## 2 Materials and Methods  
### Protection for Detector  
Our imaging detector, which we previously developed [25], utilizes an advanced microchannel plate to enhance image quality. The detector consists of a photocathode, a position-sensitive anode, and the microchannel plate, as illustrated in Figure 1a. The detector functions by converting weak light into photoelectrons through the photocathode, which are then multiplied by the V-type cascade microchannel plate to create an electron cloud. The multiplied electron cloud accelerates under the electric field and falls onto the sensitive anode, which is read by the analog front-end electronics. Then, the position decoding formula is applied to obtain the photon's position, and photon counting is achieved. After a certain integration time, the gray image of the photon signal can be obtained.  
The microchannel plate is a large array of two-dimensional distribution electron multiplier tube array. Each microchannel hollow tube can be considered a micro continuous amplification photomultiplier tube, as illustrated in Figure 1b. It is composed of leaded glass with a high quadratic emission coefficient. The current gain (\(\upmu\)) of MCP is usually determined via the ratio of channel length (L) to channel diameter (d), which can be approximated as:  
\[\mu=e^{\delta\times\frac{\mathrm{L}}{2}}, \tag{1}\]  
where \(\mu\) is the current gain of MCP and \(\delta\) is the secondary electron emission coefficient of MCP channel wall, which is determined by channel material and electric field intensity.  
If charged particles of space radiation penetrate the shielding layer and impact the inner wall of the microchannel plate at a certain energy and angle, secondary electrons will be excited in a manner similar to real photoelectrons. These secondary electrons then multiply and eventually form electron clouds at the output end, which can lead to false counts. As seen from Formula (1), the closer the position where the charged particles hit the inner wall of the microchannel plate is to the input end (as marked in red in Figure 1a), the larger the length-diameter ratio, the more times electron multiplication takes place, the higher the photoelectric pulse, and the easier it is for electronics to recognize it as a count. On the other hand, when the charged particles interact with the side wall of the microchannel plate near the output end, the length-diameter ratio will be relatively small and the gain will also decrease, potentially preventing it from being identified by electronics.  
In Figure 2, the radiation protection structure of the detector is depicted. The detector is enveloped by an Al shell that provides electromagnetic shielding and acts as the first layer of radiation shielding. The optical system is connected to the front end of the detector, which only covers a portion of the detector's total area and should not directly face the incident particles. A low-noise electronic readout circuit is connected to the front end, while the processing circuits can be placed at a slight distance. When illuminated from the side of the MCP, the diameter is equal to the thickness, resulting in an increase in the detection efficiency [26]. Consequently, a shielding ring is added to the side wall of the detector input end--the most sensitive area. The width of the shielding ring (Ws), shown in blue in Figure 2b, can be determined based on disturbance and weight constraints.  
Figure 1: Schematic structure of a photon counting imaging detector. (**a**) Principle of imaging detectors; (**b**) single channel section of MCP.  
Figure 2: (**a**) The structure of detector assemblies; (**b**) profile of an imaging detector module.