2004-10-22 82th kps meeting 1 radon detector calibration and detection efficiency study for the kims...
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2004-10-22 82th KPS meeting
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Radon Detector Calibration and Detection Efficiency Study for the KIMS
Experiment
Lee, Myeong Jae
DMRC, Seoul national universityFor the KIMS collaboration
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KIMS experiments Korea Invisible Mass Search Y2L : Yang-Yang underground Laboratory
Opened at 2003.4. Located YangYang pumped water power plant
Dark matter (WIMP) search experiment using CsI crystal detector
Recoil energy : ~ few keV Low gamma background need
s Radon monitoring is necess
ary to check the systematic effect of the gamma background generated from the radon daughter elements
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Decay chain of Radon
Able to detect 6.00. 6.78, 7.59, 8.79MeV alpha separately
Rock U : < 0.5 ppm Th : ~5.6 ppm
Air : 220Rn < ~10 % of 222Rn Consider 222Rn only
232Th232Th1.4e10yr1.4e10yr
238U238U3.5e9yr3.5e9yr
4.014.01 4.204.20228Ra228Ra5.75yr5.75yr
234Th234Th24.10d24.10d
228Ac228Ac6.15h6.15h
234Pa234Pa6.7h6.7h
228Th228Th1.9yr1.9yr
234U234U2.5e5yr2.5e5yr
5.425.42 4.774.77224Ra224Ra3.65d3.65d
230Th230Th7.5e4yr7.5e4yr
5.695.69 4.694.69220Rn220Rn55.6s55.6s
226Ra226Ra1600y1600y
rr6.296.29 4.784.78
216Po216Po0.1450.145
ss
222Rn222Rn3.82d3.82d
6.786.78 5.495.49212Pb212Pb10.6h10.6h
218Po218Po3.05m3.05m
(~36%(~36%))
212Bi212Bi60.6m60.6m(~64%(~64%
))
6.006.00
6.056.05 212Po212Po60.3u60.3u
ss
214Pb214Pb26.8m26.8m
208Tl208Tl3.05m3.05m
8.798.79 214Bi214Bi19.7m19.7m
208Pb208PbStableStable
214Po214Po164us164us
7.697.69210Pb210Pb22.3yr22.3yr
210Bi210Bi5.01d5.01d
210Po210Po138.4138.4
dd5.305.30
206Pb206PbStableStable
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Radon Detection Scheme
222Rn
218Po (115keV)
α (5.49MeV)Stripping effect
218Po+,+2
Electrostatic collection near photodiode
Decay
α (6.00MeV)
DetectDetect
Attach to water, aerosol, oxyzen
(chemically stable)Decay
214PbDecay214Bi
214Po
α (7.69MeV)
Electrostatic collection near photodiode
Decay
Decay
2004-10-22 82th KPS meeting
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Voltage Divider,Preamp
Photodiode
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
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Radon Detector Operation
Radondetector
Moisture trap
Vacuum pump
VoltageDivider
Preamplifier
Shapingamplifier
ADC
DAQ & Controlcomputer
HV Supply
Humidity, Temperature, Pressure, HV monitor
Controller
HV
Air
Signal
Control
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Typical signal and Background
Air, 2658 event/hour
N2, 26.8 event/hour
12h DAQ for typical air
6h DAQ for typical air and nitrogen6.00MeV
(218Po)7.69MeV
(214Po)
2004-10-22 82th KPS meeting
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Radon Contamination Measurement
Nitrogen test for ~9days: Time evolution of alpha counts
Nitrogen test for ~9days: (218Po count) / (214Po count)
Radon contamination of radon detector does not matter in this experiment setup (1h DAQ each time)
Equilibrium
214Po count218Po count
2004-10-22 82th KPS meeting
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Standard Radon Source/ Calibration Setup
226Ra in 5% HNO3 50g Max. Activity : 130.3 Bq (at ~65days)
= ~3500pCi = 50pCi/L * 70L Made at 2004.4.17 at KRISS Used at 2004.5.27 (34days after) : ~130.03B
q
Radon detector
Pump
X Flow meter Flow meter Filter
Temp. Humidity Sensor
X300cc/min 300cc/min Silicagel 280g, 452cc
Grain size : Φ3mm
11.5 liter/min
222Rn 130.3Bq
2004-10-22 82th KPS meeting
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Calibration data analysis
Typical Event Shape
Data Before Cut
Mean time
ADC
Data After Cut
Mean time
ADC
Data Histogram
ADC
Energy resolution is not good, But we can distinguish two
alpha particle.
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Calibration data analysisCalibration factor = (activity)/(event rate) [pCi/liter/Hz]
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Calibration data analysis(Po214 Decay Event)
Calibration factor = (activity)/(event rate) [pCi/liter/Hz]
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Calibration data analysis
Total Event Po214 Decay Event Po218 Decay Event
Calibration factor = (activity)/(event rate) [pCi/liter/Hz]
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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
2004-10-22 82th KPS meeting
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Active region
Simulation Result Positive ion repulses themselves, which
prevents gathering near the photodiode
2.59Bq 38.85Bq
120.50Bq90.65Bq
Efficiency change in time
2004-10-22 82th KPS meeting
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Humidity Dependence Test Setup
Po positive ion easily attach to water molecule ⇒ “Neutralize” Po ion ⇒ Cannot detect alpha decay
Radon detector(With know radon activity)
Pump
X Flow meter Flow meter
Temp. Humidity Sensor
X
11.5 liter/min
NaCl saturated water
X
X
~74% at 25℃
Always ~74% at 25℃
Saturated solution
Salt (ex. NaCl)
2004-10-22 82th KPS meeting
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Humidity Dependence of Efficiency
Humidity factor
Absolute humidity [kg/m3]
Humidity factor = (real event rate) / (expected event rate)
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Humidity Dependence of Efficiency
Humidity factor
Absolute humidity [kg/m3]
Humidity factor = (real event rate) / (expected event rate)