Development of a High Pressure Xenon Imager with Optimal

Energy Resolution

Azriel Goldschmidt on work with David Nygren

Instrumentation Series SeminarLBNL, March 2009

Physics Motivations

• Neutrinoless double beta decay ():– Tests Majorana nature of neutrino – Helps determine absolute neutrino mass– If observed, lepton number NOT conserved– Current situation: controversial (one claim), may

require new and richer approach

• WIMP dark matter:– Direct detection– Current situation: controversial (one claim), may

require new and richer approach This talk focuses on the

• Rare nuclear transition between same mass nuclei– Energetically allowed for even-even nuclei

• (Z,A) → (Z+2,A) + e-1 + 1 + e-

2 + 2

• (Z,A) → (Z+2,A) + e-1 + e-

2

• (Z,A) → (Z+2,A) + e-1 + e-

2 +

Double Beta Decay Spectra

50 meVOr ~ 1027 yr

Normal

Inverted

H-M Claim

0.1

1

10

100

1000

Eff

ecti

ve

M

ass

(meV

)

12 3 4 5 6 7

102 3 4 5 6 7

1002 3 4 5 6 7

1000Minimum Neutrino Mass (meV)

Ue1

= 0.866 m2

sol = 70 meV

2

Ue2

= 0.5 m2

atm = 2000 meV

2

Ue3 = 0

Inverted

Normal

Degenerate

How to look for neutrino-less decay

• Measure the spectrum of the electrons

Experiments• CANDLES 48Ca CaF2 scintillator crystals• COBRA116Cd CdZnTe crystals• CUORE 128Te TeO2 Bolometers• EXO 136Xe Liquid Xenon TPC• GERDA 76Ge Enriched Ge diode• MAJORANA 76Ge Enriched Ge diode• SNO+ 150Nd Nd loaded liquid scintillator• SuperNEMO 82Se Foils in track/calorimeter

Past Results

Elliott & VogelAnnu. Rev. Part. Sci. 2002 52:115

48Ca >1.4x1022 y <(7.2-44.7) eV

76Ge >1.9x1025 y <0.35 eV

76Ge >1.6x1025 y <(0.33-1.35) eV

76Ge =1.2x1025 y = 0.44 eV

82Se >2.1x1023 y <(1.2-3.2) eV

100Mo >5.8x1023 y <(0.6-2.7) eV

116Cd >1.7x1023 y <1.7 eV

128Te >7.7x1024 y <(1.1-1.5) eV

130Te >3.0x1024 y <(0.41-0.98) eV

136Xe >4.5x1023 y <(1.8-5.2) eV

150Nd >1.2x1021 y <3.0 eV

102

103

104

105

20202000198019601940Year

Ge-76Ge-76

Te-128

Te-128

Se-82

Ge-76

Ge-76Ca-48

Nd-150Nd-150

20

15

10

5

205020402030202020102000

Energy (KeV)

20

15

10

5

205020402030202020102000

Energy (KeV)

NIM

A52

2, 3

71 (

200

4)• H-M: Only claimed evidence of detection

with 11 kg of 86% enriched 76Ge for 13 years

•Klapdor-Kleingrothaus et al Phys.Lett.B586:198-212,2004.

T1/2~1.19x1025y<m> ~ 0.44 eV

CUORE• Cryogenic “calorimeters”

• CUORICINO: 40.7kg TeO2 (34% abundant 130Te)

– T ≥ 3.0 × 1024 yr (90% C.L.)

– <m> ≤ 0.19 – 0.68 eV

– Resolution E/E = 2 x 10-3 FWHM at 2.5 MeV

• CUORE ~1000 crystals, 720 kg 60Co (cosmogenic) decays, 2 gammas summed

“Gotthard TPC” Pioneer TPC detector for 0- decay search

– 5 bars, enriched 136Xe (3.3 kg) + 4% CH4

– MWPC readout plane, wires ganged for energy– No scintillation detection no TPC start signal!

• No measurement of drift distance

E/E ~ 80 x 10-3 FWHM (1592 keV) 66 x 10-3 FWHM (2480 keV)

Reasons for this less-than-optimum resolution are not clear…

Likely: uncorrectable losses to electronegative impurities

Possible: Undetectable losses to quenching (4% CH4)

But: ~30x topological rejection of interactions!

EXO-200 200 kg Enriched 136XeCharge & scintillation

light readout

EXO-200 expected E resolutionAnticorrelation between ionization and scintillation signals in

liquid xenon can be used to improve the energy resolution

Extrapolates to E/E = 33 10-3 FWHM @ Q0

570 keV

Energy resolution in a Xe Dual Phase (XENON)

Aprile, Paris 2008

Extrapolates to E/E = 21 10-3 FWHM @ Q0

What’s needed…

• Long lifetimes (>1025 years) require:– Large Mass of relevant isotope (>100 kg)– Small or No background:

• Clean materials• Underground, away from cosmic rays• Background rejection methods:

– Energy resolution– Event topology– Particle identification– Identification of daughter nucleus

– Years of data-taking

Why use Xe for search

• Only inert gas with a candidate• Long lifetime ~1022-1023 y (not seen yet) • No need to grow crystals• Can be re-purified in place (recirculation)• No long lived Xe isotopes• Noble gas: easier to purify• 136Xe enrichment easier (natural 8.9%):

- noble gas (no chemistry involved)

- centrifuge efficiency ~ Δm (136Xe vs. ~131Xe)

Energy partition in xenon

• When a particle deposits energy in xenon, where does the energy go?– Ionization

– Scintillation: VUV ~170 nm (1, 2 …)

– Heat

• How is the energy partitioned?– Complex responses, different for , , nuclei– Dependence on xenon density , E-field– Processes still not perfectly understood

LXe or HPXe?

With high-pressure xenon (HPXe)

A measurement of ionization alone

is sufficient to obtain

good energy resolution…

Xenon: Strong dependence of energy resolution on density!

For >0.55 g/cm3, energy resolution deteriorates rapidly

Ionization signal only

E/E ~ 1-2 10-3 FWHM

E/E ~ 35 10-3 FWHM

What is this factor “G”?

• In a very real sense:

G is a measure of the precision with which a single electron (from an ionizing track) can be counted.

Electro-Luminescence (EL) (Gas Proportional Scintillation)

– Electrons drift in low electric field region– Electrons then enter a high electric field region– Electrons gain energy, excite xenon, lose energy– Xenon generates UV– Electron starts over, gaining energy again– Linear growth of signal with voltage– Photon generation up to ~1000/e, but no ionization– Early history irrelevant, fluctuations are small– Maybe… G ~ F?

Electroluminescence in 4.5 bar of Xenon

Corresponds to 5 x 10-3 FWHMWhen naively extrapolated toQ of 2.5 MeV(compare with the Fano limited 2.8 x 10-3 FWHM best case)

Fluctuations in Electroluminescence (EL)

EL is a linear gain process

G for EL contains three terms:1. Fluctuations in nuv (UV photons per e):

2. Fluctuations in npe (detected photons/e):

3. Fluctuations in photo-detector single PE response:

G = 2 = 1/(nuv) + (1 + 2pmt)/ npe)

For G = F = 0.15 npe ≥ 10

The more photo-electrons, the better!

Equivalent noise: much less than 1 electron rms!

Virtues of an EL readout

• Immune to microphonics

• Absence of positive ion space charge

• Linearity of gain versus pressure, HV

• Isotropic signal dispersion in space

• Trigger, energy, and tracking functions accomplished with optical detectors

Detector Concept

• Use enriched High Pressure Xenon

• TPC to provide image of the decay particles

• Design to also get an energy measurement as close to the intrinsic resolution as possible

High-pressure xenon gas TPC

• Fiducial volume surface:– Single, continuous, fully active, variable,...– 100.00% rejection of charged particles

(surfaces) – TPC with t0 to place event in z coordinate

• Tracking:– Available in gas phase only– Topological rejection of single electron events

Separated Function TPC with Electroluminescence

Readout Plane A

- position

Readout Plane B- energy

Electroluminescent Layer

Electro-Luminescent Readout

For optimal energy resolution, 105 e- * 10 pe/e- = 106 photoelectrons need to be detected!

Energy readout plane is a PMT array

• electron (secondary) drift is very slow: ~1 mm/s• This spreads out the arriving signal in time - up to 100 s

for many events• The signal is spread out over the entire readout cathode-

side, 100’s of PMTs• These two factors greatly reduce the dynamic range

needed for readout of the signals No problem to read out <5 kev to >5000 keV

Single 2.49 MeV e- in 20 atm Xe(background) MC simulation

~5 cm

Using tracking information, separate single electrons (like these) from 2-electron events that should have “blobs” at BOTH ends of the combined track

Backgrounds for the search

NEXT Collaboration

Can one measure Ba++ Directly?

• Extract the ion from the high pressure into a vacuum

• Measure mass and charge directly

• A mass 136, ++ ion is a unique signature of Ba++. (Assumption is Xe++ cannot survive long enough to be a problem)

• This has been done for Ba++ in Ar gas

Sinclair, TPC Workshop Paris 2008

Barium ions are guided towards the exit orifice and focused using an asymmetric field technique. The second chamber is maintained at a pressure of ~10-30 mbUsing a cryopump and is lined with an RF carpet. An RF funnel guides the ionsTowards the RF quadrupole which is at high vacuum. The ion is identified usingTOF and magnetic rigidity

Sinclair, TPC Workshop Paris 2008

Top EL/Scint Detector (Tracking)

Bottom EL/Scint Detector (Energy)

Field Cage

EL Grid

CathodeGrids

BaChannel

Sinclair, TPC Workshop Paris 2008

HPXe and the Dark Matter search

• Liquid Xenon has the lead on this (since energy resolution is not critical), however

• HPXe offers better discrimination between nuclear recoils and electrons

• There are ideas that would enable a lower threshold in the gas phase (useful for testing the DAMA/LIBRA positive result)– Challenge at low recoil energy for both LXe and HPXe

is that the primary scintillation signal (for trigger ing and fiducialization) is Tiny

Big Impact for WIMP Search in LXe

Scintillation (S1) & Ionization (S2) are the

signals used to reject electron recoils: S2/S1

But, in LXe:

S2/S1 fluctuations are anomalously large

Bad news for discrimination power in LXe…

(though may be not critical in the presence of self shielding)

7-PMT 20 Bar Test Cell

anode + fluorescence grid

cathode

J. White, TPC08

7-PMT,20 barTest Cell

1 inch

R7378AJ. White, TPC08

Going from concept to an R&D program at LBNL

• Build a test detector large enough to demonstrate ~ E/E5 * 10-3 at ~2.5 MeV– Gas system (to take out electronegative impurities)– Energy side readout (PMTs most likely)– Enough “tracking-side” sensors to achieve energy resolution– 50 cm size chamber to house ~20 Atm of Xe

• Monte Carlo simulation for detector optimization ongoing (2 students and 0.25*yours-truly)

• Groups in Canada/US (part or EXO) and in Spain (NEXT collaboration) pursuing this line of research as well –both healthy competition and collaboration-

Stay tuned…and thanks for listening!

Other possible uses of HPXe imagers with optimal resolution

• Nuclear safeguardsCheck fuel content of fuel rods

• Homeland security Directional information from

“Compton camera” to identify U and Pu isotopes from a “source”

Summary

Backup Slides

Double beta decay

Only

2-v decays

Rate

Energy Q-value

Only

0-v decays

No backgrounds above Q-value

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

0

A scary result: adding a tiny amount of simple molecules (CH4, N2, H2 ) to HPXe quenches both ionization and scintillation for ’s

particle: dE/dx is very high

Gotthard TPC: 4% CH4

Loss(): factor of 6

For particles, what was effect on energy resolution?

Surely small but not known, and needs investigation(~25 bars)

particlesK. N. Pushkin et al, 2004

IEEE Nuclear Science Symposium proceedings

Molecular Chemistry of Xenon

• Scintillation:• Excimer formation: Xe*+ Xe Xe2* h + Xe• Recombination: Xe+ + e– Xe*

• Density-dependent processes also exist: Xe*+ Xe* Xe** Xe++ e- + heat

• Two excimers are consumed!• More likely for both high + high ionization density

– Quenching of both ionization and scintillation can occur!Xe* + M Xe + M* Xe + M + heat (similarly for Xe2*, Xe**, Xe2*+… )

Xe+ + e–(hot) + M Xe+ + e–(cold) + M*

Xe+ + e–(cold) + M + heat e–(cold) + Xe+ Xe*

Energy Resolution Factors in Xenon Gas Detectors

– Intrinsic fluctuations• Fano factor (partition of energy): small for < 0.55 g/cm3

– Loss of signal (primary):• Recombination, quenching by molecular additives (heat)

– Loss of signal (secondary):• Capture by grids or electronegative impurities

– Gain process fluctuations:• Avalanche charge gain fluctuations are large

– Gain process stability:• Positive ion effects, density and mix sensitivity,...

– Long tracks extended signals • Baseline shifts, electronic non-linearities, wall effect,...

Sensitivity Issues

• Target (from oscillations): m ~0.050 eV = 50 meV

– Masses could be higher… ∑m < 0.61 eV – There are ~109 relic neutrinos for each baryon

• the total mass could be ∑all visible matter

• Goal: 100’s to 1000’s kg active mass likely to be necessary

• Rejection level of internal/external backgrounds:– Less than one event per ~1027 atoms/year!

• Energy resolution needed: E/E <<10 x 10-3 FWHM, with gaussian behavior

Xenon: Strong dependence of energy resolution on density!

For >0.55 g/cm3, energy resolution deteriorates rapidly

Ionization signal only

TPC: Signal & Backgrounds

-HV planeReadout plane BReadout plane A

.

ionselectrons

Fiducial volume surface

Signal: event Backgrounds

*