0- ν Double-beta Decay, WIMPs, and Xenon: Will the Search Converge? David Nygren LBNL / Stockholm U
Conclusion A high-pressure xenon gas TPC can offer superior performance to LXe for both direct WIMP search and 0- ν ββ search simultaneously How can this be? Neutrino 2008 2
0- ν ββ ββ : TPC Performance Goals 1. Rejection at 100.0% level of charged backgrounds from surfaces, based on 100.0% active , 100.0% closed , deadtime-free virtual fiducial surface 2. Rejection of γ backgrounds using topological discriminants, to the highest possible level 3. Attractive scaling behavior to at least 1 ton 4. Energy resolution at 2480 keV: δ E/E <1% FWHM Neutrino 2008 3
“Conventional” TPC Geometry -HV plane Readout plane B Readout plane A Fiducial Surface . Fiducial Surface ions Neutrino 2008 4
Topology: spaghetti, with meatballs ββ events: 2 γ events: 1 Gotthard TPC: ~ x30 rejection Slide: J J Gomez - NEXT project 5
Scaling: 1000 kg Xe: ρ ~ 0.1 g/cm 3 (~20 bars) ∅ = 225 cm, L =125 cm A. Sensitive volume B. HV cathode plane C. Readout planes L D. Flange for services E. Filler and neutron absorber, polyethylene, … F. Field cages and HV insulator, (rings are exaggerated here) Neutrino 2008 6
Energy resolution: 1% FWHM, or better! δ E/E: 1% FWHM Exposure: 0.5 ton-year m ββ = 60 meV Slide: J J Gomez - NEXT project
WIMP: TPC Performance Goals 1. Recoil energy range: 0 < E recoil < 50 keV Lowest possible threshold: E min < 3 keV ? 2. Maximum rejection of γ backgrounds: Ionization/scintillation (S 2 /S 1 ) ratios are quite different for nuclear and electron recoils What do the data show? Neutrino 2008 8
Xenon10 Neutrino 2008 9 Slide: Elena Aprile
Gamma events (e - R) Why do the γ events Log 10 S2/S1 show large S 2 /S 1 fluctuations at all energies, but nuclear recoil events do not? Neutron events (N - R) Neutrino 2008 10
For electrons in LXe, fluctuations in energy partitioning are large ionization ⇔ scintillation a. Landau fluctuations ( δ -rays) lead to a highly non-gaussian distribution of ionization + b. High atomic density in LXe, and the existence of a conduction band ⇒ exaggerated fluctuations in recombination Slow-moving nuclear recoils do not create δ -rays Neutrino 2008 11
Strong anti-correlations in LXe are due to anomalously large fluctuations in energy partition between ionization and scintillation 1 kV/cm Bi-207 source ~570 keV keV ~570 EXO data EXO: predicted energy resolution: ~3.4 % FWHM @ Q( ββ ) Slide: Giorgio Gratta - EXO 12
Big Impact for WIMP Search in LXe! Scintillation (S 1 ) & Ionization (S 2 ) are the signals used to reject electron recoils: S 2 /S 1 But, in LXe, since S 2 and S 1 fluctuations are anti-correlated and anomalously large Bad news for discrimination power in LXe! Neutrino 2008 13
Gamma events (e-R) The large fluctuations Log 10 S2/S1 and anti-correlation of S 2 /S 1 leads to a reduced efficiency for nuclear recoil events Neutron events (N-R) Neutrino 2008 14
Xenon: Strong dependence of energy resolution on density! Ionization signal only For ρ >0.55 g/cm 3 , energy resolution deteriorates rapidly Neutrino 2008 15
Xenon: Strong dependence of energy resolution on density! Bad! Ionization Here, the signal only fluctuations are normal For ρ <0.55 g/cm 3 , ionization energy resolution is “intrinsic” Neutrino 2008 16
Density ρ < 0.55 g/cm 3 • Anomalous fluctuations are absent • S 2 /S 1 has normal fluctuations, much better γ rejection • Good news for direct recoil WIMP search ! • Energy resolution is “intrinsic” • What is intrinsic resolution, and how do I get it? • δ E/E = σ N /N x 2.35 (FWHM) σ N =(FN) 1/2 • F is the Fano factor, a constraint on fluctuations • N is the number of electron-ion pairs: N = E/W • Intrinsic resolution from ionization signal only • I don’t need to measure primary scintillation precisely • Good news for 0- ν ββ ββ search ! Neutrino 2008 17
“Intrinsic” Energy Resolution for Ionization at 136 Xe Q-value Q-value ( 136 Xe → 136 Ba) = 2480 KeV W = Δ E per ion/electron pair in xenon gas = 21.9 eV, but W depends on Electric field strength, might be ~24.8 eV N = number of ion pairs = Q/W N = 2480 x 10 3 eV/24.8 eV = ~ 100,000 electron/ion pairs σ N = (FN) 1/2 F is the Fano factor - constraint on fluctuations F = 0.13 - 0.17 (measured for xenon gas) - take F = 0.15 σ N = (FN) 1/2 ~ 124 electrons rms @ 2480 keV 124: This is a rather small number! Neutrino 2008 18
“Intrinsic” Energy Resolution for Ionization at 136 Xe Q-value δ E/E = 2.35 x (FW/Q) 1/2 δ E/E ~2.8 x 10 -3 FWHM @ 2480 keV (xenon gas - ionization intrinsic fluctuations only) Germanium diodes, in practice , δ E/E = 1 - 2.4 x 10 -3 FWHM ⇒ δ E/E ≥ 33 x 10 -3 FWHM Fano factor for LXe: ~20 Neutrino 2008 19
High-pressure xenon gas TPC • Fiducial volume surface: – Single, continuous, fully active, variable,... – 100.000% rejection of charged particles (surfaces) – but: TPC needs t 0 to place event in z coordinate • Excellent tracking capability: – Available in gas phase only! – Topological discrimination against single electrons (meatballs) – X-ray fluorescence can tag γ photo-conversion events • Can HPXe TPC deliver the energy resolution? Neutrino 2008 20
Generalization • If fluctuations are uncorrelated, then σ N = ((F + L + G)N) 1/2 F = Fano factor = 0.15 L = loss of primary ionization (set to 0) G = fluctuations & noise in gain process Goal: Make sure that G is smaller than F If F = 0.15, Is this possible ?? Neutrino 2008 21
Avalanche Charge Gain • Avalanche: exponential amplification Early fluctuations are amplified exponentially 0.6 < G < 0.9 * • for wire (E ~1/r) • σ N = ((0.15 + 0.8)N) 1/2 = 328 • δ E/E = ~7.0 x 10 -3 FWHM The benefit from a small Fano factor is lost! Micromegas should do better, but, in general… Avalanche devices can’t deliver G < F! * Alkhazov G D Nucl. Inst. & Meth. 89 ( 1970) 155 (for cylindrical proportional counters) Neutrino 2008 22
HPXe TPC: G < F ? • Answer: Yes, with electroluminescence! δ E/E = 4 x 10 -3 FWHM (in principle, @ 2480 keV) Why Electroluminescence? Amplification is linear with Voltage Fluctuations are very small • With EL, it is possible to realize G = ~0.1 • Each electron is counted with ~10% precision Neutrino 2008 23
Electroluminescence (EL) H. E. Palmer & L. A. Braby Nucl. Inst. & Meth. 116 (1974) 587-589 Neutrino 2008 24
55 Fe Resolution: 8.4 % FWHM Neutrino 2008 25
55 Fe Resolution: 8.4 % FWHM From this spectrum: G ~0.19 Neutrino 2008 26
Fluctuations in EL G for EL contains three terms: Fluctuations in n uv (UV photons per e): σ uv = K / √ n uv 1. – n uv ~ HV/E γ = 6600/10 eV ~ 660 K < 1 ? Fluctuations in n pe (detected photons/e): σ pe = 1/ √ n pe 2. – n pe ~ solid angle x QE x WLS x n uv = 0.1 x 0.25 x 0.5 x 660 ~ 8 3. Fluctuations in PMT single PE response: σ pmt ~ 0.5 G = σ 2 = 1 /(n uv ) + (1 + σ 2 pmt )/ n pe ) The more photo-electrons, the better! Neutrino 2008 27
Fluctuations in EL G for EL contains three terms: Fluctuations in n uv (UV photons per e): σ uv = K / √ n uv 1. – n uv ~ HV/E γ = 6600/10 eV ~ 660 K < 1 ? Fluctuations in n pe (detected photons/e): σ pe = 1/ √ n pe 2. – n pe ~ solid angle x QE x WLS x n uv = 0.1 x 0.25 x 0.5 x 660 ~ 8 3. Fluctuations in PMT single PE response: σ pmt ~ 0.5 G = σ 2 = 1 /(660) + (1 + σ 2 pmt )/8) ~ 0.19 The more photo-electrons, the better! Neutrino 2008 28
Beppo-SAX satellite: a HPXe TPC in space!
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Electro-Luminescent Readout • To keep G < F = 0.15, then: G = (1 + 0.5)/n pe n pe > 10/electron For 2480 keV, N e = 1 x 10 5 ⇒ Σ n pe > 1,000,000 Neutrino 2008 32
Electro-Luminescent Readout How to detect this much signal? Readout planes are PMT arrays Answer : Use both TPC readout planes – If EL signal is generated in plane “A” – do “tracking” in Plane “A” – but: record “energy” in plane “B” Neutrino 2008 33
Electro-Luminescent Readout How to detect this much signal? Readout planes are PMT arrays Answer : Use both TPC readout planes – If EL signal is generated in plane “B” – do “tracking” in Plane “B” – but: record “energy” in plane “A” Neutrino 2008 34
TPC Signal Transparent -HV plane Readout plane B Readout plane A record EL signal energy . created here signal here… • ions Signal: WIMP or ββ event Neutrino 2008 35
How to generate EL @ 20 bars? • Answer: multi-wire plane with ~5 mm pitch – Optical gain of 300, but no charge gain – Wire diameter: 0.2 - 0.3 mm - robust! – Luminous region: <1 mm – 1/r field: negligible degradation of resolution! – G ~0.08 may be possible… δ E/E < 4 x 10 -3 FWHM @ 2480keV Neutrino 2008 36
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