Calibration study of the Gamma-Ray Monitor onboard the SVOM satellite

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Abstract

The Gamma-Ray Monitor (GRM), which is one of the four main scientific instruments onboard the Space-based multi-band astronomical Variable Objects Monitor (SVOM) satellite, is dedicated to the spectrum measurement and study of Gamma-Ray Bursts (GRBs) in the 15 keV to 5 MeV energy band, as well as to perform real-time triggering and localization of the transient bursts in orbit. In order to fully understand the performance of the developed spectrometer and optimize the calibration scheme for the flight model (FM), several calibration campaigns, such as energy–channel relation and energy resolution, detection efficiency, temperature response, high-voltage effect and the non-uniformity tests of detector crystal, have been implemented during the development phase of the qualification model (QM) of GRM, utilizing radionuclides, hard X-ray beam and synchrotron radiation facility. The data analysis results show good agreement with the expected output by Monte Carlo simulations. The detailed optimization scenario for FM calibration is also presented in this paper.

Introduction

The SVOM mission, developed by Chinese and French scientists [1], [2], is dedicated to conduct researches on Gamma-Ray Bursts (GRBs), cosmology, and Multi-messenger astrophysics [3], [4]. The satellite comprises two high-energy detectors with a wide field of view (FOV), namely a coded-mask gamma-ray imager called ECLAIRs [5], [6], and a gamma-ray spectrometer called GRM (Gamma-Ray Monitor) [7], and two narrow FOV telescopes that can measure the evolution of the afterglow after a slew of the satellite: an X-ray telescope called MXT (Micropore X-ray Telescope) [8] and an optical telescope called VT (Visible Telescope) [9]. Besides, there are also two ground-based telescopes [10]: GWAC (Ground Wide Angle optical Camera) monitoring the FOV of ECLAIRs in real-time, and two 1m class robotic GFTs (Ground Follow-up Telescopes) completing follow-up observations. After the Swift project [11], SVOM will be one of the most promising missions for real-time GRB trigger and localization, GRB afterglow measurement over many energy bands, high-redshift GRB detections [12]. It is scheduled to launch in 2022 at the Xichang Satellite Launch Center (XSLC) in China, and to initiate observation in a low-earth orbit (LEO) with an altitude of 625km and inclination of 30 degree for a designed lifetime of more than 3 years.

Designed with large FOV and high detection efficiency, GRM is optimized to detect GRBs in the hard X-ray and soft gamma-ray band, and transmit the information of GRB triggers and locations to other onboard payloads real-timely and the ground segment instruments through the VHF fast channel, a process which is particularly crucial for short and hard GRBs. The main design requirements of GRM are listed in Table 1. GRM is composed of three identical NaI(Tl) crystal spectrometers (GRDs) pointing in different orientations for onboard localization of GRBs, three auxiliary calibration detectors (GCDs), each one installed on the edge of the top surface of each GRD to stabilize the detector’s gain through high-voltage adjustment and to perform the on-orbit relative energy calibration, one electronics box (GEB) for scientific data collection and management and power supply for other parts of the system, and one particle monitor (GPM) for early warning of charged particle flow on orbit.

As the main detecting unit, each GRD contains NaI(Tl) crystal box, photomultiplier, magnetic shielding, high-voltage distributor, front-end electronics, and mechanical structures. The NaI(Tl) crystal, which has a diameter of 160mm and thickness of 15mm, is the sensitive material that the incoming gamma photons will interact with to produce scintillation fluorescence with a decay time of 230ns and maximum-emission wavelength of 420nm. The entrance window material of the crystal box is beryllium which was rolled to a thin thickness of 1.5mm to increase the transmittance of low-energy X-ray. The ESR reflection film is used to wrap the top and sides of the NaI(Tl) crystal, which is coupled with the quartz glass with a thickness of 15mm by silicone gel. The size of the quartz glass is matched to the PMT coupling end to improve the collection efficiency of scintillation fluorescence. The PMT R877-100 with 5-inch window, which is produced by the Hamamatsu Company, is selected to collect the scintillation fluorescence, and two magnetic shielding films are laid on the inner side of the GRD shell and the outer side of the PMT to reduce the influence of magnetic fields on the detector. The GRD electronics, including the charge-sensitive preamplifier, differential amplifier drive circuit and high-voltage power supply circuit, are responsible for amplitude signal conversion, signal amplification, and power supply for the PMT. A large dynamic range of GRD is achieved by the design of the high-gain and low-gain channels in the preamplifier. Each GCD consists of a plastic scintillator BC-440M with an embedded 241Am source inside, one silicon photomultiplier (SiPM) and readout electronics. By selecting the coincident signal of the alpha-particle event in GCD and the gamma-ray event in GRD, GRM can distinguish the gamma-ray events of 241Am from other sources. The coincident gamma-ray spectrum can be used to monitor the performance changes of GRD on orbit. Fig. 1 shows the GRD module structure with GCD installed on its top surface.

GRM can achieve a detection rate of more than 90 GRBs per year which ensures the scientific output of SVOM according to the Monte Carlo simulations. The determination of spectra and temporal properties of the detected transient events from GRM data requires very detailed knowledge of the full physical response of the detector to X-rays or gamma-rays [13]. Responses are produced via Monte Carlo simulations [14], which are supported and verified by ground-based experimental calibration measurements. Generally, continuous optimization of simulation parameters should be referred to the comparison of simulation results and the calibration data. Finally, the response model of the detector in line with experimental data will be built, which represents a complete description of the real performance of the detector. To perform the validations described above, the establishing of specific calibration procedures and detector-level verified tests have been carried out during the GRM qualification model (QM) development phase, which provides an important basis for the preparation of the calibration scenario for GRM flight model (FM). The simulated effective area of one GRD for different off-axis angles is shown in Fig. 2.

Section snippets

Calibration campaign

Radionuclides, the X-ray beam of the Hard X-ray Calibration Facility (HXCF) at the National Institute of Metrology (NIM) and the Shanghai Synchrotron Radiation Facility (SSRF) were all used in the calibration campaigns during the GRM QM phase. Two QM detectors, identified as QM01 and QM02 respectively, were involved in the campaigns. Along with some assistant equipment like the GEB and control machines, the energy response and detection efficiency of QM detectors were determined specifically.

Energy spectrum and fitting

The background should be subtracted from the measured spectrum of detector with the X-ray sources after time normalization. If TP(E) is the acquisition time of a pulse amplitude spectrum P(E), and Tbg is the acquisition time of corresponding background spectrum P(bg), then the net energy response spectrum in the ADC channel is calculated by PHA(E)=P(E)P(bg)Tbg×TP(E). A series of background-subtracted spectra at different energies measured by HXCF are shown in Fig. 8, which highlight the

Optimization of calibration scheme for GRM FM

According to the calibration process and data analysis results of QM01 and QM02, optimization and improvement of calibration methods can be done based on the existing scheme, which ensures a complete and efficient performance calibration of flight products. The detailed key points are shown as follows:

1. A test of the central position of the detector alone performed with low-activity nuclides cannot fully reflect the GRD’s overall response because of its non-uniformity. Therefore stronger

Conclusions

Preliminary calibration studies of GRM detectors, by using radionuclides, synchrotron radiation facility and hard X-ray beam, were performed during the QM phase. Several special experiments of QM01 were carried out using radioactive sources, such as temperature response and spatial non-uniformity tests. The results of data analysis indicate the proper method to select measuring points of temperature and spatial scan more efficiently during the FM phase. E–C, E-Res and detection efficiency

CRediT authorship contribution statement

Xing Wen: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data curation, Writing - original draft, Writing - review & editing. Jianchao Sun: Conceptualization, Methodology, Investigation, Resources, Data curation, Writing - review & editing. Jiang He: Formal analysis, Investigation, Data curation, Resources. Ruiqiang Song: Resources, Investigation. Eryan Wang: Resources, Investigation. Pengyue Zhou: Resources, Investigation. Yongwei Dong:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB23040400), the Joint Research Fund in Astronomy, China (Grant No. U1631242) under cooperative agreement between the National Natural Science Foundation of China (NSFC) and Chinese Academy of Sciences (CAS) and the National Natural Science Foundation of China (Grant No. 11403028, 11603026, 11503028). This work was carried out with the support of Shanghai Synchrotron Radiation

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