combined wimp and 0- decay searches with high-pressure 136 xe gas tpc dave nygren lbnl

Combined WIMP and 0- decay searches

with

High-pressure 136Xe Gas TPC

Dave Nygren

LBNL

DUSEL Workshop 2007 2

Energy partitioning in Xe()

Ionization signal only


Energy Partitioning

• The anti-correlations observed in LXe reflect a partitioning of event energy between:– Scintillation S– Ionization I– Heat

Event visible energy = (a x S) + (b x I)

• Only a fraction of the light can be detected• But: Highly non-gaussian fluctuations!• Measurement precision is compromised


Strategy:

• Use High pressure xenon gas TPC ~20 bars ( = ~0.1 gm/cm3) E/E (I) is intrinsic: =√FN F = 0.15– E/E = 2.7 x 10-3 FWHM @ 2.48 MeV

2 = (F + G + L) x E/W: (NIM A 581 (2007) 632-642)

– Best energy resolution in practice:• Gas proportional scintillation: G ~0.2• Losses: L <0.05

E/E ~4 x 10-3 FWHM @ 2.48 MeV


Combined Search: Issues

• 0- – Energy: 2.5 MeV– Resolution: Etotal

E/E <1% FWHM• Ionization signal only

– Primary Scintillation:• TPC start signal• Modest sensitivity

– Background: -rays

• WIMP– 5 < Erecoil < ~50 keV– Resolution: S2/S1

• N/ Discrimination• I + S gives energy

– Primary Scintillation:• TPC start signal• Maximum sensitivity

– Backgrounds -rays• neutrons


Scintillation Dynamic range: OK

– S1 WIMP signals are very small

• Optimize detector for maximum S1 detectiion efficiency

• Not a problem if S1 signals from events saturate PMT

– S2 signals are not very different• only ~2 mm of track is present in the PS gap track instantaneous signal is ~1000 e– (25 keV)• OK, tracks parallel to PS plane will saturate…


S2/s1 Fluctuations…

– S2/S1 resolution in LXe • degraded by anomalous large fluctuations in LXe• dominant effect in LXe• sensitivity compromised in LXe

– S2/S1 ( < 0.5 g/cm3) in HPXe:• anomalous fluctuations are surely absent for particles;

• very little recombination S2/S1 should be “large”

– S2/S1 for nuclear recoils in HPXe is unmeasured• Careful, systematic measurements need to be done

N/ discrimination might be (much) better in HPXe


Now: 7-PMT cell @ TAMU

Uncalibrated, raw,

initial data from

60 keV source

(similar to GPSC on board Beppo-SAX satellite - what is that? )

Beppo-SAX Xe Gas Proportional Counter


Beppo-SAX Gas Proportional Counter


Next: 37-PMT Cell

• 3 PMT rings added around center PMT– Total number of PMTs: 37– Diameter: ~18 cm, drift length ~18 cm

• Goal:– Demonstrate tracking of ~1 MeV particles– Determine best practical E/E resolution– Determine N/ discrimination in system


Time scales

• 7-PMT TAMU system: now 2008• 37 PMT system (DUSEL R&D proposal)

– 2008 2010– Costs: ~150 k$/year, for three years

• 200 kg system– Proposal in 2009/ reviews 2010– Construction start 2011/12– Costs: $6,782,361.49 – 1000 kg system also feasible


1000 kg Xe: = 225 cm, L =225 cm ~ 0.1 g/cm3 (~20 bars)

A. Sensitive volume

B. HV cathode plane

C. GPSC readout planes, optical gain gap is ~1-2 mm

D. Flange for gas & electrical services to readout plane

E. Filler and neutron absorber, polyethylene, or liquid scintillator, or …

F. Field cages and HV insulator, (rings are exaggerated here) likely site for photo-detectors


Two identical HPXe TPCsTwo distinct physics goals

• “ Detector”– Fill with enriched Xe

mainly 136Xe– Events include all

events + backgrounds

– Isotopic mix is mainly even-A

– WIMP events include more scalar interactions

• “WIMP Detector”– Fill with normal Xe or fill

with “depleted” Xe– Events include only

backgrounds to

– Isotopic mix is ~50% odd-A: 129Xe 131Xe

– WIMP events include more axial vector interactions

DUSEL Workshop 2007DUSEL Workshop 2007 1616


Barium daughter tagging and ion mobilities…

• Ba+ and Xe+ mobilities are quite different!– The cause is resonant charge exchange– RCE is macroscopic quantum mechanics

• occurs only for ions in their parent gases • no energy barrier exists for Xe+ in xenon • energy barrier exists for Ba ions in xenon

• RCE is a long-range process: R >> ratom

• glancing collisions = back-scatter

RCE increases viscosity of majority ions


Barium daughter tagging and ion mobilities…

– Ba++ ion survives drift: IP = 10.05 eV

– Ba++ ion arrives at HV plane, well ahead of all other Xe+ ions in local segment of track

– Ba++ ion liberates at least one electron at cathode surface, which drifts back to anode

– arriving electron signal serves as “echo” of the Ba++ ion, providing strong constraint

– Clustering effects are likely to alter this picture!


A small test chamber can show whether ion mobility differences persist at higher gas density (no data now).

This could offer an auto-matic method to tag the “birth” of barium in the decay, by sensing an echo pulse if the barium ion causes a secondary emission of one or more electrons at the cathode.


QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.


Molecular physics of xenon

– Ionization process creates regions of high ionization density in a very non-uniform way

– As density of xenon increases, aggregates form, with a localized quasi-conduction band

– Recombination is ~ complete in these regions• Complex multi-step processes exist:

• Xe+ + e– Xe* (or direct excitation)• Excimer formation: Xe*+ Xe Xe2* h + Xe

– Also: Xe*+ Xe* Xe** Xe++ e- + heat• Two excimers are consumed to make one photon!


QuickTime™ and aTIFF (LZW) decompressor

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Gamma events ()

Neutron events (NR)

Latest Xenon-10 results look better, but nuclear recoil acceptance still needs restriction

L og 1

0 S

2/S

1


“Intrinsic” energy resolutionfor HPXe ( < 0.55 g/cm3)

Q-value of 136Xe = 2480 KeVW = E per ion/electron pair = 22 eV (depends on E-field)N = number of ion pairs = Q/WN 2.48 x 106 eV/22 eV = 113,000

N2 = FN (F = Fano factor)

F = 0.13 - 0.17 for xenon gas N = (FN)1/2 ~ 130 electrons rms

E/E = 2.7 x 10-3 FWHM @ 2.48 MeV (intrinsic fluctuations only)

Compare: Ge diodes (energy per pair) √3/22 = 0.37 Compare: LXe/HPXe Fano factors: √20/.15 = 11.5 !


Pioneer HPXe TPC detector for 0- decay search

• “Gotthard tunnel TPC”– 5 bars, enriched 136Xe (3.3 kg)– MWPC readout plane, wires ganged for energy– No scintillation detection! no TPC start signal 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


Energy resolution issues in traditional gas detectors

• Main factors affecting ionization:– Intrinsic fluctuations in ionization yield

• Fano factor (partition of energy)

– Loss of signal• Recombination, impurities, grids, quenching,

– Avalanche gain fluctuations• Bad, but wires not as bad as one might imagine...

– Head-tail +ion effects corrupt the wire avalanche gain• Long tracks may really suffer from this in MWPC

– Electronic noise, signal processing, calibration• Extended tracks extended signals


Loss of signal

Fluctuations in collection efficiency introduce another factor: L

L = 1 - similar to Fano factor (assume uncorrelated errors)

N 2 = (F + L)N

– Loss on grids is small: Lgrid < F seems reasonable• If Lgrid = 5%, then E/E = ~3 x 10-3 FWHM

– Other sources of L include:• Electronegative impurities that capture electrons (bad correlations)• Escape to edges • Quenching - of both ionization and scintillation!

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*


A surprising result: adding a tiny amount of simple molecules - (CH4, N2, H2 ) quenches both ionization and scintillation ( particle)

dE/dx is very high; does this effect depend on too? (yes...) Impact for atomic recoils?…

Gotthard TPC: 4% CH4

how much ionization for particles was lost?

K. N. Pushkin et al, IEEE Nuclear Science Symposium proceedings 2004

(~25 bars)

particles


H. E. Palmer & L. A. Braby

Nucl. Inst. & Meth. 116 (1974) 587-589


From this spectrum: G ~0.19


Fluctuations in PS

• G for PS contains three terms:• Fluctuations in nuv (UV photons per e): uv = 1/√nuv

– nuv ~ HV/E = 6600/10 eV ~ 660

• Fluctuations in npe (detected photons/e): pe = 1/√npe

– npe ~ solid angle x QE x nuv x 0.5 = 0.1 x 0.25 x 660 x 0.5 ~ 8

• Fluctuations in PMT single PE response: pmt ~ 0.6

G = 2 = 1/(nuv) + (1 + 2pmt)/npe) ~ 0.17

Assume G + L = F, then

Ideal energy resolution (2 = (F + G + L) x E/W):

E/E ~4 x 10-3 FWHM


Some Issues:– Operation of PS at 20 bar? should work...

– Maintain E/P, but surface fields larger, gaps smaller…

– Large diffusion in pure xenon tracking good enough? (I think so… needs study)

– Integration of signal over area? (… needs study)– are pixels on the track edge adding signal or noise?

– Role of additives such as: H2 N2 CH4 CF4 Ne?– molecular additives reduce diffusion, increase mobility, but

– Do molecular additives quench signals? (atomic recoils?)


Surface/volume

• For given mass M, HPXe detector has ~9 x more surface area than LXe…

backgrounds 9x larger ?

– Will background rejection be >>9x?

combined wimp and 0- decay searches with high-pressure 136 xe gas tpc dave nygren lbnl

Documents

dusel workshop

xe e xe

xe ions

xe mobilities

h xe

xe ionization signal

xe e heat

compromised slide