## 2 Materials and Methods  
The neutron imaging was performed at PKUNIFTY in China [15]. The reaction Be (d, n) is selected for neutron generation at PKUNIFTY. The fast neutrons are produced by 2 MeV deuterons bombarding a beryllium target. The deuteron beam energy is designed as 2 MeV and the recent average beam current is 0.2 mA, which gives a fast neutron yield of \(1.5\times 10^{11}\) n/s. The moderators used were polyethylene (primary) and water (secondary). Its structure is shown in Figure 2.  
A 200 \(\upmu\)m \({}^{6}\)LiF/ZnS scintillator screen on the detector was used. The CCD camera the chip of which can be cooled to \(-70\)\({}^{\circ}\)C was set to acquire images of \(1024\times 1024\) pixels. The pixel size of the acquired images corresponded to 150 \(\upmu\)m. Considering both spatial resolution and time resolution, we chose an exposure time of 90 s by comparing different experimental schemes. When the neutron yield is \(1.5\times 10^{11}\) n/s and the L/D is 67, the thermal neutron flux is \(1.4\times 10^{4}\) n/cm\({}^{2}\)/s at the imaging plane. The thermal neutron spectrum can be seen in Figure 3, [16]. Figure 1: Schematic diagram of the compact accelerator-driven neutron source for PKUNIFTY. Figure 2: Moderator assembly configuration in central horizontal profile. Considering that the purpose of this paper is to demonstrate a neutron imaging method for a compact neutron source, a normal cement paste was used, while other components were mixed as a control group. Specimens were prepared with seven types of cement paste. Ordinary Portland cement type 42.5, fly ash, and marble powder were used. The exact composition of each of the seven types of paste is given in Table 1. The three-dimensional size of the samples is 25 mm \(\times\) 15 mm \(\times\) 50 mm, which were dried in a ventilated oven at 50 \({}^{\circ}\)C for 4 days until constant weight had been reached. All surfaces except the two opposite surfaces (25 mm \(\times\) 15 mm) were covered with self-adhesive aluminum foils, in order to impose unidirectional water flow when one of the two opposite surfaces was put in contact with water. Then, two flat aluminum water containers, together with all seven samples were placed in the neutron beam. We obtained the data form the seven samples at the same time during the experiment due to the field of view being large enough (150 mm \(\times\) 150 mm) and the thermal neutron flux uniformity in the field of view being better than 7% [16]. The placement of the samples is shown in Figure 4.  
First, a neutron image was taken in the dry state. Then, the water container was filled with water up to a level approximately 5 mm above the absorbing surface of the cement paste slice. Two minutes after the water container was full, we started taking images of the wet samples every 90 s. In this way, the process of water absorption for cement paste could be followed as a function of time. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **W/C** & **Cement/g** & **Fly Ash/g** & **Marble Powder/g** & **Water/g** \\ \hline PC04 & 0.4 & 100 & - & - & 40 \\ FA10 & 0.4 & 90 & 10 & & 40 \\ FA30 & 0.4 & 70 & 30 & - & 40 \\ FA60 & 0.4 & 40 & 60 & - & 40 \\ MP10 & 0.4 & 90 & - & 10 & 40 \\ MP20 & 0.4 & 80 & - & 20 & 40 \\ MP30 & 0.4 & 70 & - & 30 & 40 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Composition of the seven types of cement paste used in this project. Figure 3: Neutron spectrum at the imaging plane. Since the quality of our neutron imaging was lower than that from reactor and spallation neutron sources, image processing after the experiment was the focus and difficulty in this work. As a preliminary study, we only focused on the evolution of the shape and the average height of the water front in cement-based materials over time. All image processing and quantitative analyses were performed using the ImageJ (Version 1.53 n) [17] software packages.