 cnts/liter/day (90 % confidence level ) of neutron background inside of shield.
KPS meeting

17. Neutron flux outside of shield
Energy [MeV]
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
KPS meeting

18. Neutron flux induced by muon
Log10(Dt)
Energy [MeV]
Dt = Min(Muon trigger time – Neutron trigger time)
High energy events ( > 3 MeV) of Neutron detector is mostly from cosmic muon
Time offset of muon and neutron detector = 133 ns
- 20m delay cable for muon detector and more electronics
Time resolution = 25 ns
- Use clock pulse of 16ns width
KPS meeting

19. Coincidence neutron signal
Gamma region
Neutron region
Energy [MeV]
Energy [MeV]
Two events is strong candidates of neutron induced by muon
- 2 events for days = cnts/liter/year
Also Tagged low energy gamma induced by cosmic muon
Gamma rate = 0.9 cnts/liter/day
KPS meeting

20. Summary Estimated internal background of Neutron detector
cnts/liter/day of alpha’s from 238U chain
cnts/liter/day of alpha’s from 232Th chain
Upper limit of neutron rate inside of shield
- less than 1.8 CL
Neutron flux outside of shield
- 8x10-7/cm2/s - 15 cnts/liter/day
Flux of neutron induced by cosmic muon
cnts/liter/year
KPS meeting