Mass testing and characterization of 20-inch PMTs for JUNO

Abstract

Main goal of the JUNO experiment is to determine the neutrino mass ordering using a 20 kt liquid-scintillator detector. Its key feature is an excellent energy resolution of at least 3% at 1 MeV, for which its instruments need to meet a certain quality and thus have to be fully characterized. More than 20,000 20-inch PMTs have been received and assessed by JUNO after a detailed testing program which began in 2017 and elapsed for about four years. Based on this mass characterization and a set of specific requirements, a good quality of all accepted PMTs could be ascertained. This paper presents the performed testing procedure with the designed testing systems as well as the statistical characteristics of all 20-inch PMTs intended to be used in the JUNO experiment, covering more than fifteen performance parameters including the photocathode uniformity. This constitutes the largest sample of 20-inch PMTs ever produced and studied in detail to date, i.e. 15,000 of the newly developed 20-inch MCP-PMTs from Northern Night Vision Technology Co. (NNVT) and 5000 of dynode PMTs from Hamamatsu Photonics K. K.(HPK).

Performance evaluation of the 8-inch MCP-PMT for Jinping Neutrino Experiment

“…4a represents the charge of a single PE with negligible multi-PE contributions due to the low occupancy. A long tail is evident in the single-PE charge distribution, which was also found in the NNVT 20-inch MCP-PMTs by the JUNO collaboration [15]. Zhang et al [35] proposed a phenomenological parameterization without dedicated consideration of the multiplication process of the PEs.…”

“…Generated from photons hitting the MCP or the first dynode directly rather than the photocathode, pre-pulses appear about tens of nanoseconds earlier with smaller amplitudes [15]. The probability P pre of pre-pulses is N pre /N t − DCR • T pre , in which N pre is the pre-pulse count of the total waveforms in the interval [−100 ns,−10 ns] (T pre = 90 ns)…”

“…Ions such as H + , He + and O + produced from gaseous impurities in the vacuum bulb by the PEs drift back to the photocathode, generate new electrons and then afterpulses [15,41]. The delay times of after-pulses are proportional to the square root of the mass-to-charge ratios of the ions [21,41,42].…”

“…(estimated by the sample resolution ν 2 in Section 3.2). The latter is known as the excess noise factor [15,46,47] indicating the impact of single-PE charge smearing on energy.…”

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

“…All photomultiplier tubes are produced, tested and instrumented with waterproof potting. The final photon detection efficiency of the LPMTs turned out to be higher than designed, and its mean value now amounts to 29.6% [10].…”