ChinaKorea Workshop 2011 September 22 23 2011 KIMS

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

I. Introduction 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 Cs. I(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 1. 3. 40 K, 238 U and 232 Th series from PMTs origin - this study Modeling of the Background Radiation in Cs. I(Tl) Crystals 1. Monte Carlo Simulations based on the GEANT 4 toolkit (ver. 4. 9. 2) 2

I. Introduction Internal Background Sources 4+ 134 Cs E [ke. V] β- 27. 3% 4+ 2. 50% 3+ 70. 1% 4+ 0. 033% 2+ 0. 10% 2+ 1365 802 569 1969. 87 1038 475 242 1643. 28 795 1400. 55 1168 563 1167. 93 605 0+ 604. 70 0. 0 134 Ba Figure 1. Decay scheme of 134 Cs 7/2+ 137 Cs 94. 40% 5. 60% E [ke. V] β 11/2 - 661. 7 1/2+ 283. 5 3/2+ 0. 0 137 Ba Figure 2. Decay scheme of 134 Cs

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

I. Introduction External Background : PMT 9269 QA ~70 Graded seal section Figure 5. Dimensions of 9269 QA PMT (unit: mm) Natural K Thorium Uranium Content 58 mg 8 ug 3 ug Activity 7200 ppm 1030 ppb 420 ppb Table 1. Radiopurity of Graded seal section (provided by electron-tubes manufacturer) 5

II. Background Estimation 1. Background Data 1. SET 22 Data Set (200909 - 201005; 8 months) 2. Energy Calibration 241 Am 134 Cs 2. (59. 54 ke. V) for low energy (605 ke. V and 796 ke. V) for high energy. Monte Carlo Simulations 1. based on GEANT 4 toolkit (ver. 4. 9. 2) 2. applied same energy resolution of gamma rays with background data 3. Studied Radioisotope Internal Background 134 Cs External Background 40 K, 238 U 232 Th 4. and 137 Cs decay chains (234 Th, 234 Pa, 214 Pb, 214 Bi, 210 Pb, 210 Bi) decay chains (228 Ra, 228 Ac, 212 Pb, 212 Bi, 208 Tl) Positions of radioactive sources Internal Background distributed evenly in all Cs. I(Tl) crystals External Background the graded seal section of PMTs 6

II. Background Estimation Detector Construction 10 cm Cu shield N 2 gas Cu case (12) Cs. I(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) Figure 6. Detector Construction for Monte Carlo Simulations 7

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

II. Background Estimation Internal Background Event Selection for 134 Cs Estimation 134 Cs E [ke. V] β- 27. 3% 4+ 1969. 87 2. 50% 3+ 1643. 28 70. 1% 4+ 1400. 55 0. 033% 2+ 1167. 93 0. 10% 2+ 604. 70 0+ 0. 0 134 Ba Figure 7. Three selected cases Figure 8. Selected cases Figure 9. Selected cases Event Selection: Coincident events among at least two different crystals 1. 569 ke. V + 1401 ke. V in two different crystals 2. 569 ke. V + 796 ke. V + 605 ke. V in three different crystals 3. 802 ke. V + 563 ke. V + 605 ke. V in three different crystals 9

II. Background Estimation 134 Cs Internal Background Estimation 134 Cs Beta Decay 604. 7 ke. V gamma Events from det 0 Measured background spectrum (cut: previously described cut) Simulated spectrum of energy deposition in the Cs. I(Tl) crystals by 134 Cs (cut: same as measure background) 10

II. Background Estimation 137 Cs Left 214 Bi Beta Decay 609. 312 ke. V Right 137 Cs Estimation 137 Cs Beta Decay 661. 657 ke. V Measured background spectrum (cut: single hit event) Internal Background Simulated spectrum of energy deposition in the Cs. I(Tl) crystals by 137 Cs (cut: single hit event) 11

II. Background Estimation External Background 40 K Estimation 40 K EC 1460. 830 ke. V gamma Measured background spectrum (single hit event) Simulated spectrum of energy deposition in the Cs. I(Tl) crystals by 40 K (cut: single hit event) 12

II. Background Estimation 238 U 214 Bi External Background Chain Estimation Beta Decay 1764. 494 ke. V gamma Measured background spectrum (cut: single hit event) 214 Bi Beta Decay 1764. 494 ke. V gamma Simulated spectrum of energy deposition in the Cs. I(Tl) crystals by 238 U decay chain (cut: single hit event) 13

II. Background Estimation 232 Th 208 Tl External Background Chain Estimation Beta Decay 208 Tl 2614. 533 ke. V gamma Measured background spectrum (cut: single hit event) Beta Decay 2614. 533 ke. V gamma Simulated spectrum of energy deposition in the Cs. I(Tl) crystals by 232 Th decay chain (cut: single hit event) 14

II. Background Estimation Total Event Measured Energy Spectrum Calculated Energy Spectrum x 3 Measured background spectrum (solid area) and simulated spectra (lines) of energy depositions in the Cs. I(Tl) crystals, and their combined spectrum (purple line) (no cut: total event) 15

II. Background Estimation Single Hit Event Not identified yet, Measured Energy Spectrum but expected to be x-rays from Pb (decayed from 208 Tl) Calculated Energy Spectrum x 3 Measured background spectrum (solid area) and simulated spectra (lines) of energy depositions in the Cs. I(Tl) crystals, and their combined spectrum (purple line) (cut: single hit event) 16

II. Background Estimation Multiple Hit Event Measured Energy Spectrum Calculated Energy Spectrum x 3 Measured background spectrum (solid area) and simulated spectra (lines) of energy depositions in the Cs. I(Tl) crystals, and their combined spectrum (purple line) (cut: multiple hit event) 17

II. Background Estimation Total Event 18

II. Background Estimation Single Hit Event 19

II. Background Estimation Multiple Hit Event 20

III. Result Expected Activities Internal Background External Background Detector Number 134 Cs 137 Cs 40 K 238 U 232 Th [m. Bq/kg] [Bq] 0 7. 16 1. 00 0. 42 0. 37 0. 21 1 6. 91 0. 92 0. 31 0. 28 0. 15 2 6. 89 1. 05 0. 30 0. 26 0. 15 3 8. 64 1. 00 0. 36 0. 29 0. 17 4 6. 99 0. 88 0. 30 0. 26 0. 16 5 7. 70 1. 13 0. 30 0. 27 0. 14 6 7. 61 1. 30 0. 32 0. 14 7 12. 44 1. 10 0. 42 0. 20 8 9. 82 1. 36 0. 44 0. 49 0. 19 9 7. 09 1. 36 0. 24 0. 12 10 6. 96 3. 79 0. 24 0. 22 0. 11 11 9. 03 1. 93 0. 39 0. 46 0. 15 21

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

IV. Plan SET 22 Another method to estimate 134 Cs 208 Tl Beta Decay 2614. 533 ke. V gamma GEANT 4 Monte Carlo Simulation Data: 1970 ke. V Single Hit Events – only from its own crystal 23

IV. Plan SET 22 Another method to estimate 134 Cs 1970 ke. V 214 Bi 2204 ke. V Measured Background Spectrum (Single Hit Events) Decay Curve 200909 – 201107 (23 months) 24

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