Radon Detector Calibration and Detection Efficiency Study for the KIMS Experiment Lee, Myeong Jae DMRC, Seoul national university For the KIMS collaboration 2004-10-22 82th KPS meeting

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 needs Radon monitoring is necessary to check the systematic effect of the gamma background generated from the radon daughter elements 2004-10-22 82th KPS meeting

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 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 2004-10-22 82th KPS meeting

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) 2004-10-22 82th KPS meeting

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 2004-10-22 82th KPS meeting

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 2004-10-22 82th KPS meeting

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) 2004-10-22 82th KPS meeting

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) 2004-10-22 82th KPS meeting

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 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 2004-10-22 82th KPS meeting

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. 2004-10-22 82th KPS meeting

Calibration data analysis Calibration factor = (activity)/(event rate) [pCi/liter/Hz] 2004-10-22 82th KPS meeting

Calibration data analysis (Po214 Decay Event) Calibration factor = (activity)/(event rate) [pCi/liter/Hz] 2004-10-22 82th KPS meeting

Calibration data analysis Calibration factor = (activity)/(event rate) [pCi/liter/Hz] 2004-10-22 82th KPS meeting Total Event Po214 Decay Event Po218 Decay Event

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

Particle Generation for Simulation 2004-10-22 82th KPS meeting

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 2004-10-22 82th KPS meeting

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℃ 2004-10-22 82th KPS meeting

Humidity Dependence of Efficiency Humidity factor = (real event rate) / (expected event rate) Humidity factor Absolute humidity [kg/m3] 2004-10-22 82th KPS meeting

Humidity Dependence of Efficiency Humidity factor = (real event rate) / (expected event rate) Humidity factor Absolute humidity [kg/m3] 2004-10-22 82th KPS meeting

Y2L Radon Level 2004-10-22 82th KPS meeting

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 2004-10-22 82th KPS meeting