The steady decline of neutron imaging (NI) facilities, exacerbated by the closure of several research reactors such as Orph & eacute;e (LLB, France) and BER (HZB, Germany), has increased the importance of leveraging existing reactors for NI applications. Zero-power research reactors offer a low-cost and flexible platform for student training, materials research and development, and proof-of-principle studies that can support larger neutron facilities. Previously, no dedicated NI beamline was available at the EPFL zero-power reactor CROCUS. In this work, we demonstrate neutron imaging at the CROCUS reactor for the first time. To adapt the reactor, an air-filled aluminum channel was inserted into the water reflector to enhance neutron leakage, supplemented by aluminum flight tubes to minimize transport losses. Gold foil irradiations were performed for neutron flux measurements, and a 3He position-sensitive detector was used to map the 2D flux distribution. The neutron energy spectrum at the imaging position was subsequently calculated. A NI detector, comprising a scintillator screen coupled to a CCD camera, was installed to conduct the first imaging experiments at CROCUS. These experiments confirmed the feasibility of neutron imaging at CROCUS by capturing neutron images of a Bon religious statuette, flowers, a pacemaker and a battery, several copper and polyethylene samples, a dynamic sequence of images showing water uptake in concrete. These results provide a foundation for establishing a permanent NI facility at EPFL to complement existing regional infrastructure.

The detection of hydrogen in neutron imaging is prevalent due to its high contrast relative to other structural materials present in or around samples. This presents a significant advantage over X-ray studies, where hydrogen contrast is almost negligible. In many applications, accurate quantification of hydrogen content is essential, warranting careful consideration of the approach's limitations. This paper, grounded in practical experiences, presents both successful and unsuccessful interpretations of neutron imaging data, highlighting the challenges and nuances of hydrogen detection in this context.

The observation of inner, hidden material distributions of objects with cultural heritage importance requires methods which enable a transmission of the used radiation. Because inspection with visible light is limited for non-opaque samples, only X-rays and neutrons can be used for such investigations. Alternative kinds of radiation like electrons, micro-waves are limited in the practical transmission and the required spatial resolution. Besides a retrospective of successful project, we want to present ways how to collaborate and to emphasize the importance of personal commitment and contacts with relevant people in the museum’s community. To answer the question “how to present best our results”, a library of image data is under preparation for ancient Tibetan bronze objects, studied with neutron imaging methods at the PSI facilities.

Neutron imaging (NI) complements X-ray radiography by exploiting the strong interaction of neutrons with several light elements and a high penetration for many heavy elements. Hitherto no dedicated NI beamline currently exists at the zero-power CROCUS reactor. Establishing such a station would create a low-cost platform for student training, materials R&D, and proof-of-principle studies that feed larger facilities. Using the Serpent 2 Monte Carlo neutron transport code, we explored how to modify an existing zero power research nuclear reactor into a useful neutron source for NI. Three measures proved decisive: (i) inserting a 70 mm-diameter air (or vacuum) tube through the 300 mm water reflector, boosting channel flux by approximately factor of fifty (ii) retaining a thin water layer between fuel and tube to convert fast neutrons to thermal ones without excessive attenuation; and (iii) employing low-scatter aluminum windows and vacuum flight tubes to preserve beam intensity over source-to-detector ~5m distance. A resolution-versus-flux map identified an optimal layout that balances image sharpness and count rate for the detector planned for first radiographs in 2025. The outcomes of the simulations show that a reflector modification can deliver performance comparable to demonstrator stations at VR-1 in Czechia and AKR-2 in Germany, and they underline CROCUS’s potential as a national testbed for neutron radiography, and potentially even low-resolution computed tomography.

Neutrons are versatile probes for the non-destructive examination of various materials. To introduce the following chapters on the applications and methods, we give a general overview of the fundamental properties of neutrons and the ways of their production in research institutes. We also present neutron sources of international significance and their capabilities.

Neutron imaging is established at many neutron sources around the world as a method for noninvasive investigations of samples and object on the macroscopic scale. Similarly to X-ray imaging, it provides the possibility to “look through” materials and allows one to “see” the inner, hidden content. However, owing to the complete different interaction mechanism, neutron imaging provides very different and complementary contrasts compared to X-rays, even if the image quality often is about the same. We report about the method’s principles, describe the state of the art, and give an outlook for new trends and developments.

We have developed neutron tomography as a new tool to study the interior of ancient works of art. The present paper details the principles of the method and summarizes also some of the latest results, particularly concerning Mongolian Buddhist statues of the 17./18th century. It is shown that offering deposits made of organic and ceramic materials, even when fully enclosed in a cast metallic statue, can be examined in a non-invasive way. Within certain limits this concerns composition, size, structure and position of the interior deposits. Such studies can contribute to understand the history of ritual practices and their goals. They can also contribute to evaluate whether a statue has been properly cleaned and filled during consecration and is therefore fit for religious service. Such studies can also help to form an opinion of whether a statue is genuine.

We propose a method to analyze the characteristics of scintillator screens for neutron imaging applications. Using calculations based on the theory of cascaded linear steps as well as experimental measurements, we compared the characteristics of different lithium- and gadolinium -based scintillator screens. Our results show that, despite their much lower light output, gadolinium -based scintillators outperform lithium -based scintillators in terms of noise characteristics for a variety of imaging setups. However, the relative performance of scintillator screens is highly dependent on the other setup characteristics such as the beam spectrum, field of view, used optical lens and size of the camera sensor. Consequently, the selection of the best scintillator screen - as well as the scintillator characteristics assessment in new developments - requires a systematic consideration of all these elements, as enabled by the framework presented here.

Neutron imaging is a non-destructive testing method that functions according to principles similar to X-ray imaging. In contrast to X-rays, neutrons can generally penetrate metals rather well, but at the same time they have a high sensitivity for hydrogen. This makes neutron imaging – which includes radiography (investigations in 2D) as well as tomography (3D) – an ideal method for studying the impact of playing historical brass instruments. Playing a brass instrument creates an accumulation of moisture inside the instrument, which can eventually lead to the generation and expansion of corroded areas inside it. This moisture, along with many other products of corrosion, contains hydrogen, which provides a high degree of contrast for neutron imaging. This article explains how neutron imaging was used to monitor the condition of historical brass instruments, i.e. the changes in the internal corroded areas, by comparing 3D CT-data sets acquired before and after the instruments had been played on a regular basis over the period of fourteen months.

Modern digital imaging detectors have been implemented with scintillator screens as the key component. They capture neutrons and emit light, which is registered by suitable converts into the digital signal, used for analyses. The two established detector systems are either based on highly-efficient sensitive cameras or on amorphous silicon flat panels, which are in direct contact with the scintillator screen. Because the neutron beam intensity is quite low (e.g. compared to synchrotron light), it is essential to have scintillator screens with highest light emission per neutron. This property defines the efficiency of the detection process and the needed acquisition time for a valid image. On the other hand, highest spatial resolution is demanded in order to see smallest feature of a structure under investigation. In order to distinguish small contrast variation in the image data, a possible highest signal-to-noise-ratio is required. These conditions cannot however be satisfied in only one unique device at the same time. Although the principle neutron absorbers for the neutron detection and the most powerful light emitters have been known for many years, there is still potential for further improvements and optimization. Even exotic neutron absorbers and new scintillation materials are under investigation for the improvement of the scintillator screens, used in the neutron imaging detectors. Our report will summarize the current status of the development and describe the performance of common, commercialized materials. On the other hand, we will give the outlook for further approaches and first results of running developments.

Digital thermal neutron imaging (radiography and tomography) is a powerful non-destructive analytical tool and has demonstrated its importance in industrial and research application world-wide. The standardization process, to certify digital thermal neutron imaging as a standard practice in industry, entails standardized test phantoms to be evaluated. Through the evaluation of the phantoms the spatial resolution and contrast of a thermal neutron digital imaging system can be determined in a controlled and standardized manner by accepting good practice in terms of scanning, data processing, data visualization and evaluation. Standard test phantoms are objects with physical features designed to test facility capabilities to reveal these features without any ambiguity. The good practice enables the acceptable assessment of different international digital thermal neutron imaging facilities for spatial resolution and contrast abilities. The purpose of this contribution is to establish good practice for the experimental setup, acquiring of 2-D digital projections, the reconstruction process, the visualization and evaluation of the 3-D digital images of standard test phantoms for spatial resolution and contrast. Results obtained from applying this suggested good practice on contrast standard test phantom will be discussed.

We report on the realization of an improved concept for the detection of fast neutrons using a specific setup of scintillators with the aim to overcome limitations in spatial resolution. While fast neutron imaging (FNI) is a technique to investigate very thick material layers in transmission mode, currently there is a limit in the spatial resolution at similar to 1 mm, which hinders the performance for the detection of features like cracks, material damage or pores in large objects (>> 1 cm(3)). The improved concept presented here is based on the separation of the converter material from the scintillation layer in a suitable way, which was successfully tested and referenced to the standard devices at the NECTAR facility (FRM-2, MLZ, Garching) under realistic beamline conditions. The results imply a break-through in enabling detection of structures on the order of 0.2 to 0.5 mm (a more precise estimation is not possible due to other resolution determining factors in a realistic setup). Potential additional improvements are discussed.

This chapter explains how to use tomography methods, both with thermal and cold neutrons and also X-rays for the investigation of cultural heritage objects. With the help of these noninvasive techniques, it becomes possible to describe and analyze the whole three-dimensional structure and composition of samples in a size range between a few millimeters and several tens of centimeters in one data volume. Next to the description of the basic principles of the method and the experimental infrastructure required, we give seven examples of successful applications of tomography either with neutrons or X-rays—or with both to obtain complementary information.

A new type of scintillator screen consisting of a ZnS scintillator with a dysprosium neutron converter is explored in a joint effort between Idaho National Laboratory (INL) and Paul Scherrer Institute (PSI). In contrast with a traditional prompt(6)Li or Gd converters, a dysprosium converter generates a latent image as neutron activated dysprosium produces an isotope which decays with half-lives of 1.26 min and 2.3 h and the decay radiation excites the ZnS scintillator. The activated scintillator screen is physically transported out of the neutron beam and away from radioactive samples into the imaging apparatus and emits photons as the screen decays, which are read by a digital camera. This technology bridges the gap between traditional indirect transfer radiography and modern digital camera-based systems. This paper describes initial development of dysprosium-based scintillator screens and the results of initial tests performed at PSI. Some screen variants exhibit sufficient light output to produce good quality radiographs in a matter of minutes. The basic spatial resolution measured using a Siemens star is approximately 300 mu m. This work demonstrates for the first time that indirect digital transfer method neutron imaging is a plausible method of imaging highly radioactive sources such as irradiated nuclear fuel.

The fast neutron imaging technique with recoil proton detection harbors significant potential for imaging of thick, large-scale objects containing high-Z elements. However, the challenge to find efficient fast neutron scintillators with high spatial resolution is ongoing. The list of requirements for such scintillators is long and demanding: a proton-rich, scattering-free material combining high light yield with the absence of light reabsorption. To meet these challenges, we look for a suitable material among a rising class of 0D organic–inorganic Pb(II) halide hybrids. The use of large organic cations, e.g., trihexyltetradecylphosphonium, results in room-temperature ionic liquids that combine highly Stokes-shifted (up to 1.7 eV), reabsorption-free, and efficient emission (photoluminescence quantum yield up to 60%) from molecularly small and dense (PbX2 molar fraction up to 0.33) emitting centers. We investigate the optical properties of the resulting ionic liquids and showcase their utility as fast neutron imaging scintillators. Concomitantly with good light yield, such fast-neutron scintillators exhibit both higher spatial resolution and lower γ-ray sensitivity compared with commercial ZnS:Cu-based screens.

Just after the start into the new millennium the concept for combined neutron and X-ray imaging was introduced by extending the standard configuration of the thermal neutron imaging NEUTRA instrument with a complementary 320 kV X-ray tube setup. Using essentially the same detector configuration for both neutron and X-ray imaging enables a pixel-wise (in radiography) and a voxel-wise (in tomography) correlation and combination of attenuation data. The optimal use and analyses of such complementary data sets depend on the specific investigation and research question and range from a combinatory interpretation of separately analyzed images to full data fusion approaches. Here, several examples from more than a decade of bimodal neutron and X-ray imaging at NEUTRA at PSI shall be reviewed.