The GECAM series of satellites utilizes LaBr \(_3\) (Ce), LaBr \(_3\) (Ce,Sr), and NaI(Tl) crystals as sensitive materials for gamma-ray detectors (GRDs). To investigate the nonlinearity in the detection of low-energy gamma rays and address the errors in the calibration of the E–C relationship, comprehensive tests and comparative studies of the three aforementioned crystals were conducted using Compton electrons, radioactive sources, and mono-energetic X-rays. The nonlinearity test results of the Compton electrons and X-rays demonstrated substantial differences, with all three crystals presenting a higher nonlinearity for X/ \(\gamma\) -rays than for Compton electrons. Despite the LaBr \(_3\) (Ce) and LaBr \(_3\) (Ce,Sr) crystals having higher absolute light yields, they exhibited a noticeable nonlinear decrease in the light yield, especially at energies below 400 keV. The NaI(Tl) crystal demonstrated an "excess" light output in the 6–200 keV range, reaching a maximum "excess" of 9.2% at 30 keV in the X-ray testing and up to 15.5% at 14 keV during Compton electron testing, indicating a significant advantage in the detection of low-energy gamma rays. Furthermore, we explored the underlying causes of the observed nonlinearity in these crystals. This study not only elucidates the detector responses of GECAM, but also initiates a comprehensive investigation of the nonlinearity of domestically produced lanthanum bromide and sodium iodide crystals.
