1. Development of a High Pressure Xenon Imager with Optimal Energy Resolution
Azriel Goldschmidt on work with David Nygren
Instrumentation Series Seminar LBNL, 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
This talk focuses on the bb0n

3. Rare nuclear transition between same mass nuclei
Energetically allowed for even-even nuclei
(Z,A) → (Z+2,A) + e-1 + n1 + e-2 + n2
(Z,A) → (Z+2,A) + e-1 + e-2
(Z,A) → (Z+2,A) + e-1 + e-2 + c

4. Double Beta Decay Spectra
mention that a number of other exotic particles can also contribute to BB decay.

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
Indicate the variation in expected rate (modulo m^2) from the matrix element calculations

8. Past Results 48Ca >1.4x1022 y <(7.2-44.7) eV 76Ge >1.9x1025 y
Elliott & Vogel
Annu. Rev. Part. Sci :115
Past Results
48Ca
>1.4x1022 y
<( ) eV
76Ge
>1.9x1025 y
<0.35 eV
>1.6x1025 y
<( ) eV
=1.2x1025 y
= 0.44 eV
82Se
>2.1x1023 y
<( ) eV
100Mo
>5.8x1023 y
<( ) eV
116Cd
>1.7x1023 y
<1.7 eV
128Te
>7.7x1024 y
<( ) eV
130Te
>3.0x1024 y
<( ) eV
136Xe
>4.5x1023 y
<( ) eV
150Nd
>1.2x1021 y
<3.0 eV

9. H-M: Only claimed evidence of 0nbb detection with 11 kg of 86% enriched 76Ge for 13 years
Klapdor-Kleingrothaus et al Phys.Lett.B586: ,2004.
T1/2~1.19x1025y
<m> ~ 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.)
<mn> ≤ 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. Charge & scintillation
EXO 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 = Q0nbb

14. Energy resolution in a Xe Dual Phase (XENON)
Extrapolates to dE/E = Q0nbb
Aprile, Paris 2008

15. 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

16. Why use Xe for bb0n search
Only inert gas with a bb0n candidate
Long bb2n lifetime ~ 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)

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  >0.55 g/cm3, energy resolution deteriorates rapidly

20. DE/E ~ FWHM
DE/E ~ FWHM

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 to Qbb 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 =  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, 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

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
Cathode
Grids Ba
Channel
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 bar Test Cell
1 inch
R7378A
J. White, TPC08

41. Going from concept to an R&D program at LBNL
Build a test detector large enough to demonstrate ~ DE/E ~ 5 * 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!

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
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  < 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 < 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

50. Xenon: Strong dependence of energy resolution on density!
Ionization signal only
For  >0.55 g/cm3, energy resolution deteriorates rapidly

51. TPC:  Signal & Backgrounds
-HV plane
Readout plane B
Readout plane A
Fiducial volume surface . *
electrons
ions
Signal:  event
Backgrounds