Shingo Kazama (Nagoya University, KMI) on behalf of the XENON collaboration @TAUP2019, September 11th 2019 The XENONnT Neutron Veto Detector
This Talk! > 2500 SLPM GXe) GXe purification (120 SLPM) neutron-induced background To tag and measure in situ the detector 222 Rn emanated inside To online remove the large LXe volume (5L/min LXe, 10 times higher sensitivity compared to XENON1T To achieve fast cleaning of the Chamber ~6t Time Projection Neutron Veto Radon Distillation LXe Purification New TPC with 20 t-year exposure, reaching spin-independent � 2 XENONnT Experiment Condenser Distillation stages Piston pump Reboiler Lique fj er XENON10 (2008) 10 − 43 WIMP-nucleon σ SI [cm 2 ] 10 − 44 XENON100 (2016) 10 − 45 PandaX-II (2017) ) 1 7 0 2 ) ( k X r U o L w s i h 10 − 46 t , r y t × 1 ( T 1 WIMP-nucleon cross-section of O(1) ☓ 10- 48 cm 2 N O N E X ) n o i c t e o j 10 − 47 r P r a e y t 0 2 ( T n N O N E X t i m l i y r e v o c s i d 10 − 48 o n i r u t e n , 3 1 0 2 d r a l l i B Bagnaschi 2017 10 − 49 10 1 10 2 10 3 WIMP mass [GeV/c 2 ]
Rn is removed as planned (~1μBG/kg) 1.3±0.3 neutron events / yr in [4-50] keVr in 4 t FV without neutron veto For 20 t-year exposure, ~ 6.5 events! aim at neutron tagging efficiency > 80% neutron generator) Active neutron veto preliminary � 3 Neutron Background @ XENONnT ‣ Neutron events become the one of main BGs once ‣ PRELIMINARY estimate: See poster by D.Ramirez on NR BG@ XENONnT ‣ For the best dark matter discovery potential, ‣ Helps modeling neutron background ‣ Can check efficiency with 241 AmBe (and
* Top part will also be covered by reflectors! will be installed to detect Cherenkov photons (in total ~ 700 kg) Cherenkov detector for the muon veto from the outside of the water tank from EGADS/SK-Gd experiment in Kamioka collection efficiency and to separate optical BGs coverage & light collection efficiency with highly reflective foil. - digitized with CAEN v1730 (500MHz) � 4 Neutron Veto (nVeto) System @ XENONnT ‣ XENON1T water tank (700 t) already works as a ‣ Gd-loaded water Cherenkov detector technology ‣ Build an inner neutron veto detector with higher PMT ‣ Octagonal support structure made of SUS304L ‣ 120 8” PMTs (Hamamatsu R5912-100WA-D30) ‣ Highly reflective foil will be used to increase light
~2m n Coverage < 10% 7.6×10 5 difference compared with 1H(333 mb) PMTs Decays via gamma-cascade n-capture (0.2% Gd concentration) ~ 20μs Delay time between single scatter and γ-ray Diffuse reflector Cryostat thermalization n+155Gd→156Gd*→156Gd+γ’s n+157Gd→158Gd*→158Gd+γ’s 155Gd (14.80%): 61 kb, 8.5 MeV γ-rays 157Gd (15.65%): 254 kb, 7.9 MeV γ-rays n-capture Single-scatter � 5 nVeto Working Principle
n Single-scatter Cryostat Diffuse reflector electron Cherenkov photons PMTs thermalization n-capture γ-ray Coverage < 10% Cryostat is also covered with the reflector ~2m � 6 nVeto Working Principle
- Large PMTs with high quantum efficiency and low radioactivity - Highly reflective foil - High transparency of Gd-loaded water Requirements to nVeto@XENONnT PMTs Coverage < 10% � 7 nVeto Working Principle: Summary & Requirements ‣ High light collection efficiency
- Similar water purification system as in EGADS - 120 8” PMTs: R5912-100WA-D30 (QE~33%@380nm) inside the water tank (soak-test in Gd-water) dissolved when octahydrated: Gd 2 (S0 4 ) 3 ・8H 2 0 - Careful checks of all the detector components used - ePTFE reflector with ~99% reflectivity (details later) - > 90% of neutrons can be captured by Gd - Others are captured by H (2.2 MeV) - 3.4 t of Gd-sulfate octahydrate is necessary for 700t of the water tank - ~1Bq/PMT of 232Th, 238U, 40K radioactivity � 8 nVeto System @ XENONnT ‣ Gd is insoluble in water, but Gd sulfate can be ‣ 0.2% Gd-concentration by weight ‣ Large PMTs with high quantum efficiency and low arXiv:1908.11532 ‣ High reflective foil ‣ High transparency of Gd-loaded water
Key point: injecting Gd-sulfate while keeping good water transparency Gd-loaded water Cherenkov detector: developed by SK-Gd / EGADS to detect the supernova relic ν Lessons learned from the EGADS experiment (arXiv:1908.11532) - Gd sulfate is essentially transparent to Cherenkov light. 200 t water tank � 9 SK-Gd / EGDAS Techniques arXiv:1908.11532 ‣ After full Gd-loading (0.2%), the water transparency is within typical SK values ‣ Water filtration system can maintain good water quality (will use similar system for nT) ‣ No Gd-losses are found after more than two years operation ‣ Gd sulfate quickly dissolves and is homogeneously distributed throughout the detector ‣
preliminary > 60% tagging efficiency with 10-fold γ-ray spectrum after nCapture preliminary collaboration already implemented in our MC collaboration already implemented in our MC - 0% (pure water), 0.02%, 0.2%(nT target) γ-ray emission model concentration, we can achieve > 80% tagging efficiency PTEP 023D01 tagging efficiency with 10-fold coincidence preliminary preliminary coincidence requirement - 0% (pure water), 0.02%, 0.2%(nT target) � 10 Simulation Results: γ -ray emission model ‣ γ-ray emission model developed by ANNRI-Gd ‣ γ-ray emission model developed by ANNRI-Gd Counts [1/10 keV] 5 10 ‣ Three different cases are considered in MC 4 10 ‣ Three different cases are considered in MC 3 10 ‣ With 10-fold coincidence requirement and 0.2% 2 10 Data GLG4sim 10 ‣ Even without Gd (pure water case), can achieve Our model ‣ Even without Gd (pure water case), can achieve > 60% 1 0 1000 2000 3000 4000 5000 6000 7000 8000 Energy [keV]
Δt (TPC, nVeto) preliminary captured by hydrogen. longer for neutrons to be thermalized and then coincidence window up to 500 μs because it takes coincidence window down to 150 μs 0.94 ~1 ~1 500μs 0.84 0.98 ~1 300μs 0.62 0.85 Coincidence window 0.2% 0.02% 0% Fraction of the events vetoed 100μs 0.98 0.72 0.48 150μs 0.99 � 11 Simulation Results: Timing Information ‣ For 0.2% case, it may be possible to decrease the ‣ For the pure water case, need to increase the
plays an important role for achieving higher tagging efficiency because of many reflections. - Also it is radio-pure: U(~20ppt), Th(~30ppt) with ICP-MS test in Gd-water (0.2% concentration) preliminary Many Reflections! Before soak-test After 2-month soak-test � 12 Simulation Results: Reflectivity Dependence ePTFE reflector (1.5mm thickness) ‣ Photo-coverage of the PMTs is < 10%, thus reflectivity ‣ ePTFE is selected because of its high reflectivity: ~99% ‣ No degradation of reflectivity has been found after soak-
preliminary - preliminary preliminary events is estimated to be ~ 1event to 0.17±0.05 events/year 4t FV, e-lifetime of 1ms, drift field of 200 V/cm 10-fold coincidence,150μs window (TPC, nVeto), - with nVeto NR BG rate without nVeto � 13 Simulation Results: NR BG Rate with/without nVeto ‣ Target: 0.2% Gd concentration, and ‣ With nVeto, neutron BG can be reduced from 1.3 ‣ For 20 t-year target exposure, the # of neutron BG See poster by D.Ramirez on NR BG@ XENONnT
238U PMT Component Window [Bq/PMT] 232Th [Bq/PMT] 0.4 [Bq/PMT] 40K Body < 0.6 events is ~ 1.5 %. 1.1 be ~100Hz. 0.9 - measured with HP-Ge detector@LNGS PMTs 1.3 much BG rate 0.6 � 14 Simulation Results: Fake BG Rate in nVeto ‣ We may discard many TPC events if nVeto has too nVeto PMTs radioactivity ‣ Main contribution is from radioactive impurities in ‣ For 10-fold coincidence case, BG rate is expected to * 120 PMTs will be used ‣ With 150μs coincidence window, the loss of TPC
TPC provides trigger or delay (γ + n) - - In 57.5 %, a coincident γ-ray with an energy of 4.438MeV - Average neutron energy 4.6 MeV - Use AmBe for the calibration of the nVeto - Calibration of the TPC nVeto provides trigger Measure multiple-scattering / single-scattering neutrons � 15 Calibration • ▫ ▫ γ an ▫ γ n) nVETO: neutron tagging efficiency TPC: nuclear recoil response calibration • ‣ ▫ ▫ ‣ ‣
the dark matter run with the fully efficient nVeto. - None of them showed a bad behavior. purification plant at the beginning of 2020, then we will start sulphate at LNGS. The goal is to install the Gd-Water in the nVeto. construction, and it will start by the end of this year. - We have tested 125 8” R5912 PMTs at LNGS high transparent Gd-load water - requires high photo-coverage, highly reflective foils and technology established by SK-Gd / EGADS � 16 Summary & Outlook ‣ nVeto is essential for the dark matter discovery ‣ XENONnT nVeto will be based on Gd-loaded Water Cherenkov ‣ Neutron tagging efficiency >= 85 % is possible ‣ XENONnT nVeto design is almost finalized ‣ Installation of the nVeto will be the last step of XENONnT ‣ Then, we will start XENONnT commissioning with pure water ‣ In parallel, we are working to get the permission for Gd-
Back Up
� 18 Water Purification System arXiv:1908.11532 Figure 7: Schematic view of the band-pass system and the fast recirculation system (inside dashed line). These systems were built in cooperation with the South Coast Water company.
� 20 Geometry
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