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
The LUX Detector
VIDEO The Large Underground Xenon Experiment Video by Harvard-Smithsonian Center for Astrophysics and Learner 3
VIDEO Video by Harvard-Smithsonian Center for Astrophysics and Learner 4
S1 e - e - e -e- e e ē - e - e - - 511
top hit pattern: x-y localization S2 e - e - e -e- e e ē - e - e - - x Δt : z localization Δt 612
Photo by C. Faham 7
Dark Matter
Dark Matter: Direct Detection 10 m =100 GeV, =1.0 10 7 cm 2 r r 10 10 10 Xe A=131 Ge A= 73 Ar A= 40 0 20 40 60 80 100 120 Recoil Energy, E r [kevr] 9
The LUX Hamamatsu R8778 PMTs
Photo by C. Faham Hamamatsu R8778 11
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
Hamamatsu R8778 Single-phe (Sphe) Spectrum 2000 1800 1600 1400 BA0339 Sphe Spectrum Gain = 3.9e+06 σ/µ =0.384 ENF = 1.15 Counts 1200 1000 800 600 400 200 0 10 20 30 40 50 Sphe Area [mvns] 13
Hamamatsu R8778 QE in LUX Distribution of QE of 59 LUX R8778 PMTs 14 12 Mean 33.3% STD = 2.3% 10 Counts 8 6 4 2 25 30 35 40 QE at 175 nm [%] 14
Healthy R8778 PMT Afterpulsing Spectrum 15
R8778 exposed to He, and having a small air leak Afterpulsing Spectrum for BA0214 10 0 H+ He+ N+,O+ Normalized height 10 1 10 2 10 3 Main Pulse 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 t [µs] 16
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Hamamatsu R8778 Output Linearity % nonlinearity 20 10 0 10 20 30 2% Linearity BA0404 Nonlinearity Plot 2% nonlinearity at 14 ma 2% Hamamatsu Spec -100 C 40 0 5 10 15 20 20100506 CHF Peak Anode Current (ma) QUPID 2% nonlinearity ~1 ma 18
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, 2011 19
Radioactivity
Faking a WIMP 1) Electron Recoil Leakage 2) Single-scatter neutrons 3) Other non-gaussian rare events 21
Radioactivity Comparison 10 kbq 40 K, 14 C 10 Bq 40 K 10 mbq 238 U, 232 Th, 40 K, 60 Co 22
LUX s R8778 Measured Radioactivity 10 1 R8778 Background Counts / kev / kg / day 10 0 10 10 10 2 10 3 Energy [kev] SOLO counting facility 23
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
Implications for LUX 25
The LZS and LZD Experiments
LUX-ZEPLIN (LZ) 27
LZD 1000 3 PMTs LUX 28
The Hamamatsu R11410 MOD An ultra-low background PMT
R11410 MOD Twice the photocathode area of the R8778 QE, gain, etc. equivalent to R8778 ~x2 better anode linearity See Yoshizawa s presentation 30
Hamamatsu R11410 MOD 31
Hamamatsu R11410 MOD Sphe Spectrum 1000 900 800 700 ZK4991 Sphe Spectrum Gain = 1.4e+07 σ/µ =0.351 ENF = 1.12 Counts 600 500 400 300 200 100 0 50 100 150 200 Sphe Area [mvns] 32
Hamamatsu R11410 MOD Measured Radioactivity Counts / kev / kg / day 10 1 10 0 10 R8778 R11410 MOD Background 10 10 2 10 3 Energy [kev] 33
Hamamatsu R11410 MOD Radioactivity Results mbq/pmt Decay chain <0.4 238 U <0.3 232 Th <8.3 40 K 2 ± 0.2 60 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
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
Thank you
Extra Slides
150 LUX 0.1 Event (Summed across all channels) phe/sample 100 50 S1 52.8 phe S2 4543 phe 0 phe/sample 10 5 0 5 10 15 20 µs S1 60 40 20 S2 0 0.5 0 0.5 µs 0 17 18 µs 19 J. Chapman 01 Oct 2009 Brown Particle Astrophysics 38
Photo by J. Chapman 39
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
Co-60 and K-40 Decay Chains 5.2714 y 5+ 0 60 27 Co Q =2823.9 99.925% 7.5 <0.022% >12.9 2 0.057% 15.0 2 4+ 2+ 2+ 0+ 2.0 10-6 2505 99.9736 1173.237 E2(+M3) 0.0076 346.93 0.00111 2158.57 (E2) 0.0076 826.06 D+Q 99.9856 1332.501 (E2) 2505.766 2158.64 1332.518 0 60 28 Ni 1.1 ps 0.59 ps 0.713 ps stable 1.12 ps 2+ 11 1460.830 E2 1460.859 1.277 10 9 y 4 0 40 19 K Q EC =1504.9 10.72% 10.67% 11.6 1 stable 0+ 0 40 18 Ar 0.048% 21.0 3 41
LUX, LZS and LZD Sensitivities 42
Afterpulsing Delay - Ion Identification Afterpulsing Delay τ [µs] 1.5 1 0.5 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 AP3 0 0.025 0.026 0.027 0.028 0.029 0.03 0.031 0.032 1/ (bias)[v] 43
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