# Research on a Neutron Detector with a Boron-Lined Multilayer Converter
## 2 Physical Design and Optimization  
The BMCND's physical structure consists of a boron-lined multichip converter (BMC) and a multiwire proportional chamber (MWPC) [36; 37], which are secured by eight fasteners, as shown in Figure 1. The BMC, with a size of 100 \(\times\) 100 \(\times\) 100 mm\({}^{3}\) and composed of 34 pieces of boron-lined ultrathin glass with a 3 mm slit, is placed between the rectangular cathode and protection ring with voltages of -2500 V and 0 V, respectively. The upper section of the protection ring is the MWPC with voltages of 25-1160 V, and the MWPC could act as a position-sensitive detector through adding specific electronic readouts. The boron-lined ultrathin glass includes one piece of ultrathin glass with a thickness of 150 \(\upmu\)m and two pieces of a natural boron layer with a thickness of 20 \(\upmu\)m, which are attached to both sides of the ultrathin glass using epoxy resin [34]. A field cage surrounds the boron-lined multilayer converter to provide a uniform drift electric field. This cage comprises flexible printed circuits comprising 47 pieces of a thin copper sheet connected by the same resistance value. The MWPC consists of two sets of orthogonal gold-plated tungsten wires secured by the rectangular printed circuit board, forming cathode and anode reels. The cathode and anode reels include 64 and 32 gold-plated tungsten wires with diameters of 60 \(\upmu\)m and 20 \(\upmu\)m, respectively. All these components are sealed in an aluminum chamber with a thickness of 3 mm and filled with working gas (Ar:CO\({}_{2}\) = 90:10). Figure 2 shows the schematic diagram of the BMCND's neutron detection process. The incident neutrons enter the BMCND at an angle of about 1.72\({}^{\circ}\), which is the most excellent angle for neutron detection efficiency based on the size of the BMC, with its 100 mm length and 3 mm slit [38]. As a result, all the boron films can react with the incident neutrons, thereby increasing the neutron detection efficiency. The incident neutrons are absorbed by the boron film through the nuclear reaction \({}^{10}\)B (n, \(\alpha\))\({}^{7}\)Li, and \(\alpha\) and \({}^{7}\)Li charged particles are produced. One of these charged particles has the possibility of penetrating the boron film, entering the working gas, and ionizing and producing electrons. The electric field provided by the field cage carries these electrons to the MWPC, where a nuclear signal is generated when the electrons undergo multiplication and are collected. This study optimizes the width of the slit (WOS) for better neutron detection since it affects the electron drift efficiency (EDE) and the surface area of the boron film, both of which have an impact on neutron detection. The electron drift efficiency matrix (EDEM) was established to calculate the neutron detection efficiency at all points of the slit. One of the slit models was established, as shown in Figure 3, after considering that all the slits have the same geometric parameters. At the same time, the Y and Z axes were along the slit direction, with a total length of 10 cm. However, the X axis was in the vertical slit direction, i.e., along the WOS. The chosen WOS values were 2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, and 4.0 mm, based on experience. The Y and Z axes were discretized with a 0.5 cm space between them, while the X axis was discretized with a space of 0.1 mm, forming a three-dimensional lattice of the slit. The EDE at every point in the three-dimensional lattice was simulated one by one with Garfield++, with an initial electron kinetic energy of 0.01 eV and 10\({}^{5}\) electrons, to obtain the EDEM at the different WOS values. Figure 1: Physical structure of the BMCND. Figure 2: Neutron detection schematic diagram of the BMCND. Figure 4 shows an example of an EDEM with a 3 mm slit. All the EDEMs with different widths of slits have the same transformation law. The EDE decreases gradually as the drift distance increases from z = 0 to 10.0 cm. When the drift distance, z, is fixed, the EDE is more extensive and closer to the center.