# Research on a Neutron Detector with a Boron-Lined Multilayer Converter
## 1 Introduction  
The boron-lined gaseous detector, which is an excellent \({}^{3}\)He alternative neutron detector [1; 2], is studied by many researchers to address the \({}^{\sim}\)3 He supply crisis\({}^{\ast}\) worldwide problem [3; 4; 5]. This detector is widely applied in various fields, such as homeland security [6; 7], cosmic ray neutron sensing [8; 9], and large neutron spectrometers [10; 11; 12; 13; 14]. As a result, various boron-lined gaseous detectors with different structures have been proposed, such as boron-coated straw detectors [15; 16], multigrid detectors [17; 18], boron-lined honeycomb detectors [19; 20; 21], multiblade detectors [22; 23], and GEM-based detectors [24; 25; 26; 27]. All these detectors have the same neutron detection principle, which relies on the \({}^{10}\)B isotope capture neutron nuclear reaction \({}^{10}\)B (n, \(\alpha\))\({}^{7}\)Li, which can convert neutrons into charged particles. The abundance of \({}^{10}\)B nuclides is 19.8%, and the cross section of \({}^{10}\)B atoms to thermal neutrons is approximately 3844 barns. The capture reaction equation is as follows:\[n+{{}^{10}B}\rightarrow\left\{\begin{array}{l}\alpha+{{}^{7}Li}+2.792 MeV,6.1\%\\ \alpha+{{}^{7}Li}^{*}+2.310 MeV,93.9\%.\end{array}\right. \tag{1}\]  
\[{{}^{7}Li}^{*}\rightarrow{{}^{7}Li}+\gamma+0.478 MeV\]  
During the process of thermal neutron detection, \(\alpha\) and \({{}^{7}Li}\) charged particles are produced in the boron film as the incident neutrons are being absorbed by the \({{}^{10}B}\) atoms, distributing the reaction energy (2.792 and 2.310 MeV with branching ratios of 6.1% and 93.9%, respectively) according to the conservation of energy and momentum. In addition, the \(\alpha\) and \({{}^{7}Li}\) particles move in a backward direction because the thermal neutron kinetic energy (25.3 meV) is far smaller than the reaction energy. Therefore, only one particle has the possibility of penetrating the boron film and entering the working gas, which could produce electrons after being ionized by charged particles. Then, a nuclear signal is produced when these electrons are multiplied and collected by the anode. Based on the aforementioned neutron detection principle, in order to achieve a higher neutron detection efficiency, the incident neutrons should penetrate the multilayer boron film, which is attached to the aluminum substrate. In this way, the incident neutrons cannot help but penetrate the multilayer substrate, leading to an increase in the number of scattered neutrons and deteriorating the signal-to-background ratio [28]. Therefore, a new detector structure has been proposed to detect the incident neutrons directly, avoiding the neutrons penetrating multilayer substrates. This structure comprises a boron-lined honeycomb neutron converter and an electron multiplier named the boron-lined honeycomb neutron detector [20; 21]. However, the boron-lined honeycomb neutron converter's substrate is aramid paper, making it challenging to ensure that the structural parameters of each honeycomb hole are equal. This issue can lead to a deterioration in the value and uniformity of the neutron detection efficiency. This study proposes a new design for the boron-lined gaseous neutron detector, called the boron-lined multilayer converter neutron detector (BMCND), to solve the above problem. The BMCND's structure parameters were optimized using Geant4 [29; 30; 31] and Garfield++(Version 2023.4) [32; 33]. Based on the optimized parameters, the BMCND was constructed in conjunction with our previous boron-lined method study [34; 35]. To test the BMCND, we used a \({{}^{241}Am^{-239}Pu}\) mixture \(\alpha\) source and a photo-neutron source. The results show that the BMCND is equipped with the capability to detect neurons, indicating that the BMCND has the potential to be an outstanding \({{}^{3}He}\)-alternative neutron detector.