1. Radon Detector Calibration and Detection Efficiency Study for the KIMS Experiment
Lee, Myeong Jae
DMRC, Seoul national university
For the KIMS collaboration
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2. KIMS experiments Korea Invisible Mass Search
Y2L : Yang-Yang underground Laboratory
Opened at
Located YangYang pumped water power plant
Dark matter (WIMP) search experiment using CsI crystal detector
Recoil energy : ~ few keV
Low gamma background needs
Radon monitoring is necessary to check the systematic effect of the gamma background generated from the radon daughter elements
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3. Decay chain of Radon
232Th
1.4e10yr
238U
3.5e9yr
4.01
4.20
228Ra
5.75yr
234Th
24.10d
228Ac
6.15h
234Pa
6.7h
228Th
1.9yr
234U
2.5e5yr
5.42
4.77
224Ra
3.65d
230Th
7.5e4yr
5.69
4.69
220Rn
55.6s
226Ra
1600yr
6.29
4.78
216Po
0.145s
222Rn
3.82d
6.78
5.49
212Pb
10.6h
218Po
3.05m
(~36%)
212Bi
60.6m
(~64%)
6.00
6.05
212Po
60.3us
214Pb
26.8m
208Tl
8.79
214Bi
19.7m
208Pb
Stable
214Po
164us
7.69
210Pb
22.3yr
210Bi
5.01d
210Po
138.4d
5.30
206Pb
Able to detect , 7.59, 8.79MeV alpha separately
Rock
U : < 0.5 ppm
Th : ~5.6 ppm
Air : 220Rn < ~10 % of 222Rn
Consider 222Rn only
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4. Radon Detection Scheme
222Rn
218Po (115keV)
α (5.49MeV)
Stripping effect
218Po+,+2
Electrostatic collection
near photodiode
Decay
α (6.00MeV)
Detect
Attach to water, aerosol, oxyzen
(chemically stable)
214Pb
214Bi
214Po
α (7.69MeV)
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5. Radon Monitoring Detector
Electrostatic alpha spectroscopy
Use Si(Li) photodiode
30mm X 30mm, ETRI
Photodiode – chassis potential difference : -500V
Photodiode bias : -70V
Photodiode calibration : 210Po, 241Am
Neutralization of 218Po
: Humidity control is necessary
Volume : ~ 70 liter
Voltage Divider,
Preamp
Photodiode
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6. Radon Detector Operation
Moisture trap
Vacuum
pump
Voltage
Divider
Preamplifier
Shaping
amplifier
ADC
DAQ & Control
computer
HV Supply
Humidity, Temperature,
Pressure, HV monitor
Controller HV
Air
Signal
Control
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7. Typical signal and Background
Air, 2658 event/hour
N2, 26.8 event/hour
12h DAQ for typical air
6h DAQ for typical air and nitrogen
6.00MeV
(218Po)
7.69MeV
(214Po)
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8. Radon Contamination Measurement
214Po count
218Po count
Nitrogen test for ~9days
: Time evolution of alpha counts
Nitrogen test for ~9days
: (218Po count) / (214Po count)
Equilibrium
Radon contamination of radon detector does not matter in this experiment setup (1h DAQ each time)
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9. Standard Radon Source / Calibration Setup
226Ra in 5% HNO3 50g
Max. Activity : Bq (at ~65days)
= ~3500pCi = 50pCi/L * 70L
Made at at KRISS
Used at (34days after) : ~130.03Bq
Radon detector
Pump X
Flow meter
Filter
Temp. Humidity
Sensor
300cc/min
Silicagel 280g, 452cc
Grain size : Φ3mm
11.5 liter/min
222Rn 130.3Bq
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10. Calibration data analysis
Data Histogram
ADC
Typical Event Shape
Data Before Cut
Mean time
ADC
Data After Cut
Mean time
ADC
Energy resolution is not good,
But we can distinguish two alpha particle.
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11. Calibration data analysis
Calibration factor = (activity)/(event rate) [pCi/liter/Hz]
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12. Calibration data analysis (Po214 Decay Event)
Calibration factor = (activity)/(event rate) [pCi/liter/Hz]
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13. Calibration data analysis
Calibration factor = (activity)/(event rate) [pCi/liter/Hz]
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Total Event
Po214 Decay Event
Po218 Decay Event

14. Electric Field Simulation
For 1pCi/L , 1.23x106 particle exist : N-body simulation is impossible.
Particle number(1000) restricted, charge-factor is given for each particle.
Electric field from each particle is calculated
Drift motion is assumed
Mobility = μ = 1.94x10-4 m2/V sec for new-born 218Po
(S.C.Yoon, Radiation Measurement v.23(1994))
Diffusion coefficient D = μkT/e
-1V
-20V
-25V
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15. Particle Generation for Simulation
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16. Efficiency change in time
Active region
Simulation Result
Positive ion repulses themselves, which prevents gathering near the photodiode
2.59Bq
38.85Bq
120.50Bq
90.65Bq
Efficiency change in time
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17. Humidity Dependence Test Setup
Po positive ion easily attach to water molecule
⇒ “Neutralize” Po ion
⇒ Cannot detect alpha decay
Always ~74% at 25℃
Saturated solution
Salt (ex. NaCl)
Radon detector
(With know radon activity)
Pump X
Flow meter
Temp. Humidity
Sensor
11.5 liter/min
NaCl saturated water
~74% at 25℃
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18. Humidity Dependence of Efficiency
Humidity factor = (real event rate) / (expected event rate)
Humidity factor
Absolute humidity [kg/m3]
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19. Humidity Dependence of Efficiency
Humidity factor = (real event rate) / (expected event rate)
Humidity factor
Absolute humidity [kg/m3]
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20. Y2L Radon Level
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21. Conclusion Radon monitoring detector setup completed in Y2L
Radon contamination level monitoring in the air of Y2L is ongoing
Humidity dependence of Radon detection is studied.
Gamma background reduction from radon daughter elements in KIMS experiments is possible
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