Sep. 22, 2011 Seoul National University Jae Keum Lee KIMS Background 1 China-Korea Workshop 2011 September 22-23, 2011

1.Purpose of This Study To understand the background in KIMS experiment 2.Major Background Sources in the KIMS Experiment 1.Internal Background Sources 1.Cesium radioisotopes ( 134 Cs and 137 Cs ) in CsI(Tl) crystals - this study 2.Doped Thallium is also possible source (Pb x-rays) - next step 3.Internal 238 U and 232 Th chains (very small contribution - Dr. Myung’s study) – next step 4.Iodine radioisotope ( 129 I ) – next step 5.Surface alpha (Dr. S.C. Kim’s study) - next step 2.External Background Sources K, 238 U and 232 Th series from PMTs origin - this study 3.Modeling of the Background Radiation in CsI(Tl) Crystals 1.Monte Carlo Simulations based on the GEANT4 toolkit (ver ) Introduction 2 I. Introduction

134 Cs 134 Ba E [keV] β-β- 27.3% 2.50% 70.1% 0.033% 0.10% Figure 1. Decay scheme of 134 Cs Internal Background Sources 137 Cs 137 Ba 11/2- 7/2 + 1/2 + 3/ E [keV] β-β % 5.60% Figure 2. Decay scheme of 134 Cs I. Introduction

External Background Sources 4 Figure 3. Uranium Series Figure 4. Thorium Series I. Introduction

External Background : PMT 9269QA Table 1. Radiopurity of Graded seal section (provided by electron-tubes manufacturer) Graded seal section Natural KThoriumUranium Content58 mg8 ug3 ug Activity7200 ppm1030 ppb420 ppb ~70 Figure 5. Dimensions of 9269QA PMT (unit: mm) 5 I. Introduction

1.Background Data 1.SET22 Data Set ( ; 8 months) 2.Energy Calibration 241 Am (59.54 keV) for low energy 134 Cs (605 keV and 796 keV) for high energy. 2.Monte Carlo Simulations 1.based on GEANT4 toolkit (ver ) 2.applied same energy resolution of gamma rays with background data 3. Studied Radioisotope Internal Background 134 Cs and 137 Cs External Background 40 K, 238 U decay chains ( 234 Th, 234 Pa, 214 Pb, 214 Bi, 210 Pb, 210 Bi) 232 Th decay chains ( 228 Ra, 228 Ac, 212 Pb, 212 Bi, 208 Tl) 4.Positions of radioactive sources Internal Backgrounddistributed evenly in all CsI(Tl) crystals External Backgroundthe graded seal section of PMTs Background Estimation II. Background Estimation 6

Detector Construction 10 cm Cu shield N 2 gas Cu case (12) CsI(Tl) crystal / Teflon (12) PMT / PMT vacuum (24) 3 mm Cu plate (12) 10 mm Cu plate (1) 8 mm Cu pipe / Ethyle Alcohol (6) 53 mm Polyethylene plate (1) 7 Figure 6. Detector Construction for Monte Carlo Simulations II. Background Estimation

Detector Construction Side View Top View Front View 8 II. Background Estimation

Event Selection: Coincident events among at least two different crystals keV keV in two different crystals keV keV keV in three different crystals keV keV keV in three different crystals Event Selection for 134 Cs Estimation 9 Figure 7. Three selected cases 134 Cs 134 Ba E [keV] β-β- 27.3% 2.50% 70.1% 0.033% 0.10% Figure 8. Selected cases Figure 9. Selected cases II. Background Estimation Internal Background

134 Cs Estimation II. Background Estimation Internal Background 10 Measured background spectrum (cut: previously described cut) Simulated spectrum of energy deposition in the CsI(Tl) crystals by 134 Cs (cut: same as measure background) 134 Cs Beta Decay keV gamma Events from det0 134 Cs Beta Decay keV gamma

137 Cs Estimation 11 Left 214 Bi Beta Decay keV Right 137 Cs Beta Decay keV II. Background Estimation Internal Background Measured background spectrum (cut: single hit event) Simulated spectrum of energy deposition in the CsI(Tl) crystals by 137 Cs (cut: single hit event) 137 Cs Beta Decay keV

40 K Estimation K EC keV gamma II. Background Estimation External Background Simulated spectrum of energy deposition in the CsI(Tl) crystals by 40 K (cut: single hit event) Measured background spectrum (single hit event) 40 K EC keV gamma

238 U Chain Estimation Bi Beta Decay keV gamma II. Background Estimation External Background Measured background spectrum (cut: single hit event) Simulated spectrum of energy deposition in the CsI(Tl) crystals by 238 U decay chain (cut: single hit event) 214 Bi Beta Decay keV gamma

232 Th Chain Estimation Tl Beta Decay keV gamma II. Background Estimation External Background Measured background spectrum (cut: single hit event) Simulated spectrum of energy deposition in the CsI(Tl) crystals by 232 Th decay chain (cut: single hit event) 208 Tl Beta Decay keV gamma

Total Event 15 II. Background Estimation x3 Measured Energy Spectrum Calculated Energy Spectrum Measured background spectrum (solid area) and simulated spectra (lines) of energy depositions in the CsI(Tl) crystals, and their combined spectrum (purple line) (no cut: total event)

Single Hit Event 16 Not identified yet, but expected to be x-rays from Pb (decayed from 208 Tl) II. Background Estimation x3 Measured Energy Spectrum Calculated Energy Spectrum Measured background spectrum (solid area) and simulated spectra (lines) of energy depositions in the CsI(Tl) crystals, and their combined spectrum (purple line) (cut: single hit event)

Multiple Hit Event 17 Measured Energy Spectrum Calculated Energy Spectrum II. Background Estimation x3 Measured background spectrum (solid area) and simulated spectra (lines) of energy depositions in the CsI(Tl) crystals, and their combined spectrum (purple line) (cut: multiple hit event)

Total Event 18 II. Background Estimation

Single Hit Event 19 II. Background Estimation

Multiple Hit Event 20 II. Background Estimation

III. Result 21 Internal BackgroundExternal Background Detector Number 134 Cs [mBq/kg] 137 Cs [mBq/kg] 40 K [Bq] 238 U [Bq] 232 Th [Bq] Expected Activities

22 III. Result Activities of internal radioactive sources Activities of external radioactive sources Surface alpha ?

Another method to estimate 134 Cs 23 IV. Plan SET22 GEANT4 Monte Carlo Simulation Data: 1970 keV Single Hit Events – only from its own crystal 208 Tl Beta Decay keV gamma

Another method to estimate 134 Cs 24 Decay Curve – (23 months) Measured Background Spectrum (Single Hit Events) IV. Plan SET Cs 1970 keV 214 Bi 2204 keV

1.Internal 134 Cs re-estimation 2.Internal Thallium (Pb x-ray) 3.Internal Uranium and Thorium chain 4.Iodine radioisotopes ( 129 I) Plan 25 IV. Plan