Wide Field Of View Cherenkov Telescope Array Research Articles

Study of Atmospheric Depth Profiles at LHAASO Using the MSISE-90 Model

High Altitude Cosmic Ray Observatory (LHAASO) is located at Haizi Mountain in Daocheng County, Sichuan Province. Its Wide Field of view Cherenkov Telescope Array (WFCTA) primarily studies cosmic rays by observing the Cherenkov light signals produced during extensive air showers. Calibration, simulation, and reconstruction of WFCTA are all related to atmospheric depth. The currently used atmospheric depth model is the US Standard Atmosphere Depth Profile Model. In this study, the US Standard Atmosphere Depth Profile Model is compared with the atmospheric depth profile recorded by the infrared radiometer SABER carried by the satellite TIMED at LHAASO from 14 km to 50 km, as well as with the atmospheric depth recorded by the ground meteorological station at LHAASO. The atmospheric depth obtained from the US Standard Atmosphere Model is consistently smaller. The MSISE-90 atmospheric model describes the neutral temperature and density from the Earth's surface to the thermosphere. Further research shows good consistency between the MSISE-90 atmospheric model and the atmospheric depth recorded by TIMED/SABER and the ground standard meteorological station at LHAASO. According to the MSISE-90 atmospheric model, the mean atmospheric depth profile at LHAASO is lowest in January, followed by February, March, April, November, and December, which are also the optimal observation months for WFCTA operation. The atmospheric boundary layer is highest in April, and there is about 2% diurnal variation in atmospheric depth. Using the functional form of the US Standard Atmosphere Model, the atmospheric depth profiles from 4.4 to 100 kilometers were fitted for each month, obtaining monthly atmospheric depth profile models at the LHAASO site. And compared the lateral distribution of the Cherenkov photons produced by 100 TeV cosmic-ray protons incident at a zenith angle of 30° in the MSISE-90 atmospheric model and the US Standard Atmosphere Model,with the maximum difference reaching approximately 20%.

Properties and performance of nitrogen laser systems for calibration of LHAASO-WFCTA

The Wide Field of view Cherenkov Telescope Array (WFCTA) of the Large High Altitude Air Shower Observatory (LHAASO), located in Haizishan National Natural Reserve, Sichuan Province, China, is designed to perform nearly calorimetric measurements of extensive air showers induced by cosmic rays with energies between 1013eV–1018eV. The end-to-end calibration method will be employed in the WFCTA and nitrogen lasers have been proven to work properly in our application at the altitude of 4410 m a.s.l. for the first time.Due to extremely harsh environment, especially outdoor temperature as low as about −30 °C during operation of WFCTA, a high-stability thermotank for lasers was designed to stabilize temperature and humidity, with a thermal stability of 1 °C. In order to precisely control the beam pointing, a high-precision 3D Lifting Rotating Platform (HiRoP) with a home-made standard zero monitoring system is developed. To eliminate the error of the beam pointing by manual controls, an automatic scanning program was adopted to control the beam pointing when scanning the full field-of-view (FOV) of WFCTA. Both the on-site and remote controls are applied for the nitrogen laser systems and ∼13 million laser shots have been recorded by WFCTA from Sept. 2020 to May 2022. In this paper, the design, properties, operation and the on-site performance of the nitrogen laser systems at LHAASO are reported.

Pointing calibration of LHAASO-WFCTA telescopes using bright stars

One of the main scientific objectives of the Large High Altitude Air Shower Observatory (LHAASO) is to perform accurate measurements of the energy spectra for different cosmic ray masses, from a few TeV to 100 PeV. As one of the main sub-arrays of LHAASO, the Wide Field-of-View Cherenkov Telescope Array (WFCTA), which consists of 18 telescopes, can help in achieving this goal. The pointing accuracy of each telescope is crucial for reconstructing energy and determining mass-sensitive parameters. In this study, a method for absolute pointing calibration using ultraviolet bright stars was established. The proposed method can achieve a pointing accuracy of approximately 0.02° by using more than five stars. When more stars are used in the telescope’s field of view, the accuracy of the pointing calibration can be improved, e.g, a pointing accuracy of 0.01° can be achieved when using more than 15 stars. A high-precision inclinometer with a monitor resolution of 0.003° was installed on the camera to monitor the zenith direction of the telescope at every second. After calibration using bright stars, the absolute pointing accuracy of the inclinometer was 0.02°.

Design and development of laser temperature control system of LHAASO

The Large High Altitude Air Shower Observatory (LHAASO) is located in Haizi Mountain, Daocheng County, Sichuan Province, with an average altitude of 4410 m. The Wide Field of view Cherenkov Telescope Array (WFCTA) is one of its three observation arrays. Laser calibration system for WFCTA consists of 2 nitrogen lasers and 1 Nd:YAG laser system. During the operation of WFCTA, the outdoor temperature may dropped from 17 °C to about -30 °C, which will bring serious challenges to the operation of laser calibration system operating in the field. In order to ensure the stable working environment and remote operation of the laser, we have developed the LHAASO-WFCTA laser temperature control, remote control and data monitoring system. The fluctuation of temperature in the box is within 1 °C around the target. The laser temperature control system was officially put into operation in Sept. 2020. In this paper, the design scheme, fabrication scheme, comparison of heat dissipation coefficient and test of temperature control system are introduced. The working data shows that both nitrogen molecule lasers and Nd:YAG laser temperature control system can achieve stable and long-term operation in high-altitude extreme field.

Calibration of the air shower energy scale of the water and air Cherenkov techniques in the LHAASO experiment

The Wide Field-of-View Cherenkov Telescope Array (WFCTA) and the Water Cherenkov Detector Arrays (WCDA) of LHAASO are designed to work in combination for measuring the energy spectra of various cosmic ray species over a very wide energy range from a few TeV to 10 PeV. The energy calibration of WCDA can be achieved with a proven technique of measuring the westward shift of the Moon shadow of galactic cosmic rays due to the geomagnetic field. This deflection angle $\Delta$ is inversely proportional to the energy of the cosmic rays. The precise measurements of the shifts by WCDA allows us to calibrate its energy scale for energies as high as 35 TeV. The energy scale measured by WCDA can be used to cross calibrate the energy reconstructed by WFCTA, which spans the whole energy range up to 10 PeV. In this work, we will demonstrate the feasibility of the method using the data collected from April 2019 to January 2020 by the WFCTA array and WCDA-1 detector, the first of the three water Cherenkov ponds, already commissioned at LHAASO site.

Remote update solution for LHAASO wide field-of-view Cherenkov telescope array electronic readout system

The wide field-of-view Cherenkov telescope array (WFCTA), which consists of 18 wide field telescopes, is one of the main detectors used in the Large High Altitude Air Shower Observatory (LHAASO) . The WFCTA collects Cherenkov light and fluorescence signals from cosmic ray showers and converts them into electrical signals that are then processed by an electronic system based on a field-programmable gate array (FPGA) for later physical analysis. The high altitude of the array and the difficulties involved in disassembling the electronics inside the telescope mean that an electronic remote update solution with high fault tolerance and simple operation is required. The proposed remote update solution for the LHAASO-WFCTA electronic readout system is based on Xilinx Multiboot and is intended to implement remote updates without appended circuits. When compared with other remote update solutions based on Multiboot, the solution used on the WFCTA saves numerous resources in the original FPGA program, eases the timing constraint burden, and enhances security. This solution has been successfully implemented and used in remote updating of FPGAs in the LHAASO-WFCTA several times.

Design of analog circuit of photomultiplier tube array for wide field of view Cherenkov telescope array of LHAASO

The wide field of view Cherenkov telescope array (WFCTA) of the Large High Altitude Air Shower Observatory (LHAASO) is intended to detect atmospheric Cherenkov light or fluorescence. The photomultiplier tube (PMT) array is designed to enable atmospheric monitoring in real time and telescope calibration. This paper introduces the design of the analog circuit for the PMT array. The analog board has 16 channels, where each channel contains a shaping circuit, a high gain circuit and a low gain circuit. The shaping circuit uses a charge integrating circuit, and the amplifier module is composed of a noninverting amplifier circuit and a full differential amplifier circuit. Test results indicate that the dynamic range of the designed circuit is more than 3.2 orders of magnitude, the small signal resolution is less than 20%, the corresponding resolution of large signals is less than 4%, and the relative deviation from linearity is less than 5%. These performance indicators exceed the requirements of the project.

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.

Design and performance of analog circuit for the wide field of view Cherenkov telescope array of LHAASO

The Wide Field of View Cherenkov Telescope Array (WFCTA) is an important part of the Large High Altitude Air Shower Observatory (LHAASO) project. A high-resolution and low-relative deviation charge measurement is required over a dynamic range of 10 photoelectrons (p.e.) to 32,000 p.e. in the electronics readout system of the WFCTA. The analog circuit is one of the crucial parts in the electronics readout system. Because of the large dynamic range of the signals, a dual-gain electronics channel has been designed. In other words, for the signals from a SiPM, we designed two channels with the similar structures and different gains When these two kinds of channels are placed on two different Printed Circuit Boards (PCBs), it reduces the crosstalk between the large gain channels and the small gain channels. Tests have been conducted to evaluate the performance of the electronics readout system under different temperature conditions. Results indicate that accurate charge measurements can be achieved, ranging from 10 p.e. to 32,000 p.e., and that the system is capable of adapting to the temperature environment of the plateau where the altitude is more than 4000 m.

Performance of SiPMs and pre-amplifier for the wide field of view Cherenkov telescope array of LHAASO

The Wide Field of View Cherenkov Telescope Array (WFCTA), a main component of the Large High Altitude Air Shower Observatory (LHAASO), is designed for cosmic rays spectrum measurement from 30 TeV to several EeV. Silicon photomultipliers (SiPMs) have been adopted for the telescope cameras as they do not suffer any aging even with strong light exposure and thus they can be operated with moonlight. The physics of WFCTA requires a dynamic range of SiPMs to be within 10 p.e. (photoelectrons) to 32, 000 p.e. at any energy range. And the dynamic range of SiPMs is proportional to the total number of APDs and depends on the distribution of light hitting on the active area. This light distribution is affected by the use of light concentrator and also by the use of spherical mirrors of the telescope. The effect of these two factors on the Cherenkov light distribution has to be taken into account to evaluate the dynamic range and in this work they have been simulated with a ray-tracing software. The non-uniform light distribution caused by the light concentrator and the spherical mirror will be below 2% for 32, 000 p.e. at the condition if the SiPM has 360, 000 cells. The characteristics of SiPMs from three different producers – Hamamatsu, FBK, and SensL – are measured in laboratory, and the dynamic range of them has also been measured using a uniform light flux. The results of dynamic range correspond with the calculation by the function which describes the relationship between the number of fired APDs and the total number of APDs in the SiPM. The performance of the SiPMs and PMTs under long time duration light pulses up to 3 μs are compared, in order to investigate the stability of the gain of the SiPMs for the fluorescence mode.

Readout electronics for the Wide Field of view Cherenkov/Fluorescence Telescope Array

The aim of the Large High Altitude Air Shower Observatory (LHAASO), supported by IHEP of the Chinese Academy of Sciences, is a multipurpose project with a complex detectors array for high energy gamma ray and cosmic ray detection. The Wide Field of view Cherenkov Telescope Array (WFCTA), as one of the components of the LHAASO project, aim to tag each primary particle that causes an air shower. The WFCTA is a portable telescope array used to detect cosmic ray spectra. The design of the readout electronics of the WFCTA is described in this paper Sixteen photomultiplier tubes (PMTs), together with their readout electronics are integrated into a single sub-cluster. To maintain good resolution and linearity over a wide dynamic range, a dual-gain amplification configuration on an analog board is used The digital board contains two 16channel 14-bit, 50 Msps analog-to-digital converters (ADC) and its power consumption, noise level, and relative deviation are all tested.

Front-end electronics and data acquisition system for imaging atmospheric Cherenkov telescopes

In this paper, a front-end electronics based on an application-specific integrated circuit (ASIC) is presented for the future imaging atmospheric Cherenkov telescopes (IACTs). To achieve this purpose, a 16-channel ASIC chip, PARISROC 2 (Photomultiplier ARray Integrated in SiGe ReadOut Chip) is used in the analog signal processing and digitization. The digitized results are sent to the server by a user-defined User Datagram Protocol/Internet Protocol (UDP/IP) hardcore engine through Ethernet that is managed by a FPGA. A prototype electronics fulfilling the requirements of the Wide Field of View Cherenkov Telescope Array (WFCTA) of the Large High Altitude Air Shower Observatory (LHAASO) project has been designed, fabricated and tested to prove the concept of the design. A detailed description of the development with the results of the test measurements are presented. By using a new input structure and a new configuration of the ASIC, the dynamic range of the circuit is extended. A highly precise-time calibrating algorithm is also proposed, verified and optimized for the mass production. The test results suggest that the proposed electronics design fulfills the general specification of the future IACTs.