Radon Background of Y2L MyeongJae Lee (SNU) 2 nd workshop on Double beta decay Oct 6, 2010, KRISS

Decay chain of radon Two sources of radon 220 Rn (55.6sec) ▫ 232 Th → 220 Rn (55.6sec) → 208 Pb(stable) 222 Rn (3.82days) ▫ 238 U → 222 Rn (3.82days) → 210Pb(22yr) → 206 Pb(stable) 222 Rn is important BG source for all underground exp. ▫ Rather long life time of 222 Rn : escape from rock ▫ Long life time of 210 Pb : becomes a significant background 220 Rn (Thoron) : not important in general Oct 8, nd workshop on Double beta decay

Radioactivities of Y2L Element(ppm) Samples ABCDE U Th Experimental hallU(ppb)Th(ppb) 222 Rn(pCi/L) Boulby67±6127±10<0.14 Gran Sasso (Hall A)6800± ±740.3~2.6 Gran Sasso (Hall B)420±10062±200.3~2.6 Soudan170±60890±2050~190 Sudbury~1200~5300~3 Homestake (DUSEL)400~ ~150001~5 Y2L1770~ ~14500~1.2 Environment of Y2L is similar with GranSasso or Homestake Bed rock of Y2L : Gneiss aging ~2GY Oct 8, nd workshop on Double beta decay

Radon Monitoring Detector Photodiode : 30mm X 30mm -500V is applied at bottom side, 70V for PD bias Installed in Y2L at 2003, start DAQ at 2004, retired at 2010 Volume : 70 liter Oct 8, nd workshop on Double beta decay

How to Detect Radon 222 Rn 218 Po 218 Po Pb + 1.Blow the radon detector with air 2.Then 222 Rn comes inside the detector chamber 3.By applying negative HV to PIN diode, electric field is generated inside the detector chamber Rn α- decays to 218 Po (subsequently to 214 Po) Po atom have large kinetic energy (~115keV) Po atom becomes a positive ion, because of the striping effect Po + ions are collected near PIN diode since the electric field Po α -decays to 218 Pb 9.The α particles are detected by PIN diode 10.Energy deposition s are read by DAQ Electronics DAQ Electronics DAQ Electronics DAQ Electronics DAQ Oct 8, nd workshop on Double beta decay

Calibration Use a standard radon source (made by J.M.Lee of KRISS) Blow a known activity of radon to the detector chamber and measure the exponential decay of event rate. Oct 8, nd workshop on Double beta decay

α -Energy spectrum from calibration data Resolution : ~8%. No good as a Photo-diode, however no problem to identify two alpha decays 218 Po decay 6.00MeV 214 Po decay 7.69MeV 218 Po decays are used to measure 222 Rn activity Oct 8, nd workshop on Double beta decay

Result of Calibration Black : Data Red : Fit to exponential function Half life of 222 Rn = ± days Cf) Nuclear Data (1996) : ± (Errors : Fit procedure for event selection and half life) Oct 8, nd workshop on Double beta decay

Dependence to humidity Positive ion of 218 Po or others are easily attach to water molecule, aerosol….etc. → Neutralizing the positive ion → Cannot be collected by negative electric field → Efficiency drop Need to control the humidity ▫ Moisture trap ▫ Humidity Correction 1.Blow radon-enriched air to the detector 2.Measure the radon activity at zero humidity 3.Replace the radon source to a humidity source 4.Blow the humid air to the detector 5.Measure the radon activity with humidity Oct 8, nd workshop on Double beta decay

Correction on humidity Black dots : Data Dashed line : Exponential Fit Gray band : 1σ level from Fit Oct 8, nd workshop on Double beta decay

Systematic Uncertainties SourcesUncertainties (%) Event counting5.2 Calibration4.3 Humidity correction1.7 DAQ time0.1 Volume correction0.0 Intrinsic background6.5 TOTAL9.5 Intrinsic background : Effect of leak or contamination of the chamber Fill the chamber with pure nitrogen and measure the event rate 0.05±0.02 Hz : correspond to <0.1pCi/L Oct 8, nd workshop on Double beta decay

Radon activity for last 5 years N : number of events Cf : calibration factor Hf : Humidity correction Vf : Volume correction T : DAQ time Avg : 1.20±0.49 pCi/L Oct 8, nd workshop on Double beta decay

New Radon detector A commercial radon detector from DURRIDGE Co. Now developing DAQ programs… will be installed at Y2L in this year Cannot detect very low activity (<0.5pCi/L). Need to develop by ourselves if even such radon level is significant Oct 8, nd workshop on Double beta decay

Summary A Radon Detector had been installed at Y2L in 2003, had taken data until 2009, retired at Average radon activity in Y2L is 1.20±0.49 pCi/L ▫ A paper is being in referring internally. A new radon detector will be installed in this year in Y2L Oct 8, nd workshop on Double beta decay