Background experiment of the low energy x-ray telescope detectors on insight-HXMT

The low energy x-ray telescope (LE) is one of the main instruments of the insight-hard x-ray modulation telescope, the first x-ray astronomical satellite of China. The scientific objectives of the LE focus on the scanning and pointed observations of the x-ray sources in the soft x-ray band (1–13 keV). In order to complete the observation tasks and accurately analyze the background information of the LE, it is essential to obtain the background data of the detectors. Therefore, we designed an LE background experiment. The experiment began with an underground background experiment in the China Jinping Underground Laboratory with a rock of a thickness of 2400 m, followed by a ground background experiment. These two experiments lasted for a long time, and through comparison and analysis of the background data, it was found that underground laboratories significantly shielded cosmic rays. In addition, the background of detectors in the underground experiment was more than one order of magnitude lower than that in the ground experiment. The experiments also revealed multiple x-ray fluorescence peaks of various elements in the background, including silicon from the detector itself, erbium in the ceramic substrate, and copper in the mounting plate. The anti-coincidence design of detectors was observed to reduce the x-ray fluorescence peaks of silicon. By comparing the background flux obtained with the background flux in the orbit, it was found that the background generated by radioactive substances inside the LE detector is very low. Within the energy range of less than 7.5 keV, the flux in the orbit is about 0.012 counts/s/keV, the ground flux is ∼3 × 10−3 counts/s/keV, and the underground flux is about 1.5 × 10−4 counts/s/keV. However, the flux in the orbit significantly increases above 7.5 keV, which does not occur in both the ground and underground background experiments. These results provide reference and guidance for scientific teams and instrument teams to analyze the data of the low energy x-ray telescope.