Combined WIMP and 0-  decay searches with High-pressure 136 Xe Gas TPC Dave Nygren LBNL

DUSEL Workshop Energy partitioning in Xe(  ) Ionization signal only

DUSEL Workshop 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

DUSEL Workshop  Strategy: Use High pressure xenon gas TPC –  ~20 bars (  = ~0.1 gm/cm 3 ) –  E/E (I) is intrinsic:  =√FN F = 0.15 –  E/E = 2.7 x MeV  2 = (F + G + L) x E/W: (NIM A 581 (2007) ) –Best energy resolution in practice: Gas proportional scintillation: G ~0.2 Losses: L <0.05  E/E ~4 x MeV

DUSEL Workshop Combined Search: Issues 0-  –Energy: 2.5 MeV –Resolution: E total  E/E <1% FWHM Ionization signal only –Primary Scintillation: TPC start signal Modest sensitivity –Background:  -rays WIMP –5 < E recoil < ~50 keV –Resolution: S 2 /S 1 N/  Discrimination I + S gives energy –Primary Scintillation: TPC start signal Maximum sensitivity –Backgrounds  -rays neutrons

DUSEL Workshop Scintillation Dynamic range: OK –S 1 WIMP signals are very small Optimize detector for maximum S 1 detectiion efficiency Not a problem if S 1 signals from  events saturate PMT –S 2 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…

DUSEL Workshop S 2 /s 1 Fluctuations… –S 2 /S 1 resolution in LXe degraded by anomalous large fluctuations in LXe dominant effect in LXe sensitivity compromised in LXe –S 2 /S 1 (  < 0.5 g/cm 3 ) in HPXe: anomalous fluctuations are surely absent for  particles; very little recombination  S 2 /S 1 should be “large” –S 2 /S 1 for nuclear recoils in HPXe is unmeasured Careful, systematic measurements need to be done N/  discrimination might be (much) better in HPXe

DUSEL Workshop Now: 7-PMT 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

DUSEL Workshop

DUSEL Workshop Beppo-SAX Gas Proportional Counter

DUSEL Workshop 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

DUSEL Workshop Time scales 7-PMT TAMU system: now  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, –1000 kg system also feasible

DUSEL Workshop kg Xe:  = 225 cm, L =225 cm  ~ 0.1 g/cm 3 (~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

DUSEL Workshop Two identical HPXe TPCs Two distinct physics goals “  Detector” –Fill with enriched Xe mainly 136 Xe –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: 129 Xe 131 Xe –WIMP events include more axial vector interactions

DUSEL Workshop

DUSEL Workshop 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 >> r atom glancing collisions = back-scatter RCE increases viscosity of majority ions

DUSEL Workshop Barium daughter tagging and ion mobilities… –Ba ++ ion survives drift: IP = 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!

DUSEL Workshop 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. 

DUSEL Workshop

DUSEL Workshop

DUSEL Workshop

DUSEL Workshop

DUSEL Workshop 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  Xe 2 *  h + Xe –Also: Xe * + Xe *  Xe **  Xe + + e - + heat Two excimers are consumed to make one photon!

DUSEL Workshop Gamma events (  ) Neutron events (NR) Latest Xenon-10 results look better, but nuclear recoil acceptance still needs restriction Log10 S2/S1

DUSEL Workshop “Intrinsic” energy resolution for HPXe (  < 0.55 g/cm 3 ) Q-value of 136 Xe = 2480 KeV W =  E per ion/electron pair = 22 eV (depends on E-field) N = number of ion pairs = Q/W N  2.48 x 10 6 eV/22 eV = 113,000  N 2 = FN (F = Fano factor) F = for xenon gas   N = (FN) 1/2 ~ 130 electrons rms  E/E = 2.7 x MeV (intrinsic fluctuations only) Compare: Ge diodes (energy per pair) √3/22 = 0.37 Compare: LXe/HPXe Fano factors: √20/.15 = 11.5 !

DUSEL Workshop Pioneer HPXe TPC detector for 0-  decay search “Gotthard tunnel TPC” –5 bars, enriched 136 Xe (3.3 kg) –MWPC readout plane, wires ganged for energy –No scintillation detection!  no TPC start signal –  E/E ~ 80 x FWHM (1592 keV)  66 x FWHM (2480 keV) Reasons for this less-than-optimum resolution are not clear

DUSEL Workshop 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

DUSEL Workshop 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: L grid < F seems reasonable If L grid = 5%, then  E/E = ~3 x 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 Xe 2 *, Xe**, Xe 2 * + … ) Xe + + e – (hot) + M  Xe + + e – (cold) + M *  Xe + + e – (cold) + M + heat  e – (cold) + Xe +  Xe*

DUSEL Workshop A surprising result: adding a tiny amount of simple molecules - (CH 4, N 2, H 2 ) 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% CH 4 how much ionization for  particles was lost? K. N. Pushkin et al, IEEE Nuclear Science Symposium proceedings 2004 ( ~25 bars )  particles

DUSEL Workshop H. E. Palmer & L. A. Braby Nucl. Inst. & Meth. 116 (1974) 

DUSEL Workshop From this spectrum: G ~0.19

DUSEL Workshop Fluctuations in PS G for PS contains three terms: Fluctuations in n uv (UV photons per e):  uv = 1/√n uv –n uv ~ HV/E  = 6600/10 eV ~ 660 Fluctuations in n pe (detected photons/e):  pe = 1/√n pe –n pe ~ solid angle x QE x n uv 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/(n uv ) + (1 +  2 pmt )/n pe ) ~ 0.17 Assume G + L = F, then Ideal energy resolution (  2 = (F + G + L) x E/W) :  E/E ~4 x FWHM

DUSEL Workshop

DUSEL Workshop 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: H 2 N 2 CH 4 CF 4 Ne? –molecular additives reduce diffusion, increase mobility, but –Do molecular additives quench  signals? (atomic recoils?)

DUSEL Workshop Surface/volume For given mass M, HPXe detector has ~9 x more surface area than LXe…  backgrounds 9x larger ? –Will background rejection be >>9x?