TWO-PHASE DETECTORS USING THE NOBLE LIQUID XENON Henrique Araújo Imperial College London Rutherford Appleton Laboratory – 27th September, 2017 OUTLINE • Liquid xenon response(s) to radiation • Physics underlying the detector response • Two-phase xenon for (dark) radiation detection • Instrumenting a liquid xenon target for low background and low energy threshold • I will not talk much about dark matter searches. This technology has other applications in low energy particle physics H. Araújo (Imperial) 2 TWO-PHASE XE DETECTOR / LXe-TPC ANODE ~1 keV e- GATE CATHODE Vadim Lebedenko 1939H. Araújo – 2008 (Imperial) 3 TWO-PHASE XE DETECTOR / LXe-TPC • S1: Prompt scintillation • Light yield: >60 ph/keV (0 field) • Scintillation light: 175 nm (VUV) • Nuclear recoil threshold ~3-6 keV • S2: Ionisation via electroluminescence • Sensitive to single ionisation electrons • S1+S2 > vertex reconstruction, discrimination • Nuclear recoil threshold <1 keV • WIMP target • Dense medium (3 g/cm3), high Z (54), high A (131) • Spin independent WIMP-nucleon scattering rate dR/dE~A2 • Odd-neutron isotopes (129Xe, 131Xe) enable spin-dependent sensitivity • No intrinsic backgrounds (127Xe, 129m/131mXe, 136Xe subdominant) • Others dispersible backgrounds can be controlled (85Kr, 222/220Rn) H. Araújo (Imperial) 4 THE LUX EXPERIMENT Technical papers: arXiv:1512.03133, arXiv:1608.05381 arXiv:1610.02076, arXiv:1610.02076 250 kg active mass arXiv:1708.02566, arXiv:1709.00095 ~100 kg fiducial arXiv:1709.00800 H. Araújo (Imperial) 5 Conceptual Design Report arXiv:1509.02910 LUX-ZEPLIN (LZ) Technical Design Report arXiv:1703.09144 7,000 kg active mass ~5,600 kg fiducial H. Araújo (Imperial) lz.ac.uk6 WIMP-NUCLEUS ELASTIC SCATTERING The ‘spherical cow’ galactic model • DM halo is 3-dimensional, stationary, with no lumps • Isothermal sphere with density profile ∝ r −2 3 • Local density 0 ~ 0.3 GeV/cm (~1/pint for 100 GeV WIMPs) Maxwellian (truncated) velocity distribution, f(v) • Characteristic velocity v0=220 km/s • Escape velocity vesc=544 km/s • Earth velocity vE=230 km/s Nuclear recoil energy spectrum [events/kg/day/keV] dR vmax f (v) 0 A F 2 (q) d 3v 2 v min dER 2m A v dR R0 ER / E0r 4mW mT e , r 2 1 ~ few keV dER E0r (mW mT ) H. Araújo (Imperial) 7 XENON AS A WIMP TARGET A p Xenon Isotopes Z T1/2 or % J 122 + 9 ‘stable’ Xe 20 h 0 123Xe 2.1 h (1/2)+ 2nbb decays 124Xe 0.10% 0+ Short-lived 125Xe 17 h (1/2)+ High atomic mass (SI XS) 126 + Odd-neutron isotopes (SD XS) Xe 0.09% 0 127 + 4 Xe 36 d (1/2) M=50 GeV/c2 128Xe 1.91% 0+ -11 -47 2 129 + 3 ,SI = 10 pb (10 cm ) Xe 26.4% (1/2) 130Xe 4.1% 0+ 131 + 2 Xe Xe 21.2% (3/2) Ge 132Xe 26.9% 0+ 133 + 1 Ar Xe 5.2 d (3/2) 134Xe 10.4% 0+ integralrate, counts/tonne/year 135 + 0 Xe 9.1 h (3/2) 0 10 20 30 40 50 136Xe 8.9 % (2.2×1021 y) 0+ threshold recoil energy, keV H. Araújo (Imperial) 8 SIGNALS AND BACKGROUNDS • WIMP signal: elastic scattering producing keV-scale nuclear recoils (NR); primary recoil leads to small atomic ‘cascade’ • NRs also from neutrons and neutrinos (n-A) • Background particles interact mostly with atomic electrons producing keV-scale electron recoils (ER); longer, well defined ‘tracks’ • ERs also from gammas, betas, neutrinos (n-e) • Also some signal models NR • Our goal: To detect efficiently, and to discriminate between, NR and ER interactions in as large a detector as possible Chepel & HA, 2013 JINST 8 R04001 H. Araújo (Imperial) 9 THE EXPERIMENTAL CHALLENGE • Low-energy particle detection is easy ;) E.g. Microcalorimetry with Superconducting TES Detection of keV particles or photons with eV FWHM • Rare event searches are also easy ;) E.g. Super-Kamiokande contains 50 kT water Cut to ~20 kT fiducial mass: huge self-shielding effect • But doing both is hard! Small is better for collecting signal Large is better to shield backgrounds • And there is no trigger… H. Araújo (Imperial) 10 KEY DETECTOR REQUIREMENTS The success of the two-phase xenon relies on three technical aspects: • Low energy threshold (mostly S1) • Detectable WIMP scattering rate • Scintillation yield, VUV reflectivities, VUV QE, PMTs in the liquid phase • Accurate 3D imaging (mostly S2) • Background reduction (self-shielding) • Electroluminescence yield, electric fields, electron lifetime, PMTs in the gas phase • ER/NR discrimination (S1 + S2) • Background reduction (NR selection) • High yields, uniform detector, no spurious event topologies H. Araújo (Imperial) 11 100 LXE SELF-SHIELDING 10 • Self-shielding of external backgrounds 1 • Interesting challenge for calibration… Elastic mean interaction length, cm length, interactionmean neutrons in LXe (131Xe) Total 0.1 0.01 0.1 1 10 single scatters neutron energy, MeV <5 keVee 100 Photoelectric Compton Pair production 10 Total 1 mean interaction length, cm length, interactionmean gammas in LXe 0.1 0.01 0.1 1 10 H. Araújo (Imperial) photon energy, MeV 12 LXE RESPONSE TO RADIATION • LXe-TPC response generated from the energy transferred to the ‘electronic system’ of the target medium, for both ER and NR (atomic excitation & ionisation): • Most energy for ER, only fraction (퓛) for NR – the rest is lost as heat • Electron-ion recombination is a key parameter (퓛) 풏풆풙, 풏풊 S2 풓 S1 H. Araújo (Imperial) 13 LXE RESPONSE TO RADIATION • Recently, we have come to appreciate that the basics needed to understand the detector response are simpler than we thought • For electron recoils: 푬 = 푾 ⋅ 풏풆풙 + 풏풊 = 푾 ⋅ 풏휸 + 풏풆 −ퟏ • For nuclear recoils: 푬 = 퓛 푾 ⋅ 풏휸 + 풏풆 • “W-value”: average energy per quantum of response (excitation or electron-ion pair) – common & constant value works for LXe! 푾 = ퟏퟑ. ퟕퟎ. ퟐ 퐞퐕 ER H. Araújo (Imperial) 14 WHAT IS REQUIRED FOR A JOB WELL DONE • Understand thy detector • S1 (photon) detection • S1 response quantum (phe/phd), light collection efficiency, absolute S1 gain (g1) in detected photons per emitted photon • S2 (electron) detection • S2 response quantum, charge & light collection efficiencies, absolute S2 gain (g2) in detected photons per drifting electron • Understand LXe physics • ER scintillation & ionisation yields • and fluctuations • NR scintillation & ionisation yields S1 S2 • and fluctuations H. Araújo (Imperial) 15 DETECTING PHOTONS (mostly S1) Quartz-windowed PMTs work well at 175 nm Resolved PMT backgrounds over the last decade Must calibrate VUV QE: sometimes 2 phe/photon… • Photon counting in PMT waveforms: #phe > #phd • Improved resolution & linearity arXiv:1506.08748 Preliminary 16 DETECTING PHOTONS (mostly S1) PTFE is a magical material • Very high VUV reflectance when immersed in LXe; very low background; low outgas, compatible with LXe; great electrical properties,… We got lucky. • Ex situ measurement of the LUX PTFE • Best fit: R=98.7%(diffuse) or 98.7% (diffuse+specular) (Neves, 1612.07965) 17 DETECTING ELECTRONS (S2) • Drifting electrons: requires field, right materials, purification • Cross-phase electron emission: requires high fields (>5 kV/cm) • Electroluminescence in the vapour phase provides high gain for ionisation channel – and transduces signal to VUV photons Electroluminescence yield Emission probability 18 DETECTING ELECTRONS (S2) • Single electron response: absolute calibration of S2 channel • Electroluminescence gain depends on several parameters which can vary during a long run (pressure, fields, liquid level,…) • SE signal can be tracked to % level during run • Sources of SE signals include VUV photoionisation of impurities Single electron pulse area distribution 19 POSITION RECONSTRUCTION (S2) arXiv:1112.1481 • Mercury algorithm (ZEPLIN-III, LUX) • Data-driven Light Response Functions (LRFs) for each top-array PMT • Simultaneous fit to all PMTs with signal 20 S1 & S2 PDEs – FLATFIELDING WITH 83mKr • X,Y,Z response calibration with dispersed 83mKr radioisotope • Routine injection, decays within detector, emitting 2 CEs (T1/2=1.86 hrs) 83mKr Kr-83m calibration source: Rb-83 infused into zeolite, located within xenon gas plumbing H. Araújo (Imperial) 21 S1 & S2 PDEs – FLATFIELDING WITH 83mKr =8.1% Flat-fielding light collection – and E field (f/ 12.3%) • g1 and g2 corrected to detector centre @41.5 keV • Field is non-uniform due to leakage through grids and other factors • Varying field light yield, drift speed,… H. Araújo (Imperial) 22 S1 & S2 ABSOLUTE GAINS: ER ‘DOKE PLOT’ • Mono-energetic ER sources in the mean-yields plane (5-662 keV) • Line fit and W-value = 13.7 eV give absolute number of quanta: ⟨S1c[phd]⟩=0.117±0.003·nph , ⟨S2c[phd]⟩ = 12.1±0.8·ne 23 PROOF OF THE PUDDING: 127Xe (127I) N X-RAYS • Detection of 186 eV n-shell x-ray, with ~12 ionisation electrons • (Running out of S1 at this point) 1709.00800 24 ER TRITIUM CALIBRATION (0–18 keV) • Injection of CH3T through gas system, removal through getter • Extensive R&D at UMD followed by tests in LUX using CH4 • Progressive injections of CH3T to confirm no residual background 25 ER TRITIUM CALIBRATION (0–18 keV) • Main CH3T calibration campaign: 180,000 events • Reconstruct spectrum using Doke Plot result • Constrain detection threshold, mean ER yields, and ER recombination fluctuations arXiv:1512.03133 26 ER TRITIUM CALIBRATION (0–18 keV) • Absolute ER light and charge yields for LXe (+ recombination) • Essential to advance modelling – NEST tool (Szydagis et al) • NEST is in turn needed to really understand WIMP sensitivity LXe ABSOLUTE SCINTILLATION YIELD (ER) LXe ABSOLUTE IONISATION YIELD (ER) 105 V/cm 180 V/cm 105 V/cm 180 V/cm 27 ER TRITIUM CALIBRATION (0–18 keV) • Recombination fraction and its fluctuations for electron recoils (S1 and S2 event by event) 28 RECOMBINATION