Transcript Neutron Detector

Status of Neutron flux Analysis
in KIMS experiment
Jungwon Kwak
Seoul National University
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
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2004 10 22 KPS meeting
Backgrounds of Neutron Detector
Neutron monitoring detector
 1.2 liter BC501A liquid scintillator
 Quartz window and Teflon (CF2) container (10f x 16 cm3 )
 D7265 3 “ 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
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Probability function for g and neutron pulses
Pg i and Pn i
Pn i - Pg i
Normalized distribution of accumulated pulse shape
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)
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Simple DCC and Weighted DCC method
Weighted DCC A+/A- ratio
Simple DCC patial (60ns) / Full 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
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FOM
Comparison between two DCC methods
Energy
 FOM = ( Peak
n
– Peak
g
[MeV]
) /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
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A+/A-
Data analysis cuts
Energy
[MeV]
 Use the data of 67.4 days DAQ period
Energy Threshold 300 keV
Ratio A+/A- > 0.085 for Neutron region
Require 390 ns < peak of pulse < 430 ns
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Data taking time
[Day]
Energy spectrum
Rate: 0.1 counts/liter/s
Energy
[MeV]
Energy
[MeV]
 Inside of shield for 67.41 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 ( 33.65 cnts/liter/day )
 Lifetime limit of b – a coincidence < 1 s
- 1 g event per every 10 s ( 0.100 cnts/liter/s)
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238U
Family in
238
U
decay chain
Isotope
Lifetime
Decay
Q-value
(MeV)
U
234 Th
234 Pa*
234 Pa
4.468 E+9 y
24.10 d
1.18 m
6.70 h
Alpha
Beta
Beta (IT)
Beta
4.270
0.274
2.117 (0.08)
2.197
238
U
238
234
U
234
U
2.455 E+5 y
Alpha
4.859
230
Th
230
Th
7.538 E+4 y
Alpha
4.770
226
Ra
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
Alpha
Alpha
Alpha
Beta
Beta
Alpha
4.871
5.590
6.115
1.024
3.272
7.833
Pb
210 Bi
210 Po
22.4 y
5.013 d
138.376 d
Beta
Beta
Alpha
0.064
1.163
5.407
210Pb
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210
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232Th
Family in
232
Th Isotope
decay chain
Lifetime
Decay
Q-value
(MeV)
232
Th
232
Th
228 Ra
228 Ac
1.405 E+10 y Alpha
5.75 y
Beta
6.15 y
Beta
4.083
0.046
2.127
228
Th
228
Th
224 Ra
220 Rn
216 Po
212 Pb
1.9116 y
3.66 d
55.6 s
0.145 s
10.64 h
Alpha
Alpha
Alpha
Alpha
Beta
5.520
5.789
6.405
6.906
0.574
212
Bi
212
60.55 m
Beta (64.06%)
Alpha (35.94%)
Alpha
Beta
2.254
6.207
8.954
5.001
Bi
Po
208 Tl
212
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299 ns
3.057 m
2004 10 22 KPS meeting
b – a coincidence candidates
 238U
•
214Bi
chain
: 3.327 MeV b-decay g
– Lifetime of
214Po
214Po
: 7.833 MeV a-decay
= 0.1643 ms … 4 ms dead time 97.6% efficiency
– 100 keV Energy threshold … 90.3 % efficiency of b-spectrum
 232Th
Chain
• 212Bi : 2.254 MeV b-decay g
212Po
: 8.954 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
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214Bi
b-decay g
214Po
Coincidence time
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a-decay
[ms]
a – a coincidence candidates
 238U
•
chain
222Rn
: 5.590 MeV a-decay g
– Lifetime of
 232Th
218Po
: 6.115 MeV a-decay
= 3.10 m
Chain
• 224Ra : 5.789 MeV a-decay g
– Lifetime of
•
218Po
220Rn
220Rn
220Rn
= 55.6 s
: 6.405 MeV a-decay g
– Lifetime of
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216Po
: 6.405 MeV a-decay
216Po
: 6.906 MeV a-decay
= 0.145 s
2004 10 22 KPS meeting
222Rn
a-decay g
218Po
Coincidence time
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a-decay
[m]
220Rn
a-decay g
216Po
Coincidence time
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a-decay
[m]
Summary of Internal background
• a-a coincidence
• 553 events of
222Rn
• b-a coincidence
• 471 events of
214Bi
a-decay g 218Po a-decay
. 553 +- 44 events ( 1341 events of background )
• 26 events of 220Rn a-decay g 216Po a-decay
. 26 +- 5 events
b-decay g 214Po a-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
•
232Th
dominant contamination in
232Th
. 0.32 +- 0.06 cnts/liter/day - 0.63 ppt of
230Ra
level
chain
232Th
level
• 2722(all events) – 2851(expected alpha) = -121 +- 114
. -1.5 +- 1.4 neutron cnts/liter/day
• less than 1.8 cnts/liter/day (90 % confidence level ) of
neutron background inside of shield.
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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
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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
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Coincidence neutron signal
Neutron region
Gamma region
Energy
[MeV]
Energy
[MeV]
 Two events is strong candidates of neutron induced by muon
- 2 events for 67.41 days = 11 +- 8 cnts/liter/year
 Also Tagged low energy gamma induced by cosmic muon
Gamma rate = 0.9 cnts/liter/day
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Summary
 Estimated internal background of Neutron detector
- 33.3 cnts/liter/day of alpha’s from
- 1.92 cnts/liter/day of alpha’s from
238U
chain
232Th chain
 Upper limit of neutron rate inside of shield
- less than 1.8 cnts/liter/day @90% CL
 Neutron flux outside of shield
- 8x10-7/cm2/s - 15 cnts/liter/day
 Flux of neutron induced by cosmic muon
- 11 +- 8 cnts/liter/year
2015-07-20
2004 10 22 KPS meeting