Performance and Radioactivity Measurements of the PMTs for the LUX - - PowerPoint PPT Presentation

Carlos Hernandez Faham

Brown University

TIPP 2011, June 11

Performance and Radioactivity Measurements

• f the PMTs for the LUX and LZ Dark Matter Experiments

Carlos Faham

Brown University Particle Astrophysics Group

The LUX Detector

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The Large Underground Xenon Experiment

Video by Harvard-Smithsonian Center for Astrophysics and Learner

VIDEO

4 Video by Harvard-Smithsonian Center for Astrophysics and Learner

VIDEO

TIPP 2011 Carlos Faham, Brown University

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e- e- e- e- e- e- e- e-e-

S1

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Δt

Δt

top hit pattern: x-y localization : z localization x

S2

e- e- e- e- e- e- e- e-e-

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Photo by C. Faham

Dark Matter

TIPP 2011 Carlos Faham, Brown University

Dark Matter: Direct Detection

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20 40 60 80 100 120 10

• 10
• 10
• 10
• m=100 GeV, =1.0107 cm2

Recoil Energy, Er [keVr] rr Xe A=131 Ge A= 73 Ar A= 40

The LUX Hamamatsu R8778 PMTs

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Photo by C. Faham

Hamamatsu R8778

TIPP 2011 Carlos Faham, Brown University

Hamamatsu R8778: High Expectations

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Developed by Hamamatsu Photonics, in collaboration with XMASS, specifically for liquid xenon operation

Desired Characteristic Value Operational at LXe temperatures

• 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

TIPP 2011 Carlos Faham, Brown University

Hamamatsu R8778 Single-phe (Sphe) Spectrum

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10 20 30 40 50 200 400 600 800 1000 1200 1400 1600 1800 2000 Sphe Area [mVns] Counts BA0339 Sphe Spectrum Gain = 3.9e+06 σ/µ = 0.384 ENF = 1.15

TIPP 2011 Carlos Faham, Brown University

Hamamatsu R8778 QE in LUX

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25 30 35 40 2 4 6 8 10 12 14 Distribution of QE of 59 LUX R8778 PMTs QE at 175 nm [%] Counts Mean 33.3% STD = 2.3%

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Healthy R8778 PMT Afterpulsing Spectrum

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R8778 exposed to He, and having a small air leak

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0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 10

3

10

2

10

1

10

H+ He+ N+,O+ Main Pulse

t [µs] Normalized height Afterpulsing Spectrum for BA0214

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TIPP 2011 Carlos Faham, Brown University

Hamamatsu R8778 Output Linearity

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5 10 15 20 40 30 20 10 10 20

2% Hamamatsu Spec

2% Linearity

Peak Anode Current (mA) % nonlinearity BA0404 Nonlinearity Plot

20100506 CHF

QUPID 2% nonlinearity ~1 mA

• 100 C

2% nonlinearity at 14 mA

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LUX 20 PMT Commissioning

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All 122 PMTs scheduled to be deployed in July, 2011 Partial PMT deployment due to pressure testing of vessel

Photo by C. Faham

Radioactivity

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Faking a WIMP

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

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Radioactivity Comparison

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10 Bq

40K

10 kBq

40K, 14C

10 mBq

238U, 232Th, 40K, 60Co

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LUX’s R8778 Measured Radioactivity

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102 103 10

• 10
• 10

10

1

Energy [keV] Counts / keV / kg / day

R8778 Background

SOLO counting facility

TIPP 2011 Carlos Faham, Brown University

LUX Component Radioactivity Comparison

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• D. Malling

These PMTs are not ultra-low background. Levels have improved much since then (see R11410 MOD radioactivity levels coming up...)

TIPP 2011 Carlos Faham, Brown University

Implications for LUX

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The LZS and LZD Experiments

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LUX-ZEPLIN (LZ)

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LZD LUX

1000 3’’ PMTs

The Hamamatsu R11410 MOD

An ultra-low background PMT

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R11410 MOD

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T wice the photocathode area of the R8778 QE, gain, etc. equivalent to R8778 ~x2 better anode linearity See Yoshizawa’s presentation

TIPP 2011 Carlos Faham, Brown University

Hamamatsu R11410 MOD

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Hamamatsu R11410 MOD Sphe Spectrum

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50 100 150 200 100 200 300 400 500 600 700 800 900 1000 Sphe Area [mVns] Counts ZK4991 Sphe Spectrum Gain = 1.4e+07 σ/µ = 0.351 ENF = 1.12

TIPP 2011 Carlos Faham, Brown University

Hamamatsu R11410 MOD Measured Radioactivity

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102 103 10

• 10
• 10

10

1

Energy [keV] Counts / keV / kg / day

R8778 R11410 MOD Background

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Hamamatsu R11410 MOD Radioactivity Results

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• 60Co will be further reduced in new Hamamatsu production units

by replacing Kovar metal enclosure

• Further, 60Co always decays with correlated gammas, making the

single-scatter probability lower

• 40K only has a 10% BR to EC + gamma decay mode

mBq/PMT Decay chain <0.4

238U

<0.3

232Th

<8.3

40K

2 ± 0.2

60Co

90% CL for upper limits, 1-sigma error bars

TIPP 2011 Carlos Faham, Brown University

Conclusions

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• 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.

Thank you

Extra Slides

TIPP 2011 Carlos Faham, Brown University

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5 10 15 20 50 100 150

LUX 0.1 Event (Summed across all channels)

• J. Chapman 01Oct2009

Brown Particle Astrophysics

µs phe/sample 0.5 0.5 5 10

S1

µs phe/sample 17 18 19 20 40 60

S2

µs

52.8 phe 4543 phe

S1 S2

TIPP 2011 Carlos Faham, Brown University

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Photo by J. Chapman

TIPP 2011 Carlos Faham, Brown University

SOLO

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• 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 210Pb 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

Soudan Low-Background Counting Facility

TIPP 2011 Carlos Faham, Brown University

Co-60 and K-40 Decay Chains

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60 28Ni

0+ 1332.518 2+ 2158.64 2+ 2505.766 4+ 2 . 1

• 6

2 5 5 9 9 . 9 7 3 6 1 1 7 3 . 2 3 7 E 2 ( + M 3 ) . 7 6 3 4 6 . 9 3 . 1 1 1 2 1 5 8 . 5 7 ( E 2 ) . 7 6 8 2 6 . 6 D + Q 9 9 . 9 8 5 6 1 3 3 2 . 5 1 ( E 2 )

stable

0.713 ps 0.59 ps 1.1 ps

60 27Co

0.057% 15.02 <0.022% >12.92 99.925% 7.5 5+

5.2714 y Q =2823.9

40 18Ar

0+ 1460.859 2+ 11 1460.830 E2

stable

1.12 ps

40 19K

0.048% 21.03 10.67% 11.61 4–

1.277 109 y

QEC=1504.9

10.72%

TIPP 2011 Carlos Faham, Brown University

LUX, LZS and LZD Sensitivities

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Afterpulsing Delay - Ion Identification

43 0.025 0.026 0.027 0.028 0.029 0.03 0.031 0.032 0.5 1 1.5 Afterpulsing Delay vs. Bias Voltage, BA0217, Main Pulse ~100 pC (Anode) 1/

• (bias) [V]

Afterpulsing Delay τ [µs]

Charge/Mass Ratio: AP1 = 1.1 ± 0.6 AP2 = 4.00 AP3 = 15.2 ± 0.1

AP1 AP2 AP3

AP1 AP2 AP3

He+ H+ N+, O+

Measured by He exposure

TIPP 2011 Carlos Faham, Brown University

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