[](JUNO collaboration) 1]Yerevan Physics Institute, Yerevan, Armenia
2]Université Libre de Bruxelles, Brussels, Belgium 3]Universidade Estadual de
Londrina, Londrina, Brazil 4]Pontificia Universidade Catolica do Rio de
Janeiro, Rio de Janeiro, Brazil 5]Millennium Institute for SubAtomic Physics
at the High-energy Frontier (SAPHIR), ANID, Chile 6]Pontificia Universidad
Católica de Chile, Santiago, Chile 7]Universidad Tecnica Federico Santa Maria,
Valparaiso, Chile 8]Beijing Institute of Spacecraft Environment Engineering,
Beijing, China 9]Beijing Normal University, Beijing, China 10]China Institute
of Atomic Energy, Beijing, China 11]Institute of High Energy Physics, Beijing,
China 12]North China Electric Power University, Beijing, China 13]School of
Physics, Peking University, Beijing, China 14]Tsinghua University, Beijing,
China 15]University of Chinese Academy of Sciences, Beijing, China 16]Jilin
University, Changchun, China 17]College of Electronic Science and Engineering,
National University of Defense Technology, Changsha, China 18]Chongqing
University, Chongqing, China 19]Dongguan University of Technology, Dongguan,
China 20]Jinan University, Guangzhou, China 21]Sun Yat-Sen University,
Guangzhou, China 22]Harbin Institute of Technology, Harbin, China
23]University of Science and Technology of China, Hefei, China 24]The
Radiochemistry and Nuclear Chemistry Group in University of South China,
Hengyang, China 25]Wuyi University, Jiangmen, China 26]Shandong University,
Jinan, China, and Key Laboratory of Particle Physics and Particle Irradiation
of Ministry of Education, Shandong University, Qingdao, China 27]Institute of
Modern Physics, Chinese Academy of Sciences, Lanzhou, China 28]Nanjing
University, Nanjing, China 29]Guangxi University, Nanning, China 30]East China
University of Science and Technology, Shanghai, China 31]School of Physics and
Astronomy, Shanghai Jiao Tong University, Shanghai, China 32]Tsung-Dao Lee
Institute, Shanghai Jiao Tong University, Shanghai, China 33]Institute of
Hydrogeology and Environmental Geology, Chinese Academy of Geological
Sciences, Shijiazhuang, China 34]Nankai University, Tianjin, China 35]Wuhan
University, Wuhan, China 36]Xi’an Jiaotong University, Xi’an, China 37]Xiamen
University, Xiamen, China 38]School of Physics and Microelectronics, Zhengzhou
University, Zhengzhou, China 39]Institute of Physics, National Yang Ming Chiao
Tung University, Hsinchu 40]National United University, Miao-Li 41]Department
of Physics, National Taiwan University, Taipei 42]Charles University, Faculty
of Mathematics and Physics, Prague, Czech Republic 43]University of Jyvaskyla,
Department of Physics, Jyvaskyla, Finland 44]IJCLab, Université Paris-Saclay,
CNRS/IN2P3, 91405 Orsay, France 45]Univ. Bordeaux, CNRS, LP2i Bordeaux, UMR
5797, F-33170 Gradignan, France 46]IPHC, Université de Strasbourg, CNRS/IN2P3,
F-67037 Strasbourg, France 47]Centre de Physique des Particules de Marseille,
Marseille, France 48]SUBATECH, Université de Nantes, IMT Atlantique, CNRS-
IN2P3, Nantes, France 49]III. Physikalisches Institut B, RWTH Aachen
University, Aachen, Germany 50]Institute of Experimental Physics, University
of Hamburg, Hamburg, Germany 51]Forschungszentrum Jülich GmbH, Nuclear Physics
Institute IKP-2, Jülich, Germany 52]Institute of Physics and EC PRISMA+,
Johannes Gutenberg Universität Mainz, Mainz, Germany 53]Technische Universität
München, München, Germany 54]Helmholtzzentrum für Schwerionenforschung,
Planckstrasse 1, D-64291 Darmstadt, Germany 55]Eberhard Karls Universität
Tübingen, Physikalisches Institut, Tübingen, Germany 56]INFN Catania and
Dipartimento di Fisica e Astronomia dell Università di Catania, Catania, Italy
57]Department of Physics and Earth Science, University of Ferrara and INFN
Sezione di Ferrara, Ferrara, Italy 58]INFN Sezione di Milano and Dipartimento
di Fisica dell Università di Milano, Milano, Italy 59]INFN Milano Bicocca and
University of Milano Bicocca, Milano, Italy 60]INFN Milano Bicocca and
Politecnico of Milano, Milano, Italy 61]INFN Sezione di Padova, Padova, Italy
62]Dipartimento di Fisica e Astronomia dell’Università di Padova and INFN
Sezione di Padova, Padova, Italy 63]INFN Sezione di Perugia and Dipartimento
di Chimica, Biologia e Biotecnologie dell’Università di Perugia, Perugia,
Italy 64]Laboratori Nazionali di Frascati dell’INFN, Roma, Italy 65]University
of Roma Tre and INFN Sezione Roma Tre, Roma, Italy 66]Institute of Electronics
and Computer Science, Riga, Latvia 67]Pakistan Institute of Nuclear Science
and Technology, Islamabad, Pakistan 68]Joint Institute for Nuclear Research,
Dubna, Russia 69]Institute for Nuclear Research of the Russian Academy of
Sciences, Moscow, Russia 70]Lomonosov Moscow State University, Moscow, Russia
71]Comenius University Bratislava, Faculty of Mathematics, Physics and
Informatics, Bratislava, Slovakia 72]Department of Physics, Faculty of
Science, Chulalongkorn University, Bangkok, Thailand 73]National Astronomical
Research Institute of Thailand, Chiang Mai, Thailand 74]Suranaree University
of Technology, Nakhon Ratchasima, Thailand 75]Department of Physics and
Astronomy, University of California, Irvine, California, USA

# Mass Testing and Characterization of 20-inch PMTs for JUNO

Angel Abusleme  Thomas Adam  Shakeel Ahmad  Rizwan Ahmed  Sebastiano Aiello
Muhammad Akram  Abid Aleem  Tsagkarakis Alexandros  Fengpeng An  Qi An
Giuseppe Andronico  Nikolay Anfimov  Vito Antonelli  Tatiana Antoshkina  Burin
Asavapibhop  João Pedro Athayde Marcondes de André  Didier Auguste  Weidong
Bai  Nikita Balashov  Wander Baldini  Andrea Barresi  Davide Basilico  Eric
Baussan  Marco Bellato  Antonio Bergnoli  Thilo Birkenfeld  Sylvie Blin  David
Blum  Simon Blyth  Anastasia Bolshakova  Mathieu Bongrand  Clément Bordereau
Dominique Breton  Augusto Brigatti  Riccardo Brugnera  Riccardo Bruno  Antonio
Budano  Jose Busto  Ilya Butorov  Anatael Cabrera  Barbara Caccianiga  Hao Cai
Xiao Cai  Yanke Cai  Zhiyan Cai  Riccardo Callegari  Antonio Cammi  Agustin
Campeny  Chuanya Cao  Guofu Cao  Jun Cao  Rossella Caruso  Cédric Cerna  Chi
Chan  Jinfan Chang  Yun Chang  Guoming Chen  Pingping Chen  Po-An Chen
Shaomin Chen  Xurong Chen  Yixue Chen  Yu Chen  Zhiyuan Chen  Zikang Chen  Jie
Cheng  Yaping Cheng  Yu Chin Cheng  Alexey Chetverikov  Davide Chiesa  Pietro
Chimenti  Artem Chukanov  Gérard Claverie  Catia Clementi  Barbara Clerbaux
Marta Colomer Molla  Selma Conforti Di Lorenzo  Daniele Corti  Flavio Dal
Corso  Olivia Dalager  Christophe De La Taille  Zhi Deng  Ziyan Deng  Wilfried
Depnering  Marco Diaz  Xuefeng Ding  Yayun Ding  Bayu Dirgantara  Sergey
Dmitrievsky  Tadeas Dohnal  Dmitry Dolzhikov  Georgy Donchenko  Jianmeng Dong
Evgeny Doroshkevich  Marcos Dracos  Frédéric Druillole  Ran Du  Shuxian Du
Stefano Dusini  Martin Dvorak  Timo Enqvist  Heike Enzmann  Andrea Fabbri
Donghua Fan  Lei Fan  Jian Fang  Wenxing Fang  Marco Fargetta  Dmitry Fedoseev
Zhengyong Fei  Li-Cheng Feng  Qichun Feng  Richard Ford  Amélie Fournier
Haonan Gan  Feng Gao  Alberto Garfagnini  Arsenii Gavrikov  Marco Giammarchi
Nunzio Giudice  Maxim Gonchar  Guanghua Gong  Hui Gong  Yuri Gornushkin
Alexandre Göttel  Marco Grassi  Vasily Gromov  Minghao Gu  Xiaofei Gu  Yu Gu
Mengyun Guan  Yuduo Guan  Nunzio Guardone  Cong Guo  Jingyuan Guo  Wanlei Guo
Xinheng Guo  Yuhang Guo  Paul Hackspacher  Caren Hagner  Ran Han  Yang Han
Miao He  Wei He  Tobias Heinz  Patrick Hellmuth  Yuekun Heng  Rafael Herrera
YuenKeung Hor  Shaojing Hou  Yee Hsiung  Bei-Zhen Hu  Hang Hu  Jianrun Hu  Jun
Hu  Shouyang Hu  Tao Hu  Yuxiang Hu  Zhuojun Hu  Guihong Huang  Hanxiong Huang
Kaixuan Huang  Wenhao Huang  Xin Huang  Xingtao Huang  Yongbo Huang  Jiaqi Hui
Lei Huo  Wenju Huo  Cédric Huss  Safeer Hussain  Ara Ioannisian  Roberto
Isocrate  Beatrice Jelmini  Ignacio Jeria  Xiaolu Ji  Huihui Jia  Junji Jia
Siyu Jian  Di Jiang  Wei Jiang  Xiaoshan Jiang  Xiaoping Jing  Cécile Jollet
Jari Joutsenvaara  Leonidas Kalousis  Philipp Kampmann  Li Kang  Rebin
Karaparambil  Narine Kazarian  Amina Khatun  Khanchai Khosonthongkee  Denis
Korablev  Konstantin Kouzakov  Alexey Krasnoperov  Nikolay Kutovskiy  Pasi
Kuusiniemi  Tobias Lachenmaier  Cecilia Landini  Sébastien Leblanc  Victor
Lebrin  Frederic Lefevre  Ruiting Lei  Rupert Leitner  Jason Leung  Daozheng
Li  Demin Li  Fei Li  Fule Li  Gaosong Li  Huiling Li  Mengzhao Li  Min Li
Nan Li  Nan Li  Qingjiang Li  Ruhui Li  Rui Li  Shanfeng Li  Tao Li  Teng Li
Weidong Li  Weiguo Li  Xiaomei Li  Xiaonan Li  Xinglong Li  Yi Li  Yichen Li
Yufeng Li  Zepeng Li  Zhaohan Li  Zhibing Li  Ziyuan Li  Zonghai Li  Hao Liang
Hao Liang  Jiajun Liao  Ayut Limphirat  Guey-Lin Lin  Shengxin Lin  Tao Lin
Jiajie Ling  Ivano Lippi  Fang Liu  Haidong Liu  Haotian Liu  Hongbang Liu
Hongjuan Liu  Hongtao Liu  Hui Liu  Jianglai Liu  Jinchang Liu  Min Liu  Qian
Liu  Qin Liu  Runxuan Liu  Shubin Liu  Shulin Liu  Xiaowei Liu  Xiwen Liu  Yan
Liu  Yunzhe Liu  Alexey Lokhov  Paolo Lombardi  Claudio Lombardo  Kai Loo
Chuan Lu  Haoqi Lu  Jingbin Lu  Junguang Lu  Shuxiang Lu  Bayarto
Lubsandorzhiev  Sultim Lubsandorzhiev  Livia Ludhova  Arslan Lukanov  Daibin
Luo  Fengjiao Luo  Guang Luo  Shu Luo  Wuming Luo  Xiaojie Luo  Vladimir
Lyashuk  Bangzheng Ma  Bing Ma  Qiumei Ma  Si Ma  Xiaoyan Ma  Xubo Ma  Jihane
Maalmi  Jingyu Mai  Yury Malyshkin  Roberto Carlos Mandujano  Fabio Mantovani
Francesco Manzali  Xin Mao  Yajun Mao  Stefano M. Mari  Filippo Marini
Cristina Martellini  Gisele Martin-Chassard  Agnese Martini  Matthias Mayer
Davit Mayilyan  Ints Mednieks  Yue Meng  Anselmo Meregaglia  Emanuela Meroni
David Meyhöfer  Mauro Mezzetto  Jonathan Miller  Lino Miramonti  Paolo Montini
Michele Montuschi  Axel Müller  Massimiliano Nastasi  Dmitry V. Naumov  Elena
Naumova  Diana Navas-Nicolas  Igor Nemchenok  Minh Thuan Nguyen Thi  Feipeng
Ning  Zhe Ning  Hiroshi Nunokawa  Lothar Oberauer  Juan Pedro Ochoa-Ricoux
Alexander Olshevskiy  Domizia Orestano  Fausto Ortica  Rainer Othegraven
Alessandro Paoloni  Sergio Parmeggiano  Yatian Pei  Luca Pelicci  Nicomede
Pelliccia  Anguo Peng  Haiping Peng  Yu Peng  Zhaoyuan Peng  Frédéric Perrot
Pierre-Alexandre Petitjean  Fabrizio Petrucci  Oliver Pilarczyk  Luis Felipe
Piñeres Rico  Artyom Popov  Pascal Poussot  Ezio Previtali  Fazhi Qi  Ming Qi
Sen Qian  Xiaohui Qian  Zhen Qian  Hao Qiao  Zhonghua Qin  Shoukang Qiu
Gioacchino Ranucci  Neill Raper  Alessandra Re  Henning Rebber  Abdel Rebii
Mariia Redchuk  Bin Ren  Jie Ren  Barbara Ricci  Mariam Rifai  Mathieu Roche
Narongkiat Rodphai  Aldo Romani  Bedřich Roskovec  Xichao Ruan  Arseniy
Rybnikov  Andrey Sadovsky  Paolo Saggese  Simone Sanfilippo  Anut Sangka
Utane Sawangwit  Julia Sawatzki  Michaela Schever  Cédric Schwab  Konstantin
Schweizer  Alexandr Selyunin  Andrea Serafini  Giulio Settanta  Mariangela
Settimo  Zhuang Shao  Vladislav Sharov  Arina Shaydurova  Jingyan Shi  Yanan
Shi  Vitaly Shutov  Andrey Sidorenkov  Fedor Šimkovic  Chiara Sirignano
Jaruchit Siripak  Monica Sisti  Maciej Slupecki  Mikhail Smirnov  Oleg Smirnov
Thiago Sogo-Bezerra  Sergey Sokolov  Julanan Songwadhana  Boonrucksar
Soonthornthum  Albert Sotnikov  Ondřej Šrámek  Warintorn Sreethawong  Achim
Stahl  Luca Stanco  Konstantin Stankevich  Dušan Štefánik  Hans Steiger
Jochen Steinmann  Tobias Sterr  Matthias Raphael Stock  Virginia Strati
Alexander Studenikin  Jun Su  Shifeng Sun  Xilei Sun  Yongjie Sun  Yongzhao
Sun  Zhengyang Sun  Narumon Suwonjandee  Michal Szelezniak  Jian Tang  Qiang
Tang  Quan Tang  Xiao Tang  Eric Theisen  Alexander Tietzsch  Igor Tkachev
Tomas Tmej  Marco Danilo Claudio Torri  Konstantin Treskov  Andrea Triossi
Giancarlo Troni  Wladyslaw Trzaska  Cristina Tuve  Nikita Ushakov  Vadim Vedin
Giuseppe Verde  Maxim Vialkov  Benoit Viaud  Cornelius Moritz Vollbrecht
Cristina Volpe  Katharina von Sturm  Vit Vorobel  Dmitriy Voronin  Lucia
Votano  Pablo Walker  Caishen Wang  Chung-Hsiang Wang  En Wang  Guoli Wang
Jian Wang  Jun Wang  Lu Wang  Meifen Wang  Meng Wang  Meng Wang  Ruiguang Wang
Siguang Wang  Wei Wang  Wei Wang  Wenshuai Wang  Xi Wang  Xiangyue Wang
Yangfu Wang  Yaoguang Wang  Yi Wang  Yi Wang  Yifang Wang  Yuanqing Wang
Yuman Wang  Zhe Wang  Zheng Wang  Zhimin Wang  Zongyi Wang  Apimook
Watcharangkool  Wei Wei  Wei Wei  Wenlu Wei  Yadong Wei  Kaile Wen  Liangjian
Wen  Christopher Wiebusch  Steven Chan-Fai Wong  Bjoern Wonsak  Diru Wu  Qun
Wu  Zhi Wu  Michael Wurm  Jacques Wurtz  Christian Wysotzki  Yufei Xi  Dongmei
Xia  Xiang Xiao  Xiaochuan Xie  Yuguang Xie  Zhangquan Xie  Zhao Xin  Zhizhong
Xing  Benda Xu  Cheng Xu  Donglian Xu  Fanrong Xu  Hangkun Xu  Jilei Xu  Jing
Xu  Meihang Xu  Yin Xu  Yu Xu  Baojun Yan  Taylor Yan  Wenqi Yan  Xiongbo Yan
Yupeng Yan  Changgen Yang  Chengfeng Yang  Huan Yang  Jie Yang  Lei Yang
Xiaoyu Yang  Yifan Yang  Yifan Yang  Haifeng Yao  Jiaxuan Ye  Mei Ye  Ziping
Ye  Frédéric Yermia  Na Yin  Zhengyun You  Boxiang Yu  Chiye Yu  Chunxu Yu
Hongzhao Yu  Miao Yu  Xianghui Yu  Zeyuan Yu  Zezhong Yu  Cenxi Yuan
Chengzhuo Yuan  Ying Yuan  Zhenxiong Yuan  Baobiao Yue  Noman Zafar  Vitalii
Zavadskyi  Shan Zeng  Tingxuan Zeng  Yuda Zeng  Liang Zhan  Aiqiang Zhang  Bin
Zhang  Binting Zhang  Feiyang Zhang  Guoqing Zhang  Honghao Zhang  Jialiang
Zhang  Jiawen Zhang  Jie Zhang  Jin Zhang  Jingbo Zhang  Jinnan Zhang  Mohan
Zhang  Peng Zhang  Qingmin Zhang  Shiqi Zhang  Shu Zhang  Tao Zhang  Xiaomei
Zhang  Xin Zhang  Xuantong Zhang  Xueyao Zhang  Yinhong Zhang  Yiyu Zhang
Yongpeng Zhang  Yu Zhang  Yuanyuan Zhang  Yumei Zhang  Zhenyu Zhang  Zhijian
Zhang  Fengyi Zhao  Jie Zhao  Rong Zhao  Runze Zhao  Shujun Zhao  Dongqin
Zheng  Hua Zheng  Yangheng Zheng  Weirong Zhong  Jing Zhou  Li Zhou  Nan Zhou
Shun Zhou  Tong Zhou  Xiang Zhou  Jiang Zhu  Jingsen Zhu  Kangfu Zhu  Kejun
Zhu  Zhihang Zhu  Bo Zhuang  Honglin Zhuang  Liang Zong  Jiaheng Zou
<EMAIL_ADDRESS>[  [  [  [  [  [  [  [  [  [  [  [  [  [  [  [
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[  [  [  [  [  [  [

(Received: date / Accepted: date)

###### 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 5,000 of dynode PMTs from Hamamatsu Photonics K. K.(HPK).

###### keywords:

photon detectors for UV, visible and IR photons (vacuum) (photomultipliers,
HPDs, others), neutrino detectors, JUNO, PMT, MCP-PMT

##  1 Introduction

The Jiangmen Underground Neutrino Observatory (JUNO) experiment JUNOCDR ;
JUNOphysics  is a new large-volume multi-purpose liquid-scintillator
experiment currently under construction in southern China, located in a cavern
with a 700 m rock overburden. Its main goal is to determine the neutrino mass
ordering from the neutrino oscillation spectrum of two close-by nuclear power
plants at a distance of 53 km each, with a sensitivity better than three
standard deviations after six years of data taking JUNOphysics ; JUNOyufeng ;
JUNOliang . The central detector (CD) of JUNO JUNOCDlombardi ; JUNOCDzhimin ;
JUNOCDyuekun ; JUNOdetector  (Fig. 1) consists of an acrylic sphere with a
diameter of 35.4 m and is filled with 20 kt of LAB-based liquid scintillator
(LS) JUNO-LS-optimization . A high transparency LS, high optical coverage
(>75%) with high quantum efficiency photomultiplier tubes (PMTs), and low
background/noise levels are needed to achieve an energy resolution of at least
3 % at 1 MeV JUNOdetector ; JUNOCalibration , which is essential for realizing
the physics goals of JUNO. The high coverage of the CD is achieved by closely
packing 17,612 high quantum efficiency 20-inch PMTs (Large PMTs or LPMTs) and
25,600 3-inch PMTs (Small PMTs or SPMTs) in the gaps between the LPMTs, placed
at a distance of 19.8 m to the detector’s center. All of them are mounted on a
stainless steel lattice shell (diameter of 40.1 m) outside the acrylic sphere.
The whole construction is further embedded into a cylindrical pure water pool
with a diameter of 43.5 m and depth of 44 m (water filled to 43.5 m),
instrumented with another about 2,400 20-inch PMTs. The water pool will be
used as an active Cherenkov veto against cosmic muons traversing the detector,
but also acts as a shielding buffer from the surroundings.

Figure 1: Layout of the JUNO detector system JUNOdetector . The zoomed in
picture on the right is illustrating the proposed PMT arrangement of the JUNO
central detector, with the 3-inch PMTs placed in the gaps between the 20-inch
PMTs.

In large volume LS detectors as JUNO, optimizing the energy resolution
specifically concerns all parts related to the light collection and detection,
including in particular the performance of the light-sensitive devices in the
range from single photo-electrons (SPE) to hundreds of photo-electrons per
channel. Aiming to achieve an unprecedented energy resolution, JUNO is
designed with a set of 1 GHz waveform sampling electronics to record the
pulses of the 20-inch PMTs for a precise neutrino event detection and
reconstruction JUNOdetector ; JUNOElectronics ; JUNOEmbedded . Playing the key
role in this process, the PMTs’ photon detection efficiency, dark count rate
(DCR), and timing characteristics are most important to achieve the best
possible detector performance JUNOdetector . At the same time, good
performance in other parameters is needed for better event reconstruction,
background reduction, and a successful long-term operation: this concerns the
gain as a function of high voltage, charge resolution, peak-to-valley ratio,
pulse shape features, correlated pulses (pre-/after-pulses), and, in
particular, the photocathode response uniformity and Earth’s magnetic field
(EMF) sensitivity of the large volume PMTs. Detailed knowledge of the PMT
characteristics moreover provides valuable input for precise detector
simulations leading to a better understanding of the to-be-running experiment
JUNOdetector ; JUNOSimulation ; JUNO-Rec-QLiu ; JUNO-Rec-Zhen ; JUNO-Rec-
Marcel . All LPMTs used in JUNO have been measured in detail and checked
against a list of preassigned requirements (see sec. 2.2). These tests also
include consistency checks between bare and waterproof potted PMTs111See sec.
3.1 as well as Qin_Neutrino ; JUNOPMTinstr  for details about the potted
PMTs..

Concerning the mechanical safety which is relevant for the large vacuum glass
bulbs of PMTs working in pure water at a maximum depth of 44 m, a careful
inspection is performed on each PMT to identify any mechanical/structural
defects. Furthermore, following many valuable, previously performed studies on
different PMT parameters, such as general studies on the large area PMT
performance largePMTlei ; largePMTcomp ; wavesamplingPMT ; largePMTxia ;
JUNOPMTlinearity ; JUNOPMTgain ; PMTrelativeCE , temperature effects lowTPMT ;
lowTPMT2 ; HamManual ; largePMTtemperature , and the lifetime of the newly
developed MCP-PMT by Northern Night Vision Technology Co. (NNVT)
lifetimeMCPPMT ; lifetimeMCPPMT2 ; lifetimeMCPPMT3 ; argonneMCPPMT , follow-up
studies on the DCR as a function of temperature and the long-term stability of
the MCP-PMTs are evaluated.

In this work, all details are presented about the measured characteristics of
all accepted JUNO 20-inch PMTs, as well as brief introductions to the setup of
the mass testing systems and the performed testing procedures. The selected
20-inch PMT types of JUNO will be described in sec. 2. The testing facilities
and performed procedures will be presented in sec. 3. The testing results of
the bare PMTs (full 20,000 LPMT sample) will be shown in detail in sec. 4.
Expected features of waterproof potted PMTs and differences with respect to
testing results from the acceptance tests of the bare PMTs will be discussed
in sec. 5. Finally, a summary is given in sec. 6. Meanwhile, it is valuable to
stress that several other PMT characterization campaigns were performed in the
past; we mention those of the IceCube icecubePMT , MiniBooNE MiniBooNEPMT ,
IMB IMBPMT , Borexino BorexinoPMT ; BorexinoPMT2 , Daya Bay DayabayPMT , Chooz
ChoozPMT , SNO SNOPMT , Double Chooz doubleChoozPMT ; doubleChoozPMT2 , DAMA
DAMAPMT , Auger AugerPMT ; AugerPMT2 , ANNIE ANNIEPMT , YBJ YBJ8inch , LHAASO
LHAASOPMT ; LHAASOPMT2 , Super-K sk-SUZUKI1993299 , Hyper-Kamiokande HKPMT ;
HKPMT2 , and KM3Net KM3NeTPMT ; KM3NeTPMT2  experiments. For completeness, it
should be noted here that the 3-inch PMTs used for JUNO are investigated as
well in a separate campaign JUNODCmiao2017 ; JUNO3inchPMT .

##  2 20-inch photomultiplier tubes for JUNO

###  2.1 Selected 20-inch PMTs of JUNO

It is theoretically and practically significant to build a detector with
maximum physics potential at minimal costs. To date, PMTs offer the best
compromise between single-photon sensitivity and acceptance in noise and cost
per unit area around room temperature, and large area PMTs are the first
choice of photon sensors for large LS- or water-based neutrino experiments
which require high photocathode coverage. Besides photon detection efficiency
(PDE), other characteristics such as dark count rate (DCR), transit time
spread (TTS), the radioactive background of the glass, peak-to-valley ratio
(P/V), etc., will affect the photon detection and event reconstruction, and
thus impact the physics measurements. Considering all related PMT parameters,
costs and risks, evaluated with the physics goals of JUNO, a selection
strategy for PMTs was proposed and applied, leading to a set of requirements
for the performance of JUNO 20-inch PMTs JUNOPMTliangjian . With the bidding
following the selection strategy, there are two types of 20-inch PMTs selected
by JUNO: approx. 5,000 box and linear-focused dynode-PMTs (R12860-50 HQE or
R12860) from Hamamatsu Photonics K. K. (HPK) HPK-R12860  and approx. 15,000
MCP-PMTs (GDB-6201 or N6201) from North Night Vision Technology Co. (NNVT)
YWang_newMCP ; LiuMCP ; NNVT-GDB6201-improvement ; NNVT-GDB6201-note . The
dimensions of the two types of PMTs are shown in Fig. 2.

Figure 2: Technical drawing of selected PMTs for JUNO: NNVT 20-inch MCP-PMT
(type GDB-6201 NNVT-GDB6201-note , left) and HPK 20-inch dynode PMT (type
R12860 HPK-R12860 , right).

###  2.2 Acceptance criteria

Based on joint considerations of the physics requirements discussed above and
the PMT manufacturers, a list of combined performance criteria was defined for
all ordered 20,000 20-inch PMTs of JUNO. Each of the received PMTs must be
tested at least once to assure that all predefined requirements are satisfied.
In Tab. 1 the required nominal values as well as lower or upper limits for
part of the parameters are summarized. Each of the listed parameters will be
discussed separately and in detail within this paper. The photon detection
efficiency (PDE), as one critical parameter required by JUNO and discussed in
detail in sec. 4.4, is not listed directly in Tab. 1, but appears as a
combined parameter of the quantum efficiency (QE), the collection efficiency
(CE), and the effective area ratio (EAR)222The PDE is defined as PDE
($\lambda$) = QE ($\lambda$) $\times$ CE $\times$ EAR. The Effective Area
Ratio (EAR), which is from the manufacture, is the relative effective area
ratio, where the collection efficiency is greater than 95.6% when the diameter
of a PMT is 508 mm.. It is finally requested with an averaged value of $\geq
27\,\%$ for all the PMTs, and a minimum value of $\geq 24\,\%$ for a single
PMT (in particular for use in the CD), both defined at a wavelength of 420 nm
to match the typical emission spectra of JUNO liquid scintillator JUNO-LS-
optimization . A catalog index of the content and all the checked parameters
is listed in Tab. 2.

Table 1: Main acceptance criteria for JUNO 20-inch PMTs.

| HPK R12860-50 | NNVT GDB-6201
---|---|---
Parameter | Average (Limit) | Average (Limit)
QE |  $30.3\,\%$ ($\geq 27\,\%$) |  $28.5\,\%$ ($\geq 26.5\,\%$)
CE | $95.6\,\%$ |  $98\,\%$ ($\geq 96\,\%$)
Effective area ratio |  $96\,\%$ ($93\,\%$) |  $97\,\%$ ($\geq 96\,\%$)
Gain | $10^{7}$ | $10^{7}$
HV (for a $10^{7}$ gain) | 2000 V ($\leq 2500$ V) | 2500 V ($\leq 2800$ V)
QE uniformity |  $5\,\%$ ($\leq 15\,\%$ inside $70^{\circ}$) |  $8\,\%$ ($\leq 10\,\%$)
|  $20\,\%$ ($\leq 30\,\%$ inside $80^{\circ}$) |
TTS (FWHM) | 2.7 ns ($\leq 3.5$ ns) | 12 ns ($\leq 15$ ns)
P/V ratio | 3 ($\geq 2.5$) | 3.5
Pre-pulse ratio |  |
(80 ns window, |  $0.8\,\%$ ($\leq 1\,\%$) |  $0.5\,\%$ ($\leq 1\,\%$)
main pulse $\sim$160 p.e.) |  |
After-pulse ratio |  |
(0.5$\sim$20 µs window, |  $10\,\%$ ($\leq 15\,\%$) |  $10\,\%$ ($\leq 15\,\%$)
main pulse $\sim$160 p.e.) |  |
Dark count rate | 10 kHz ($\leq 50$ kHz) |  $\leq 50$ kHz (if $24\,\%\leq$ PDE $<27\,\%$)
(0.25 p.e., 22∘C) |  |  $\leq 60$ kHz (if $27\,\%\leq$ PDE $<28\,\%$)
|  |  $\leq 80$ kHz (if $28\,\%\leq$ PDE $<29\,\%$)
|  |  $\leq 100$ kHz (if $29\,\%\leq$ PDE)
Glass radioactivity |  238U: $<400$ ppb |  238U: $<75$ ppb
|  232Th: $<400$ ppb |  232Th: $<75$ ppb
|  40K: $<40$ ppb |  40K: $<30$ ppb
Pressure tolerance |  $\geq 0.8$ MPa |  $>1$ MPa
Dimension tolerance1 |  $508\,(\pm 3$ mm) (diameter) |  $508\,(\pm 3$ mm) (diameter)
|  $<10$ mm (height) |  $<10$ mm (height)
Lifetime2 |  $\geq 20$ years |  $\geq 25$ years

  * 1

The tolerance is $\pm$3 mm with the lower limit required by the optical
coverage and the upper limit constrained by the detector installation. It is
larger than the specification of the NNVT PMT shown in Fig. 2.

  * 2

Gain decrease less than 50% in 20 years with the same HV.

Table 2: Catalog index of content and checked parameters.

Category | Measurement/Test | Section
---|---|---
Testing system | Facilities | 3.1
Procedure | 3.2
| Mechanical Safety
---
& Physical Characteristics
Visual Inspection, Diameter, Weight | 4.1
Performance for precise detection and reconstruction | Gain/Operating Voltage | 4.2
SPE Amplitude | 4.3.2
SPE Pulse Shape | 4.3.3
SPE Charge Resolution | 4.3.4
SPE Peak-to-Valley Ratio (P/V) | 4.3.5
Photon Detection Efficiency(PDE) | 4.4
Dark Count Rate (DCR) | 4.5
Transit Time Spread (TTS) | 4.6
Correlated Pulses | 4.7
Anode Non-linearity | 4.8
| Earth Magnetic Field (EMF) Sensitivity | 4.9
| Photocathode Response Uniformity | 4.10
Long-term Stability | DCR, Gain, Light Intensity | 4.11
Potted PMT | Gain | 5.1
DCR | 5.2

##  3 Testing setup and procedure

###  3.1 Testing facilities

To concentrate all actions related to PMT testing, a warehouse with around
4500 $\mathrm{m}^{2}$ was rented since 2017 at Zhongshan Pan-Asia Electric
Co., Ltd., Guangdong Province, China, which is about 150 km away from the JUNO
experimental site. In this warehouse, named _Zhongshan Pan-Asia 20-inch PMT
testing and potting station_ , all relevant steps of preparing the 20,000
20-inch PMTs for JUNO were processed, including receiving (database management
for recording and statistics), acceptance tests, storage, and waterproof
potting. The warehouse runs in a controlled environment with a temperature of
$25^{\circ}$C (in a range of $\pm 3^{\circ}$C) and relative humidity of $50$ %
(in a range of 30%-70%), and is maintained by a local team which is also in
charge of the daily PMT testing routine and all storage operations. At the
same time, most of the 20-inch PMT testing shifts focusing on data acquisition
and analysis are shared by the whole JUNO collaboration.

Based on the specific criteria of the selected 20-inch PMTs, a set of
acceptance testing facilities were developed to realize a process of semi-
automatic PMT testing covering most of the parameters listed in Tab. 1333Glass
radioactivity JUNOPMTradioactivity  and pressure tolerances are investigated
in separate tests, since those tests may require to destroy the PMTs as part
of the measurement process.. Main units are a commercial-container-based
multi-channel testing system developed and installed to perform full
characterizations for each of the 20,000 20-inch PMTs, as well as a scanning
station system designed and built aiming to sample about $5\,\%$ of all the
PMTs to check i.e. their photocathode uniformity. All PMT tests are performed
under a shielding against Earth’s magnetic field (EMF) with a remaining
strength of less than $10\,\%$ of EMF (which is about 5 µT).

All PMTs are tested with JUNO-optimized high voltage (HV) dividers using a
positive HV JUNOPMTinstr ; JUNOPMTsignalover ; JUNOPMTsignalopt ;
JUNOPMTflasher . The initial tests of the full sample of PMTs used a pluggable
HV divider with an integrated HV-signal de-coupler on board. Later, a large
sub-sample of the PMTs were tested again with the final JUNO version of the HV
divider directly soldered to the PMT pins, where the entire PMT and HV divider
were encapsulated with a waterproof housing JUNOPMTinstr ; JUNOCDPMT . In the
latter case the PMT pulses are picked up from a stand-alone HV-signal
decoupler through a 2 m (central detector PMTs) or 4 m (water pool PMTs)
cable444Where another 2 m extension cable is used for the connection between
potted PMT and testing electronics. Although these versions differ in their
mechanical setup, they feature the same HV-divider ratio and direct current
(DC). However, the first few thousand bare NNVT PMTs were measured by another
HV-divider555Based on the PMT testing results and feedback from the
manufacturer, the design of the HV dividers for NNVT PMTs was improved over
the testing campaign and updated to its final version after testing the first
few thousand bare PMTs. which only affects the rise- and fall-time of the
pulses of these PMTs.

Two containers and two scanning stations are built and used for the bare PMT
testing. Another two containers of slightly modified interior design are set
up, one dedicated to long-term stability tests of PMTs, and another to the
testing of waterproof potted PMTs together with JUNO (final front-end) 1F3
electronics JUNOdetector .

####  3.1.1 PMT mass testing: container systems

The main setup of the individual (acceptance) tests of all 20,000 20-inch JUNO
PMTs is the so-called container system. This system has been described
extensively in a stand-alone paper JUNOPMTcontainer , where all details about
mechanical setup, data taking electronics, measurement process and accuracies
can be found. To enhance understanding of the later presented results from the
PMT mass testing, the setup will be briefly described here as well.

The container system consists of four 20-feet high-cube reefer containers,
which act as darkrooms and are able to host 36 (containers $\\#$A and $\\#$B)
or 32 (containers $\\#$C and $\\#$D) PMTs in optically separated measurement
channels (shelf system with drawer boxes), see Fig. 3. Each container is
passively shielded against the EMF by a multi-layer silicon-iron shielding and
equipped with a high power HVAC (heating, ventilation, and air conditioning)
unit, granting environmental control over the container’s interior. All drawer
boxes are further equipped with temperature sensors, which allow to measure
the relationship between dark count rate and temperature of the surroundings.
Containers $\\#$A and $\\#$B are equipped with commercial data taking
electronics (main unit is a CAEN V1742 switched-capacitor digitizer CAEN-FADC
), controlled and supervised by a custom-made data acquisition software (DAQ)
based on LabView. This DAQ performs a sequence of individual measurements to
enable a full PMT characterization within 24 hours, covering absolute PDE,
DCR, gain, TTS666Due to mechanical issues at container $\\#$A, precise values
for the TTS by laser could be measured only in container $\\#$B and only for a
representative sub-sample of all PMTs, see sec. 4.6 for more details., P/V,
charge resolution, pulse characteristics such as rise-/fall-time, pre-pulse
ratio and signal-to-noise ratio. These containers are intended to perform the
acceptance tests of the full LPMT sample as well as functionality tests of a
large sub-sample of potted PMTs. Container $\\#$C is equipped with a similar
set of commercial electronics and intended to act as long-term stability test
setup, while container $\\#$D is modified to host the JUNO 1F3 underwater
boxes JUNOdetector , allowing additional functionality and performance tests
of waterproof potted PMTs together with the final JUNO electronics.

Each drawer in all four containers is equipped with a LED device from company
HVSys HVSYS-LED , together with a collimator, attenuator, diffuser, and a
cylindrical reflector to illuminate the full photocathode uniformly for SPE-
and multi-p.e.-related measurements at optimized light intensities
PMTintensity  and uniformity JUNOPMTcontainer  as shown at the bottom of Fig.
3. These devices are pulsed LEDs (at 420 nm) featuring a feedback loop on a
small micro-controller, which stabilizes the light intensity over time to a
calibrated level (based on a dedicated calibration campaign prior to the
testing campaign, see sec. 4.2.1 and 4.4 for details). Each of the containers
$\\#$A and $\\#$B are further equipped with a picosecond laser flashing system
based on a PiLas 420X picosecond laser from Advanced Laser Systems (A.L.S.)
container-laser . This device can produce short light pulses with a wavelength
of 420 nm and a width of $\sim 80$ ps, which are distributed by a fiber
splitter over all channels and used for precise TTS measurements at SPE
level777The laser light does not pass the diffuser, however the opening angle
of the emitted light from the fibers still covers most of the PMT’s
photocathode. TTS measurements using the LED system are performed as well
(after ADC jitter correction of container LED measurement), but only for a
relative check, see also sec. 4.6 and 4.10.4.. In container $\\#$C
specifically, an additional set of LEDs was introduced into the drawer boxes
pulsing with different frequencies and light intensities in order to simulate
an accelerated aging of the PMTs due to higher stress.

The performance and stability of each container (except for container $\\#$C)
is monitored by a small number of 20-inch PMTs (“reference PMTs” or
“monitoring PMTs”, consisting of up to three HPK and two NNVT PMTs per
container), which are included in every measurement run with one staying at a
fixed drawer and the others being circulated over all channels. The containers
achieve a stable performance over time as well as a sufficient accuracy of
e.g. $\leq 1\,\%$ for the absolute PDE and $\leq 1$ ns for the timing
parameters such as the TTS (only container $\\#$B) JUNOPMTcontainer  – this
will be discussed also in secs.4.4 and 4.6. Hence, the system can provide
reliable and comparable results for all available 20-inch PMTs characterized
in this PMT testing campaign. More details about this system can be found also
in JUNOPMTcontainer ; TietzschDiss ; Tietzsch_Neutrino .

Figure 3: Pictures of the PMT container system, located at Zhongshan Pan-Asia
20-inch PMT testing and potting station, China. Top row: Conceptual and actual
picture of the four testing containers; the sketch drawing shows the shelf
system mounted inside. Bottom left: Interior of one of the testing containers,
with a PMT loaded onto a drawer. Bottom right: Schematic view of the inside of
a drawer box.

####  3.1.2 PMT sample testing: scanning stations

The second setup for the individual testing of a large sub-sample of 20-inch
JUNO PMTs is the so-called scanning station system. Also this system has been
described already in other publications Tietzsch_Neutrino ; JUNOPMTstation ,
where details about mechanical setup, electronics, and measurement process can
be found. Similar to before, this setup shall be briefly described, to enhance
understanding of the later presented results.

The scanning station system constitutes a complementary system, which is able
to investigate possible inhomogeneities of characteristics along the PMT’s
photocathode surface (in complementary to the container system, which is not
sensitive to local inhomogeneities). This can be achieved using a rotatable
arch with seven stabilized pulsed LEDs mounted at different zenith angles (#1
(pole), …, #7 (equator)) covering all 360∘ azimuthal angles in 15∘ steps and
thus the full photocathode surface. The light sources used by the scanning
stations are LEDs also from HVSys company (similar to the ones in the
container systems) but only equipped with collimators and attenuators
(detected light intensity is 1 $\sim 1.5$ p.e. per flash). Two of these
scanning stations are set up for mass sampling and operated in separate
darkrooms (Fig. 4). Due to an active magnetic compensation of the scanning
stations provided by Helmholtz coils installed within the walls, floor, and
ceiling of the housing darkrooms of each station, they also allow for the
testing of a PMT’s magnetic field sensitivity in a range from -50 µT to +50 µT
as well as a PMT characterization in a completely EMF-free environment888The
residual magnetic field applied for the general testing is $\lesssim 1\,$µT..
The scanning station uses a DRS4-based ADC DRS4-web  with an additional
$\times$ 10 amplifier to acquire full PMT waveforms. The scanning procedure
and data analysis are fully automated in a four to six hours cycle per PMT and
station. The absolute light intensities of all LEDs in both scanning stations
are frequently calibrated and monitored using a small calibration PMT (head-
on-type linear-focused $1\frac{1}{8}$-inch R1355 PMT with a PDE of $23.9\pm
1.0\,\%$ at 420 nm). The calibrated LED intensity only shows a spread of less
than 2% relatively over the whole running period and thus proves the high
stability in light output of the LEDs over time.

The scanning stations allow the characterization of individual PMTs in all
relevant parameters by scanning the PMT’s photocathode (i.e. local gain and
average number of detected photo-electrons), and enable a deeper understanding
of their performance based on the PMT’s uniformity (see also sec. 4.10). This
may help to further understand the PMTs’ performance and identify potential
problems not detectable by only using the containers as testing facilities.
The scanning stations are moreover used to define the weighted photon
detection efficiency (PDE) of the whole 20-inch photocathode surface, which is
one of the key parameters to be discussed in sec. 4.4. Since the
characterization process is more complex, and is complementary to the
container system each scanning station can take only one PMT to be tested,
these features will be obtained only for a sub-sample $\sim$ 5% of all 20,000
PMTs (originally aimed to cover about 5 % randomly selected, added by cross-
checks between containers and scanning stations).

Figure 4: Pictures of the scanning station system. Top: the darkrooms used for
the two scanning stations; bottom: mechanical setup and rotating support of a
single scanning station.

###  3.2 Receiving, cataloging and testing procedures

Starting from early 2017, the first batches of JUNO 20-inch PMTs arrived at
Zhongshan Pan-Asia 20-inch PMT testing and potting station, and continued to
do so over a period of about four years (usually in batches of several hundred
PMTs) (see Fig. 5). After their arrival, a checklist was applied step by step
to realize a detailed characterization and full documentation of all PMTs.
This includes the whole process of checking-in, multiple testings (including
visual inspections as well as performance tests with the containers and
scanning stations), waterproof potting, and storage until further processing.
Barcodes representing the PMT ID are used as a keyword to identify all
individual records. The management of all PMTs is handled using a database
with a web page link JUNOrecieving  following the scheme in Fig. 6, which
includes the vendor data of each PMT, inspection results, measurement records
of both container and scanning station systems, measurement results
management, waterproof potting status, potted PMT testing results, and storage
location in individual charts, everything tagged by the individual unique PMT
ID. Generally, it takes about three to six months to complete the whole
process following the PMT receiving and storage. This cautiously executed
procedure then leads to a final classification (accepted to use in JUNO or
rejected). Since PDE and DCR are critical parameters, they will be cross-
checked between containers and scanning stations if the initially measured
value is around the boundary of the requirements as listed in Tab. 1 and
indicated in Fig. 6).

Figure 5: Statistics of PMTs received at Zhongshan Pan-Asia 20-inch PMT
testing and potting station and later tested within the testing systems. In
total 22,414 PMTs were delivered. Figure 6: Measurement and classification
procedure performed by the two testing systems. Details of the full PMT
characterization process performed by the container system can be also found
in JUNOPMTcontainer .

During the whole PMT testing campaign, 22,414 PMTs have been received and
stored in the warehouse, more than 52,000 individual PMT characterizations
with the containers and more than 5,200 PMT scans with the scanning stations
have been completed. Some PMTs have been tested multiple times including the
systems’ calibrations, bare PMT testing, tests of the monitoring PMTs, potted
PMT testing (using commercial electronics and JUNO 1F3 electronics) and
special testing campaigns (see Fig. 7). In total, 20,065 PMTs have finally
passed all testing criteria and then are selected to operate in the JUNO
experiment.

(a) PMT testing statistics (containers)

(b) PMT testing statistics (scanning stations)

Figure 7: Time evolution of the PMT testing. Left: more than 52,000 individual
PMT characterizations were performed in about 1,600 runs using containers
$\\#$A, $\\#$B and $\\#$D (some PMTs have been tested multiple times including
calibration runs, acceptance tests, re-tests, and continuous measurement
campaigns). Right: 4,656 valid characterizations from more than 5,200 scans
(including special testings, and calibration) were performed using the
scanning stations $\\#$1 and $\\#$2.

##  4 PMT testing and characterization: bare PMT results

With the designed testing systems, each of the received PMTs has been tested
at least once for the required parameters. Their performance will be discussed
in detail in the following sections, leading to the selection of a sample of
individually qualified PMTs for JUNO. During the daily testing, the reference
PMTs (compare sec. 3.1.1) are loaded into the containers in each run together
with the untested PMTs to monitor the comparability of the measured parameters
(and i.e. the stability of the container performance) over time.

###  4.1 Visual inspection

When a 20-inch PMT is working in up to 44 m water depth, there is an implosion
risk for its evacuated glass bulb SKPMTaccident ; sk-PhysRevD.78.032002 ; sk-
PhysRevD.81.092004 . To protect the PMTs from a possible implosion chain
triggered by any accident or defective PMT, all PMTs will be capsuled using a
protection cover design featuring an acrylic shell on its upper half and a
stainless steel shell around its bottom half JUNOPMTwang ; JUNOdetector . The
protection cover was optimized considering its structural strength, a good
transparency, dimension limitations from PMT photocathode coverage and
installation clearance, a compatibility with pure water, and a low
radioactivity JUNOPMTwang . The smaller diameter of PMT will affect the
physics coverage and the fix structure of the installation, while the larger
diameter is possible to introduce some conflicts among PMTs. In order to
satisfy the needs for the protection covers and assure an installation
tolerance on the $\sim 3$ mm level, a precise knowledge of the dimensions of
all individual PMTs is required. Therefore, the dimensions (dameters)s) of the
PMTs’ glass bulbs were measured for all individual PMTs. The typical diameter
of HPK dynode PMTs was found to be 510 mm in a range of 505 to 512 mm; the
typical diameter of NNVT MCP-PMTs was found to be 509 mm in a range of 506 to
511 mm (Fig. 8). Only four HPK PMTs are out of the tolerance range, which has
negligible impact on both physics and installation.

Figure 8: Measured diameter in mm of all checked 20-inch PMTs. Some PMTs were
rejected before the measurement after visual inspection, therefore the total
number of tested PMTs is slightly reduced. Left: HPK; Right: NNVT.

The glass thickness999Only a small sample of the PMTs was measured directly
for the thickness of its glass bulb using an ultrasonic thickness gauge to
find possible weak points. is assumed to be proportional to the PMT weight
statistically, thus measuring the PMTs’ weight helps to evaluate possible
weaknesses, where the lighter PMTs can be cross-checked further in more
detail101010Some of the glass bulbs from NNVT featuring a very low weight are
further tested under different water pressures, resulting in all PMTs
satisfying the structural strength requirements for JUNO.. According to the
measurement on a sub-sample of all PMTs, the typical weight of an HPK dynode
PMT is about 7.4 kg in a range of 6.5 to 8.4 kg; the typical weight of an NNVT
MCP-PMT is about 7.5 kg in a range of 5.2 to 9.1 kg (see Fig. 9).

Figure 9: Measured weight in kg of all checked 20-inch PMTs. Weighing was
performed only with a sub-sample of all PMTs. Left: HPK; right: NNVT.

A high-quality glass bulb is essential to avoid any dangerous defect related
to its structural strength, especially for JUNO since all PMTs are closely
packed. Even though both vendors have improved their glass bulb’s strength via
an optimized geometry as well as the identification of any small bubbles,
cracks, weaknesses on the glass thickness, etc., a detailed visual check on
every PMT is still required. Thus, all individual PMTs were visually checked
for mechanical related issues (including cracks, bumps, scratches), bubbles
and open bubbles, broken holes (glass notches) on the outside glass surface,
defects at the sealing or KOVAR section111111The KOVAR section of the NNVT
MCP-PMTs includes the KOVAR plate for sealing the vacuum glass bulb and the
output pins which feed the signals into the glass bulb, as well as the glass
transition section, see also the end part of the PMT neck with output pins on
the left of Fig. 2. (including a gap between the KOVAR plate and the glass,
dislocation of the KOVAR plate, broken glass or glass shedding at the
transitional section of NNVT PMTs, and issues related to the socket), glass
devitrification, gas leakage, and all other possible defects. Furthermore,
internal impurities such as metal contaminants in the photocathode or glass
were checked since they might affect the electronic features of the PMTs. The
results of the visual inspection are listed in Tab. 3 including several PMT
defects.

Table 3: Main defects of all 20-inch PMTs rejected after the visual inspection. Defect | HPK | NNVT
---|---|---
| # (% to total) | # (% to total)
Impurity inside | 5 (0.09%) | 304 (1.78%)
Mechanical related | 6 (0.11%) | 86 (0.50%)
(Open) bubbles | 353 (6.52%) | 38 (0.22%)
Broken glass hole | 0 (0.00%) | 29 (0.17%)
Seal (KOVAR) section | 1 (0.02%) | 61 (0.36%)
Gas leakage | 6 (0.11%) | 9 (0.05%)
Glass devitrification | 0 (0.00%) | 87 (0.51%)
others | 2 (0.04%) | 29 (0.17%)
Total | 373 (6.9 %) | 643 (3.8 %)

The visual inspection is a primary step of the PMTs’ acceptance testing
procedure. All PMTs with good tags are transferred to the following
performance checks. In total, 5,041 PMTs of 5,414 delivered HPK PMTs, and
16,357 PMTs from 17,000 delivered NNVT PMTs were selected as they satisfied
the requirements of the visual inspection for JUNO.

###  4.2 Working high voltage and gain

####  4.2.1 Charge spectrum calculation

Using the containers, all PMT waveforms are acquired by a 1 GS/s (samples per
second) Analog-Digital Converter (ADC, type CAEN V1742) in 512 ns long window,
synchronized to the auto-stabilized low intensity LED light pulses. To
calculate the charges of the PMT pulses, each waveform is integrated in a time
window [-20,+55] ns relative to the peak location of the signal (see Fig.
10(a)), considering the external trigger timing, an input impedance of 50
$\Omega$ of the electronics waveAnalysisHaiqiong . An example waveform and
charge spectrum in pC is shown in Fig. 10. The single photo-electron (SPE)
spectrum can be de-convoluted with Poisson statistics as in Eq. 1, where µ is
the mean p.e. number received by the first dynode or MCP, and $P(n)$ is the
probability to have $n$ p.e.s. When the value of µ is around 0.1, the
probability $P(1)$ of 1 p.e. events is $0.090$ (and $P(2)\sim 0.005$ for 2
p.e. events etc.), thus single p.e. events will dominate the signal
distribution in this case. The resulting distribution or SPE response spectrum
(SPR) can be used in the following analyses of e.g. the gain (using a
comparable and constant light intensity of µ $\sim 0.1$ for all LEDs) or the
PDE (using individual light intensities and a calibration function for a set
DAC of each LED, which is calibrated using a small sample of 20-inch reference
PMTs121212These calibration PMTs are not the same PMTs as the ones introduced
as monitoring PMTs in sec. 3.1.1. with known PDE).

$P(n)=\frac{e^{-\mu}\mu^{n}}{n!}$ (1)

In the resulting charge spectrum (see Fig. 10(b)), the pedestal represents all
trigger/signal events with no light; it will be fitted in the range
[-0.15,+0.15] p.e. ($Q_{\textit{ped}}$ and $\sigma_{\textit{ped}}$). The
second peak right of the pedestal peak corresponds to single p.e. events,
located around 1.602 pC for a gain of $1\times 10^{7}$ (as assumed in this
example). This SPE peak will be fitted in the range [0.5,1.4] p.e.131313The
range is selected to enable a unique definition for all the PMTs considering
also the long tail effect of MCP-PMTs JUNOPMTgain . ($Q_{sig}$ and
$\sigma_{sig}$). All the fitting mentioned here is done with a Gaussian
function.

(a) Example of a waveform of a single PMT as recorded by the ADC.

(b) Typical charge distribution of a single PMT for µ$\sim 0.1$ p.e.

Figure 10: Typical plots of an averaged waveform from a single PMT (left, also
showing the typical definitions used for the following analysis) and the
corresponding integrated charge spectrum from these waveforms (right). The SPE
peak is located around 1.602 pC in the case of a gain of $1\times 10^{7}$.

####  4.2.2 Gain and HV determination

To calculate the PMT gain $G$ at a set voltage, the information from the
charge spectrum as shown before in Fig. 10(b) can be used as shown in Eq. 2:

$G=\frac{Q_{\textit{sig}}-Q_{\textit{ped}}}{e}$ (2)

The voltage to be applied for a gain of $1\times 10^{7}$ is determined from a
series of measurements with different high voltages (HV), performed between
-150 V to +150 V (executed in 50 V steps) relative to the HV value proposed by
the manufacturer. The optimal working HV for a $1\times 10^{7}$ gain is then
identified using a fit between applied HV and measured gain, as shown in Fig.
11(a). The used fitting function is defined in Eq. 3 with $G$ the measured
gain, which is consistent with the applied fitting function used for dynode
PMTs in the selected range HamManual . Additional information about the HV
determination can be found in JUNOPMTcontainer . A discussion about different
methods for the gain determination and their effects can be found also in
separate papers PMTgainmodel1994 ; PMTgainmodel2017 ; JUNOPMTgain .

$\log_{10}\,(G)=A~{}\times~{}\textit{HV}+B$ (3)

The fitted correlation factor $A$ between HV and gain $G$ of all the measured
PMTs is shown in Fig. 11(b). The mean value of $A$ is $0.0013$ for HPK and
$0.0022$ for NNVT, which means the working gain will be doubled when
increasing the HV by additional $\sim$ 230 V for HPK PMTs and $\sim$ 140 V for
NNVT PMTs around this working gain range.

(a) Example plot and fitting of the HV vs. measured gain, with corresponding
fit results for the data.

(b) Gain factor $A$ for the gain of the qualified PMTs. Red: NNVT; blue: HPK.

Figure 11: Example plot for a HV vs. gain fit (left, for a single PMT) and the
distribution of the fitted gain factor $A$ (as introduced in Eq. 3) of all
qualified PMTs (right).

With having tested the monitoring PMTs hundreds of times during the whole
campaign, the standard deviation of the determined working HV repeatability
was found to be about 1 % (15$\sim$18 V) for both HPK and NNVT PMTs on
average, and the standard deviation of the determined $1\times 10^{7}$ gain is
$2$ % for HPK dynode PMTs and $3$ % for NNVT MCP-PMTs, which indicates the
uncertainty in determining the working HV and gain with the container system.

The determined working HV of all qualified bare PMTs producing a gain of
$1\times 10^{7}$ is shown in Fig. 12(a); the determined mean HV is about
$1863$ V for HPK, and $1748$ V for NNVT PMTs. The determined HV for a $1\times
10^{7}$ gain is a slightly biased with respect to the values suggested by the
manufacturers of both PMT types (the working HV is $20$ V higher for HPK PMTs
and $20$ V lower for NNVT PMTs on average compared to the supply voltages
suggested by the manufacturers), even though they show a very good correlation
(the fitted slope is 0.99 for HPK PMTs and 1.01 for NNVT PMTs), as illustrated
in Fig. 13. This difference could arise from different testing methods, cable
attenuations, used HV dividers and electronics. However, all PMTs are reaching
the requirements for the working HV at a $1\times 10^{7}$ gain, i.e. the NNVT
PMTs show a much lower working HV value than the allowed maximum value as
fixed in Tab. 1.

The final gain applied in the container system during the PMT characterization
is cross-checked once more as a part of the regular analysis. The results show
that the set gains do not exactly match a value of $1\times 10^{7}$ in all
cases, but follow a distribution with a width of 2-4 % (which is consistent to
the estimation about the gain determination uncertainty from the monitoring
PMTs) as well as an offset of $3\,\%$ for the NNVT MCP-PMTs towards a higher
gain, as presented in Fig. 12(b). This is mainly introduced by the used HV-
gain model in the data analysis, which is more applicable for the dynode PMTs
than for the MCP-PMTs.

(a) Determined HV of all qualified bare PMTs for a gain of $1\times 10^{7}$.

(b) Measured gain distribution of all qualified PMTs during the PMT
characterization.

Figure 12: Determined HV and gain of all qualified PMTs. Measured gains not
contained in $[0.95,1.15]\times 10^{7}$ lead to a retest of the corresponding
PMT. Black: all PMTs; red: NNVT; blue: HPK.

Figure 13: Comparison between the working HV for a gain of $1\times 10^{7}$ of
all qualified bare PMTs, measured by JUNO and the suggested HV values from the
manufacturers. Left: HPK; right: NNVT.

###  4.3 Single photo-electron features

With the determined HV for a $1\times 10^{7}$ gain, approximately 20,000
waveforms are acquired for each PMT in the SPE mode (with µ$\simeq 0.1$ p.e.)
using an external trigger synchronizing the LED pulses to the data
acquisition. The recorded waveforms are analyzed for their characteristic
features in order to extract the SPE spectrum and a number of important
parameters characterizing the PMT’s performance. These parameters and the
resulting distribution for the full data set will be presented in following
sections.

####  4.3.1 Signal-to-noise ratio (S/N)

The signal-to-noise (S/N) ratio in the container measurements is defined as a
control parameter to ensure a valid measurement of the charge resolution and
peak-to-valley ratio, as well as for a reliable threshold setting in the
following PDE and DCR measurements. The S/N ratio is defined in Eq. 4, where
$\sigma_{\textit{ped}}$, $Q_{\textit{sig}}$ and $Q_{\textit{ped}}$ all are
calculated as described in sec. 4.2.1. The S/N ratio is required to be larger
than 10 in all individual SPE measurements, corresponding to a noise level
smaller than 0.1 p.e. At the container systems, the mean S/N ratio for both
HPK and NNVT PMTs is around around 13, as depicted in Fig. 14(a).
For comparison, the scanning station has reached a S/N ratio of $20$ in its
measurements due to the use of an additional amplifier (see sec. 3.1.2). This
also allows high quality cross-checks of the PDE and DCR measurement with the
scanning station.

$S/N=\frac{Q_{\textit{sig}}-Q_{\textit{ped}}}{\sigma_{\textit{ped}}}$ (4)

(a) S/N ratio of the container systems.

(b) SPE amplitude of container systems.

Figure 14: The S/N ratio of all the measurements with qualified PMTs (left).
The SPE amplitude of all qualified PMTs (right). Black: all PMTs; red: NNVT;
blue: HPK. Note: the amplitude is only selected within [4,10] mV.

####  4.3.2 Amplitude of single photo-electron

In case of a $1\times 10^{7}$ working gain, the pulse amplitude of SPE events
is picked out from the recorded waveforms as described in Fig. 10(a), and
corresponds to the minimum value of the negative SPE pulse form. Typical
values for each PMT are shown in Fig. 14(b); HPK dynode PMTs have a slightly
smaller SPE amplitude of about 6.5 mV compared to about 7.5 mV for the NNVT
MCP-PMT, whereas HPK PMTs have a narrow distribution (with a standard
deviation (STD) of $0.4$ mV) compared to NNVT PMTs (STD of $2.5$ mV). The
different mean values of SPE amplitudes of the two PMT types are mainly
influenced by the different pulse shapes related to the PMTs itself and the
designs of the HV dividers.

####  4.3.3 Rise-time, fall-time and full width at half maximum of SPE pulses

Rise-time, fall-time and full width at half maximum (FWHM) of a typical pulse
can be extracted from the full sample of recorded waveforms of each PMT,
following the definition illustrated in Fig. 10(a). As commonly known, the
rise-time and fall-time are more related to the PMT itself, i.e. using the
JUNO optimized version of the HV divider for the NNVT tubes, the rise-time was
observed to be slower than in the case of using an early design version of the
HV divider (which was used also for early testings). All qualified PMTs were
tested in the containers with different pluggable JUNO optimized HV dividers
JUNOPMTsignalover ; JUNOPMTsignalopt ; JUNOPMTflasher . Their SPE pulses show
typical values for rise-time, fall-time, and FWHM of $6.9$ ns, $10.2$ ns and
$11.6$ ns for HPK PMTs, and $4.9$ ns, $17.3$ ns, and $7.9$ ns for NNVT PMTs,
respectively.

####  4.3.4 Charge resolution

The charge resolution of the PMTs’ SPE response is calculated using Eq. 5,
where the $\sigma_{\textit{sig}}$ originates from the $\sigma$ of the Gaussian
fitting of the PMT’s SPE charge spectrum as introduced in sec. 4.2.1. The
distribution of the SPE charge resolution for the full PMT sample as measured
by the container system is shown in Fig. 15(a), where the mean values are
around 27.9 $\%$ for the HPK PMTs and around 33.2 $\%$ for the NNVT PMTs.
However, the characteristic PMT charge response does not necessarily follow a
Gaussian distribution AugerPMT , especially in case of the NNVT PMTs which
normally show a long tail in their charge spectrum JUNOPMTgain . Nevertheless,
the use of a Gaussian fit is justified since the parameter is used only as a
relative control for charge response spread. The SPE charge resolution is
required to be less than 40% for all individual PMTs of the JUNO CD, while it
releases for a few NNVT tubes for the JUNO VETO detector.

$\textit{Res.}\,(\%)=\frac{\sigma_{\textit{sig}}}{Q_{\textit{sig}}-Q_{\textit{ped}}}\times
100$ (5)

(a) SPE charge resolution.

(b) Measured P/V.

Figure 15: Left: SPE charge resolution of all qualified PMTs; right: measured
P/V of all qualified PMTs. Black: all PMTs; red: NNVT; blue: HPK. Please note
that the requirement of the charge resolution is relaxed for a few NNVT tubes
for the JUNO veto detector, which is higher than 40%.

####  4.3.5 Peak-to-valley ratio (P/V)

The peak-to-valley ratio (P/V) is a parameter to estimate the PMT’s ability to
distinguish between noise and photo-electron signals which can be calculated
using Eq. 6 and is based on the measured SPE charge spectrum (see Fig. 10(b)),
where the valley $N_{v}$ is defined as the level of the local minimum between
pedestal and SPE peak and is fitted by a parabolic function, while the peak
$N_{p}$ describes the level of the SPE peak picked out from their Gaussian
fit. The measurement results from the container systems of all qualified PMTs
are shown on the right of Fig. 15(b), where its typical values are around 3.8
for HPK PMTs and 3.9 for NNVT PMTs. The mean of the P/V of both PMT types
satisfies the defined acceptance criteria (larger than 3 of HPK PMT and 3.5 of
NNVT PMT, respectively), while some of individual PMTs show much smaller
values ($<2.5$), especially the NNVT PMTs.

$P/V=\frac{N_{p}}{N_{v}}$ (6)

####  4.3.6 Excess noise factor (ENF) and gain excess normal distribution
factor (gENF)

A previous study AugerPMT  also mentioned an excess noise factor (ENF),
defined in Eq. 7 in order to effectively represent the measured charge spread.
The larger the ENF under a similar light intensity, the broader the
distribution of output signals will be for the same input141414If the input is
following a Poisson distribution, and the smearing is following a Gaussian
distribution (in an ideal case), it is that
$\textit{ENF}=1+\textit{Resolution}_{\textit{ideal}}^{2}$.. $N_{\textit{pe}}$,
$\sigma_{\textit{spec}}$ and $M_{\textit{spec}}$ in Eq. 7 are the measured
number of p.e.s (mean number of p.e. from a measurement as µ in Eq. 1), the
standard deviation and the mean of the whole charge spectrum distribution
including the pedestal151515The pedestal’s mean value is shifted to zero.,
respectively. The ENF distribution of all qualified PMTs is shown in Fig.
16(a), where the typical values are 1.17 for HPK PMTs and 1.55 for NNVT PMTs.
The distribution indicates that the variation of output signals of NNVT PMTs
for the same input is much larger than in case of HPK PMTs.

$\textit{ENF}=N_{\textit{pe}}~{}\times~{}\left(\frac{\sigma_{\textit{spec}}}{M_{\textit{spec}}}\right)^{2}$
(7)

The gain excess normal distribution factor (gENF), as defined in Eq. 8 and
discussed in PMTgainmodel2017 ; JUNOPMTgain , is further characterizing the
mismatch between the determined charge in p.e. at a gain $G$ (determined from
the SPE peak based on a normal distribution as in Eq. 2) and the expected
charge based on the Poisson distribution (as in Eq. 1), where
$Q_{\textit{pC}}$, $Q_{\textit{pe}}^{\textit{G}}$ and
$Q_{\textit{pe}}^{\textit{Poisson}}$ are the measured charge161616With
$Q_{\textit{pe}}^{\textit{Poisson}}=Q_{\textit{pe}}^{G_{\textit{avg}}}$,
compare JUNOPMTgain . of the PMT pulses in pC, the charge in p.e. expected for
a gain $G$ and the individually expected charge of a PMT pulse in p.e. based
on Poisson statistics. $Q_{\textit{pe}}^{\textit{Poisson}}$ is only defined
with a pulsed light source (such as the used LED) taken under a synchronized
external trigger. Using the gENF factor, the expected charge in p.e. of a PMT
measurement can be calculated as defined Eq. 9 for a spectrum on average or
for a single signal.

$\textit{gENF}=\frac{Q_{\textit{pe}}^{G}}{Q_{\textit{pe}}^{\textit{Poisson}}}=\frac{Q_{\textit{pC}}/G}{Q_{\textit{pe}}^{\textit{Poisson}}}$
(8)

$Q_{\textit{pe}}^{\textit{exp}}=\frac{1}{\textit{gENF}}~{}\times~{}Q_{\textit{pe}}^{G}$
(9)

(a) ENF distribution

(b) gENF distribution

Figure 16: Left: ENF distribution of all qualified bare PMTs. Right: gENF
distribution of all qualified bare PMTs. Red: NNVT; blue: HPK.

With the container systems, the gENF factor was measured and calculated for
all qualified PMTs as shown in Fig. 16(b). One can find that the gENF factor
of the HPK dynode PMTs is smaller than 1 ($0.93$), while that of NNVT MCP-PMTs
is bigger than 1 ($1.28$). The expected charge in p.e. is little larger than
the calculated charge at a gain $G$ for HPK dynode PMTs, while it is smaller
for NNVT MCP-PMTs. This effect is caused mainly by the imperfect amplification
process and correlated noise of the dynode PMT on one hand, and by the long
tail of the charge spectrum which is typical for MCP-PMTs on the other hand
(see the SPE amplitude spectra in Sec. 4.5.1); those effects are discussed in
more details also in JUNOPMTgain .

###  4.4 Photon detection efficiency (PDE)

Instead of setting a requirement on the QE of the PMTs, JUNO uses the photon
detection efficiency (PDE) to measure the sensitivity of the PMTs. Here, the
PDE of the 20-inch PMTs is defined as the photon detection efficiency for
light impinging vertically on the PMT surface averaged over the whole
photocathode area with a surface area weight. This can be measured directly by
the scanning station by calculating a surface weighted average of all its
measurement points across the PMT area171717The average PDE here is calculated
as
$\textit{PDE}(\lambda)=\sum_{\textit{spot}\\_\textit{i}}\textit{PDE}_{\textit{spot}\\_\textit{i}}(\lambda,\vec{r})\times\textit{Weight}_{\textit{spot}\\_\textit{i}}$.
considering the geometry differences of HPK and NNVT PMTs, where a reference
PMT (HPK R1355 as mentioned in sec. 3.1.2) is used for normalization.
Following the QE measurements for samples of both types of 20-inch PMTs in
largePMTlei ; largePMTxia ; waveAnalysisHaiqiong ; MCPPMTTTSsen ; MCPPMT2016 ;
MCPPMT2018 , a special comparison study on the relative collection efficiency
among PMTs with different collection structure PMTrelativeCE  has concluded
that the reference PMT’s QE can be dealt as its PDE, assuming a $100\,\%$
collection efficiency in case of a 5 mm light spot impinging on the PMT
center. Furthermore, the measured PDE of the container systems is normalized
drawer by drawer to the scanning station using a set of 20-inch PMTs that have
been measured in both the container and scanning station systems. This set of
20-inch PMTs with known PDE was also used to individually calibrate the light
intensities in the drawers to enable a reliable PDE measurement in the first
place.

####  4.4.1 PDE measurement

The PDE measurements of the containers and scanning stations are realized by
employing the photon counting method on the charge spectra obtained with the
stabilized pulsed LEDs operated at low intensity, where the effects of
threshold and dark count rate are checked and considered as uncertainty.
Considering the observed noise level and charge resolution, a threshold of
around 0.25 p.e. was chosen to separate the pedestal from photon induced
events. The light intensity used in both container and scanning station system
is a low multi-photon level (with $1-2$ p.e.s observed by the PMTs) to
minimize the uncertainty from system and statistics as suggested in
PMTintensity . Based on the repeatability of the daily measurements of the
monitoring PMTs (see sec. 3.1.1), a measurement uncertainty of better than 1 %
of the absolute PDE ($3$ % relative PDE)181818Considering the later discussed
light source aging in sec.4.4.2, the acceptable uncertainty is enlarged to
$1.5$ % abs. ($4-5$ % relative) for the directly measured PDE values for both
containers before applying any aging corrections. is achieved for both
containers JUNOPMTcontainer  as well as for both scanning stations (see sec.
3.1.2).

The PDE distribution of all qualified PMTs measured by the container system is
shown in Fig. 17(a), where the mean PDE is $28.1\,\%$ for HPK PMTs, and
$28.9\,\%$ for NNVT PMTs. The sample of qualified NNVT PMTs can be classified
as 4,609 NNVT low-QE PMTs (early version of the MCP-PMTs and identified by
production dates) with a mean PDE of $26.8\,\%$, and 10,456 NNVT high-QE PMTs
(latest version of the MCP-PMTs with higher QE, again identified by production
dates) with a mean PDE of $29.9\,\%$ (Fig. 17(b)). Following the proposed
selection to distribute the qualified PMTs between JUNO CD and veto detector,
the mean PDE for the 17,612 PMTs selected for the CD is $29.1\,\%$ (including
all HPK PMTs191919Due to their clearly better timing performance (see
sec.4.6), all selected HPK PMTs will be used for the CD instrumentation.),
while the mean PDE for the 2,400 PMTs selected to be used in the veto detector
is $25.6\,\%$.

(a) Measured PDE of all qualified PMTs

(b) Measured PDE of NNVT PMTs only

Figure 17: Measured PDE distribution of all qualified PMTs (left), and of all
qualified NNVT PMTs (right). Black: all PMTs; red: NNVT; blue: HPK; Right:
dark green: High_QE; deeppink: Low_QE.

####  4.4.2 Light system aging and PDE correction

Within the $\sim$four years of operation of the two container systems, a
decrease in the measured PDE of the monitoring PMTs was found for both of
container $\\#$A and $\\#$B as shown on top of Fig. 18. Both HPK and NNVT
monitoring PMTs show a similar trend, which however required a long period of
observation to be confirmed within the stated PDE uncertainty. The results
were checked in detail based on the results of the monitoring PMTs as well as
based on additional cross-checks with additional PMTs on the consistency among
drawers, between the two containers, between the containers and scanning
stations, as well as between the scanning stations themselves. It is concludes
that the decrease is mainly caused by the LED light system202020This includes
LED, collimator, attenuator (neutral density filter), PTFE diffuser plate and
reflector (light shaping tube in the drawer boxes). itself used in the
containers rather than by direct PMT aging. The effect directly depends on the
container drawer as well. The scanning stations do not observe such a
decreasing effect in the same period, according to the calibration and
monitoring by the reference PMTs.

(a) Container #A before correction

(b) Container #B before correction

(c) Container #A after correction

(d) Container #B after correction

Figure 18: Relative variation of the measured PDE of the monitoring PMTs (HPK
PMTs tagged by ”EA”, NNVT PMTs tagged by ”PA”) at the container system, before
and after a correction based on a recalibration at the end of the regular
testing period. Indicated variation is relative to the beginning of the
testing period, which is used to compare among the PMTs. Top: directly
measured PDE (with top left: container $\\#$A; top right: container $\\#$B).
Bottom: PDE after aging correction (with bottom left: container $\\#$A; bottom
right: container $\\#$B).

Based on a dedicated recalibration of all individual container channels at the
end of the regular testing campaign and the observations of the monitoring
PMTs from all individual drawers, a correction factor for the measured PDE was
specified and applied for each drawer box of both containers, using a linear
assumption for the PDE decrease relative to the testing date. The correction
scale on average of all drawers was found to be $6.5\times 10^{-5}$/day for
container $\\#$A and $\sim 7.6\times 10^{-5}$/day for container $\\#$B and was
applied for both HPK and NNVT PMTs. Finally, the corrected PDE results show a
stable behaviour of the monitoring PMTs in both container systems, as well as
consistent results with the scanning stations and between both containers.

The largest residual factor after the aging correction of all drawers is
considered as the final bias uncertainty of the container system, which is
about 2.3% relatively for the tubes tested after 1,000 days with an aging
factor of $2.3\times 10^{-5}$/day, translating to an absolute value of $0.7$ %
for tubes with 30% PDE. After applying the aging correction of each drawer to
the measured PDE of the container systems, the distributions of the corrected
PDE results from the measurements of the monitoring PMTs (as shown at the
bottom of Fig. 18) are checked again as an uncertainty control. The variations
are now within the aimed uncertainty range of $\lesssim 1\,\%$ absolute PDE,
in particular when taking the spread of the corrected PDE distribution of each
monitoring PMT into account. The largest variation of container #B is provided
by the fixed drawer monitoring PMT that is not used for daily measurements,
which also includes a possible variation of the PMT itself.

The distributions of the corrected PDE results of all qualified PMTs are
updated and depicted in Fig. 19(a), where the averaged values now increased to
$28.5\,\%$ for the HPK PMTs and $30.1\,\%$ for NNVT PMTs. The average PDE of
the qualified NNVT low-QE PMTs was updated to $27.3\,\%$, and $31.3\,\%$ for
the qualified NNVT high-QE PMTs respectively212121Since the difference between
the corrected and original PDE is related to the testing date, the HPK PMTs
are affected less as they were tested at an early stage, while the NNVT high-
QE PMTs were tested later and thus are affected more., as shown in Fig. 19(b).
Furthermore, the corrected results of the proposed PMTs for JUNO CD and veto
detector are also updated, with the averaged PDE increase to $30.1\,\%$ in the
CD and to $26.1\,\%$ in the veto detector.

(a) Corrected PDE of all qualified PMTs.

(b) Corrected PDE of NNVT PMTs only.

Figure 19: Corrected PDE distribution of all qualified PMTs (left), and of all
NNVT PMTs (right). Black: all PMTs; red: NNVT; blue: HPK; dark green: high-QE
MCP-PMTs; deep pink: low-QE MCP-PMTs.

###  4.5 Dark count rate (DCR)

Using the container system, the dark count rate (DCR) of each individual PMT
was measured under a $1\times 10^{7}$ gain. Data acquisition was performed
using a CAEN V830AC scaler together with a CAEN V895B leading-edge
discriminator, as described in JUNOPMTcontainer . The measured DCR value can
be affected by the applied signal threshold, the time the PMT initially stayed
in darkness in order to stabilize its dark rate prior to the measurement
(“cooling time”), the photocathode’s temperature, and the applied gain. The
effect of each factor was checked in detail and will be discussed in the
following.

####  4.5.1 Amplitude threshold

A quarter p.e. threshold was selected to consider the noise level of the
system, the SPE charge resolution of the PMTs, as well as possible future JUNO
conditions. The target threshold in charge corresponds to a signal threshold
of about 3 mV for the pulse amplitude of both HPK and NNVT PMTs. The optimal
threshold is not located precisely at the same level for both PMT types due to
pulse shape differences between HPK and NNVT PMTs – moreover, it is expected
to be specific for each PMT rather than only refer to the mean amplitude value
for an SPE pulse at a gain of $1\times 10^{7}$. Since there are operational
constraints set by the electronics used in the test222222The used CAEN V895
discriminators provide a minimum step size for the threshold setting of only 1
mV, see also CAEN-disc ., a threshold of 3 mV for the pulse amplitude was
selected for the DCR measurements and was found to be a good compromise
between measurement accuracy and comparability on one side, and mentioned
constraints and present noise level on the other side, as illustrated in Fig.
20.

Figure 20: Effect of the set amplitude threshold on the DCR measurement of a
single, typical PMT. The cut distributions (in blue) represent all pulses with
a charge above the threshold of 0.25 p.e., matching well to a pulse amplitude
threshold of 3 mV. Left: HPK pulse amplitudes; right: NNVT pulse amplitudes.

As indicated in Fig. 6, PMTs failing the DCR acceptance criteria in the
containers have been reevaluated in another container run or finally using the
scanning station to exclude any other possible uncertainties on threshold or
noise level. Since the scanning stations are using a factor $\times 10$
amplifier prior to the counting electronics (see sec. 3.1.2 again), they can
apply i.e. an adequate DCR threshold with much higher precision than possible
in the container systems.

####  4.5.2 Cooling time

Large area PMTs such as the used 20-inch PMTs of JUNO generally need several
hours in a dark environment to stabilize their dark rate after applying the
HV. This is valid i.e. after the PMT was exposed to ambient light. As
discussed in JUNOPMTcontainer , all PMTs tested in the container system are
resting for at least 12 hours in darkness before the start of the PMT
characterization to ensure an effective (and significant) DCR
measurement232323This cooling time also considers operational constraints for
an optimized testing cycle (max. of 24 hours including the reloading of the
containers).. During this time, the DCR will be frequently monitored as
exemplarily presented in Fig. 21. The concluding DCR measurement is usually
performed after about 16 hours in darkness, as explained in JUNOPMTcontainer
242424Some container runs, i.e. the ones performed over the weekends, are
performed with extended cooling times, in order to investigate the
stabilization behavior of PMTs after a longer time in darkness, as well as to
use the best possible conditions for a significant DCR measurement. With the
monitoring data, it was confirmed that the bare HPK PMTs usually show a stable
performance after $\sim$12 hours cooling time while the bare NNVT PMTs usually
show a stable performance after $\sim$25 hours of cooling time.. The gain has
been set to a level of $1\times 10^{7}$ following the procedure described in
sec. 4.2.1 prior to this measurement.

Figure 21: Measured DCR vs. cooling time for a typical HPK and NNVT PMT. The
measured DCR values are normalized to the value taken after 12 h (time after
loading the PMT into the container). The indicated time of $\sim 16$ h marks
the usual time when the final DCR measurement is performed as part of the
regular PMT characterization process.

####  4.5.3 Temperature

As discussed in HamManual ; lowTPMT2 ; largePMTlei ; largePMTtemperature , the
actual DCR of a PMT is a function of temperature. We thereby assume that the
photocathode’s temperature is in equilibrium with the temperature of the
surrounding air, i.e. after a several hours stay in the drawers. To estimate
the effect of different air temperatures on the DCR results of the JUNO PMTs
during their characterizations, data from the temperature monitoring system
within the containers was taken into account to the observed DCR of the PMTs.
This was done for a large PMT sub-sample containing more than 1,800 HPK PMTs
and more than 11,900 NNVT PMTs. Although every PMT acts uniquely, a slight
trend could be observed for each PMT type, indicating a mild temperature
dependence of about 0.5 kHz/∘C for the combined HPK PMT sub-sample, and of
about 3.0 kHz/∘C for the combined NNVT PMTs sub-sample within a temperature
range of 19∘C to 29∘C as present during the mass characterization campaign,
see also Fig. 22.

(a) HPK PMTs (bare and potted)

(b) NNVT PMTs (bare and potted)

Figure 22: Observed correlation between air temperature in the container and
measured DCR of a large sub-sample of PMTs. The black dots indicate the mean
and STD of all DCR measurements within the corresponding $0.25^{\circ}$ C
slice.

Additionally, specific temperature surveys were performed using the container
system with its HVAC unit to gain a better understanding of the temperature
dependence of the measured DCR. In these surveys, the effect of temperature to
the measured DCR was investigated by varying the temperature in a range from
14∘C to 28∘C (with a rate of 0.5∘C/h) and monitoring the DCR in parallel. Such
a testing cycle containing 26 HPK and 26 NNVT PMTs is shown in Fig. 23, where
the DCR of every measurement was normalized to the value at 22∘C of each PMT
individually. The results of these surveys are confirming a stronger
temperature dependence of the NNVT PMTs than for HPK PMTs, which is on average
about 0.2 kHz/∘C ($\sim$2 %/∘C relative DCR change) for the combined HPK PMT
sample, and about 4.8 kHz/∘C ($\sim$10 %/∘C relative DCR change) for the
combined NNVT PMT sample. These values are consistent with the before
mentioned estimation between 19∘C and 28∘C in a first order approx. The
results further indicate, that the observed DCR is significantly increasing
for temperatures higher than 22∘C (particularly for the NNVT PMTs). It should
also be noted, although not visible in the plot, that PMTs showing a higher
absolute DCR at 22∘C are more sensitive to temperature changes, compared to
PMTs with a lower absolute DCR at 22∘C.

(a) Sample of 26 HPK PMTs

(b) Sample of 26 NNVT PMTs

Figure 23: Measured DCR versus temperature in a specific temperature survey.
The black dots represent the averaged trend of all measured PMTs under the
same temperature. The discrete points far from the trend are generated by
noise during the tests.

####  4.5.4 Measured dark count rate (DCR)

The measured DCR distributions (as shown in Fig. 24) of all qualified PMTs
were tested by the container systems using the discussed amplitude threshold
of 3 mV (p.e. threshold of 0.25, refer to Fig. 20), with a cooling time of
$\sim 16$ hours, and at an average air temperature of $25\pm 3^{\circ}$C. The
mean value is around 15.3 kHz for HPK PMTs and 49.3 kHz for NNVT PMTs. The
NNVT DCR distribution shows a long tail which is cut at 100 kHz according to
the requirement of JUNO as listed in Tab. 1. Also, both low- and high-QE MCP-
PMTs show a consistent DCR distribution. In the final JUNO detector, an even
lower average DCR is expected based on the considerations in sec. 4.5.3, since
the water temperature in the veto pool (where the PMTs will be embedded in) is
predicted to be at about $21\pm 1^{\circ}$C.

Figure 24: Measured DCR distribution of qualified PMTs. Left: black solid: all
PMTs; red solid: NNVT; blue solid: HPK; Right: dark green solid: High_QE; deep
pink solid: Low_QE. Note: the DCR of NNVT PMTs is selected with $\leq$ 100
kHz.

###  4.6 Transit time spread (TTS)

The transit time spread (TTS) describes the timing resolution of the PMT. It
represents the spread of different photo-electron transit times within the PMT
bulb, which depend on the interaction point and emission angle of the photo-
electron, as well as on the energy of the released photo-electron and the
focusing electric field distribution PMTbook . For JUNO, the timing resolution
of the HPK dynode PMTs is crucial for a precise event reconstruction in the
detector. For these PMTs, the distribution of transit times (TT)252525The
relative “transit time” (TT) calculates in this case from the difference
between the trigger time of the laser light pulse and the time when the PMT
pulse arises (“hit time”) as shown in Fig. 10(a). Its absolute number is
arbitrary here, since it depends also on cable lengths and individual response
times of the data taking electronics. follows a Gaussian distribution as shown
in Fig. 25(a), where the TTS is defined as the $\sigma$ of this distribution,
with $\sigma\simeq\mathrm{FWHM}/(2\sqrt{2\ln 2})$ HamManual . In case of the
NNVT MCP-PMTs, this relative TT distribution is much more complex and contains
a varying number of substructures (sub-peaks), see Fig. 25(b). These features
are related to the PMT design and MCP structures MCPPMTTTSsen  and are a
consequence of the optimization of the MCP-PMTs for a maximum collection
efficiency, which leads to a worse timing resolution TTSQianS  (will be
discussed in sec. 4.10.4). As a consequence, the TTS can not be described
using a Gaussian fit, but rather will be described here as the mean standard
deviation of the TT distribution with the edges cut away (only the quantiles
$\left[Q_{03}:Q_{97}\right]$ are taken into account) to remove outliers and
noise events262626This method has been approved by comparing the TTS results
of about 200 HPK PMTs analyzed with both methods (fit and mean std. dev. of
cut distribution), showing highly consistent results.
$\left[Q_{03}:Q_{97}\right]$ are the quantiles of first 3% and 97% of all the
entries from left to right..

(a) Typical HPK PMT

(b) Typical NNVT PMT

Figure 25: Typical relative transit time (TT) distributions for an HPK (left)
and an NNVT PMT (right, indicated “hit times”). The PMT’s TTS is determined
using a Gaussian fit in case of the HPK PMTs, and using the std. dev. of a cut
distribution (use only quantiles $Q_{03}$ to $Q_{97}$) in case of the NNVT
PMTs. Both methods produce consistent results if applied on the data of HPK
PMTs.

All valid TTS results discussed in this section are from measurements with the
container system using the pico-second laser fiber system272727The container
LED system only features light pulses with a pulse width of $\sigma\sim 2$ ns,
which is too large to resolve the TTS of the HPK PMTs, thus it can be used
only for a relative comparison, particularly for NNVT PMTs. JUNOPMTcontainer ;
TietzschDiss  and similar settings as used for the SPE features in sec. 4.3
(i.e. a light intensity of µ$\simeq 0.1-1$ p.e.), but with larger statistics
of up to 50,000 trigger events. The triggers for the data acquisition and
light sources are provided by a Keysight 33512B arbitrary waveform generator
featuring a jitter of only $\sim 50$ ps containerGen . Unfortunately only
container $\\#$B has a sufficient timing resolution, with a minimum measurable
TTS of $\sim 0.8$ ns (that’s the reason why there is a left cut around 0.8 ns
in Fig.26(b)) – due to a mechanical incident, the fiber system of container
$\\#$A can not provide a timing resolution of $<2$ ns anymore. The final
accuracy for the TTS measurements achieved with container $\\#$B’s laser is
less than 0.2 ns (RMS 0.2 ns) for HPK PMTs and less than 0.6 ns (RMS 1.4 ns)
for NNVT PMTs 282828Different measurement accuracies are due to the different
analysis methods related to the PMT type. Values describe
repeatability/accuracy for multiple measurements of the same PMT. (also see
Fig. 26(a)). Systematic effects introduced by the data taking electronics and
the fiber system were measured to be only about 0.6 ns on average for all
channels and were corrected in order to achieve a fair estimation of the PMTs’
TTS.

(a) TTS reproducibility of HPK and NNVT monitoring PMTs.

(b) TTS distribution of a sub-sample of qualified bare PMTs.

Figure 26: Left: TTS results of the monitoring PMTs in container #A. Combined
result distribution of all monitoring PMTs (3 HPK, 2 NNVT, tested in all
channels) shows a good reproducibility of the results. Right: TTS distribution
of a sub-sample of qualified bare PMTs containing 837 HPK and 3,610 NNVT PMTs,
measured with the ps-Laser system (red: NNVT; blue: HPK).

Since only one of containers $\\#$A and $\\#$B provides a sufficient timing
resolution, the TTS was evaluated only for a large sub-sample of all tested
PMTs. The results of this sub-sample are representative for the full sample of
qualified PMTs for JUNO as presented in the other sections. Results are
available for a total of 837 HPK and 3,610 NNVT PMT of qualified bare PMTs
tested with the laser system in container $\\#$B, covering about 16.7 % of the
full qualified HPK PMT sample, and about 24.0 % of the full qualified NNVT PMT
sample292929In total, TTS data of 1198 qualified HPK and 4042 qualified NNVT
PMTs tested in container $\\#$B was analyzed, including monitoring PMTs and
potted PMT, while the distribution of the full analyzed sample is highly
consistent with Fig. 26(b).. The results’ distribution is depicted in Fig.
26(b). The mean TTS is about 1.3 ns in $\sigma$ as required for HPK dynode
PMTs, with only about $1\,\%$ of the examined PMTs clearly failing the timing
performance requirements for JUNO, which is at an acceptable level. Results
are also consistent with another study performed earlier, see JUNOTTS2  for
details. The mean TTS of NNVT MCP-PMTs is about 7.0 ns, see also Fig. 26(b).

The relative transit times’ distribution can be calculated also from the data
shown in secs. 4.3.2 and 4.3.3 using the LED system of the containers (SPE
light intensity, 20,000 waveforms)303030The relative TT calculation using the
LEDs is analogue to the one using the ps-laser system.. This is shown in Fig.
27(a) for an example, single HPK and NNVT PMT. Comparing to the results shown
in Fig. 25(b) using the laser system, the sub-peaks in the relative TT
distribution of NNVT MCP-PMTs are smeared out by the container LEDs – this
will be further discussed in sec. 4.10.4 in the scope of the scanning station
(which also uses LEDs). When calculating the relative TTS of each PMT measured
with the LED system of the containers313131In case of the LED measurement, the
‘relative TTS’ describes the combined timing resolution of PMT and light
system, in contrast to the laser measurement where the TTS described purely
the PMT performance. The timing resolution of the LED itself can refer to
HVSYS-LED ., the relative TTS can be described by the sigma of a directly
Gaussian fit on the relative TT distribution, as shown in Fig. 27(a). Using
the monitoring PMTs for checking the repeatability of these measurements, the
final accuracy for the relative TTS measurements using the LEDs achieved with
containers $\\#$A and $\\#$B is around $\sigma\sim 0.2$ ns (RMS $1.9$ ns) for
HPK PMTs and around $\sigma\sim 0.7$ ns (RMS $1.6$ ns) for NNVT PMTs, which is
consistent to the laser measurements. The typical values of the relative TTS
from the LED measurements for all qualified PMTs is $2.6$ ns for HPK and $8.4$
ns for NNVT PMTs, as shown also in Fig. 27(b). The results for both PMT types
are larger than the values acquired from the laser measurements in container
$\\#$B as expected from the smearing due to the LED pulses and differences of
the photon distribution on the photocathode between laser and LED system.
Therefore, the results acquired using the LED system will be used only for a
relative comparison and systematic control, and for comparison with the
results of the scanning stations (will be presented in sec. 4.10.4).

(a) Examples of relative TT, measured with the LED for single HPK or NNVT PMT.

(b) Relative TTS, measured with the LED of a sub-sample of qualified bare PMTs

Figure 27: Relative TT distribution from LED measurements of single PMT and
the relative TTS distribution of sub-sample of all qualified bare PMTs
measured with LED in the container system. Dark-green, NNVT PMT; blue, HPK
PMT.

###  4.7 Pre-pulse and after-pulse ratio

Pre-pulses originate mainly from photons which hit the first dynode or MCP
directly rather than the photocathode. Hence, they have a smaller pulse
amplitude than the main pulses and show up before the main pulse by a few to
tens of nanoseconds. With the recorded waveform from the container systems, a
charge ratio is checked between the pre-pulse and main pulse in a window of
[-80,-10] ns before the peak of the main pulse. The general results are shown
in Fig. 28. The pre-pulse ratio is <1% for HPK PMTs and around 1% for NNVT
PMTs, while there is a larger uncertainty for NNVT MCP-PMTs due to their
larger TTS smearing.

Figure 28: Pre-pulse ratio of all qualified PMTs. Black: all PMTs; red: NNVT
PMTs; blue: HPK PMTs.

Another possible issue of PMTs is the contamination of the vacuum bulb with
gases. Molecules inside the PMT glass envelope can be ionized when the photo-
electrons are passing through. These ions will travel back and hit the
photocathode, ejecting more electrons. Such events will cause an after-pulse,
which features a charge ratio that is proportional to the initial pulse while
the after-pulse will be delayed in time by hundreds of nanoseconds to tens of
microseconds, depending on the gas molecule, the dimension of the glass bulb
and the strength of the applied electric field. The after-pulse ratio of
20-inch PMTs is calculated in a window from 500 ns to 20,000 ns after an
initial pulse of about 100 p.e. with a special LED configuration. This ratio
is expected to be less than 15% of the initial pulse charge.
There are already distinct studies about after-pulses HamManual ; largePMTxia
; YBJ8inch ; APU2008 ; APKorea2011 . This study only measured a sample of
several $\sim$ 100s PMTs using the scanning station as shown in JUNOPMTapYu .
The mean after-pulse charge ratio is calculated from 150 NNVT MCP-PMTs and 7
HPK dynode PMTs respectively. This sample leads to the result of 6.7 $\%$ for
NNVT MCP-PMTs and 12.0 $\%$ for HPK dynode PMTs. At the same time, it was
found that the HPK dynode PMTs and NNVT MCP-PMTs have very different after-
pulse features, as indicated for an example PMT shown in Fig. 29: the selected
MCP-PMT from NNVT was measured with a $\sim$7.0% after-pulse charge ratio,
showing four typical after-pulse peaks in time around 0.8 µs (ratio 3.1 $\%$),
3.2 µs (ratio 1.4$\%$), 4.6 µs (ratio 1.7$\%$) and 17.0 µs(ratio 0.8$\%$, only
a hint), while the selected HPK dynode PMT was measured with an after-pulse
charge ratio of 13.8% and typical peaks at 0.8 µs (ratio 1.1$\%$), 3.9 µs
(ratio 8.7 $\%$) and 14.2 µs (ratio 4.0 $\%$). The intensity in charge of a
single after-pulse is mainly at the SPE level, while it can reach up to
several O(10) p.e.s as shown in Fig. 30: so in the case of this particular HPK
PMT, the charge ratio of the after-pulses at 3.9 µs is larger than the pulse
at 14.2 µs despite the count rates are vice versa (compare Fig. 29 again).

Figure 29: Typical time features of after-pulses for two typical PMTs. Top:
NNVT; bottom, HPK (Hamamatsu). Figure 30: After-pulse intensity in p.e. versus
time after initial pulse. Left, HPK; Right, NNVT.

###  4.8 Non-linearity

Neutrino events detected in JUNO typically correspond to a signal strength of
up to few hundreds of p.e.s in each of the 20-inch PMTs. Furthermore, there
are huge signals expected from physics or background events with much higher
energy such as cosmic muons. A dynamic range in linearity of the PMT response
with the designed HV divider is required to reach 1,000 p.e.s with less than
10$\%$ distortion. The HV divider design for the 20-inch PMTs of JUNO
described in JUNOPMTsignalover ; JUNOPMTsignalopt ; JUNOPMTflasher ;
JUNOPMTinstr  was designed to provide a DC of $\sim$100 µA for HPK PMTs and
$\sim$180 µA for NNVT PMTs at a working gain of $1\times 10^{7}$. A few
samples of both PMT types were measured by the double-LED (LED A, LED B, C is
when LED A and B flashing at the same time) method (C/(A+B)) JUNOPMTlinearity
in a pulsed mode, and a cross-checking method between the 20-inch PMT and the
3-inch PMT. One resulting curve of a NNVT MCP-PMT is shown in Fig. 31, where
the PMT response with the designed HV divider can satisfy the requirements,
and the results of both methods are consistent. The current design can satisfy
the requirement on the linearity response. Another standalone paper will be
prepared on this topic.

Figure 31: Measured linearity of a 20-inch PMT by double-LED method (C/(A+B))
(red rectangular) and cross checking with 3-inch PMT (SPMT monitored) (blue
triangle).

###  4.9 Earth magnetic field (EMF) effect

PMTs with large vacuum bulbs are significantly affected by magnetic fields
when the photoelectron drifts to the collection dynode or MCP, in fact, their
performance is even sensitive to the Earth magnetic field (EMF, $\sim$50 µT at
the JUNO’s experimental site). This requires a more thorough characterization
on the magnetic field effect to PMTs. Using the scanning station, the EMF’s
strength around PMT’s location can be surveyed in the range -50µT to +50µT by
changing the DC configuration of the Helmholtz coils in the housing dark
rooms. As shown in Fig. 32, the variation of the performance of PMTs is
negligible (<1%$\sim$3%) when the residual field is less than 10$\%$ of EMF.
The direction of the magnetic field is perpendicular to the central axis of
the PMT, which is considered as the worst condition, and followed by the
direction of the collection box of dynode PMTs or the direction of micro-
channel of MCP-PMT. The container system and the scanning stations are
operating with the magnetic field suppressed to a level of $<5$ µT. Following
these measurements, the requirement on the residual magnetic field in the JUNO
detector is specified to be $\lesssim$5µT. As one can see, NNVT PMTs are
affected more strongly than HPK PMTs, which could be explained by differences
in the photoelectron focusing process. For more details, see JUNOstationIlya .

Figure 32: Averaged PMT PDE versus remaining magnetic field (MF) strength
tested with 9 HPK and 15 NNVT PMTs.

###  4.10 Uniformity

For a 20-inch PMT, the anode uniformities along the full photocathode are
significant for a better understanding of the PMT itself and the detector’s
response. An exemplary PMT surface scan performed by the scanning station is
shown in Fig. 33 with 15∘ steps along the azimuthal angle ($\phi$) by the
seven LEDs distributed in zenith angle ($\theta$), the online integrated
charge spectrum of one spot, and the scanned map on PDE and gain. Thanks to
the powerful capability of the scanning stations, detailed studies were made
using a few thousand PMTs (585 HPK PMTs, 2,658 NNVT PMTs, including 939 high-
QE PMTs, and 1,719 low-QE PMTs) to characterize the uniformity, which will be
discussed in detail as follows.

Figure 33: Exemplary data of a PMT scan with the scanning station. Top left:
scanning process indication; top right: integrated charge spectrum of selected
area element; bottom: measured maps of an NNVT MCP-PMT (PA2004-1007) on PDE
(bottom left) and gain (bottom right).

####  4.10.1 Uniformity of gain and gain excess normal distribution factor
(gENF)

The charge response along the whole photocathode surface can be described
firstly by its gain. As known, there will be a large response deformation
around the equator ($\theta\sim$90∘) of the glass bulb, compared to the top
pole ($\theta\sim$0∘) of the photocathode. Following the gain definition in
sec. 4.2.1 and applying the determined HV for a gain of $1\times 10^{7}$, the
gain is determined for each LED spot measurement individually. The uniformity
along the whole photocathode in the zenith ($\theta$) and azimuthal ($\phi$)
angles is shown in Fig. 34: each point is the average of all the measurements
of the full, tested PMT sample with the same zenith angle or azimuthal angle
individually, and the uncertainty bar in the vertical axis is calculated from
the error of the average. The gain of HPK PMTs is more uniform for
$\theta$<70∘ along the zenith angle, but it decreases sharply by $\sim$30%
towards the equator. Along the azimuthal angle, the gain of HPK PMTs shows
systematically smaller values ($\sim$5%) and some sub-structures (peak-to-peak
variation $\sim$10%) around its collection box in a rectangle shape, including
the peaks before ($\phi\sim$200∘), around $\phi\sim$270∘, and $\phi\sim$0∘ in
particular. On the other hand, the gain of NNVT PMTs is continuously
increasing as the zenith angle increases up to $\theta\sim$70∘ before drops at
the equator with a total variation range of $\sim$10%. Along the azimuthal
angle ($\phi$), the NNVT PMTs are more uniform than the HPK PMTs and one can
observe a two-cycles-oscillation with the valleys located at around
$\phi\sim$50∘ and $\phi\sim$230∘ with a peak-to-peak variation of $\sim$3%,
which should be related to the direction of the MCP channels. The averaged
gain of the full photocathode of all PMTs tested by the scanning station is
1.07$\times 10^{7}$ for HPK PMTs and 1.09$\times 10^{7}$ for NNVT PMTs, and
they are systematically higher than the gain measured by the container
($1.00\times 10^{7}$ for HPK PMTs and $1.03\times 10^{7}$ for NNVT PMTs,
respectively). The difference is assumed from the amplification factor of the
stations, which is not exactly equal to the claimed $\times$10.

Figure 34: Uniformity of gain along the zenith angle ($\theta$, left) and
azimuthal angle (($\phi$, right). HPK: blue; NNVT: red.

As a correlated factor of the gain, the uniformity of gENF (defined in sec.
4.3.6) is also checked with the same data set as the gain uniformity, which is
shown in Fig. 35 in the zenith ($\theta$) and azimuthal angles ($\phi$).
Firstly, the absolute value difference between HPK and NNVT PMTs is related to
the PMT itself as discussed in sec. 4.3.6. The gENF on average of the whole
photocathode of the full, scanned sample is 0.89 for HPK PMTs and 1.38 for
NNVT PMTs. Comparing to the values acquired by the containers, the discrepancy
is mainly assumed from the averaging scheme and the contribution of the gENF
at larger $\theta$. Except the absolute value difference, the gENF of both PMT
types is more uniform along the zenith angle (when $\theta$<80∘) and azimuthal
angle. At the same time, there are a few sub-structures worth mentioning:

  * (1)

the gENF value shows a larger change for $\theta$>80∘.

  * (2)

NNVT PMTs show a variation ($\sim$6%) sub-structure for $\theta$<80∘ (peak at
$\theta\sim$50∘ and valley at $\theta\sim$70∘).

  * (3)

the gENF factor of HPK PMTs along the azimuthal angle shows a small structure
($\sim$4%), which could be related to that in Fig. 34.

  * (4)

the gENF factor of NNVT PMTs along the azimuthal angle shows a two-cycles-
oscillation (variation $\sim$1.5%) at its gain but with opposite phases (peaks
at $\phi\sim$60∘ and $\phi\sim$210∘).

Figure 35: Uniformity of gENF along the zenith angle ($\theta$, left) and
azimuthal angle (($\phi$, right). HPK: blue; NNVT: red.

####  4.10.2 Uniformity of resolution and excess noise factor (ENF)

Following the SPE charge resolution definition in sec. 4.3.4, the uniformity
of the SPE charge resolution on the whole photocathode is shown in Fig. 36 in
the zenith ($\theta$) and azimuthal ($\phi$) angles. Each point is directly
averaged from all PMTs tested by the scanning station with the same zenith
angle or azimuthal angle individually. The total mean charge resolution of the
whole photocathode and for all the scanned PMTs is 0.28 for HPK and 0.32 for
NNVT. Comparing the uniformity results of HPK and NNVT PMTs, the charge
resolution is almost constant when $\theta$<70∘ but getting larger towards to
the equator for both of them (but in particular for HPK PMTs). Along the
azimuthal angle, the charge resolution of both HPK and NNVT PMTs shows a
similar trend but in opposite phase at the gain as shown on the right of Fig.
34: it is systematically higher than average around $\phi\sim$270∘ and small
fluctuations can be observed (variation $\sim$14%) for HPK PMTs and a two-
cycles-oscillation (variation $\sim$10%) for the NNVT PMTs with peaks located
at around $\phi\sim$50∘ and $\phi\sim$230∘.

Figure 36: Uniformity of the SPE charge resolution along the zenith angle
($\theta$, left) and azimuthal angle ($\phi$, right). HPK: blue; NNVT: red.

As a correlated factor to the SPE charge resolution, the uniformity of ENF
(defined in sec. 4.3.4) along the whole photocathode is determined for the
same data set as done for the charge resolution analysis, see Fig. 37. The ENF
on average over the whole photocathode of all scanned PMTs is 1.08 for HPK
PMTs and 1.41 for NNVT PMTs. The HPK PMTs show a more uniform performance
along both zenith angle and azimuthal angles. The NNVT PMTs reach $\sim$1.5%
as the zenith angle increases and also show a two-cycles-oscillation with a
variation of $\sim$1.5% along the azimuthal angle with the peaks located at
around $\phi\sim$50∘ and $\phi\sim$230∘. The NNVT PMTs show a similar trend as
in case of the gENF factor and charge resolution along the zenith angle to a
max. change of $\sim$18%, while a clear dependence to azimuthal angles also
with a two-cycles-oscillation with peak-to-peak variation of $\sim$2% peaks
located at around $\phi\sim$50∘ and $\phi\sim$230∘.

Figure 37: Uniformity of ENF along the zenith angle ($\theta$, left) and
azimuthal angle ($\phi$, right). HPK: blue; NNVT: red.

####  4.10.3 Uniformity of photon detection efficiency (PDE)

The measurement of the PDE uniformity along the whole photocathode of all
scanned PMTs is shown in Fig. 38 and divided in three data sets: HPK
(Hamamatsu), High-QE NNVT (HiQE) and Low-QE NNVT (NNVT). The PDE along the
zenith ($\theta$) and azimuthal ($\phi$) angle is the averaged value of all
the measured spots with the same zenith and azimuthal angle of all the scanned
PMTs. The high-QE NNVT PMTs show a better performance than HPK PMTs in both
directions: a better uniformity towards the equator and a larger absolute
value in both zenith and azimuthal direction. The low-QE NNVT PMTs show a
smaller PDE which has slightly better uniformity than for the high-QE PMTs
along $\theta$. The HPK PMTs’ PDE sharply drops from $\theta\sim$80∘ to
$\theta\sim$90∘ by about 40%, and a clear asymmetry (max. $\sim$10%) is
observed along azimuthal angle related to its focusing box direction around
$\phi\sim$270∘. Both the low-QE and high-QE NNVT PMTs show a two-cycles-
oscillation along azimuthal angle (peak-to-peak variation is $\sim$3%) and
peaks located at around $\phi\sim$110∘ and $\phi\sim$300∘.

Following the requirements, the non-uniformity of PDE (defined as the ratio of
the distribution of the PDE of each light spot to their average) should not
exceed 15%. In Fig. 39, the distributions for 320 NNVT and 245 HPK randomly
selected PMTs are presented. These samples allow an estimation of the total
number of PMTs failing the requirements as (0.5 - 1.7)% @ C.L. = 68% for NNVT
and (0.0 - 0.5)% @ C.L. = 68% for HPK PMTs. Considering such a small number of
non-uniform PMTs for the general sample there is no strict necessity to test
all the PMTs on the scanning system and no PMT is rejected by the non-
uniformity. These distributions will be useful for further detector
simulations to evaluate the PMTs’ non-uniformity contribution to the JUNO
energy resolution.

Figure 38: Uniformity of PDE along the zenith angle ($\theta$, left) and
azimuthal angle ($\phi$, right). Blue,: HPK PMTs; red: NNVT low-QE PMTs;
green: NNVT high-QE PMTs.

Figure 39: PDE non-uniformity distributions for randomly sampled PMTs. Black
lines show the threshold for PDE non-uniformity (15%), numbers in brackets
indicate failed PMTs. Left: HPK (Hamamatsu) PMTs; right: NNVT PMTs.

####  4.10.4 Uniformity of relative transition time spread (TTS) and
transition time (TT)

The scanning station only implements the stabilized-LED flashing in pulse
mode. The LED flashing intensity detected by PMT is 1$\sim$1.5 p.e. for the
PDE measurement, where the timing resolution can only reach $\sigma\sim$2 ns
to realize a relative TTS comparison along the photocathode HVSYS-LED . Fig.
40 shows an example of the hit time (relative TT) distribution of a single HPK
and NNVT PMT measurement analyzed with constant fraction discrimination (CFD).
The NNVT PMT shows a broader distribution than the HPK PMT. Additionally, a
sub-peak structure can be observed, that is similar to the right plot of Fig.
40. There is a clear dependence of the typical relative TT value and its
distribution spread on the location of the LED spots as shown on the right of
Fig. 40. We can distinguish some sub-peaks among LED_1, LED_3, and
particularly LED_5: the sub-peak structure (as introduced before in Fig.
25(b)) of the NNVT PMT’s relative TT is related to the location of the photon
hitting on the photocathode due to its relation on the focusing electric field
and MCP itself.

Figure 40: PMT hit time (relative TT) distribution of a single NNVT and HPK
PMT directly measured in the scanning station with different LEDs (locations)
at azimuthal angle $\phi$ = 0. Left: HPK (blue: 5th LED; orange: 7th LED.);
right: NNVT (blue: 1st LED; orange: 3rd LED; green: 5th LED.).

A Gaussian fitting is applied to the relative TT spectrum (Fig. 40) around its
maximum peak ($\pm$20 ns) on each light spot of the scanning station
measurements, the fitted mean is treated as the relative TT and the $\sigma$
as the relative TTS. Fig. 41 shows the average of the relative TTS of all
scanned PMTs along the respective zenith or azimuthal angle. The relative TTS
average over the whole photocathode of all scanned PMTs is 2.3 ns for HPK PMTs
and 6.6 ns for NNVT PMTs, where the discrepancy to the results of the
container LEDs is mainly due to the averaging scheme and ignoring the later
discussed relative TT difference. HPK dynode PMTs show a better uniformity
(max. variation $\sim$0.1 ns) along the zenith angle for $\theta$<80∘. On the
other hand, the NNVT MCP-PMTs show a larger TTS than HPK PMTs and a clear
zenith angle dependence: the larger $\theta$, the larger the relative TTS
(relative TTS increases 0.25 ns/10∘ when $\theta$<80∘ on average with a linear
assumption). The HPK PMTs also show some tiny structures along the azimuthal
angle in the range of 0.15 ns and a a single-cycle-oscillation related to its
focusing box direction at $\phi\sim$270∘.

Figure 41: Uniformity of the relative TTS ($\sigma$) along the zenith
($\theta$, left) and azimuthal angle ($\phi$, right) on average of all scanned
PMTs with the same zenith or azimuthal angle. Blue: HPK PMTs; red: NNVT PMTs.

At the same time, the uniformity of the transit time (TT) is also determined
for both HPK and NNVT PMTs, which could also contribute to the TTS, i.e. for
NNVT PMTs. Considering the absolute difference on TT between HPK and NNVT PMTs
(as shown on the right of Fig. 40), as well as variations among the PMTs, the
relative TT (fitted mean of the PMT hit time spectrum as previously discussed)
normalized to LED_1 angle_0 ($\phi$ = 0) of each scanned PMT itself is
averaged and determined for all the scanned PMTs. The relative TT non-
uniformity in average of the whole photocathode of all scanned PMTs is 0.6 ns
for HPK PMTs and 4.8 ns for NNVT PMTs. The NNVT PMTs show a clear dependence
on the zenith angle ($\theta$) as shown on the left of Fig. 42 (relative TT
increases by a factor of $\sim$5 between $\theta\sim$30∘ to $\theta\sim$66∘),
while for the HPK PMTs the relative TT is almost constant and shows a sharp
rise for $\theta$>80∘. Along the azimuthal angle, the HPK PMTs show a two-
cycles-oscillation (peak-to-peak amplitude 0.6 ns) and peaks located at around
$\phi\sim$15∘ and $\phi\sim$190∘), while the NNVT PMTs remain almost flat
across the range of azimuthal angles.

Figure 42: Uniformity of relative TT (normalized to the LED_0 angle_0 of each
scanned PMT itself) along the zenith angle ($\theta$, left) and azimuthal
angle ($\phi$, right) on average of all measurements with the same zenith
angle or azimuthal angle of all the scanned PMTs. Blue: HPK PMTs; red: NNVT
PMTs.

####  4.10.5 Uniformity of PDE vs. Earth magnetic field (EMF)

Another concern of JUNO is raised regarding the correlation of the PDE
uniformity versus the magnetic field strength when JUNO’s EMF shielding system
has an imperfect shielding near the edges and corners of the detector, in
particular for the PMTs used in the JUNO VETO system. The PDE uniformity
versus magnetic field strength is further determined with the same data set as
done for the PDE variation in sec. 4.9. The sigma ($\sigma_{\textit{PDE}}$) of
the 168 scanned PDEs’ distribution of a single PMT scanning is used to
represent the PDE uniformity. The relative ratio of $\sigma_{\textit{PDE}}$ to
that with zero-residual MF is used to represent the uniformity of PDE versus
the remaining EMF. The averaged relative ratio of all the tested PMTs (9 HPK
and 15 NNVT) is shown in Fig. 43. In contrast to the PDE versus MF, it seems
that the uniformity of PDE of NNVT MCP-PMTs has a better tolerance than HPK
dynode PMTs.

Figure 43: PDE uniformity versus magnetic field strength. Indicated data
points represent means and standard deviations of the tested samples (9 HPK
PMTs and 15 NNVT PMTs).

###  4.11 PMT long term stability

A slow aging of the PMTs used in JUNO is one of the crucial features required
by JUNO for a long-term running over in a 20 to 30 years period. Long term
stability measurements are of interest especially for the newly developed NNVT
MCP-PMTs. Lots of studies are performed on the MCP or MCP-PMT lifetime, even
with atomic layer deposition (ALD) protections lifetimeMCPPMT3 ;
MCPPMTlifeBelleII ; lifetimeMCPPMT ; lifetimeMCPPMT2 . Concerning this issue,
some R&D work had been done in JUNOMCPPMTagingWenwen , providing an
expectation for a use over 20 years under JUNO conditions: maximum anode
output charge is about 54.6 C (assuming a gain of $1\times 10^{7}$, $\sim$50
kHz p.e.s from dark noise and signals per PMT, $\sim$4 Hz muon through JUNO CD
LS and averaged $\sim$1,000 p.e.s per single PMT per muon). At the Zhongshan
Pan-Asia 20-inch PMT testing and potting station, the container $\\#$C (as
previously discussed) is re-configured to test the aging effect of HPK and
NNVT PMTs under different configurations. The first loading completed its run
after approx. 5 months and was intended partially for system debugging, the
second loading intended for PMT long-term testing is still ongoing for more
than 7 months now, where 32 PMTs (4 HPK PMTs + 28 NNVT PMTs) are separated
into four groups:

  * 1.

Constant light to simulate 1 MHz DCR and additional 10 Hz pulses to generate
1,000 p.e.s/pulse (0.140 C/day).

  * 2.

10 Hz pulses (1,000 p.e.s/pulse) to simulate accelerated aging (assuming PMT
itself with 50 kHz DCR, 0.008 C/day).

  * 3.

Constant light to simulate 1 MHz DCR (0.138 C/day).

  * 4.

1 Hz pulsed LEDs (1,000 p.e.s) to simulate the future JUNO detector (assuming
50 kHz DCR, 0.007 C/day).

Fig. 44 shows the daily monitored DCR, gain, and light intensity (proportional
to the PDE, configured with constant light source intensity). The DCR here is
derived daily from the random coincidence of dark counts to the acquired
waveform time window, which is not sensitive to instantaneous variations of
the DCR. A linear fit versus the testing day is applied to each monitored
curve. No obvious degradation of the DCR and measured light intensity are
identified so far ($\leq$0.0002/day, within the monitoring uncertainty), while
the monitored gain of group 1 (output charge up to max. 19.6 C in total) shows
a clear decreasing trend of 0.001/day (monitoring uncertainty 0.0002/day),
which is as expected from the aging of the MCP plates MCPPMTlifeBelle ;
MCPPMTlifeBelleII ; MCPPMTlifeDIRC ; JUNOMCPPMTagingWenwen ; lifetimeMCPPMT ;
lifetimeMCPPMT3 . On the other hand, the monitored gain of other groups hints
at a decreasing trend of $\sim$0.0002/day (monitoring uncertainty 0.0002/day),
which still needs further monitoring. Even if the gain shows an obvious
degradation in case of the NNVT MCP-PMTs of group 1, the requirement is still
satisfied for a runtime of 20 years running (<50% in 20 years) and deviations
can be compensated by slightly increasing the working voltage of PMTs.

Figure 44: Monitored DCR (left), gain (middle) and light intensity (measured
number of p.e.s in µ, proportional to PDE, configured with constant light
intensity, right) versus operating time under different configurations. There
was an artificial shift introduced between each testing group on the plots to
improve differentiation of the trend of each configuration: the first curve
from top in black is group 4, the second curve from top in blue is group 3,
the third curve from top in green is group 2, and the fourth curve from top in
red is group 1, The y-axis is normalized to the individual starting points.

##  5 Waterproof potted PMT

All JUNO PMTs, regardless of their placement in the CD or veto, will be
operated in pure water. Therefore, the JUNO PMTs must be potted (waterproof
encapsulated with the PMT base firmly soldered to the PMT) before their
installation JUNOdetector ; JUNOPMTinstr . The potted PMTs not only need to be
tested once more to guarantee the functionality after the potting process, bet
also need to be crosschecked for any change in characteristics.

There are several differences between the testing configurations of bare PMTs
and potted ones:

  * (1)

The HV divider will be soldered to PMT pins rather than using a pluggable HV
divider, even though their designs are the same.

  * (2)

The HV-signal decoupler (specified for a positive HV) will be realized in
another separate circuit, housed in a small aluminum box (as used for the
functionality tests with the potted PMTs in containers #A and #B), and
integrated as part of the 1F3 underwater electronics (container #D, as used in
the JUNO detector) rather than being integrated into the pluggable HV divider.

  * (3)

Another 2 m (CD PMT) or 4 m (veto PMT) extension of the HV/signal cable
(including an additional SHV connector – the bellow length of the potting
structure is 0.5 m shorter than the individual cables; another 2 m extension
cable is used during all the testings for easy connection) is necessary, in
contrast to the bare PMTs’ testing.

The potted PMTs were tested with the same containers as the bare PMTs
(containers $\\#$A and $\\#$B), using again the commercial data acquisition
electronics (tested 1,121 HPK and 3,708 NNVT PMTs in total). Some of them were
tested also together with the 1F3 electronics (in container $\\#$D, tested 732
HPK and 1112 NNVT PMTs in total), representing the same configuration as in
the JUNO detector later on JUNOdetector ; JUNOEmbedded . Since most of the
investigated parameters are consistent with the results for the bare and
potted measurements, only parameters only parameters which have been affected
after potting will be discussed further in the following subsections.

###  5.1 HV of potted PMTs

After the potting, the working HV of the PMTs to apply a gain of $1\times
10^{7}$ is re-calibrated following the procedure presented in sec. 4.2.2. A
comparison of the new HV working point with the measured values of the bare
PMTs show a good correlation (see Fig. 45). The HV increases by 30 V for HPK
PMTs on average, and decreases by 30 V for NNVT PMTs on average based on the
testing results from containers #A and #B. The determined HV values after
potting are closer to the suggested values by the manufacturers. The variation
is assumed to arise from the resistance change during production compared to
the pluggable HV dividers for the HPK PMTs, and from shorter wires between HV
divider and PMT pins in case of the NNVT PMTs.

Figure 45:  Working HV of potted PMTs for a gain of 1$\times$107. Left: HV of
bare vs. potted HPK PMTs; right: HV of bare vs. potted NNVT PMTs.

###  5.2 Dark count rate (DCR) of potted PMTs

It was found that after the waterproof potting of the PMTs, the measured DCR
was decreased by a factor of about 0.59 on average for NNVT PMTs (Fig. 46),
while HPK PMTs keep their mean value stable. This effect was verified in
detail for the noise level, temperature and other conditions. The mean DCR for
NNVT PMTs decreases to 31 kHz tested with containers #A and #B. We conclude
that this may be caused by the connection difference of the final soldered
dividers and the used pluggable HV dividers, which could generate some
additional noise for bare NNVT PMTs and increase the HV working point.

Figure 46: DCR of potted PMTs at a gain of 1$\times$107, measured in
containers $\\#$A and $\\#$B. Black: all PMTs; blue: HPK; and red: NNVT.

###  5.3 Rise-time, fall-time and FWHM of potted PMTs

SPE pulses are extracted from the waveforms of each potted PMT as mentioned in
sec. 4.3 and illustrated in Fig. 10(a). As known, the rise- and fall-time are
more related to the PMT itself, while they also can be regulated by the HV
divider design, for example, towards a slower behaviour. Fig. 47 shows the
rise-time, fall-time and FWHM of SPE pulses for all tested potted PMTs,
measured with the container system. The average values for HPK PMTs and NNVT
PMTs individually are 6.2 ns and 3.6 ns for rise-time, 9.6 ns and 15.3 ns for
fall-time, and 9.6 ns and 9.9 ns for FWHM. The multi-peak structure (around 17
ns and 21 ns) of the fall-time distribution of NNVT PMTs is the joint
contribution from the impedance mismatch among some of the PMTs, HV dividers,
cables, SHV connectors and electronics. On the other hand, the HPK PMTs show a
more uniform distribution of their waveforms among the tested PMTs.

Figure 47: Measured rise-time (left), fall-time (middle) and FWHM (right) of
potted PMTs at a gain of 1$\times$107 from containers #A and #B. Black: all
PMTs; blue: HPK; red: NNVT.

##  6 Summary

The performance of the JUNO 20-inch PMT system is one of the critical items to
reach the designed 3% energy resolution at 1 MeV. In this article, we have
described the facilities, the testing procedure and the characterization of
more than 20,000 20-inch PMTs qualified for use in the JUNO detector,
including both PMT types: HPK dynode PMTs and NNVT MCP-PMTs.

The testing systems, the container system and the scanning stations, have been
running successfully over the whole testing campaign, achieving a satisfying
noise level, measurement uncertainty, consistency between the testing systems,
and monitored stability. With these systems, the absolute PDE was studied and
measured based on the photon counting method, and most of the related PMT
parameters were measured from the 20-inch PMTs, including the HV, gain, S/N,
SPE amplitude, DCR, risetime, falltime, FWHM, charge resolution, ENF, gENF,
TTS, pre-pulse ratio, after-pulse ratio, and the uniformity along the whole
photocathode for various parameters such as the gain, charge resolution, PDE,
TTS, and the EMF sensitivity.

Table 4: Main parameters of qualified 20-inch PMTs Parameters | PMT type | Bare
---|---|---
|  | (Mean)
Gain ($\times\,10^{7}$) | HPK | 1.00
NNVT | 1.03
HV (V) | HPK | 1863
NNVT | 1748
S/N | HPK | 13.0
NNVT | 13.4
SPE amplitude (mV) | HPK | 6.5
NNVT | 7.5
PDE (%)(corrected) | HPK | 28.5
NNVT | 30.1
(High-QE/Low-QE) | (31.3/27.3)
DCR (kHz) | HPK | 15.3
NNVT | 49.3
(potted) | (31.2)
P/V | HPK | 3.8
NNVT | 3.9
Risetime (ns) | HPK | 6.9
NNVT | 4.9
Falltime (ns) | HPK | 10.2
NNVT | 17.3
FWHM (ns) | HPK | 11.6
NNVT | 7.9
TTS ($\sigma$,ns) (container laser) | HPK | 1.3
NNVT | 7.0

All received 20-inch PMTs from HPK and NNVT have completed their qualification
checks during 2017-2021. All tested PMTs have shown excellent performance,
also the newly developed MCP-PMTs, and most of the PMTs reached the JUNO
requirements. As a result, about 2,400 PMTs (9.3%), out of the tested 22,400
PMTs have been rejected due to failing the specifications required by JUNO for
visual inspection and/or electrical parameters. Except the visual defects,
most of them are failures due to a low PDE (i.e. for the early version of low-
QE MCP-PMTs), or due to a high DCR. The photocathode uniformity was also
checked in sub-samples of PMTs with a performance around the requirements
boundary, as well as for a randomly selected sample totally covering about 12%
of all HPK PMTs and about 18% of all NNVT PMTs. Finally, a total of 20,062
PMTs were successfully qualified for use in JUNO (typical parameters shown in
Tab. 4), with a mean PDE of 28.5% for HPK dynode PMTs and 30.1% for NNVT MCP-
PMTs (low-QE PMTs 27.3%, and high-QE PMTs 31.3%), and the expected mean PDE of
the JUNO CD (17,612 PMTs, the PMTs will be randomly distributed by type
(Dynode / MCP) as well as by performance in the CD.) and veto detector (2,400
PMTs) is 30.1% and 26.1% with the proposed schema, respectively. All the
measured parameters of the qualified PMTs are stored in the JUNO PMT database,
so that they can be accessed and used by the collaboration for detector
simulations, during installation and commissioning, and eventually for the
data analysis.

Furthermore, about 5,000 of the waterproof potted PMTs (encapsulated with the
PMT base firmly soldered to the PMT) are tested again after the potting in the
testing facilities. Additionally, dedicated tests including JUNO final
electronics have been performed as well to study the long-term behaviour of
the JUNO PMTs under normal conditions and higher strain to simulate and
investigate possible aging effects.

## Acknowledgments

We are grateful for the ongoing cooperation from the China General Nuclear
Power Group. This work was supported by the Chinese Academy of Sciences, the
National Key R&D Program of China, the CAS Center for Excellence in Particle
Physics, Wuyi University, and the Tsung-Dao Lee Institute of Shanghai Jiao
Tong University in China, the Institut National de Physique Nucléaire et de
Physique de Particules (IN2P3) in France, the Istituto Nazionale di Fisica
Nucleare (INFN) in Italy, the Italian-Chinese collaborative research program
MAECI-NSFC, the Fond de la Recherche Scientifique (F.R.S-FNRS) and FWO under
the “Excellence of Science – EOS” in Belgium, the Conselho Nacional de
Desenvolvimento Científico e Tecnològico in Brazil, the Agencia Nacional de
Investigacion y Desarrollo and ANID - Millennium Science Initiative Program -
ICN2019_044 in Chile, the Charles University Research Centre and the Ministry
of Education, Youth, and Sports in Czech Republic, the Deutsche
Forschungsgemeinschaft (DFG), the Helmholtz Association, and the Cluster of
Excellence PRISMA+ in Germany, the Joint Institute of Nuclear Research (JINR)
and Lomonosov Moscow State University in Russia, the joint Russian Science
Foundation (RSF) and National Natural Science Foundation of China (NSFC)
research program, the MOST and MOE in Taiwan, the Chulalongkorn University and
Suranaree University of Technology in Thailand, University of California at
Irvine and the National Science Foundation in USA.

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