# The Bimodal Neutron and X-ray Imaging Driven by a Single Electron Linear Accelerator
## 1 Introduction  
Both X-ray imaging and neutron imaging have proven capabilities in non-destructive assays (NDAs) [1]. Unlike the cross sections of photons sensitive to the atomic number, neutrons are sensitive to nuclides but not elements. In the imaging process, whether for photons or neutrons, the attenuation is determined by the product of (1) the inspected object's mass thickness (g/cm\({}^{2}\)) and (2) its mass attenuation coefficient (cm\({}^{2}\)/g). In most cases, the inspected object's mass thickness is unknown and material identification cannot be accomplished merely by the photon or the neutron attenuation information. In this context, some researchers began investigating fusing X-ray imaging and neutron imaging technologies to enhance the capability for identifying materials [2]. Combining the two imaging technologies can be beneficial in the study of renewable energies [3], biology [4], paleontology [5], porous media [6], and cultural heritage [7]. This bimodal imaging approach was first implemented by [PERSON] in 2006 by two separate facilities for X-ray and neutron radiography [8; 9; 10; 11]. Analogous facilities were later constructed by many institutes such as NIST and ILL [12; 13]. All the studies are conducted on large neutron sources (reactor or spallation neutron sources) with the addition of a 100-200 kV X-ray tube. However, material identification based on the fusion of neutron and X-ray imaging technology can hardly be applied in situ for the following two reasons: (1) due to the high cost, the construction of a large neutron source for industrial applications is less probable; and (2) the neutron and X-ray imaging cannot share the same imaging beam geometry. The different imaging beam geometries introduce a difficulty to form a bivariate histogram for the pixel-wise comparison between the images from two modalities [9]. Usually, the computerized tomography for both neutrons and photons is necessary to align the two images via linear translation, rotation, scaling, or skew. Using two different imaging systems within a state-of-the-art bimodal imaging system also requires oversight to avoid interference. For example, the neutron detector perhaps shadows the photons before they penetrate the object being inspected [9]. Although thisproblem could be partially resolved by increasing the distance between the detector and the sample, the deterioration of imaging sharpness caused by penumbra blurring for photons and neutrons must be considered. Thus, using a one-source-one-detector system, instead of a two-source-two-detector system, would reduce such inaccuracies and simplify data processing. Besides the research reactor and the spallation neutron sources, a variety of low energy accelerators, in which the charged particles are accelerated to decades of MeV, are used to deliver neutrons. The accelerated charged particles could be hadrons, such as the proton, deuteron, or other heavier nuclei, or the lepton, mainly the electron. As the hadrons are usually not the so-called MIPs (minimum ionizing particles), whose kinetic energies are about three times of their rest masses, they transfer most of their kinetic energies to the target atom's orbital electrons but not the target nuclei, and the short range in the target limits their neutron yield (n/hadron) within the range of 1/1000 to 1/100, far smaller than the typical neutron yield (20 n/GeV proton) of the spallation neutron source [14]. When the e-LINAC (electron linear accelerator), which accelerates the electrons, is used to produce neutrons, energetic bremsstrahlung photons are first generated and then the photoneutron reaction will be involved to liberate neutrons from the target nuclei. Due to the competing of the photoatomic reactions, the photoneutron reaction is not the dominating reaction when energetic photons interact with matter.