DPF 2015 Aug 5 2015 Ann Arbor Status

DPF 2015, Aug. 5, 2015, Ann Arbor Status of the KIMS-Na. I Experiment Kim, Kyungwon Seoul National University On behalf of the KIMS-Na. I collaboration

Motivation of KIMS-Na. I DAMA/LIBRA (Na. I) 9. 3 σ detection Eur. Phys. J. C 73: 2648 (2013) Low mass WIMP search KIMS-Cs. I Preliminary Direct check for DAMA/LIBRA using Na. I(Tl) crystal 2

KIMS-Na. I Experiment @ Yangyang underground laboratory, Minimum depth: 700 m Shield structure Mineral Oil (30 cm) Lead (15 cm) Polyethylene (5 cm) Copper (10 cm) Setup Cs. I detectors Na. I KIMS-Na. I detector Na. I(Tl) PMT § Na. I(Tl) crystals are installed among Cs. I(Tl) crystal array § Hamamatsu R 12669 PMTs are attached. quantum efficiency: ~35% @ 420 ㎚ 3

KIMS-Na. I crystals 6 Na. I(Tl) crystals are tested at Y 2 L. Na. I-001 Na. I-002 Na. I-003 Na. I-004 Na. I-005 Na. I-006 Mass (kg) 8. 26 9. 15 3. 3 9. 16 11. 44 Diameter×Length 5”× 7” 4. 2”× 11” 4. 5”× 3. 5” 4. 5”× 11” 110㎜× 200㎜ Powder AS AS SA-AG SA-CG AS-WSⅡ AS-CG Crystal AS AS AS BH Arrival 2013. 9 2014. 1 2014. 8 2014. 12 2015. 1 LY (PE/ke. V) 15. 60± 1. 41 15. 51± 1. 41 13. 26± 1. 28 3. 85± 0. 38 12. 14± 1. 14 4. 36± 0. 39 Company § AS: Alpha Spectra § SA: Sigma Aldrich § BH: Beijing Hamamatsu Powder grade § AG: Astro Grade § CG: Crystal Grade § WSⅡ: WIMP ScintⅡ Na. I 005 Na. I 003, 004 4

PMT noise rejection § Charge ratio X 1 x 1: slow component x 2: fast component Noise rejection using same parameter as used in DAMA X 2 2 ke. V ≤ E < 4 ke. V DAMA noise gamma NIM A 592, 297 (2008) KIMS-Na. I noise gamma Astropart. Phys. 62, 249 (2015) Backgrounds 55 Fe x-ray source 5

PMT noise rejection § Charge Asymmetry gamma noise § Charge Asymmetry vs Charge ratio noise signal We need additional cut to reject noise events after charge ratio cut. black: single hit red: multiple hit 6

Internal background – 40 K Measured by coincidence with Cs. I detectors ~ 3 ke. V 1460 ke. V K (ppb) Powder Na. I-001 41. 4± 2. 99 AS Na. I-002 49. 3± 2. 43 AS Na. I-003 25. 43± 3. 57 SA-AG Na. I-004 116. 70± 6. 78 SA-CG Na. I-005 40. 1± 4. 2 AS-WSⅡ Na. I-006 127. 12± 6. 45 SA-CG DAMA, K: 10~20 ppb 7

Internal background – 40 K K (ppb) Crystal Powder grade Na. I-001 41. 4± 2. 99 - AS Na. I-002 49. 3± 2. 43 - AS Na. I-003 25. 43± 3. 57 25. 07 SA-AG Na. I-004 116. 70± 6. 78 ~200 SA-CG Na. I-005 40. 1± 4. 2 - AS-WSⅡ Na. I-006 127. 12± 6. 45 ~200 SA-CG § K: AG (Astro grade) powder ≪ CG (Crystal grade) powder → K contamination level is determined in powder. § Crystal using SA-AG 10 ppb level of K powder (ICP-MS measurement) will be tested. § Powder purification to reduce K is ongoing. 8

Internal background – 238 U, 232 Th Na. I-001 (m. Bq/kg) Na. I-002 (m. Bq/kg) 238 U (214 Bi) <0. 0003 <0. 0015 228 Th (216 Po) <0. 013 0. 002± 0. 001 210 Pb 3. 28± 0. 01 1. 76± 0. 01 alpha gamma Total alphas 3. 29± 0. 01 1. 77± 0. 01 DAMA, 238 U: 0. 009 -0. 13 (m. Bq/kg) 232 Th: 0. 002 -0. 03 210 0. 005 -0. 03 ΔT(α Pb: – α) ΔT(β – α) 232 Th 238 U No 214 Bi decay component Only 216 Po component The contamination levels of U and Th are small. 9

Internal background – 210 Pb § Most of alphas are coming from 210 Pb § We can estimate crystal manufactured date using alpha rate change. Na. I-003 Na. I-005 crystal manufactured date → crystal growing? Crystal manufactured at Jul. Na. I-006 before crystal growing → powder? Crystal manufactured at Oct. 10

Internal background – 210 Pb Total alpha (m. Bq/kg) Powder Na. I-001 3. 29± 0. 01 AS Na. I-002 1. 77± 0. 01 AS Na. I-003 2. 43± 0. 011 SA-AG Na. I-004 - SA-CG Na. I-005 0. 48± 0. 004 AS-WSⅡ Na. I-006 1. 53± 0. 007 SA-CG DAMA, 210 Pb: 0. 005 -0. 03 (m. Bq/kg) § Na. I-003, 4, 5; better treatment on air contact during crystal growing. § Na. I-005; better treatment of chemical process on powder. Need to understand how 210 Pb was contaminated → Powder purification with various resin Measurement of alpha in powder 11

Background spectrum § Backgrounds level is ~ 2. 2 dru at 6 ke. V. § Significant internal backgrounds at low energy are 40 K, 210 Pb. § Additional backgrounds caused by external(including cosmogenic) Our goal for background level: ~ 1 dru 12

Background reduction – Liquid Scintillator veto system § Veto efficiencies in the low energy region(0, 10 ke. V) of U, Th and K are over 80% (based on Geant-4 simulation) Full setup Prototype 4 x 4 Na. I(Tl) array Liquid Scintillator Na. I(Tl) detector § Prototype test is just started. External and internal background can be reduced by LS veto. 13

Pulse Shape Discrimination analysis PSD parameter: § Nuclear recoil: A 300 m. Ci Am/Be neutron source § Electron recoil: 137 Cs gamma ray source § Small test crystal (same ingot with Na. I-002) 14

Pulse Shape Discrimination analysis PSD parameter: § Nuclear recoil: A 300 m. Ci Am/Be neutron source § Electron recoil: 137 Cs gamma ray source § Small test crystal (same ingot with Na. I-002) ar. Xiv: 1503. 05253 ideal detector α ~ 1, β ~ 0 K≪ 1 Good pulse shape discrimination capabilities 15

Pulse Shape Discrimination analysis PSD parameter: § Na. I-002 (9. 15 kg), 133. 7 days data § Electron recoil: Multiple hit events We are working on evaluation of limit with PSD analysis. 16

Prospects Goal: Background level: 2. 2 dru → 1 dru § Internal backgrounds 40 K: 20 ppb → 10 ppb 210 Pb: 0. 5 m. Bq/kg → 0. 2 m. Bq/kg § External backgrounds – 0. 5 dru □ □ § Ultra-Pure crystals (internal backgrounds) – Powder purification – More crystals will be tested. § Shielding design (external backgrounds) – LS veto prototype test is ongoing. § We plan to 200 kg experiment after reaching 1 dru background level. 17

Sensitivity § Expected sensitivity with annual modulation analysis § 200 kg, 3 years data (2 ke. V threshold) Spin independent WIMP-nucleon cross section 18

Summary • 6 Na. I(Tl) crystals are tested at Y 2 L. • Current low energy background level is 2. 2 dru. • Powder purification is ongoing to reduce internal background. • LS veto system will reduce external background. • We achieved good PSD power of Na. I crystal due to high light yield. Evaluation of limit with PSD analysis is ongoing. • KIMS-Na. I will start data taking end of this year. 19

Backup Slides

Pulse Shape Discrimination analysis PSD parameter: § Underground data, Electron recoil: Large crystal § Nuclear recoil: Small crystal Differences will be considered as a systematic uncertainty. 21

Sensitivity § Expected sensitivity with PSD analysis § 60 kg, 1 year data § 4 dru (2 -6 ke. V), 2 dru (> 6 ke. V) 22

Validation of PMT noise rejection cut Test dummy: • Teflon cylinder (same dimension with Na. I-002) 29. 43 days Most PMT events are rejected by noise rejection cuts. 23

Background – Cosmic excitation: 22 Na Measured by coincidence with Cs. I detectors ~ 0. 8 ke. V Na. I-001 1. 20 ± 0. 14 counts/day 1270 ke. V Na. I-002 1. 49 ± 0. 16 counts/day This can be used to study 1 ke. V energy signal. 24