## 5 Prototype Performance Test  
The fast neutron beam test was carried out under a 14 MeV D-T neutron generator of the China Academy of Engineering Physics. It was a moveable imaging facility with a compact accelerator D-T neutron source. It consisted of a compact accelerator, two different collimators (one for thermal neutrons and the other for fast neutrons), a sample-bearing platform, and a detection system, which was mounted on the platform for mobility. Figure 10 shows the sketch map of the facility [29]. The maximum neutron yield was 1.7 \(\times\) 10\({}^{11}\) neutrons/s under a deuterium beam current of 1.5 mA.  
In the test, a stainless-steel sample was selected as the imaging object, as shown in Figure 11. The sample consisted of a substrate with a thickness of 1 cm, which incorporated four stepped pieces with a thickness of 3, 5, 5, and 15 mm. The distance between the outer four-step piece and the edge of the substrate was 4 mm. These four-step pieces formed three slits with a width of 1, 3, and 10 mm. The detector system was shielded by two shield materials, namely, the lead shield with a thickness of about 5 mm and boron with a thickness of 3 mm containing polyethylene in the outer layer. This structure was designed to shield gamma and stray neutron backgrounds. The detector prototype system was powered by a 5 V/2 A power supply. The SiPM array operated under 29.5 V. The acquired data were transferred to a personal computer via the RJ45 interface for data processing.  
Figure 10: A sketch of the neutron imaging facility.  
Figure 9: SiPM arrays and readout electronics: (**a**) 1: SiPM array and compression circuit; 2: data acquisition; 3: preamplifier, and (**b**) the encapsulated detector system.  
The detector system was placed on a platform with the stainless-steel sample placed in front of the detector. After the irradiation for 5 min, a two-dimensional image was obtained, as shown in Figure 12. Figure 12a shows a raw 2-D image of stainless-steel samples with the sample shape overlay on it (open beam). The four gaps in the sample were basically visible. It can be observed that there were four peaks in Figure 12b related to different slit spacings of the stainless-steel sample. The second, third, and fourth peaks were related to the slits in the sample with a gap of 1, 3, and 10 mm, respectively. In addition, the three-peak valleys were related to three pieces of stainless-steel samples with a thickness of 3, 5, and 5 mm, respectively. The edge of the final pieces of stainless-steel samples with a thickness of 15 mm was not observed because the position of the sample was placed too far to the right. The neutron intensity contrast in the picture corresponding to different thicknesses of four steps of the steel sample cannot be observed as expected. This may be caused by the non-collimation of the neutron source, the inconsistency in the gain of each SiPM pixel, and the inhomogeneity intensity distribution of the neutron source. The edges of the 2-D sample image also showed some imaging compression. These image distortions should be eliminated through position calibration and correction. The sample image quality could also be improved by the normalization based on the no-sample neutron exposure data. However, the no-sample data cannot be used because of the neutron count saturation of the detector system.  
Figure 11: Stainless-steel sample. (**a**) Side view; (**b**) top view; and (**c**) oblique view.  
The spatial resolution of the detector was evaluated using the edge diffusion function of the sample. The minimum spatial resolution was 1.2 mm.