# The XTRA Option at the NEUTRA Facility--More Than
## 1 Introduction-Method and Instrumentation
### Neutron vs. X-ray Imaging: Physics and Cross-Section Data  
Both X-rays and neutrons can transmit through many objects and be recorded subsequently by position-sensitive area detectors, providing corresponding radiographic images. This allows for non-destructive examinations of the interior of samples. Such imagingfacilities are quite easy: the beam from the source is sent to the sample via a collimator and the transmitted part of the beam is registered by the radiation imaging detector. Today, most image data are obtained in a digital format, i.e., a discrete pixel data set of the spatial intensity distribution can be generated (see below). The image formation of both types of radiation follows the Beer Lambert law, however, the attenuation characteristics of both radiations are significantly different and often complementary. The energy and wavelength-dependent attenuation coefficients of neutrons and X-rays can be calculated from tabulated cross sections [2; 3]. Most laboratory X-ray sources are of the tube type and provide a continuous spectrum of X-ray photons depending on the tube voltage. However, X-ray imaging is also performed at synchrotron light sources [4].  
X-rays are electromagnetic radiation that mainly interacts with the electrons in the atomic shell. This leads to a distinct dependence on the atomic number Z. The more electrons there are in a chemical element, the stronger it interacts with the X-ray radiation. Therefore, X-rays do not easily penetrate metals while biological materials, mainly consisting of hydrogen and carbon, are quite transparent.  
Neutrons, on the other hand, are charge-free particles existing in the atomic nuclei together with the protons. Free neutrons result from nuclear reactions, e.g., fission of heavy nuclei like uranium or plutonium or from bombarding of heavy nuclei, e.g., lead, with a highly energetic proton beam, inducing spallation. Neutron imaging needs quite powerful neutron sources, i.e., a fission research reactor like FRM-2 in Munich [5], a spallation neutron source like SINQ [6], or the European Spallation Source, to reach the neutron flux required for acquiring single images within a few seconds and with sufficiently high spatial resolution, comparable to that of an X-ray tube. The energy range most useful for neutron imaging and scattering investigations is in the range of thermal (25 meV) and cold neutrons (<25 meV). At these energies, neutrons have favorable properties to interact with condensed matter, e.g., due to wavelengths (according to [PERSON]'s relation) of a few Angstrom-matching interatomic distances.  
In addition, at such energies they have a high probability to be captured by relevant isotopes such as \({}^{3}\)He, \({}^{10}\)B, \({}^{6}\)Li, and \({}^{157}\)Gd, which thereby produce charged particles inducing, for example, light emissions in scintillator materials. Thus, the detection efficiency is particularly high in this spectral range.  
In contrast to X-rays, neutrons, which carry no charge, interact with the atomic nuclei and thus there is no systematic dependence of cross sections on the atomic number. There are even strong variations in neutron-matter interactions depending on the isotope considered, e.g., \({}^{1}\)H and \({}^{2}\)D, \({}^{6}\)Li and \({}^{7}\)Li, or \({}^{10}\)B and \({}^{11}\)B. Even high Z materials like lead or uranium are transmitted relatively well, whereas hydrogen has a high scattering cross section for thermal and cold neutrons. Therefore, combining X-ray and neutron imaging in a dual modality setup allows for the use of their complementary contrast mechanisms for a better understanding of the hidden structures of samples subject to radiographic inspection.  
This is underlined by the example in Figure 1 with the same image quality and spatial resolution but very different contrasts for the involved components and structures. How far the involved attenuation properties can deviate for X-rays and neutrons is exemplified by the data in Table 1.