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

2. 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 neutrino/s !!)
How to identify neutrinos from background events in JUNO ?
19/3/2013

3. Electron antineutrino detection
Electron antineutrinos detected by Inverse Beta Decay (IBD) :
Energy spectrum
Energy threshold: 1.8 MeV
E(e+) = E(ν) 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

4. 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

5. 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

6. Rate of accidental background vs FV cut

7. 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 %
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

8. 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

9. 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

10. 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

11. 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

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

13. Material radioactivity: liquid scintillator
LS requirements:
40K: g/g
238U: g/g
232Th: 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

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

15. Material radioactivity: acrylic sphere
Acrylic requirements:
40K: g/g
238U: g/g
232Th: 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

16. 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

17. 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

18. 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 Bq/m3
→ negligible compared to 0.2 Bq/m3 requirement
7.7 MeV alpha counts/day
19/3/2013

19. 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

20. 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

21. 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