The payloads of each satellite include 25 gamma-ray detectors (GRDs), 8 charged particle detectors (CPDs), and Electronics module (EBOX) (see Dome module below, where the round one represents GRD and the square one represents CPD). The 25 GRD modules are all installed in the dome of the satellite, pointing to different directions. Such a design provides GRD with wide monitoring field of view. Among the eight CPDs, six CPDs are installed in the dome of the satellite and two remained CPDs are installed on the side of payload Electronics module. CPD is used to measure the flux changes of charged particles in the space environment, and can also be used to identify gamma-ray burst and charged particles event in the space in combination with GRD. Positioned in payload Electronics module, EBOX is used for acquiring and processing GRD and CPD data, realizing on-orbit triggering and positioning of gamma-ray burst, completing data storage, packaging, transmission and the interaction with satellite platform, and providing power supply for the entire payload.
Figure: Structure of payloads of GECAM
With the on-ground and in-flight calibrations, the relative time accuracy among all GRD and CPD detectors of GECAM-B is about 0.12 μs (1σ) and the absolute time difference between GECAM-B and Fermi/GBM is about 3± 6 μs (1σ).
Figure: The distribution of the maximum time difference for 20 000 shower events recorded by different detectors on board GECAM-B. The width of bin is 0.03 (Xiao, 2022)
Figure: The Crab pulse profiles in the 20–500-keV energy band obtained with GECAM-B and GBM from UTC 2021-05-01T00:00:00 to UTC 2021-05-31T23:59:59. The phase resolution is 0.0001. (Xiao, 2022)
Gamma-ray detectors (GRDs)
As the main detector of GECAM satellite payload with large area and field of view and high detection efficiency, GRD can be used for triggering and positioning of gamma-ray burst and observing the light-curve and energy spectrums. Its main functions are as follows:
1.To conduct all-sky monitoring on gamma-ray burst within the wave band from X-ray to gamma ray (covers the energy range from 8 to 2000 keV);
2.To quickly trigger and position gamma-ray burst in the field of view;
3.To measure the light-curve and energy spectrums of various gamma-ray bursts;
4.To detect gravitational wave gamma-ray burst in combination with ground gravitational wave detectors;
5.To identify gamma-ray burst and charged particle burst together with CPD payload.
The characteristic is shown below.
Detection energy range
8 keV– 2 MeV
> 40 cm2 (for each GRD)
< 4 μs (normal event)
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Detection efficiency for Gamma-rays
> 60%@8 keV
The GRD onboard the GECAM satellite is mainly composed of the encapsulated LaBr3 crystal unit, SiPM array, pre-amplification electric circuit electronics system and mechanical structure, its internal structure is shown below. For more details, refer to https://link.springer.com/article/10.1007/s41605-021-00289-y.
Figure: Schematic diagram of GRD
Figure: SiPM array of GRD.
Figure: The detector energy spectrums of GRD at simulated gamma ray incident energies of 8 keV (a), 59.5 keV (b), 662 keV (c), and 1 MeV (d), respectively
Charged particle detectors (CPDs)
The CPD mainly applies to detect space electrons at 300 keV–5 MeV. By monitoring the charged particle flow intensity in the space environment, CPD can identify gamma-ray bursts and space charged particle events to achieve the identification of space particle bursts. In addition, CPD can study the onboard background of GECAM. To meet the detection requirement for CPD, a design scheme was adopted where the plastic scintillator is used as a sensitive material for detection and silicon photomultiplier (SiPM) as an optical readable device. For more details, please refer to: https://link.springer.com/article/10.1007/s41605-021-00298-x.
Figure: The shape and the structure diagram of CPD.
Figure: Simulation results of the detection efficiency of GRD and CPD for gamma rays and charged particles.