Development of a High Pressure Xenon Imager with Optimal Energy Resolution

1
Development of a High Pressure Xenon Imager with
Optimal Energy Resolution

• Azriel Goldschmidt on work with David Nygren
• Instrumentation Series SeminarLBNL, March 2009

2
Physics Motivations

• Neutrinoless double beta decay (bb0n)
• 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 bb0n

3

• Rare nuclear transition between same mass nuclei
• Energetically allowed for even-even nuclei
• (Z,A) ? (Z2,A) e-1 n1 e-2 n2
• (Z,A) ? (Z2,A) e-1 e-2
• (Z,A) ? (Z2,A) e-1 e-2 c

4
Double Beta Decay Spectra
5
H-M Claim
Inverted
50 meV Or 1027 yr
Normal
6
How to look for neutrino-less decay

• Measure the spectrum of the electrons

7
bb0n Experiments

• CANDLES 48Ca CaF2 scintillator crystals
• COBRA 116Cd 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

8
Past Results
Elliott Vogel Annu. Rev. Part. Sci. 2002 52115
48Ca gt1.4x1022 y lt(7.2-44.7) eV
76Ge gt1.9x1025 y lt0.35 eV
76Ge gt1.6x1025 y lt(0.33-1.35) eV
76Ge 1.2x1025 y 0.44 eV
82Se gt2.1x1023 y lt(1.2-3.2) eV
100Mo gt5.8x1023 y lt(0.6-2.7) eV
116Cd gt1.7x1023 y lt1.7 eV
128Te gt7.7x1024 y lt(1.1-1.5) eV
130Te gt3.0x1024 y lt(0.41-0.98) eV
136Xe gt4.5x1023 y lt(1.8-5.2) eV
150Nd gt1.2x1021 y lt3.0 eV
9

• H-M Only claimed evidence of 0nbb detection with
  11 kg of 86 enriched 76Ge for 13 years

T1/21.19x1025y ltmgt 0.44 eV
NIM A522, 371 (2004)
10
CUORE

• Cryogenic calorimeters
• CUORICINO 40.7kg TeO2 (34 abundant 130Te)
• T0n1/2 3.0 1024 yr (90 C.L.)
• ltmngt 0.19 0.68 eV
• Resolution DE/E 2 x 10-3 FWHM at 2.5 MeV
• CUORE 1000 crystals, 720 kg

60Co (cosmogenic) decays, 2 gammas summed
11
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!

12
EXO-200 200 kg Enriched 136Xe
Charge scintillation light readout
13
EXO-200 expected E resolution
Anticorrelation between ionization and
scintillation signals in liquid xenon can be used
to improve the energy resolution
570 keV g
Extrapolates to dE/E 33 10-3 FWHM _at_ Q0nbb
14
Energy resolution in a Xe Dual Phase (XENON)
Extrapolates to dE/E 21 10-3 FWHM _at_ Q0nbb
Aprile, Paris 2008
15
Whats needed

• Long lifetimes (gt1025 years) require
• Large Mass of relevant isotope (gt100 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

16
Why use Xe for bb0n search

• Only inert gas with a bb0n candidate
• Long bb2n 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 ?m (136Xe vs. 131Xe)

17
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

18
LXe or HPXe?

• With high-pressure xenon (HPXe)
• A measurement of ionization alone
• is sufficient to obtain
• good energy resolution

19
Xenon Strong dependence of energy resolution on
density!
Ionization signal only
For ? gt0.55 g/cm3, energy resolution deteriorates
rapidly
20
DE/E 1-2 10-3 FWHM DE/E 35 10-3 FWHM
21
(No Transcript)
22
(No Transcript)
23
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.

24
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?

25
Electroluminescence in 4.5 bar of Xenon
Corresponds to 5 x 10-3 FWHM When naively
extrapolated toQbb of 2.5 MeV (compare with the
Fano limited 2.8 x 10-3 FWHM best case)
26
Fluctuations in Electroluminescence (EL)

• EL is a linear gain process
• G for EL contains three terms
• Fluctuations in nuv (UV photons per e)
• Fluctuations in npe (detected photons/e)
• 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!

27
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

28
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

29
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

30
Separated Function TPC with Electroluminescence
Readout Plane A - position
Readout Plane B - energy
Electroluminescent Layer

31
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, 100s of PMTs
• These two factors greatly reduce the dynamic
  range needed for readout of the signals
• ? No problem to read out lt5 kev to gt5000 keV

32
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
33
Backgrounds for the bb0n search
NEXT Collaboration
34
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
35
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 mb Using a cryopump and is lined with an
RF carpet. An RF funnel guides the ions Towards
the RF quadrupole which is at high vacuum. The
ion is identified using TOF and magnetic rigidity
Sinclair, TPC Workshop Paris 2008
36
Top EL/Scint Detector (Tracking)
EL Grid
Field Cage
Ba Channel
Cathode Grids
Bottom EL/Scint Detector (Energy)
Sinclair, TPC Workshop Paris 2008
37
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

38
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)

39
7-PMT 20 Bar Test Cell
cathode
anode fluorescence grid
J. White, TPC08
40
7-PMT,20 barTest Cell
J. White, TPC08
41
Going from concept to an RD program at LBNL

• Build a test detector large enough to demonstrate
  DE/E 5 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 0.25yours-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 tunedand thanks for listening!
42
Other possible uses of HPXe imagers with optimal
resolution

• Nuclear safeguards Check fuel content of fuel
  rods
• Homeland security Directional information from
  Compton camera to identify U and Pu isotopes
  from a source

43
Summary
44
Backup Slides
45
Double beta decay
Only 2-v decays
Only 0-v decays
Rate
No backgrounds above Q-value
0
Energy
Q-value
The ideal result is a spectrum of all ?? events,
with a 0-? signal present as a narrow peak,
well-separated from 2-?
46
? particles
K. N. Pushkin et al, 2004 IEEE Nuclear Science
Symposium proceedings
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)
47
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

48
Energy Resolution Factors in Xenon Gas Detectors

• Intrinsic fluctuations
• Fano factor (partition of energy) small for ? lt
  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,...

49
?? Sensitivity Issues

• Target (from oscillations) ?m??? 0.050 eV 50
  meV
• Masses could be higher ?m? lt 0.61 eV
• There are 109 relic neutrinos for each baryon ?
• the total ? mass could be ? ?all visible matter
• Goal 100s to 1000s 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 ltlt10 x 10-3 FWHM, with gaussian behavior

50
Xenon Strong dependence of energy resolution on
density!
Ionization signal only
For ? gt0.55 g/cm3, energy resolution deteriorates
rapidly
51
TPC ?? Signal Backgrounds
-HV plane
Readout plane B
Readout plane A
Fiducial volume surface
.

ions
electrons
Signal ?? event
Backgrounds
52
(No Transcript)