# The Bimodal Neutron and X-ray Imaging Driven by a Single Electron Linear Accelerator
## 2 Materials and Methods  
### System Overview  
In Figure 1, we illustrate the principle to produce neutrons and photons used for the bimodal imaging driven by a single e-LINAC system (Appendix A.1). The 9 MeV electrons are very energetic, and hence the bremsstrahlung photons generated on the tungsten target are forward emitted. A heavy water convertor (Appendix A.2) is placed ahead of the tungsten target to generate both the imaging neutrons and photons. Heavy water is chosen as the material to convert bremsstrahlung photons to neutrons due to (1) the low (\(\gamma\),n) threshold of \({}^{2}\)H (E\({}_{\text{th}}\) = 2.223 MeV) and (2) the superb neutron moderation capability of \({}^{2}\)H and \({}^{16}\)O. The neutron moderation (Appendix A.3) capability is critical in this study because the fast photoneutrons produced by the \({}^{2}\)H(\(\gamma\),n)\({}^{1}\)H reaction, in general, should be decelerated to slow neutrons to improve the imaging sensitivity. The orbital electrons of the \({}^{16}\)O atoms and those of the \({}^{2}\)H atoms can induce the scattering of bremsstrahlung photons. When the emitting angle of scattered photons is chosen as 90\({}^{\circ}\), the energy of incident photons interrogating the inspected object will typically be less than 511 keV (due to the Compton scattering), as shown in Figure 2a. Photons with such an energy spectrum mainly interact with atoms via Compton scattering, which shows an almost constant mass attenuation coefficient for different elements [15], and hence are helpful to analyze the mass thickness of the inspected object compared with X-ray tube measurements, as shown in Figure 2b.  
Figure 1: **(a)** The principle to produce imaging neutrons and photons simultaneously. Energetic electrons are delivered by a 9 MeV e-LINAC and bombard the tungsten target to produce bremsstrahlung photons. The bremsstrahlung photons then hit the \({}^{2}\)H nuclei to produce the photoneutrons via the \({}^{2}\)H(\(\gamma\),n)\({}^{1}\)H reaction and the electrons surrounding the \({}^{16}\)O nuclei to produce scattered photons, respectively. Both the neutrons and photons will penetrate the inspected object and be collected by a neutron-sensitive microchannel plate (\({}^{n}\)MCP) detector and converted to electron clouds, leading to a scintillation on the scintillation screen and formation of the photon and neutron images successively. A complementary metal oxide semiconductor (CMOS) camera then registers the two images with the aid of an optical system composed of the mirror and lenses. **(b)** Photograph of the bimodal imaging system driven by a 9 MeV e-LINAC.  
The neutron emission direction should be the same as the photons to conform to the photon imaging geometry. In fact, the energy spectrum of emitted neutrons is not sensitive to the emission direction due to the almost isotropic moderation process of neutrons within the heavy water converter. In Figure 3, we present the neutron energy spectrum measured with the time of flight (TOF) method by a 4-atm \({}^{3}\)He counter placed 10 m away from the heavy water converter at an angle of 90\({}^{\circ}\). The simulated neutron energy spectrum is also shown, and the two spectra match fairly well. The experimental results show that when the 9 MeV e-LINAC works at a 100 uA current, a 2500 neutron/cm\({}^{2}\)/s thermal neutron flux 10 m away can be acquired for the neutron imaging. Its counterpart for photons is 10\({}^{8}\) photon/cm\({}^{2}\)/s.