PPrepared for submission to JINST
International Conference on Instrumentaion for Colliding Beam Physics (INSTR-2020)February, 24-28,2020Budker Institute of Nuclear Physics, and Novosibirsk State University, Novosi-birsk, Russia
Study of the water Cherenkov detector with high dynamicrange for LHAASO
K. Jiang, a , b Z. Tang, a , b , X. Li, a , b , and C. Li a , b a State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China,Hefei 230026, China b Department of Modern Physics, University of Science and Technology of China, Hefei 230026, China
E-mail: [email protected] , [email protected] Abstract: The Large High-Altitude Air Shower Observatory (LHAASO) is being built at Haizimountain, Sichuan, China at an altitude of 4410 m. One of its main goals is to survey the northern skyfor very-high- energy (above 100 GeV) gamma ray sources via its ground-based Water CherenkovDetector Array (WCDA). WCDA is 78000 m in dimension and consists of 3120 water detectorcells divided into 3 water ponds. A hemispherical 8-inch photomultiplier tube (PMT) CR365-02-1from Beijing Hamamatsu Photon Techniques INC. (BHP) is installed at the bottom-center of eachcell of the first water pond to collect Cherenkov light produced by air shower particles crossingwater. This proceeding includes the technical design of WCDA, the design of a high dynamic rangebase for CR365-02-1, the PMT test system and test results of 997 PMTs.Keywords: Photon detectors for UV, visible and IR photons (vacuum); Large detector systems forparticle and astroparticle physics; Cherenkov detectors Corresponding author. a r X i v : . [ phy s i c s . i n s - d e t ] S e p ontents The Large High Altitude Air Shower Observatory (LHAASO) [1] is a unique and new generationcosmic ray station being built at the Haizi Mountain in Daocheng of Sichuan Province with analtitude up to 4400 meters above the sea level. As shown in figure 1, the LHAASO experimentmainly includes three detector arrays [2]: 1 km array (KM2A) composed of electromagneticparticle detectors (EDs) and muon detectors (MDs), 78,000 m water Cherenkov detector array(WCDA), and 20 wide field-of-view air Cherenkov telescopes (WFCTA). Figure 1 . The layout of LHAASO detectors.
As one of the major detectors in the LHAASO project, the WCDA focuses on surveying thenorthern sky for gamma ray sources at the energy between 100 GeV and 20 TeV. Figure 2 showsthe schematic diagram of the overall layout of WCDA. It consists of 3 water ponds and covers aneffective area of 78000 m with an effective water depth of 4.5 m. Each water pond is dividedinto 5 m × ±
100 V, < α ) Mean ± > > < > / < − < > > ± Table 1 . Specification requirements of CR365-02-1. The working gain is 3 × . neighboring cells. The first two water ponds with an effective area of 150 × m contain 900detector cells each. The third water pond with an area of 300 × m contains 1320 detectorcells. There are 3120 detection cells in total. A hemispherical 8-inch photomultiplier tube (PMT)CR365-02-1 from Beijing Hamamatsu Photon Techniques INC. (BHP) is installed at the bottom-center of each cell of the first water pond to collect Cherenkov light produced by air shower particlescrossing water. A 1.5-inch PMT is placed aside each 8-inch PMT to extend the dynamic range. Figure 2 . The schematic diagram of the overall layout of WCDA.
According to the Monte Carlo simulations on gamma ray from Crab nebula, large-sized PMTs forWCDA are required to have good single photoelectron (SPE) resolution, large linearity up to 4000photoelectrons (PEs) and fast timing characteristics [3, 4]. The specifications for 8-inch PMTs arelisted in Table 1. A special high dynamic range base circuit for PMT CR365-02-1 is designed tomeet these requirements(see figure 3). Signals from PMT are read out from two outputs: one fromthe anode and the other one from the 8-th dynode [5, 6]. The ratio of the gain of the two outputsis specially tuned to about 50 to balance the dynamic range and overlapping range. The voltage– 2 –istribution on the electrodes of the PMT is tapered in order to improve the linearity of the PMT.The damping resistor connected to the 8-th dynode has been fine tuned to minimize the oscillationof the pulse. The assembly of the CR365-02-1 is shown in figure. 3. There are 3 30-meters-longcables, one for high voltage supply and two for signal outputs, in bundle covered by waterproofmaterial.
Figure 3 . The PMT CR365-02-1 from BHP and the base circuit used for the LHAASO WCDA.
Figure 4 . The PMT test facility.
The PMT test system is setup at University of Science and Technology of China in Hefei. Itconsists of dark boxes, light sources, front end electronics , and data acquisition (DAQ) system. Thesystem is designed to test 16 PMTs simultaneously during one test run. Two calibrated PMTs stayin the setup to server as reference and also monitor the stability of the whole system. The measuredPMT parameters include: high voltage response, SPE spectrum, transit time spread (TTS), relativequantum efficiency, dark noise rate, after pulse rate [7], nonlinearity and anode to dynode chargeratio. The test procedure is pretty much automatic. Each test run takes about 15 hours and thesystem can test 14 new PMTs per day. Until recently, 997 PMTs have been tested.– 3 – ntries 997Mean 1104RMS 43.93
HV (V)
800 1000 1200 1400 N u m b e r o f P M T s Entries 997Mean 1104RMS 43.93
Figure 5 . Distribution of the working high voltage HV opt for a gain of 3 × . Entries 997Mean 7.294RMS 0.1253 b N u m b e r o f P M T s Entries 997Mean 7.294RMS 0.1253 Entries 997Mean 3.696RMS 0.4924
Peak to valley N u m b e r o f P M T s Entries 997Mean 3.696RMS 0.4924 Entries 997Mean 3.037RMS 0.2523
Transit time spread (ns) N u m b e r o f P M T s Entries 997Mean 3.037RMS 0.2523Entries 997Mean 0.2574RMS 0.01034
Relative quantum efficiency N u m b e r o f P M T s Entries 997Mean 0.2574RMS 0.01034 Entries 997Mean 1508RMS 521.4
Dark noise rate (Hz) N u m b e r o f P M T s Entries 997Mean 1508RMS 521.4 Entries 997Mean 0.9948RMS 0.4605
Afterpulse rate (%) N u m b e r o f P M T s Entries 997Mean 0.9948RMS 0.4605Entries 997Mean 47.99RMS 3.496
Anode/Dynode
30 40 50 60 70 N u m b e r o f P M T s Entries 997Mean 47.99RMS 3.496 Entries 997Mean 1643RMS 121.6
Anode dynamic range (PE)
500 1000 1500 2000 N u m b e r o f P M T s Entries 997Mean 1643RMS 121.6 Entries 997Mean 7695RMS 1980
Dynode dynamic range (PE) N u m b e r o f P M T s Entries 997Mean 7695RMS 1980
Figure 6 . Distributions of the measured PMT parameters at a gain of 3 × . – 4 –igure. 5 shows the distribution of the working high voltage HV opt for a gain of 3 × . Themean value of HV opt is 1104 V. 17 PMTs (1.7%) failed to conform the specification for HV opt ( mean ±
100 V). Due to a relative long tail on the right hand side of the HV opt distribution, all failedPMTs have a HV opt larger than 1204 V. The distributions of the other measured parameters at a gainof 3 × are shown in figure. 6. Among all the tested PMTs, one PMT failed in conforming thespecification for amplification voltage coefficient β ( mean ± . HV opt ( mean ±
100 V), 32 PMTs (3.2%) failed in A/D ( mean ± HV opt andA/D. To meet the requirements of LHAASO-WCDA, a PMT voltage divider circuit with high dynamicrange has been designed. A PMT batch test system for LHAASO project is designed and built.Until now, a total of 997 8-inch CR365-02-1 have been tested for LHAASO-WCDA, with 46 (4.6%)unqualified.
Acknowledgments
The research presented in this proceeding has received strong support from LHAASO collaborationand National Natural Science Foundation of China (No. 11675172, 11775217). We would liketo specially thank Prof. Zhen Cao, Huihai He, Zhiguo Yao, Mingjun Chen, Bo Gao and othermembers of the LHAASO Collaboration for their valuable support and suggestions.
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