next: fnal 20121 next a high-pressure xenon gas tpc: how superior energy resolution benefits both 0-...

NEXT: FNAL 2012 1

NEXTA High-pressure Xenon Gas TPC:

How superior energy resolution benefits both 0- decay in 136Xe and WIMP searches

David Nygren

LBNL

NEXT: FNAL 2012 2

Outline

• What’s NEXT?• Xenon gas TPC: new R&D results!• Both WIMP & 0- decay searches?• Electroluminescence (EL): a neglected tool• The bigger picture: EL with tracking• Intended US role in NEXT

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“Neutrino Experiment Xenon TPC”

NEXT is an approved & funded search for 0- decaybased on a high-pressure xenon gas (HPXe) TPC

NEXT will be constructed in Spain, in the new, improved Canfranc Underground Laboratory.

NEXT has been funded by Spanish Funding Agencies at the level of € 6M+

NEXT R&D phase is nearing completion, construction to start in FY2012

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are needed to see this picture.

Spain provides:

Most of the collaborators

Most secured funding

Host Laboratory - LSC

Key contributions from international groups

Engineering and integration

TPC expertise

high-pressure gas detectors

Xenon supply & enrichment

ISU

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US groups involved in new DOE proposal (in preparation):

LBNL: Azriel Goldschmidt (NSD), John Joseph (Elec. Eng.), Tom Miller (Mech. Tech.), David Nygren (Physics), Josh Renner (student), Derek Shuman (Mech. Eng.)

Texas A&M: James White (Faculty), Clement Sofka (student)

Iowa State University: John Hauptman (Faculty) + students TBD

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Laboratorio Subterraneo de Canfranc

Waiting for NEXT!

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Double beta decay spectra

Only

2-v decays

Rate

( electron energy) Q-value

Only

0-v decays

No backgrounds above Q-value

The ideal result: a spectrum of only events, with a 0- signal present as a narrow peak, well-separated from 2-

0

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Energy resolution in Xenon: Strong dependence on density!

Very large fluctuations

between light/charge!

F ~ 20

WIMPs: S2/S1

suffers!

Here, the fluctuations are normal

F = 0.15

Unfolded resolution:

E/E ~0.6% FWHM

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

Ionization signal only!

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What does a search for 0- require?

Sensitivity and Background Rejection

1. High sensitivity large mass of candidate isotopeNEXT has 100 kg of enriched xenon: ~85% 136Xe

2. Extremely good background rejection!1. Shielding, radio-purity, excellent energy resolution, event topology are critical

2. High Q-value of 136Xe, 2457 keV, places signal above most -rays

3. NEXT energy resolution: E/E <0.7 % FWHM expected at E = Q-value

4. The TPC monolithic fiducial volume presents a fully active surface

5. Good 3-D tracking in high-pressure xenon gas reveals event topology– Excellent discrimination between 1- and 2- electron events

– All charged particles from surfaces will be rejected

– Neutrons not an important background

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What does a search for WIMPs require?

Sensitivity and Background Rejection

1. High sensitivity large sensitive massNEXT has 100 kg of enriched xenon: ~85% 136Xe

A large component of neon can be added for better match to low-mass WIMPs

• Extremely good background rejection!• NEXT offers superior discrimination between nuclear and electron recoils,

Huge S2/S1 fluctuations degrade discrimination in LXe, but not in HPXe

• NEXT will exploit the TPC idea to realize a monolithic fully active fiducial volume,

Essentially all charged particle background events excluded.

1. NEXT will possess good 3-D tracking in high-pressure xenon gasEvent topology reveals single & mulitple-site interactions, reject gammas & neutrons

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The requirements have similarities...

• At TAMU, Moscow, and LBNL, near-intrinsic energy resolution has been been shown in HPXe TPCs, using -rays of 60, 122, and 662 keV

• Our new result is a world record for Xe-based detectors

• An electroluminescent gain stage is the key concept.

• We assert: “0- and direct detection WIMP searches can be made simultaneously in one detector, without compromise to either search, and with superior performance”

NEXT: FNAL 2012 12

NEXT Asymmetric TPC“Separated function”

Transparent -HV plane

Readout plane BReadout plane A

.

ions

energy & primary scintillation signals recorded here, with PMTs

Field cage: reflective teflon (+WLS)

EL signal created here

Tracking performed here, with

“SiPMT” array

Fiducial surface

Operating pressure: 10 -15 bars

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New: World’s best energy resolution for 137Cs -rays in xenon!

Best results, to show off our approach

Tight fiducial volume cut imposed here

I will explain...

662 keV, ionization signal only

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Full 137Cs -ray Spectrum with looser fiducial volume cut low threshold includes fluorescence x-rays

no correction applied for known radial dependence of signal

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Peak spectral region for 137Cs -rays: LBNL-TAMU HPXe TPC, 15 bars pure xenon

Note suppressed zero!

This spectrum taken with the “normal” fiducial volume, as in last slide

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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


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


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

10ns/sample

Primary Scintillation (S1)T0 of event

Electroluminescence (S2)Structure suggests topology due to Compton scatters

Drift Time:z-position (~0.01mm/sample) Drift velocity ~1 mm/ms

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Complex topologies are common!

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A Diagonal Muon Track! - “reconstructed”;

Signal depends on radius in chamber

~ 14 cm

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Attenuation of electrons during drift is very low

correction forattenuation ismodest, and introduces insignificant error to energy

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What is the Intrinsic Energy Resolution?

N = √FN = √FQ/w

F Fano factor: F = 0.15 (HPXe) (LXe: F ~20 !!)

w Average energy per ion pair: w ~ 25 eV

Q Energy deposited in xenon: 137Cs -rays: 662 keV

E/E = 2.35N /N = 2.35 (Fw/Q)1/2 FWHM

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The Intrinsic Energy Resolution @ 662 keV

E/E = 2.35 (Fw/Q)1/2

E/E = 0.56% FWHM (HPXe)

We are about a factor of ~2 from this value

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The basic signal

For 137Cs:

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

This is a very small number!

How can this signal be detected with minimal degradation?

What are the main degrading factors?

NEXT: FNAL 2012 27

Energy resolution in Xenon: Strong dependence on density!

Very large fluctuations

between light/charge!

F ~ 20

WIMPs: S2/S1

suffers!

Here, the fluctuations are normal

F = 0.15

Unfolded resolution:

E/E ~0.6% FWHM

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

Ionization signal only!

NEXT: FNAL 2012 28

Energy Partitioning in LXe

Anomalously large fluctuations in energy partition between ionization and scintillation generate the large Fano factor in LXe

The large fluctuations in LXe are caused by delta-rays, zones of very high ionization density, but few in number, and with “Landau” fluctuations

Within zones of both high ionization and atomic density, nearly full recombination leads to light creation at the expense of ionization.

The recombination process amplifies the non-Poisson statistics of the energy loss process of electrons in LXe...

But not for xenon gas!

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1 kV/cm

Strong anti-correlations in LXe!

~570 keV~570 keVBi-207 source

EXO data

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Gamma events (e - R)

Neutron events (N - R)

Why do events show large S2/S1 fluctuations at all energies, not improving with energy?L o

g 10

S2/

S1

Xenon10 data

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Anti-correlation of Q & L

• For fixed energy, such as Q = 2457 keV, energy resolution can be restored, in principle, by measuring both Q & L and forming the right linear combination.

• In practice, this doesn’t work very well because only a few % of the light is detected; statistical precision is poor.

• EXO predicted energy resolution @ Q (with light signal): – 3.4 % FWHM

• EXO measured energy resolution (ionization signal only)– 10.6% FWHM @ 2615 keV

NEXT: FNAL 2012 32

Double beta decay spectra and 136Xe

Only

2-v decays

Rate

( electron energy) Q-value

Q = 2457 keV for 136Xe

The ideal result: a spectrum of only events, with a 0- signal present as a peak, width dictated by resolution

0

NEXT: FNAL 2012 33

Energy resolution at Q

E/E = 2.35 (FW/Q)1/2

– F Fano factor (HPXe) : F = 0.15 – W Average energy per ion pair: W ~ 25 eV

– Q Energy deposited from 136Xe --> 136Ba: 2457 keV

E/E = 0.28% FWHM intrinsic!

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

NEXT: FNAL 2012 34

Energy resolution in Xenon gas:Gain & noise

Impose a requirement on gain stage:

(noise + fluctuations) N

Simple charge detection can’t meet this goal

Need gain with very low noise/fluctuations!

Electroluminescence (EL) is the

key!

NEXT: FNAL 2012 35

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 (Poisson: JCP = 1)

• Optimal EL conditions: JCP = 0.01

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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

NEXT: FNAL 2012 37

Gain noise & resolutionF Fano constraint due to fixed energy deposit

= 0.15Let “G” represent noise/fluctuations in EL gain

Uncorrelated 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 primary electron

2PMT 2 (due to after-pulsing!)

G 3/Npe

Npe > 20 per electron so that G ≤ F = 0.15

E/E = 0.9% FWHM (137Cs: 662 keV)

NEXT: FNAL 2012 38

1.04% FWHM 0.9% FWHM?

• The primary reasons we have not reached E/E =

0.9% FWHM with our prototype are that: – Our photoelectron yield ne is less than 20.

– Accurate radial correction requires real tracking.

• Addition of a tracking plane will make possible an accurate radial correction, and increase efficiency

• Tracking with EL is a primary R&D goal in FY 12

NEXT: FNAL 2012 39



Operating pressure: 10 - 15 bars

decay:

“spaghetti with two meatballs”

NEXT: FNAL 2012 40

Tracking plane

• Previous HPXe TPC (Gotthard Tunnel) showed that a factor of >30 reduction in background is possible with event topology.

– A larger factor may be possible, under study...

• To reveal topology, a new tracking plane for our HPXe TPC is needed– The tracking plane can be installed without major surgery to our HPXe TPC

• Tracking plane will be an x-y grid with MPPCs spaced at ~1 cm pitch– Hamamatsu 1 mm2 SiPM: MPPC s10362-11-100P

• Electronics for the tracking plane is a joint development with UPV– Simple low-power circuitry to shape, digitize, and time-stamp waveforms

NEXT: FNAL 2012 41

Silicon Photomultiplier “SiPM”

SiPM from Hamamatsu, “MPPC”

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SiPM photoelectron spectrum

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NEXT: FNAL 2012 44



Nb <4 x 10-4

counts/keV kgy

If backgrounds are as low as we calculate, then NEXT will be more than competitive!

Backgrounds are the limiting factor!

NEXT: FNAL 2012 45

Summary: 0- search

• HPXe electroluminescent TPC concept was developed at LBNL•

• HPXe EL TPC offers superb energy resolution: 0.5% FWHM?

• Event topology provides background rejection: >30

• HPXe EL TPC has been embraced by NEXT.

• 6M€+ funds provided by Spain to NEXT project

• US makes vital contributions to NEXT, plus move toward 1 ton

NEXT: FNAL 2012 46

Direct Dark Matter Search

• Neon nuclear mass 20 is a very good match to alleged low-mass WIMPs (consonance with DAMA-LIBRA et al.?).

• Lots of neon can be added to HPXe without adverse effects.

• Simultaneous 0-v decay WIMP searches appear possible.

• The xenon gas still provides shielding for low energy -rays;

• High energy -rays typically have multiple substantial Compton scatters

• A WIMP search in NEXT has not yet been thoroughly simulated.

• R&D goal in FY 12: neon and neutrons in our TPC

NEXT: FNAL 2012 47

WIMPS: Discrimination between electronic and nuclear recoils with S2(charge)/S1(light)

• In LXe, large energy partitioning fluctuations between L and Q – Intrinsic to LXe, absent in HPXe

• These huge fluctuations enter directly in the ratio S2/S1, – electron and nuclear recoil event discrimination compromised

• In HPXe, S2/S1 discrimination is expected to be hugely better– This potential needs to be demonstrated in our setup

• The highest optical detection efficiency is desired to capture S1.– Wavelength shifters: Nitrogen ?, plastic bars ?, TMA,...

NEXT: FNAL 2012 48

Predecessor: 7-PMT, 20 bar

TAMU HPXe TPC

1 inch

R7378AJ. White, TPC08, (D. Nygren, H-G Wang)

NEXT: FNAL 2012 49

Nr Discrimination in HPXe with TAMU 7-PMT TPC

neutrons

gammas

NEXT: FNAL 2012 50

Beppo-SAX satellite: a HPXe TPC in space!

NEXT: FNAL 2012 51

Electroluminescence in 4.5 bar of Xenon

2.2% FWHM resolution corresponds to

E/E = 5 x 10-3 FWHM

-- if naively extrapolated toQ of 2.5 MeV

NEXT: FNAL 2012 52

R&D Summary

• The energy resolution of the HPXe EL TPC has been demonstrated.

• Direct WIMP detection with excellent discrimination appears possible.

• Primary FY2012 HPXe TPC R&D Goals

– Tracking plane for event topology

• learn to do the radial correction properly

• Reconstruct gamma-ray events.

– Nuclear/electron recoil discrimination

• add neon

• expose chamber to neutrons

NEXT: FNAL 2012 53

NEXT construction summary

The US groups propose construction contributions for NEXT:– Energy Plane mechanics - LBNL– Tracking Plane electronics - LBNL– Engineering, design, and integration - LBNL– TPC structures - TAMU– Energy resolution/calibration - ISU

– Additional US Collaborators desirable

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Perspective

• Both 0- and WIMP searches can be done - WIMP sensitivity comes “free”, but WIMP performance needs demonstration.

• Optical detection efficiency for S1 has to be maximized to capitalize on the superb intrinsic resolution - WLS research

• Molecular additives such as tri-methyl amine (TMA) might offer much lower Fano factor, with WLS properties to 300 nm range

• A future ~1000 kg detector for simultaneous 0- and WIMP searches could be located at SURF...

58NEXT: FNAL 2012

next: fnal 20121 next a high-pressure xenon gas tpc: how superior energy resolution benefits both 0-...

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enriched xenon

excellent energy resolution

raysnext energy resolution

intrinsic energy resolution

unfolded resolution

large fluctuations

decay searches

new rd results