Water Cherenkov Detector Array Research Articles

The First LHAASO Catalog of Gamma-Ray Sources

We present the first catalog of very-high-energy and ultra-high-energy gamma-ray sources detected by the Large High Altitude Air Shower Observatory. The catalog was compiled using 508 days of data collected by the Water Cherenkov Detector Array from 2021 March to 2022 September and 933 days of data recorded by the Kilometer Squared Array from 2020 January to 2022 September. This catalog represents the main result from the most sensitive large coverage gamma-ray survey of the sky above 1 TeV, covering decl. from −20° to 80°. In total, the catalog contains 90 sources with an extended size smaller than 2° and a significance of detection at >5σ. Based on our source association criteria, 32 new TeV sources are proposed in this study. Among the 90 sources, 43 sources are detected with ultra-high energy (E > 100 TeV) emission at >4σ significance level. We provide the position, extension, and spectral characteristics of all the sources in this catalog.

Analysis of Moon shadow from 2021-2022 LHAASO WCDA Data

The Large High Altitude Air Shower Observatory (LHAASO) is a major ground-based array of cosmic-ray (CR) detectors in the Sichuan Province in China. The Water Cherenkov Detector Array (WCDA) is an essential component of LHAASO, which is designed to measure ~1-50 TeV CRs and gamma rays. As observed on Earth, the Moon blocks some CRs to create a shadow. Although CRs are mostly protons, WCDA can probabilistically identify some constituents (mostly electrons and photons) of CRs which produce electromagnetic (EM) showers in the atmosphere, using the “compactness” (C) of the showers as the criterion for which the higher C indicates the higher probability of an event being EM. The observed CR Moon shadow positions must be shifted from that of the actual Moon due to the deflection of charged particles by the Earth’s magnetic field. We analyze the LHAASO WCDA data from March 2021 to December 2022 to study the properties of the CR Moon shadows for events with C < 10 and C > 10, such as the shifted position and the size of the shadow at different energies. We found that i) CR shadows’ shifts decrease when energy increases, and ii) the higher the energy, the size of the CR shadows are more compact. This analysis is useful for detector calibration, event selection study, and understanding CR trajectories in the geomagnetic field.

A Pulsar Wind Nebula Origin of the Ultra-high-energy Source 1LHAASO J1929+1846

1LHAASO J1929+1846 is one of the ultra-high-energy (UHE, E > 0.1 PeV) sources in the first catalog of γ-ray sources detected by the Large High Altitude Air Shower Observatory. It has been detected fluxes at 3 TeV for 2.48 ± 0.11 × 10−13 TeV−1 cm−2 s−1 with the Water Cherenkov Detector Array and at 50 TeV for 0.64 ± 0.06 × 10−16 TeV−1 cm−2 s−1 with the Kilometer Squared Array, but the origin of its UHE emission remains unknown. The pulsar wind nebula (PWN) G54.1+0.3, which is powered by the pulsar PSR 1930+1852 with a period of 136 ms and it is detected within a 0.°29 region around the centroid of 1LHAASO J1929+1846. We explore whether G54.1+0.3 is capable of producing the UHE radiation of 1LHAASO J1929+1846. First, the data with Fermi Large Area Telescope for the PWN is analyzed to obtain the GeV fluxes. Second, the multiband non-thermal fluxes for the nebula are investigated based on a one-zone time-dependent model. In the model, we assume the spin-down energy of the pulsar is persistently injected into particles (electrons and positrons) and magnetic field in the PWN, and these high-energy particles produce multiband non-thermal radiation from radio to γ-rays via synchrotron radiation and inverse Compton scattering. We reproduce the spectral energy distribution of PWN G54.1+0.3 with reasonable parameters. The result indicates that the UHE γ-ray source 1LHAASO J1929+1846 possibly originates from PWN G54.1+0.3.

Performance comparison of water Cherenkov detectors with different geometries

In the framework of the development of the SWGO experiment, we have performed a detailed study of the single unit of an extensive air shower observatory based on an array of water Cherenkov detectors. Indeed, one of the possible water Cherenkov detector unit configurations for SWGO consists of tanks, and to reach a high detection efficiency and discrimination capability between gamma-ray and hadronic air showers, different tank designs are under investigation. In this study, we consider and compare double-layer tanks with several sizes, shapes and number of photo-multiplier tubes (PMTs). Muons, electrons, and gamma-rays with energies typical of secondary particles in extensive air showers have been simulated entering the tanks with zenith angles from 0 to 60 deg. The comparison between different tank geometries was evaluated considering the number of photoelectrons produced by the PMTs, the detection efficiency, and the time resolution of the measurement of the first photon. We will show that with circular and hexagonal tanks better performance can be obtained than with the square ones, and that it improves for smaller tanks as expected.

Chapter 1 LHAASO Instruments and Detector technology * *Supported by National Natural Science Foundation in China (NSFC) (U2031103, U1831208, 11805209, 11775233), NSFC for Distinguished Young Scholars (12025502), the Science and Technology Department of Sichuan Province (2021YFSY0031) and the International Partnership Program of Chinese Academy of Sciences (113111KYSB20170055)

The Large High Altitude Air Shower Observatory (LHAASO) (Fig. 1) is located at Mt. Haizi (4410 m a.s.l., 600 g/cm2, 29° 21’ 27.56” N, 100° 08’ 19.66” E) in Daocheng, Sichuan province, P.R. China. LHAASO consists of 1.3 km2 array (KM2A) of electromagnetic particle detectors (ED) and muon detectors (MD), a water Cherenkov detector array (WCDA) with a total active area of 78,000 m2, 18 wide field-of-view air Cherenkov telescopes (WFCTA) and a newly proposed electron-neutron detector array (ENDA) covering 10,000 m2. Each detector is synchronized with all the other through a clock synchronization network based on the White Rabbit protocol. The observatory includes an IT center which comprises the data acquisition system and trigger system, the data analysis facility. In this Chapter, all the above-mentioned components of LHAASO as well as infrastructure are described.

Chapter 3 Extra-galactic gamma-ray sources * *Supported by the National Key R & D program of China (2018YFA0404203), National Natural Science Foundation Of China (12121003, 11625312) and China Manned Space Project (CMS-CSST-2021-B11)

Extra-galactic gamma-ray sources, such as gamma-ray bursts, active galactic nuclei, starburst galaxies, are interesting and important targets for LHAASO observations. In this chapter, the prospects of detecting these sources with LHAASO and their physical implications are studied. The upgrade plan for the Water Cherenkov Detector Array (WCDA), which aims to enhance the detectability of relatively lower energy photons, is also presented. In addition, a study on constraining the extragalactic background light with LHAASO observation of blazars is presented.

Design and Testing of the Front-End Electronics of WCDA in LHAASO

Water Cherenkov detector array (WCDA) is one of the key parts of the Large High Altitude Air Shower Observatory (LHAASO), the construction of which was completed by the end of 2020. The WCDA covers a 78 000-m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area and there exist 3120 large size photomultiplier tubes (PMTs) in three ponds: 8-in PMTs are used in WCDA pond No. 1 and 20-in PMTs are used in ponds No. 2 and No. 3. The front-end electronics (FEE) system based on multigain measurement technique is designed to achieve both high-precision time and charge measurements over a large dynamic range from single photon electron (S.P.E.) to 4000 P.E. (for water pond No. 1)/1800 P.E. (for water ponds No. 2 and No. 3). To achieve a high-quality clock distribution and phase alignment as well as mixed transmission of data, clock, and commands in one fiber over a long distance, an enhanced white rabbit (WR) technique is used. Testing of all the 350 FEE modules for the WCDA is presented in this article. Test results indicate that the charge resolution is better than 20% at S.P.E. and 1% at 1800/4000 P.E. and the time resolution is better than 300 ps root mean square (rms), which successfully meets the application requirement. All the FEE modules have been fabricated and installed for the LHAASO WCDA from 2018 to 2020, and the initial commissioning operation indicates that the FEEs function well.

Qualification tests of 997 8-inch photomultiplier tubes for the water Cherenkov detector array of the LHAASO experiment

The Large High-Altitude Air Shower Observatory (LHAASO) is being built at Haizi Mountain, Sichuan province of China at an altitude of 4410 m. One of its main goals is to survey the northern sky for very-high-energy gamma ray sources via its ground-based water Cherenkov detector array (WCDA). 900 8-inch photomultiplier tubes (PMTs) CR365-02-1 from Beijing Hamamatsu Photon Techniques INC. (BHP) are installed in the WCDA, collecting Cherenkov photons produced by air shower particles crossing water. The design of the PMT base with a high dynamic range for CR365-02-1, the PMT batch test system, and the test results of 997 PMTs are presented in this paper.

Simulation study of scaler mode at large high altitude air shower observatory

A large high altitude air shower observatory (LHAASO) located at Daocheng in Sichuan province, China, with an altitude up to 4410 m above the sea level, takes the function of hybrid technology to detect cosmic rays. It is composed of three sub-arrays: a 1.3 km&lt;sup&gt;2&lt;/sup&gt; ground-based particle detector array (KM2A) for γ-ray astronomy and cosmic ray physics, a 78000 m&lt;sup&gt;2&lt;/sup&gt; water Cherenkov detector array (WCDA) for γ-ray astronomy, and 18 wide field-of-view air Cherenkov/fluorescence telescopes array (WFCTA) for cosmic ray physics. As the major array of LHAASO, KM2A is composed of 5195 electromagnetic particle detectors (EDs, each with 1 m&lt;sup&gt;2&lt;/sup&gt;) and 1188 muon detectors (MDs, each with 36 m&lt;sup&gt;2&lt;/sup&gt;). In the ground-based experiments, there are two common independent data acquisition systems, corresponding to the shower and scaler operation modes. Up to now, the KM2A array operates only in shower mode with the primary energy threshold of about 10 TeV. In the scaler mode, it is not necessary for too many detectors to be hit at the same time. The energy threshold of the experiment can be greatly lowered. In order to learn more about the scaler mode in LHAASO-KM2A, we adopt the CORSIKA 7.5700 to study the cascade processes of extensive air showers in the atmosphere, and employ the G4KM2A (based on Geant4) to simulate the detector responses. The KM2A-ED array is divided into dozens of clusters. For one cluster (composed of 64 EDs), the event rates of showers having a number of fired EDs ≥ 1, 2, 3, 4 (in a time coincidence of 100 ns) are recorded. The average rates of the four multiplicities are ~88 kHz, ~1400 Hz, ~220 Hz, and ~110 Hz, respectively. The particle multiplicities &lt;i&gt;m&lt;/i&gt; ≥ 3 are almost completely due to cosmic ray secondary particles. The corresponding primary energies and effective areas are also given in this paper. According to our simulations, the energy threshold of the scaler mode can be lowered to 100 GeV, and the effective areas reach up to ~100 m&lt;sup&gt;2&lt;/sup&gt;. The simulation results in this work are helpful in the online triggering with the scaler mode, and provide information for the subsequent data analysis in LHAASO-KM2A.

Study of the water Cherenkov detector with high dynamic range for LHAASO

The Large High-Altitude Air Shower Observatory (LHAASO) is being built at Haizi mountain, Sichuan, China at an altitude of 4410 m. One of its main goals is to survey the northern sky for very-high- energy (above 100 GeV) gamma ray sources via its ground-based Water Cherenkov Detector Array (WCDA). WCDA is 78000 m2 in dimension and consists of 3120 water detector cells divided into 3 water ponds. 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 particles crossing water. This proceeding includes the technical design of WCDA, the design of a high dynamic range base for CR365-02-1, the PMT test system and test results of 997 PMTs.

Prospects for a multi-TeV gamma-ray sky survey with the LHAASO water Cherenkov detector array * * Supported by National Natural Science Foundation of China (11761141001, 11635011, 11873005). The LHAASO project is supported by the National Key R & D Program of China (2018YFA0404200), the Chinese Academy of Sciences, the Key Laboratory of Particle Astrophysics, IHEP, CAS

The Water Cherenkov Detector Array (WCDA) is a major component of the Large High Altitude Air Shower Array Observatory (LHAASO), a new generation cosmic-ray experiment with unprecedented sensitivity, currently under construction. WCDA is aimed at the study of TeV -rays. In order to evaluate the prospects of searching for TeV -ray sources with WCDA, we present a projection of the one-year sensitivity of WCDA to TeV -ray sources from TeVCat using an all-sky approach. Out of 128 TeVCat sources observable by WCDA up to a zenith angle of , we estimate that 42 would be detectable in one year of observations at a median energy of 1 TeV. Most of them are Galactic sources, and the extragalactic sources are Active Galactic Nuclei (AGN).

LHAASO-WCDA++ electronic readout system

The Dynamic Range Expansion System of the Water Cherenkov Detector Array (WCDA++) is an important component of the Large High Altitude Air Shower Observatory (LHAASO). We have designed an electronic readout system for it. Using a multi-channel scheme, the electronic system can process multiple detector signals. High and low gains of the system have been included to process the output signals from the photomultiplier tube (PMT) anode and dynode, respectively, and photo-electrons, p.e., with energies ranging from 1 p.e. to 3000 p.e. have been covered. Using multistage resistor-capacitor (RC)m filtering circuit, the electronic resolution is better than 10%@1 p.e. and 2%@3000 p.e., while the relative deviation of integral linear is better than 5%@1 p.e. and 2%@3000 p.e. At 50 MHz clocked frequency, the measurement accuracy of the electronic jitter is better than 20 ns. The system uses four pairs of differential lines with a 50 Mbps transmission limit per line pair to transmit data in parallel, thus enabling the system to acquire and transmit signal at 10 kHz.

LHAASO-WCDA time calibration using high-statistic low-energy cosmic rays

The Large High Altitude Air Shower Observatory (LHAASO) is a multi-component experiment to study cosmic ray physics, which is under construction near Mt. Haizishan (4410 m a.s.l.) in Southwest China. The 78,000 m2 Water Cherenkov Detector Array (WCDA), which is one of the main components of the LHAASO, will be able to survey the gamma-ray sky continuously in the energy range from 100 GeV to PeV. The precision of the time measurement in the detector cells of WCDA determines the sensitivity to the direction of gamma ray sources, necessitating a hardware calibration system to be deployed in the WCDA. Effects such as environment temperature variations will introduce time offsets between detector cells. In order to calibrate the time offset, a method using high-statistics low-energy cosmic rays has been developed. The detector cells are divided into clusters of 6×6 cells which are calibrated locally. This paper then describes the technique combining all the detector cells and demonstrates that the performance meets the requirements. The high-statistics of low-energy cosmic rays makes it possible to calibrate the detector in real time.

Expected energy spectrum of cosmic ray protons and helium below 4 PeV measured by LHAASO * * Supported by the National Key R&D Program of China (2018YFA0404201, 2018YFA0404202), the Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, CAS (Y5113D005C) and National Natural Science Foundation (NSFC) (11563004, 11775248)

The Large High Altitude Air Shower Observatory (LHAASO) is a composite cosmic ray observatory consisting of three detector arrays: kilometer square array (KM2A), which includes the electromagnetic detector array and muon detector array, water Cherenkov detector array (WCDA) and wide field-of-view Cherenkov telescope array (WFCTA). One of the main scientific objectives of LHAASO is to precisely measure the cosmic rays energy spectrum of individual components from eV to eV. The hybrid observation will be employed by the LHAASO experiment, in which the lateral and longitudinal distributions of extensive air shower can be observed simultaneously. Thus, many kinds of parameters can be used for primary nuclei identification. In this paper, high purity cosmic ray simulation samples of the light nuclei component are obtained using multi-variable analysis. The apertures of 1/4 LHAASO array for pure proton and mixed proton and helium (H&He) samples are and , respectively. Prospect of obtaining proton and H&He spectra from 100 TeV to 4 PeV is discussed.

Photomultiplier tube performance of the WCDA++ in the LHAASO experiment

In the Large High Altitude Air Shower Observatory (LHAASO), the main physics objective of the water Cherenkov detector array (WCDA) is to survey the sky for gamma-ray sources in the energy range of 100 GeV to 30 TeV. In order to extend the dynamic range of the WCDA, a 1.5-inch photomultiplier tube (PMT) is placed aside the 8-inch PMT in each cell of the WCDA. All of these 1.5-inch PMTs (900 in total) consist of the WCDA dynamic extended system (WCDA++). These PMTs are required to maintain linearity within four orders of magnitude. The performance of the 1.5-inch PMTs with a specially designed bi-readout voltage divider is tested with a PMT test system. Accordingly, the effects of the working high voltage and signal width on the dynamic range of the PMTs are studied. The test results show that the dynamic range with a 5% non-linearity for a driven signal width of 5.5 ns is more than 200 kPEs (photoelectrons). The dark noise count rate is less than 200 Hz for a 1 mV threshold at a PMT gain of 2×105. These results confirm that the PMT performance meets the WCDA++ requirements.

Study of PMT performance in the high dynamic range at LHAASO

In the Large High Altitude Air Shower Observatory (LHAASO), the Water Cherenkov Detector Array (WCDA) and Muon Detector (MD) employ a large number of photomultiplier tubes for cosmic ray detection. In order to explore the cosmic ray sources at very high energy level, these photomultiplier tubes require not only good single photoelectron resolution, but also large linear dynamic range: up to 4000 photoelectrons for WCDA and over 10000 muons for MD. In order to meet these requirements, we design a high dynamic range base for three photomultiplier tube candidates (R5912, CR365 and XP1805) for WCDA and R5912 for MD, and test their performance. The results show that the characteristics of all three candidates with a charge dynamic range up to 1–4000 photoelectrons satisfy the demands of WCDA. For R5912 of MD, the equivalent current non-linearity of DY7 within 5% is up to 1.87A, satisfying the dynamic range requirement of MD.

A multichannel front end ASIC for PMT readout in LHAASO WCDA

Time and charge measurements over a large dynamic range from 1 Photo Electron (P.E.) to 4000 P.E. are required for the Water Cherenkov Detector Array (WCDA), which is one of the key components in the Large High Altitude Air Shower Observatory (LHAASO). To simplify the circuit structure of the readout electronics, a front end ASIC was designed. Based on the charge-to-time conversion method, the output pulse width of the ASIC corresponds to the input signal charge information while time information of the input signal is picked off through a discriminator, and thus the time and charge information can be digitized simultaneously using this ASIC and a following Time-to-Digital Converter (TDC). To address the challenge of mismatch among the channels observed in the previous prototype version, this work presents approaches for analyzing the problem and optimizing the circuits. A new version of the ASIC was designed and fabricated in the GLOBALFOUNDRIES 0.35 μm CMOS technology, which integrates 6 channels (corresponding to the readout of the 3 PMTs) in each chip. The test results indicate that the mismatch between the channels is significantly reduced to less than 20% using the proposed approach. The time measurement resolution better than 300 ps is achieved, and the charge measurement resolution is better than 10% at 1 P.E., and 1% at 4000 P.E., which meets the application requirements.

Test bench for fibers of the LHAASO-WCDA time calibration system

The Water Cherenkov Detector Array (WCDA), which is one of the main components of the Large High Altitude Air Shower Observatory (LHAASO), functions in surveying the northern sky for high-energy gamma ray sources at the energy range around of 100 GeV–30 TeV. The precision of the time measurement for shower particles hitting every detector in the array is directly associated with the detection sensitivity for the sources. The calibration precision of the time offsets among the detector cells should be less than 0.1 ns to obtain less than 0.1̂ pointing error of the detector to any point source. In this regard, “cross calibration” is employed for the detector array, with 180 bundles of fiber systems guiding the lights of LEDs to each PMT. A test bench comprising fast PMTs and an one-dimensional slide platform is set up in the laboratory to test the fiber bundles. The test bench and the test procedure is described in this paper, and the test results are presented.

Trigger and noise filtering algorithms for the LHAASO water Cherenkov detector array

The Large High Altitude Air Shower Observatory (LHAASO) will be constructed at Mt. Haizishan, Sichuan Province, China. Among several detector components of the LHAASO, the Water Cherenkov Detector Array (WCDA) is of great importance for gamma-ray astronomy from 100 GeV to 30 TeV. The single-channel counting rate of a photo- multiplier tube can reach as high as 30 -35 kHz, most of them are background noise hits from the low energy cosmic ray showers, bringing a big challenge on data transferring, data storage and event reconstruction. In this paper, a dedicated trigger scheme and a fast noise filtering method aiming to deal with these high rate background noise hits are introduced. These methods are tested with some Monte Carlo simulation data, showing a fair efficiency in filtering background noise hits, while most of the real shower signals are kept. This method is proposed to be applied in a stage of the online processing just after the data are acquired in LHAASO-WCDA.

Prototype of the Readout Electronics for WCDA in LHAASO

In the Large High Altitude Air Shower Observatory (LHAASO), the Water Cherenkov Detector Array (WCDA) is one of the key parts. The WCDA consists of 3600 Photomultiplier Tubes (PMTs) scattered in a 90000 m2 area, and both high precision time and charge measurements are required over a large dynamic range from 1 to 4000 Photo Electrons (P.E.). To achieve time measurement precision better than 500 ps RMS, high quality clock distribution and automatic phase compensation are needed among the 400 Front End Electronics (FEE) modules. To simplify the readout electronics architecture, clock, data, and commands are transferred simultaneously over 400-meter fibers, while high speed data transfer interface is implemented based on TCP/IP protocol. Design and testing of the readout electronics prototype for WCDA is presented in this paper. Test results indicate that a charge resolution better than 10% RMS @ 1 P.E. and 1% RMS @ 4000 P.E., and a time resolution better than 300 ps RMS are successfully achieved over the whole dynamic range, beyond the application requirement.