DPG Frühjahrstagung 2013, Dresden

PMT series tests for the XENON1T Water Cherenkov Muon Veto

Christopher W. Geis

Johannes-Gutenberg Universität MainzInstitut für Physik

2013/03/05

on behalf of the XENON collaboration

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Outline

● The XENON1T Muon Veto● PMT test facility at JGU Mainz

● Gain● Darkrate● Afterpulses

● Summary

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Goal of XENON1T:

Reach SI WIMP-Nucleus cross section of

σ ≈ 2 ∙ 10-47 cm²

The XENON1T Experiment

Increase of two orders of magnitude in sensivity requires a similar reduction in

background

2010-201x:XENON1T

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The XENON1T Muon Veto

Tagging of muons via their Cherenkov radiation emitted during their transit through the water tank.

Hamamatsu R5912 HQE8" PMTs

- 750m³ Volume- Walls cladded with highly reflective foil

84 PMTs located in three arrays: - Top: Ring of 24 PMTs at 4.5m radius, looking down - Lateral: 3 times 12 PMTs in equaly spaced rings - Bottom: Ring of 24 PMTs at 4.5m radius, looking up

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Muon Veto PMTs

→Performance of each PMT has to be tested before final mounting in the Muon Veto!

PMT holding construction(Stainless steel and POM)

Photocathode with high sensitivy to Cherenkov-UV light

- Cherenkov radiation is UV.- Nearly 100% reflection of foil for λ > 380nm

→Use of PMT with high QE for 350-450nm

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Test facility in Mainz

Tests take place in 4 Aluminium boxes

- 60 x 60 x 60 cm - painted black inside - feedthroughs for HV, Signal and LED

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LED calibration

PMMA sheathed fiber bundle with 4 tails - Litetec, United Kingdom - 2.2mm diameter, 1mm core - polished flat ends

Fiber bundle

470nm LED BNC input connector

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Data acquisition logic

Pulse generator LED Box

ADC

PMTTest

Boxes

HV PowersupplyDAQ PC

Trigger Signal

CAEN V1724 fADC4 channels

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Test protocol

Tests done for each PMT:

- Gain for nominal voltages ± 50, ± 100, ± 150 V

- Dark rate for nominal voltages ± 50, ± 100, ± 150 V

- Afterpulses for nominal voltage

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Gain calculation

Used fit formula:

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Gain vs. different HV

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Dark rate

1 s.p.e = 20.8 ADC channels

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Afterpulses

We define an afterpulse as a pulse with two consecutive samples over a certain threshold.

Afterpulses occur, when photoelectrons ionizing residual gas molecules inside the photomulti-plier tube.

Baseline window: 0 to 0.8 µsPrimary peak window: Pos - 0.1µs to Pos + 0.4µsAfterpulses window: Pos + 0.4µs to Pos + 20µs

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Afterpulses charge

Charges of all afterpulses in one event are summed up.

Fraction afterpulses/primary pulse is here 8%

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Afterpulses charge

Charge fraction Afterpulse/Primary pulse calculated for each PMT:

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Summary and Outlook

Almost all 90 PMTs tested so far (in Mainz and Bologna)

Test results will lead to decisions which PMT will be mounted in the Muon Veto and which will be used as spare

Just one single PMT with unexpected behaviour observed so far.

→Additional long term test with this PMT started

→ Construction of water tank starting in April 2013. Mounting PMTs during autumn!

→ First dark matter data taking by XENON1T expected in 2014