Principle verification of the calibration light source subsystem for the calorimeter in herd experiment

Purpose: The High energy cosmic-radiation detection facility (HERD) is a space astronomy and particle astrophysics experiment planned to be installed on the Chinese space station. The core detector of HERD, a three-dimensional imaging calorimeter, uses LYSO crystals as the detection medium, with wavelength-shifting fibers readout by image intensifier and CMOS imaging chips. In order to monitor the performance of the calorimeter in orbit, a highly integrated calibration light source subsystem is required to test the pulse linearity of the readout system, including the optical fibers. Method: To meet this critical requirement, a Calibration Light Source Subsystem was proposed based on the dual-light-source method, and a test circuit was set up to prove the principle. Results and conclusions: Using a photomultiplier tube and waveform sampling module, the entire system’s linear response range was measured, which is equivalent to 1–900 Minimum Ionizing Particles (MIPs), with a linearity better than ± 5%. After adding an attenuator (transmittance of 6%) to the front end of the photomultiplier tube, the linear response range expands to 14–28,000 MIPs with a linearity better than ± 5%. This result demonstrated the significant contribution of the PMT’s saturation to the linearity measurement. The addition of the attenuator reduces the adverse effects of PMT saturation, and therefore the linearity of the Calibration Light Source Subsystem itself is better than ± 5% within the required dynamic range. In addition, testing of a single light source showed that long-term stability can be maintained within a range of ± 0.5% under conditions of 10 – 20 °C. The calibration method described in this paper has the characteristics of high integration and independent control of channels, providing an efficient and flexible calibration solution for the readout of calorimeter in HERD experiment.