# The Bimodal Neutron and X-ray Imaging Driven by a Single Electron Linear Accelerator
## 1 Introduction
The microscopic cross section of the total photoatomic reactions is 100 to 1000 times higher than that of the photonuclear reactions; then, given that a photon-to-neutron convertor with sufficient thickness (the scale of mean free path of photons in the convertor) is used, the neutron yield (n/e' or n/\(\gamma\)) for a photoneutron source will also be within the range of 1/1000 to 1/100, almost the same as the low energy hadrons-based neutron sources. Despite the same neutron yield, the e-LINAC driven photoneutron sources might be a more attractive neutron source for the bimodal imaging application, in which the flux of photons should be several orders of magnitude higher than that of neutrons, because the X-rays present in the bimodal imaging have the typical energy of hundreds of keV, and for the detector, the relating microscopic cross sections of photoatomic reactions cannot be as high as that for thermal neutrons with some special nuclides (\({}^{3}\)He, \({}^{6}\)Li, \({}^{10}\)B, \({}^{113}\)Cd, or \({}^{155,157}\)Gd). The mechanism of the photoneutron production guarantees the X-rays' flux is at least two orders of magnitude higher than the photoneutrons' flux. Another benefit provided by the e-LINAC driven photoneutron source is that, benefiting from the high penetrating capability of energetic electrons, the photon-to-neutron convertor can be placed outside the e-LINAC and bombarded by the electrons penetrating a thin foil window (for example, 50 \(\upmu\)m Ti) of the e-LINAC, leading to a flexible neutron convertor/moderator design and a more effective heat dissipation of the power deposited by bombarding electrons, which is a key technical problem to improve the intensity of neutrons by increasing the power of the charged particles bombarding the target. In this article, we propose a technology that realizes both neutron imaging and photon imaging within one single e-LINAC driven system. The experimental results demonstrate that both the photon image and the neutron image can be acquired successively with a short time delay, ranging from 50 \(\upmu\)s to 10 ms, and be almost free from the different imaging beam geometries at the current spatial resolution. The pixel-wise matching of the two images can be efficiently conducted to form the inspected object's bivariate histogram to identify different materials of various mass thicknesses. In the case that the inspected sample is evolving with time or stochastic processes, the traditional two-source-two-detector system may inevitably introduce an error in fusing the two images, because each imaging mode may take several hours or even longer and the inspected sample is perhaps not identical for the two imaging modes. In this study, the time delay between the two imaging modes can be as small as 10 ms (when the e-LINAC works at the repetition rate of 100 Hz), which ensures that the inspected sample is identical for the two imaging modes. Consequently, being suitable to inspect a sample evolving with time repetitively would be an intrinsic advantage with the one-source-one-detector system presented in this study.