Low background challenges in the JUNO experiment Frédéric Perrot CENBG, University of Bordeaux FCPPL, Marseille, May 22th-25th, 2018

How to identify neutrinos from background events in JUNO ? Neutrino events expected in JUNO Main goal : determination of mass hierarchy at 3σ sensitivity in 6 years 105 events required → 20,000 tons of liquid scintillator number of photoelectrons ~1200 pe/MeV → large photon coverage (75%) and high liquid scintillator transparency (>20 m) Source Neutrino flux JUNO Detector Baseline : 53 km Reactor at 3 GWth ~ 1020 ν/s (26.6 GWth available in 2020) Only 60 neutrinos/day (or 0.0007 neutrino/s !!) How to identify neutrinos from background events in JUNO ? 19/3/2013

Electron antineutrino detection Electron antineutrinos detected by Inverse Beta Decay (IBD) : Energy spectrum Energy threshold: 1.8 MeV E(e+) = E(ν) - 0.8 MeV → visible energy Neutrino signature : Prompt signal from positron: ionization+annihilation in 2γ (1-10 MeV) Delayed signal from neutron: capture on 1H (2.2 MeV) Time correlation < 1 ms 19/3/2013

Origin of the backgrounds Natural radioactivity from the surrounding materials (U/Th/K). Random coincidence between 2 gammas mimicking an IBD → need to be well controlled by material radiopurity selection Accidental background Prompt-like Delayed-like 700 m overburden Top Tracker Water Pool Muon (3 Hz) Correlated background Volume vetoed around muon track for 9Li/8He rejection (β-neutron events) Events Muon-induced backgrounds Natural radioactivity IBD prompt energy Energy (MeV) 0.1 1 10 100 19/3/2013

Rate of accidental background Origin : natural radioactivity of the JUNO materials in 238U/232Th/40K Scenario : 2 events in coincidence that mimic the prompt and delayed signal γ Criteria for IBD identification: Prompt signal : 0.7 MeV < Ep < 12 MeV Delayed signal : 1.9 MeV < ED < 2.5 MeV Time coincidence ΔT < 1 ms Distance between two vertex < 1.5 m Number of events/day for E>0.7 MeV Rate of radioactivity events in the full active volume (20 ktons!) must be <100 Hz to have <1 event/day of accidental background ! 19/3/2013

Rate of accidental background vs FV cut

Natural radioactivity : some numbers Primordial radioactivity in Earth’s crust (responsible of geothermal power, geoneutrinos and … traces of radioactivity in all materials) Radionuclide Half-life (years) Natural abundance Concentration (mg/kg) Activity (Bq/kg) 40K 1.3*109 0.0117 % 2.8 720 238U (chain) 4.5*109 99.3% 3.0 40 232Th (chain) 14*109 100% 9.0 100m2 garden with 1m depth 0.5 kg of 238U ! Activity: 20 MBq ! Human body (70 kg) 0.2 mg of 40K Activity: 3500 Bq ! 19/3/2013

Radioactivity of the rock in JUNO Rock samples of the JUNO site has been measured both by IHEP and CENBG laboratories using low background gamma spectrometry. The results are consistent and have shown a high activity of the JUNO rock Radionuclide Earth’s crust (Bq/kg) JUNO rock 40K 720 1320 238U (chain) 40 120 232Th (chain) 106 Rock sample on a Ge detector at CENBG → dimensions of the Water Pool designed to have enough water thickness to attenuate the gamma flux from the rock to the liquid scintillator 19/3/2013

Radon from the JUNO rock Radon is a radioactive noble gas (T1/2~4 days) belonging to the 238U chain and the direct daughter nucleus from the radium 226 Already observed a long time ago… 1904 ! Radon diffusion 19/3/2013

Radon from the rock Risk of Radon diffusion in the water pool and then transportation of 222Rn near the acrylic sphere Requirement in water: Activity(222Rn) < 0.2 Bq/m3 (to be <15% of the accid. bckg) Radon diffusion 5mm PE liner has been chosen as a promising material to be tight for Rn diffusion from the walls to the water pool Samples of 2.5mm PE plates have been sent to CPPM in order to measure the induced Radon suppression factor (collab. With University of Avignon) 19/3/2013

Radon diffusion through PE liner Setup : 2 chambers separated by the sample to be measured Rn source Rn in air or water Rad 7 PE plate Cu gasket First test performed with Aluminum plate  Rn tight Second test with the 2.5mm HDPE liner  seems not to be Rn tight enough (>10% of Rn is diffusing through) Need to be confirmed with a new PE sample Sample Rn source (kBq/m3) Rn in water 2mm Al 470 0.025 2.5mm PE 1.44 19/3/2013

Material radioactivity: liquid scintillator LS requirements: 40K: 10-15 g/g 238U: 10-15 g/g 232Th: 10-15 g/g JUNO 20,000 tons A(40K)=5,4 Bq 0.070 ton A(40K)=3500 Bq << 19/3/2013

Material radioactivity: liquid scintillator LS requirements: 40K: 10-15 g/g 238U: 10-15 g/g 232Th: 10-15 g/g Critical material: no possibility to remove the background with fiducial volume cut → pilot plant at Daya Bay site to test the purification system with 20 tons of LS Strategies to control the radiopurity Neutron Activation Analysis (NAA) of small samples for K/U/Th OSIRIS pre-detector to control the U/Th radioactivity of the whole LS before JUNO filling 19/3/2013

Material radioactivity: acrylic sphere Acrylic requirements: 40K: 10-12 g/g 238U: 10-12 g/g 232Th: 10-12 g/g JUNO 570 tons A(40K)=150 Bq 0.070 ton A(40K)=3500 Bq << 19/3/2013

Material radioactivity: acrylic sphere Acrylic requirements: 40K: 10-12 g/g 238U: 10-12 g/g 232Th: 10-12 g/g Critical material in contact with the LS → radiopurity of acrylic samples measured by Neutron Activation Analysis at the required level (at Milano-Bicocca) Strategies to keep the radiopurity under control after each step of the acrylic panel production: Casting Thermoforming Machining Node bonding Annealing 19/3/2013

Material radioactivity: 20’’ PMT glass MCP-PMT (NNVT) : 13,000 PMTs Dynode-PMT (Hamamatsu) : 5,000 PMTs PMT glass bulb requirements: 40K < 2.0*10-9 g/g  < 0.5 Bq/kg 238U < 200*10-9 g/g  < 2.5 Bq/kg 232Th < 120*10-9 g/g  < 0.5 Bq/kg PMT glass bulb guaranteed: 40K < 40*10-9 g/g  < 10 Bq/kg 238U < 400*10-9 g/g  < 5 Bq/kg 232Th < 400*10-9 g/g  < 3.3 Bq/kg ~100 tons A(40K)~50,000 Bq 0.070 ton A(40K)=3500 Bq ~38 tons A(40K)~380,000 Bq 19/3/2013

Material radioactivity: 20’’ PMT glass Relative high activity of the 20’’ PMT glass in K/U/Th compared to other materials Solution found by JUNO: to put the PMTs in water at a distance of 1.8m from the LS sphere  water acts as a shielding for gamma rays emitted by PMTs Liquid Scintillator Ultrapure water 1.8 m Huge efforts done by IHEP colleagues to improve the radiopurity of the NNVT glass bulb on site by: Selecting a low activity glass Protecting the raw material during storage Cooling the glass with deionized water (40K free) Discharging the liquid glass from the bottom of glass-taking pool every week to avoid radioactivity isotope accumulation PMTs FV cut Glass sample measured every week by γ spectrometry Each sample exceeding the requirements is rejected → well-controlled production 19/3/2013

20’’ PMT glass: Radon issue ? Non negligible 226Ra (U) activity in PMT glass: risk of Radon emanation in water? 20’’PMTs (NNVT and Hamamatsu) measured at CENBG Bordeaux with a unique facility in the JUNO collaboration Large Radon emanation chamber 2 NNVT 20’’ PMTs with final glass Data consistent with background setup → emanation rate lower than 400 Rn at/s/PMT ! Contribution of 15,000 NNVT PMTs to Radon activity in water will be lower than 0.002 Bq/m3 → negligible compared to 0.2 Bq/m3 requirement 7.7 MeV alpha counts/day 19/3/2013

Other critical measurements achieved Characterization of the 3’’ PMT glass radioactivity from HZC company at IHEP and Bordeaux (low background gamma spectrometry and Radon emanation) → radiopurity level achieved for 25,000 x3’’ PMTs ! γ measurement of crushed glass Radon measurement Characterization of SS samples for the metallic truss in JUNO High sensitivity performed at Modane Underground Laboratory to reach the sensitivity level of ~1 mBq/kg → radiopurity level achieved for JUNO ! 19/3/2013

Conclusions The number of neutrino rate expected in JUNO (~60/days) required to have a high purity detector From MC simulations, background rate below 100 Hz in the full volume is needed to fulfill the physics requirements (mass hierarchy, …) Stringent constrains have been put on critical materials in JUNO such as LS, acrylic panels, PMTs, SS truss, ultrapure water… huge efforts ! A collaboration between IHEP and CENBG allowed to select and qualified the radioactivity of NNVT 20’’ PMTs and HZC 3’’ PMTs by low-background gamma spectrometry and Radon emanation measurements  requirements fulfilled for more than 45,000 PMTs ! Radon diffusion through HDPE liner is also under investigation at CPPM in order to validate the choosen material for Water Pool liner 200 µs 19/3/2013

Collaboration This program started in 2016 in the framework of FCPPL IHEP: Jie Zhao, Miao He, Xuantong Zhang, Lianghong Wei CENBG: Frédéric Perrot, Cédric Cerna CPPM: José Busto 19/3/2013