# Online Track Vertex Reconstruction Method Based on an Artificial Neural Network for MPGD
## 2 Materials and Methods  
### Detector Simulation  
The structure of the detector in the simulation is shown in Figure 1. The detector contains a cathode plate, a 0.3 mm-thick polyethylene conversion layer, a 5 mm-deep drift region filled with 75% argon and 25% carbon dioxide, a 100 \(\upmu\)m-thick and 300 mm \(\times\) 300 mm-large metallic mesh with 40% optical transparency, and a readout plate. The electric field in the drift region is 1 kV/cm. The size of the readout plate is 300 mm \(\times\) 300 mm, it is located 100 \(\upmu\)m below the metallic mesh, and it adopts the strip electrode readout method in both the X and Y direction. The anodic strips are arranged in the upper and lower layers of the printed circuit board (PCB) with a period of 0.75 mm, where the width of the anodic strips is 0.5 mm and the gap between the anodic strips in the arrangement is 0.25 mm. The upper and lower layers each contain 400 anodic strips, and the gap in the upper layer is hollowed out. The structure of the readout plate is shown in Figure 2. The range of fast neutron energies used in the simulation is 13.5 MeV to 14.5 MeV. Fast neutrons are injected vertically into polyethylene where (n, p) reactions take place to produce recoil protons, which enter the drift region at an angle and produce primary electrons that drift towards the anode under the action of the electric field in the drift region and diffuse laterally. The final location of the electrons that collect by the readout plate depends on the location of their initial ionization and the degree of lateral diffusion. The primary electrons near the cathode end of the plate drift longer and spread more laterally than the electrons near the metallic mesh end of the plate. The distribution is more decentralized. Figure 3 shows the position of the electron distribution at the anode after the incidence of a particle with a vertex of (32.76 mm, \(-\)4.04 mm), and it is clearly shown that the lateral spread of electrons is smaller for shorter drift times.  
Figure 1: Schematic diagram of the detector in the simulation.  
The time that it takes for the primary electrons to be collected by the readout plate depends on the location of the z-directional generation and the degree of longitudinal diffusion. The drift velocity of an ionized electron is about 40 \(\upmu\)m/ns and thus it takes more than 100 ns in a 5 mm thick drift region. The APV-25 chip that we used is a highly integrated chip that incorporates a large number of preamplifiers and analog-to-digital converters, and its operating frequency can be set. For instance, the APV-25 chip works at a frequency of 40 MHz, which leads to a 25-ns FEE period, so the entire track signal is sustained for several periods. Figure 4 shows the signal for one of the sampling periods of an event.  
Figure 3: Electron position distribution. The length of 300 mm in the x and y direction of the anode is defined as (\(-\)150 mm, 150 mm), and the color indicates the number of electrons.  
Figure 2: Diagram of the strip electrode readout anode.  
The relationship between the electrode channel number and its representative position is as follows:  
\[\text{xpos}=0.75\times(\text{Channel}-0.5)-150 \tag{1}\]  
In this formula, xpos represents the position where a certain readout channel is located. The channel is an integer value between 1 and 400. For example, in Figure 4, the channel with the highest signal value is 242, which represents a position of 31.125 mm.