## 3 Detector Simulation and Optimization
### Neutron Conversion Efficiency  
The EJ200 organic scintillator is widely used in fast neutron detection. The technical specification of the EJ200 is listed in Table. The fast-timing properties make it suitable for large-area and fast neutron imaging systems. In this study, the conversion efficiency ratio was defined as the ratio of the number of neutrons that collided with a hydrogen nucleus to produce a recoil proton to the total number of incident neutrons. The fast neutron conversion efficiency within the scintillator was proportional to the thickness of the scintillator. The EJ200 with a cross-section area of 1 mm \(\times\) 1 mm was irradiated by a 14 MeV neutron source in the simulation. The variation in the neutron detection efficiency with the thickness of the scintillator was recorded.  
The energy of recoil protons produced by the interaction between neutrons and hydrogen nuclei was deposited and caused scintillating photons. When the thickness of the scintillator increased, the amount of recoil protons increased. This was the main reason for an increase in the detection efficiency. However, the increase in the detection efficiency may be limited because the number of scintillating photons would decrease when the thickness of the EJ200 exceeded a certain value. This can be attributed to the light loss of the thick scintillator. Figure 4 shows the relationship between neutron detection efficiency and the thickness of the EJ200. Detection is defined as the fraction of the incident neutrons producing more than 100 photons on the SiPM. When the thickness of the EJ200 exceeded 25 mm, the fast neutron detection efficiency started to drop. This may be because the scintillating photons at the front of the EJ200 were attenuated or self-absorbed during transmission. The signals of the SiPM could not trigger the readout electronics effectively.  
The scintillator attenuation length given in the manual (Table 1) was the intrinsic light transmission attenuation length. It was determined by only the self-absorption characteristics of the scintillator. The geometric factors of the scintillator were excluded. For a scintillator constrained to a finite size, because of the effects of its geometry, such as reflection, refraction, and total reflection, as well as with the action of external conditions, the attenuation length for light transmission became shorter. The light attenuation length of the EJ200 with a section of 1 mm \(\times\) 1 mm was also simulated, and it was related to its geometry and light shielding craft. Figure 5 shows the change in the photon number  
Figure 4: The variation in detection efficiency with a plastic flash length.  
from one side of the cuboid scintillator to the other side with an increase in the scintillator thickness. This result indicated that the number of photons decreased when the thickness of the EJ200 exceeded 20 mm. This was consistent with the simulation result of the detection efficiency.  
So, the thickness of the EJ200 should be large enough to maintain high detection efficiency and small enough to maintain the scintillating photon number to the SiPM readout. Eventually, a thickness of 25 mm was chosen for the EJ200.