Radioactivity control strategy for the JUNO detector

Abstract

JUNO is a massive liquid scintillator detector with a primary scientific goal of determining the neutrino mass ordering by studying the oscillated anti-neutrino flux coming from two nuclear power plants at 53 km distance. The expected signal anti-neutrino interaction rate is only 60 counts per day (cpd), therefore a careful control of the background sources due to radioactivity is critical. In particular, natural radioactivity present in all materials and in the environment represents a serious issue that could impair the sensitivity of the experiment if appropriate countermeasures were not foreseen. In this paper we discuss the background reduction strategies undertaken by the JUNO collaboration to reduce at minimum the impact of natural radioactivity. We describe our efforts for an optimized experimental design, a careful material screening and accurate detector production handling, and a constant control of the expected results through a meticulous Monte Carlo simulation program. We show that all these actions should allow us to keep the background count rate safely below the target value of 10 Hz (i.e. ∼1 cpd accidental background) in the default fiducial volume, above an energy threshold of 0.7 MeV.

Jiangmen Underground Neutrino Observatory (JUNO): on the way to physics data

“…Since then, the total reactor thermal power available to JUNO decreased of ∼26% (from 35.8 GW to 26.6 GW) because only two of the planned four Taishan reactors have been built; moreover, the expected muon flux is now ∼33% higher (from 3 to 4 Hz) because of the shallower location of the JUNO experimental hall due to the underground water problem. On the other hand, we expect a ∼3% improvement of the detector energy resolution (from 3% to 2.9% at 1 MeV), thanks to the higher photon detection efficiency of the LPMTs, to an improved understanding of the PMT optical model, and to a more accurate simulation of the detector geometry; moreover, we expect a ∼10% improvement of the muon veto efficiency (from 83% to 91.6%), and we have more realistic estimations of the expected backgrounds from the detector materials [8]. Finally, we expect an improved reactor spectral shape uncertainty thanks to the combined analysis with the Taishan Antineutrino Observatory (TAO) [11], a 1 ton LS satellite detector placed at 30 m distance from one of the Taishan reactor cores to measure the unoscillated antineutrino flux with high precision.…”

“…Due to its dimensions, it is divided into 265 panels which are produced separately at the company and bonded onsite. There are very stringent requirements on the light transparency of the acrylic, which must be higher than 96% to reach the target energy resolution of JUNO, and on its radiopurity: the concentration of each of the natural contaminants ( 238 U, 232 Th, 40 K) must be below 1 ppt to keep the accidental background rate within JUNO requirements [8].…”

“…The LS is a Linear Alkyl Benzene (LAB) solvent mixed with 2.5 g/L PPO fluor and 3 mg/L bis-MSB wavelength shifter. To fulfil JUNO demanding requirements, it must have a very high light yield (> 1345 p.e./MeV), a long attenuation length (> 20 m), and extremely high radiopurity (< 10 −17 g/g for 238 U and 232 Th, < 10 −18 g/g for 40 K, < 10 −24 g/g for 210 Pb) [8]. An industrial scale purification process was designed, and is under construction at the JUNO site.…”

“…8 B solar neutrinos will be measured by JUNO via elastic scattering on electrons with the unprecedented energy threshold of 2 MeV. In addition, by exploiting the 200 t of 13 C of the LS, JUNO will detect 8 B neutrinos also via charged current and neutral current interactions on 13 C, and thus will study 8 B neutrinos in a model independent way. The sensitivity of JUNO to lower energy solar neutrinos (i.e.…”

Low radioactive material screening and background control for the PandaX-4T experiment

“…The gamma radiation occurring during the reaction could be thus measured by a HPGe detector. NAA can achieve substantially greater sensitivity than direct γ ray counting, typically at ppt and sub-ppt levels [26].…”