TIPP 2011 1

High-Pressure Gaseous Xenon TPCfor 0-v Search in 136Xe

Azriel Goldschmidt, Tom Miller, David Nygren, Josh Renner, Derek Shuman,

Helmuth Spieler, Jim SiegristLBNL

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Motivations• Xenon gas at high pressure offers excellent energy resolution - in principle

(within a factor 3 of best Ge diodes! )

• Electroluminescence provides linear gain with extremely low fluctuations - helps to preserve this high intrinsic energy resolution.

• HP Xe TPC can provide total energy and image of the particle tracks for topological discrimination of event type (Gotthard TPC: x30 rejection)

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Context

• R&D is focused on the NEXT Collaboration, now preparing for a 100 kg 136Xe TPC detector for Canfranc Underground Laboratory, Spain.

NEXT is funded by Spain at ~5M € for constructionSpain-Portugal-Colombia-France-Russia-US collaboration

• Applications may include -ray imaging for Homeland security/non-proliferation, medical physics/imaging

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Xenon: Strong dependence of energy resolution on density!

For <0.55 g/cm3, ionization energy resolution is “intrinsic”

Ionization signal onlyHere, the

fluctuations are normal

Large fluctuations

between light/charge

WIMPs: S2/S1

suffers!

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Intrinsic energy resolutionE/E = 2.35 (FW/Q)1/2

– F Fano factor: F = 0.15 (HPXe) (LXe: F ~20)– W Average energy per ion pair: W ~ 25 eV– Q Energy deposited, e.g. 662 keV from Cs137 -rays:

E/E = 0.56% FWHM (HPXe)

N = Q/W ~26,500 primary electronsN = (FN)1/2 ~63 electrons rms!

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Intrinsic energy resolutionE/E = 2.35 (FW/Q)1/2

– F Fano factor: F = 0.15 (HPXe) (LXe: F ~20)– W Average energy per ion pair: W ~ 25 eV– Q Energy deposited, e.g. 2457 keV from 136Xe --> 136Ba:

E/E = 0.28% FWHM (HPXe)

N = Q/W ~100,000 primary electronsN = (FN)1/2 ~124 electrons rms!

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Gain and noiseImpose a requirement:

(noise + fluctuations) N(noise + fluctuations) 124 e—

Simple charge detection can’t meet this goal

Need gain with very low noise/fluctuations!

Electroluminescence (EL) is the key

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Electro-Luminescence (EL) (aka: Gas Proportional Scintillation)

• Physics process generates ionization signal• Electrons drift in low electric field region• Electrons enter a high electric field region• Electrons gain energy, excite xenon: 8.32 eV• Xenon radiates VUV (175 nm, 7.5 eV)• Electron starts over, gaining energy again• Linear growth of signal with voltage• Photon generation up to ~1000/e, but no ionization• Sequential gain; no exponential growth fluctuations are very

small• NUV = JCP N1/2

• Optimal EL conditions: JCP = 0.01 (Poisson: JCP = 1)

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Gain and noiseF constraint due to fixed energy deposit =

0.15Let “G” represent noise/fluctuations in EL

gainUncorrelated fluctuations can add in

quadrature: n = ((F + G)N)1/2

EL: G = JCP/NUV + (1 + 2PMT)2/Npe

Npe = number of photo-electrons per electronG 1.5/Npe

Npe > 10 per electron for G ≤ FE/E = 0.9% FWHM 137Cs 662 keV

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Virtues of Electro-Luminescence in HPXe

• Linearity of gain versus pressure, HV• Immunity to microphonics• Tolerant of losses due to impurities• Absence of positive ion space charge• Absence of ageing, quenching of signal• Isotropic signal dispersion in space• Trigger, energy, and tracking functions are

accomplished with optical detectors

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TPC with Electroluminescence: Total Energy and Track Imaging

Readout Plane A

- position

Readout Plane B- energy

Electroluminescent Layer

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Pressure vessel design study at 15 bars for 100 kg NEXT

~120

cm

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Laboratorio Subterraneo de Canfranc

Waiting for NEXT...

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LBNL-TAMU TPC Prototype

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Field cages/Light cagePTFE with copper stripes

Electroluminescence region10 kV across a 3 mm gap

19 PMTs and PMT bases

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PMT Array: inside the pressure vesselQuartz window 2.54 cm diameter PMTs

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Inserting the TPC...

carefully!

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A Diagonal Muon Track - “reconstructed”

~ 14 cm

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A typical 137Cs waveform (sum of 19 PMTs)~300,000 detected photoelectrons

10ns/sample

Primary Scintillation (S1)T0 of event

Electroluminescence (S2)Structure indicates topology due to Compton scatters

Drift Time:z-position (~0.01mm/sample)

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Charge vs Drift Timeelectron lifetime of 900 ms

~5% charge loss forlongest drift of 60 ms (8 cm)

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HPXe @ 10 Atm, 137Cs 662 keVDrift time correction applied

keV

Coun

ts

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HPXe @ 10 Atm, 137Cs 662 keV

1.8% FWHM

29-36 keV Xe x-rays escape

keV

Coun

ts

Drift Time and Position corrections applied

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Conclusions and Outlook• Successful operation of xenon EL TPC in 10-15 Bar range• We have achieved E/E = 1.8% FWHM @ 662 keV (10 Atm)

– If F = G, and if no other effects, we expect: E/E = 0.9% FWHM• We do not yet claim to understand this factor of 2, but...• We do expect that this gap will be substantially reduced

– Likely contributing factors:• localization - dependence of signal on radius • calibration of gain and QE of each PMT• dissociative attachment of electrons in EL region to water/oxygen• mesh flatness• PMT after-pulsing

• NEXT is starting to happen! 100 kg 136Xe awaits us!