Performance and Radioactivity Measurements of the PMTs for the LUX and LZ Dark Matter Experiments

Transcription

1 Performance and Radioactivity Measurements of the PMTs for the LUX and LZ Dark Matter Experiments Carlos Hernandez Faham Brown University Carlos Faham Brown University Particle Astrophysics Group, June 11

2 The LUX Detector

3 VIDEO The Large Underground Xenon Experiment Video by Harvard-Smithsonian Center for Astrophysics and Learner 3

4 VIDEO Video by Harvard-Smithsonian Center for Astrophysics and Learner 4

5 S1 e - e - e -e- e e ē - e - e

6 top hit pattern: x-y localization S2 e - e - e -e- e e ē - e - e - - x Δt : z localization Δt 612

7 Photo by C. Faham 7

8 Dark Matter

9 Dark Matter: Direct Detection 10 m =100 GeV, = cm 2 r r Xe A=131 Ge A= 73 Ar A= Recoil Energy, E r [kevr] 9

10 The LUX Hamamatsu R8778 PMTs

11 Photo by C. Faham Hamamatsu R

12 Hamamatsu R8778: High Expectations Developed by Hamamatsu Photonics, in collaboration with XMASS, specifically for liquid xenon operation Desired Characteristic Operational at LXe temperatures Value -110 C min. temperature High QE at 175 nm (UV) ~33% High CE 90% Single-photon sensitive, good single phe resolution ~35% sphe sigma/mu (ENF ~1.15) High peak anode current linearity 2% at 14 ma (~100 kevee S2) Low afterpulsing < 5% (charge) for new PMTs 12

13 Hamamatsu R8778 Single-phe (Sphe) Spectrum BA0339 Sphe Spectrum Gain = 3.9e+06 σ/µ =0.384 ENF = 1.15 Counts Sphe Area [mvns] 13

14 Hamamatsu R8778 QE in LUX Distribution of QE of 59 LUX R8778 PMTs Mean 33.3% STD = 2.3% 10 Counts QE at 175 nm [%] 14

15 Healthy R8778 PMT Afterpulsing Spectrum 15

16 R8778 exposed to He, and having a small air leak Afterpulsing Spectrum for BA H+ He+ N+,O+ Normalized height Main Pulse t [µs] 16

17 17

18 Hamamatsu R8778 Output Linearity % nonlinearity % Linearity BA0404 Nonlinearity Plot 2% nonlinearity at 14 ma 2% Hamamatsu Spec -100 C CHF Peak Anode Current (ma) QUPID 2% nonlinearity ~1 ma 18

19 LUX 20 PMT Commissioning Photo by C. Faham Partial PMT deployment due to pressure testing of vessel All 122 PMTs scheduled to be deployed in July,

20 Radioactivity

21 Faking a WIMP 1) Electron Recoil Leakage 2) Single-scatter neutrons 3) Other non-gaussian rare events 21

22 Radioactivity Comparison 10 kbq 40 K, 14 C 10 Bq 40 K 10 mbq 238 U, 232 Th, 40 K, 60 Co 22

23 LUX s R8778 Measured Radioactivity 10 1 R8778 Background Counts / kev / kg / day Energy [kev] SOLO counting facility 23

24 LUX Component Radioactivity Comparison D. Malling These PMTs are not ultra-low background. Levels have improved much since then (see R11410 MOD radioactivity levels coming up...) 24

25 Implications for LUX 25

26 The LZS and LZD Experiments

27 LUX-ZEPLIN (LZ) 27

28 LZD PMTs LUX 28

29 The Hamamatsu R11410 MOD An ultra-low background PMT

30 R11410 MOD Twice the photocathode area of the R8778 QE, gain, etc. equivalent to R8778 ~x2 better anode linearity See Yoshizawa s presentation 30

31 Hamamatsu R11410 MOD 31

32 Hamamatsu R11410 MOD Sphe Spectrum ZK4991 Sphe Spectrum Gain = 1.4e+07 σ/µ =0.351 ENF = 1.12 Counts Sphe Area [mvns] 32

33 Hamamatsu R11410 MOD Measured Radioactivity Counts / kev / kg / day R8778 R11410 MOD Background Energy [kev] 33

34 Hamamatsu R11410 MOD Radioactivity Results mbq/pmt Decay chain < U < Th < K 2 ± Co 90% CL for upper limits, 1-sigma error bars 60 Co will be further reduced in new Hamamatsu production units by replacing Kovar metal enclosure Further, 60 Co always decays with correlated gammas, making the single-scatter probability lower 40 K only has a 10% BR to EC + gamma decay mode 34

35 Conclusions LUX employs 122 Hamamatsu R8778 for signal detection. These PMTs fulfill all performance benchmarks for physics requirements. They are the dominant source of radioactivity in LUX. However, measured radioactivity levels yield <1 WIMP-like event in 300 days. New ultra-low background Hamamatsu R11410 MOD PMTs have been measured to have < 1 mbq/pmt combined U/Th. Co remains at 2.0 ± 0.2 mbq, but will be removed by Hamamatsu in future productions by changing Kovar enclosure K, at 10% gamma decay BR, has negligible effects in backgrounds Performance of R11410 MOD is identical to the thoroughly tested R8778 PMTs. The LZS and LZD experiments will greatly benefit from using these PMTs. This new technology is the best available in PMTs, and has equivalent radioactivity levels to those of QUPIDs. Background reduction in photodetectors beyond current limits will not result in further gains for dark matter experiments, as coherent atmospheric neutrino scattering will remain the limiting background signal. 35

36 Thank you

37 Extra Slides

38 150 LUX 0.1 Event (Summed across all channels) phe/sample S phe S phe 0 phe/sample µs S S µs µs 19 J. Chapman 01 Oct 2009 Brown Particle Astrophysics 38

39 Photo by J. Chapman 39

40 SOLO Soudan Low-Background Counting Facility 0.6 kg HPGe detector, 0.15 cm copper shield Located at the Soudan Underground Laboratory (2000 mwe) >30 cm lead shielding The inner 5 cm lining of the chamber is comprised of ancient lead, with 210 Pb activity measured below 50 mbq/kg A mylar shell and 2.5 slpm nitrogen gas purge are used to eliminate gaseous radon from the chamber 40

41 Co-60 and K-40 Decay Chains y Co Q = % 7.5 <0.022% > % E2(+M3) (E2) D+Q (E2) Ni 1.1 ps 0.59 ps ps stable 1.12 ps E y K Q EC = % 10.67% stable Ar 0.048%

42 LUX, LZS and LZD Sensitivities 42

43 Afterpulsing Delay - Ion Identification Afterpulsing Delay τ [µs] Afterpulsing Delay vs. Bias Voltage, BA0217, Main Pulse ~100 pc (Anode) Charge/Mass Ratio: AP1 = 1.1 ± 0.6 AP2 = 4.00 AP3 = 15.2 ± 0.1 Measured by He exposure N +, O + AP3 He + AP2 H + AP1 AP1 AP2 AP / (bias)[v] 43

44 44