# Evaluating the Capture Efficiency of Microchannel Plates Through Photoelectron Detection
## 1 Introduction  
A microchannel plate (MCP) is a common detector for essential micro-particles such as electrons, ions, and even neutrons [1, 2, 3]. It is composed of millions of ultrathin conductive glass capillaries that typically range from 4 \(\upmu\)m to 25 \(\upmu\)m in diameter and 0.20 mm to 1.0 mm in length and are tightly assembled in a thin plate. When a high voltage is applied to the MCP in a vacuum [4], each capillary becomes a continuous dynode electron multiplier. Electrons or ions entering the channels strike the capillary walls to form secondary electrons and initiate an electron avalanche. Finally, an anode collects the multiplied electrons emitted from the channels to generate an electrical signal. MCPs have shown some attractive features, including fast response, high gain, and high spatial resolution, so they are widely used in photodetectors and mass spectrometers [5, 6, 7, 8]. Although the MCP has a large capture area, it cannot detect incident electrons/ions that do not enter the channels. Capture efficiency (CE) is a critical parameter for evaluating the performance of MCP. It refers to the ratio of electrons/ions detected by the MCP to the total number of incident electrons/ions [9]. Obviously, increasing CE can improve incident ion capture and detection sensitivity. In advanced applications such as mass spectrometry analysis, low-light imaging, or high-energy particle detection, improvements in CE can substantially enhance sensitivity and signal quality. For the MCP productionindustry, establishing accurate protocols for CE measurements is essential for assessing MCP structures and optimizing materials and surface treatments. Factors influencing CE include the structure of the MCP, the geometry of the capillary channels, and the incident angle. It is easy to deduce that the open-area ratio (OAR) of an MCP determines the upper limit of its CE [10, 11, 12]. Previous studies have demonstrated a direct correlation between the CE and OAR of an MCP [13]. Therefore, efforts have been made to exceed the OAR limit by geometry modification [14]. A practical solution is to manufacture tapered MCPs with expanded channel entrances while leaving the inner capillary walls unchanged [15, 16, 17]. In this scenario, implementing rapid CE evaluation for the fabricated tapered MCP will facilitate timely parameter adjustments during production and support the iterative refinement of manufacturing protocols. On the other hand, for users, understanding the CE parameters of MCPs may aid in optimizing the design of the experimental setup and provide deeper insights for result interpretation [6]. Theoretically, to obtain the CE of MCP, one needs to measure the total incident ion numbers and the number of detected ions separately. The former can be easily measured by a Faraday cup, while the latter is difficult to obtain by directly detecting the ion current output by the MCP. This is because such a signal is not only related to the number of ions entering the channels but also affected by other factors, including the multiplication factor of the microchannel and the state of the incident ions (e.g., mass to charge, kinetic energy, incident angle) [18, 19], most of which are difficult to quantify. Another practical obstacle is that the ion currents output by the Faraday cup and the MCP are generally not within comparable orders of magnitude. As an alternative and more practical solution, the number of ions reaching MCP is usually measured in the counting mode. In such a mode, to prevent signal saturation [20], the total number of incident ions needs to be reduced, which can be achieved by slit interception or pulsed laser-induced photoelectron emission [21]. This strategy has been a recognized method for assessing the efficiency of MCP, but its application is somewhat limited due to its stringent hardware requirements and relatively low measurement throughput. Recently, we have focused on fabricating MCPs with varying OARs and channel geometries.