2005-2-18 Workshop for Underground experiments and astroparticle physics

1

Radon Environment of Underground Experiment and Detection of Radon

Lee, Myeong Jae

DMRC, Seoul national universityFor the KIMS collaboration

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Contents

Property of Radon Radioactivity of the rock for other

experiments Radon detection for the KIMS experiments Radon-Free techniques Conclusion

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Radon

Odorless, tasteless, invisible gas Chemically inert and essentially non-reactive Heaviest noble gas with highest melting and boiling point Highly soluble in non-polar solvents Moderately soluble in cold water Able to diffuse through rock and soil Decays by alpha particle emission (T1/2 = 3.8 days) Exists in the typical atmosphere and specially in the unde

rground environment outside : 0.2 ~ 0.7 pCi/L inside : ~ 1.25 pCi/L residential recommendation : 4 pCi/L

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Decay chain of Radon 232Th232Th

1.4e10yr1.4e10yr238U238U

3.5e9yr3.5e9yr 4.014.01 4.204.20

228Ra228Ra5.75yr5.75yr

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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Why Radon is Important?

(From “Univ. of Minesota”)

Can cause lung cancer !!

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Why Radon is Important?(2)

Radioactive contamination of experimental setup Radon dissolves in water and contaminates all other

material Po ion easily attaches to other material

Can cause b.g. event For the neutrino experiment radon daughter beta deca

y is significant bg source

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Radioactivity data for other experiment

(From UKDMC Homepage)

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Radioactive environment of Y2L

U 3.46 <0.5 <0.5 <0.5

Th 1.89 2.20 9.02 5.58

Element(ppm) A B C D

Sample

<0.5

4.67

Avg.

Y2L

호상편마암(~2Gyr)

중생대 화성암(~200Myr)

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Radon Detection Method

No way to detect radon directly : Noble gas Decay is detectable

alpha and its emitter detection Lucas cell, SSNTD, electrostatic alpha spectroscopy, filter me

thod, ionization detector beta and its emitter detection

Liquid scintillator, beta spectroscopy, gamma counting

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Radon Detection Scheme (electrostatic alpha spectroscopy)

222Rn

218Po (115keV)

α (5.49MeV)Stripping effect

218Po+,+2

Electrostatic collection near photodiode

Decay

α (6.00MeV)

DetectDetect

Attach to water, aerosol, oxygen

(chemically stable)Decay

214PbDecay214Bi

214Po α (7.69MeV)

Electrostatic collection near photodiode

Decay

Decay

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Voltage Divider,Preamp

Photodiode

Radon Monitoring Detector Electrostatic alpha spectroscopy Use Si(Li) photodiode

30mm X 30mm 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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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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Typical signal and Background

Air, ~0.74 Hz

N2, ~0.0074 Hz

12h DAQ for typical air

6h DAQ for typical air and nitrogen6.00MeV

(218Po)7.69MeV

(214Po)

Recent BG measurement = ~0.037Hz

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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) : ~130.03Bq

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Calibration data analysis

Total Event Po214 Decay Event Po218 Decay Event

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

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

~1.8pCi/L/Hz= ~ 47Bq/Hz

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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 Flow meter

Temp. Humidity Sensor

X

11.5 liter/min

NaCl saturated water

X

X

~74% at 25℃

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Humidity Dependence of Efficiency

Humidity factor = (real event rate) / (expected event rate)

Somewhat different with other experiment measurement

Absolute humidity [kg/m3]

Humidity factor

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Y2L Radon Level

Days elapsed since 2003.1.1

(pC

i/Lite

r)

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Radioactivity data for other experiment

Mine air~2.71200SNOSudbury

~10UKDMCBoulby

80~100CDMSSoudan

~1~ 500KIMSYYL

DAMA detector0.08-0.14

Mine air. Ventilation dependence0.5-11180~670DAMA, CRESST, Borexino

GranSasso

Mine air. Seasonal dependence5.4-54

Control room0.8-1.3S-K, KamLand

Kamioka

Rn222 contents(pCi/L)

U contents(ppb)

Exp.Location

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Radon-Free techniques

Blocking Filtering Ventilation

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Radon Blocking capability for

various material

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Radon Blocking material :Example of KamLand

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Ventilation dependence of Radon

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Summary

Radon monitoring detector setup completed in Y2L

Radon contamination level monitoring in the air of Y2L is ongoing

Radioactivity in the rock is similar and radon activity is much lower than other experiment

Gamma background reduction from radon daughter elements in KIMS experiments is possible