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

2004-10-22 82th KPS meeting

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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

2004-10-22 82th KPS meeting

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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

2004-10-22 82th KPS meeting

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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

2004-10-22 82th KPS meeting

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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

2004-10-22 82th KPS meeting

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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.

2004-10-22 82th KPS meeting

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Calibration data analysisCalibration factor = (activity)/(event rate) [pCi/liter/Hz]

2004-10-22 82th KPS meeting

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Calibration data analysis(Po214 Decay Event)

Calibration factor = (activity)/(event rate) [pCi/liter/Hz]

2004-10-22 82th KPS meeting

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Calibration data analysis

Total Event Po214 Decay Event Po218 Decay Event

Calibration factor = (activity)/(event rate) [pCi/liter/Hz]

2004-10-22 82th KPS meeting

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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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Particle Generation for Simulation

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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)

2004-10-22 82th KPS meeting

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Y2L Radon Level

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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