Understanding, preserving and presenting the heritage of the past is one of the most important tasks of museums. Understanding is, however, sometimes easier said than done. For example, frequently there is no possibility or reason to execute informative but harmful or invasive investigations for invaluable objects. This perfectly illuminates the cases when the role of newly developed non-invasive and non-destructive experimental techniques of investigation can be appreciated. Neutron tomographic imaging is a powerful technique for investigations of valuable, complex samples. [1] Looking through and into objects made of varied materials, created by complex manufacturing processes and used for different purposes is a common objective. [2] The methodology and the application range of neutron imaging, similarly to x-ray imaging, has been significantly improved in the last decades. Advanced imaging methods, such as tomography, are now routine tools at several facilities worldwide. With the advent of the digital era, sophisticated image processing tools can be applied. Moreover, digitalization could serve as a last bastion to preserve the memories of the past. Based on their transmission images, neutron and x-ray radiography (2D) [3] and tomography (3D) [4] can provide information about the inner structure of the sample by detecting the attenuation of an incident beam as it passes through matter. The complementary character of the two radiations can easily be understood. With neutrons, hydrogenous materials deliver high contrast, and many metals can easily be penetrated. With x-rays, light elements, e.g., organic materials, have low contrast, and objects containing heavy elements, e.g., metals, are difficult to penetrate. However, it is our intention to emphasize that neutron imaging is not all powerful, and its complementary application together with x-ray imaging and other neutron techniques (diffraction, elemental analysis) usually gives the most complete set of answers in an investigation. Laboratory-based x-ray radiography systems are more easily accessible, being available in many labs, while neutron-imaging facilities have been established only at a few dedicated laboratories. Being interdisciplinary, cultural heritage science already relies on the practice of many disciplines, from the natural sciences such as physics and chemistry, to art history and restoration. Therefore, introducing this new technique fits into the present pattern of diversity. fig. 1 A modern radiography/tomography setup consists of a neutron sensitive, visible-light emitting scintillator screen, a mirror, optics, a digital camera and image-processing software and hardware (not shown here). The sample is put in front of the screen for radiography and rotated by a sample stage for tomography.

PRINCIPLES: NEUTRON ABSORPTION RADIOGRAPHY AND TOMOGRAPHY Neutrons are electrically neutral particles with no strong electromagnetic properties; they easily penetrate the sample, and the interactions that take place there cause the attenuation of the incoming neutron beam (lowering its intensity along straight trajectories from the source towards the detector). Radiography literally means 'draw with radiation'. It is a direct imaging technique, where the visual representation (a planar projection) of an object is obtained non-destructively by detecting the attenuation of an incident beam as it passes through matter. Radiography transforms the interaction of an invisible radiation with the material into tangible images. The visibility of an image depends on the contrast, i.e., the difference between the intensities (greyscale values) observed at two adjacent areas of the image. It is important to point out that given a complete knowledge of the material of an object, the contrast can be calculated, but the reverse calculation of finding the material without doubt only from its attenuation is impossible, because various materials in various thicknesses can cause the same attenuation. fig. 2 Four projection images out of the total of 1126, which were acquired sequentially at increasing rotational angles during the neutron tomography of the horse. In the middle of the figure one can see the horse placed in front of the imaging system. The object was supported by crumpled aluminium foil, which gives negligible contrast in neutron imaging.

Tomography is an extension of radiography, where the 3D visualization of the object is achieved by using computational algorithms from a series of radiographic projections acquired as the object is rotated in small angular increments. [5] Through digital image processing, a virtual-reality representation (a map) can be created by reconstruction and rendering of the 3D image. The map shows from point to point the attenuation power of the material independently of the thickness of the penetrated layer. A modern radiography/tomography setup is shown in fig. 1. Using both lower and higher energy (so-called cold and thermal) neutrons gives better detectability and good contrast for most materials. A beam geometry with larger collimation ratio improves the spatial resolution, usually at the expense of a longer exposure time. Nowadays a routinely achievable spatial resolution at the most advanced facilities is in the order of several tens of microns. [6] The light-sensitive CCD or sCMOS chip in the camera collects the incoming light in its pixels during the exposure, and after readout it provides a set of greyscale values, i.e., the intensity of the neutron attenuation at each (x, y) pixel. The image-processing software and hardware execute the necessary calculations with the image treated as a data matrix. During data collection it is also possible to follow on-going dynamic processes in real time, e.g., water absorption, by consecutive recording of snapshot images.

EXPERIMENT: THE FACILITY AND THE IMAGE PROCESSING The measurements were carried out at the radiography/tomography (RAD) station of the Budapest Research Reactor (BRR). [7] The Hungarian Academy of Sciences offers access to this scientific infrastructure for the domestic and international users. The radiography station (RAD) can be used for bimodal imaging using either the thermal neutron beam of BRR or a portable x-ray source. An example of four typical images seen in the neutron tomography scan is shown in fig. 2. For the setup used, the spatial resolution is about 250 µm and the exposure time for one projection was in the range of a few seconds. The image processing and analysis were completed with the latest versions of the FIJI, Octopus, and VGStudio MAX codes. At the end, images of various parts of the object can usually be separated based on their greyscale values, and during the mathematical manipulation of the 3D dataset there is a possibility to show any interesting range of the values or any interesting part of the object. No x-ray tomography was acquired because the available x-ray tube voltage value was too low for a proper penetration of the whole object from all angular directions necessary for tomographic reconstruction. Instead, 2D projections, i.e., x-ray radiography images, were taken at several rotational angles. These images were useful tools for the comparison of the neutron and x-ray attenuation by the horse. Any object put in a neutron beam will absorb some of the traversing neutrons. This absorption contributes to the shadow image in the detector. During the imaging experiment, the absorbed neutrons, unlike x-rays, can generate radioactive isotopes by a reaction called neutron activation. A few days of radioactive decay after the experiments is usually sufficient to get rid of most of the radioactivity induced in the object. Since the number of affected atoms is negligibly low to begin with, activation has no macroscopic effect on the physical integrity of the object, nor on its composition.

**NOTES** The authors wish to thank Dr Péter Barkóczy metallurgical engineer (R&D manager, FUX Zrt., Miskolc) and Csaba Laczik sculptor for helping with their comments on the production of casting.
1 Neutron Imaging and Activation Group, Neutron Imaging at the Spallation Source SINQ, Villigen 2016, https://www.psi.ch/sinq/neutra/neutron- imaging-brochure.arrow_upward

2 Veronika Szilágyi, Zoltán Kis, and László Szentmiklósi, 'Neutron Imaging for Archaeometry', Archeometriai Műhely 13, no. 3 (2016), pp. 157–72, http://www.ace.hu/am/2016\_3/AM-2016-3-SZV.pdf.arrow_upward

3 Nares Chankow, Neutron Radiography, in Nondestructive Testing Methods and New Applications, Mohammad Omar (ed.), Rijeka 2012, pp. 73–100, http://www.intechopen.com/books/nondestructive-testing-methods- and-new-applications/neutron-radiography.arrow_upward

4 Eberhard H. Lehmann and Nikolay Kardjilov, Neutron Absorption Tomography, in Advanced Tomographic Methods in Materials Research and Engineering, John Banhart (ed.), Oxford 2008, pp. 375–408.arrow_upward

5 Ian S. Anderson, Robert L. McGreevy, and Hassina Z. Bilheux, Neutron Imaging and Applications. A Reference for the Imaging Community, New York 2009, https://doi.org/10.1007/978-0-387-78693-3.arrow_upward

6 Anders P. Kaestner, Pavel Trtik, Mohsen Zarebanadkouki, Daniil Kazantsev, Michal Snehota, Katherine J. Dobson, and Eberhard H. Lehmann, 'Recent Developments in Neutron Imaging with Applications for Porous Media Research', Solid Earth 7, no. 5 (2016), pp. 1281–92, https://doi.org/10.5194/se-7-1281-2016.arrow_upward

7 Zoltán Kis, László Szentmiklósi, Tamás Belgya, Márton Balaskó, László Z. Horváth, and Boglárka Maróti, 'Neutron Based Imaging and Element-­Mapping at the Budapest Neutron Centre', Physics Procedia 69 (2015), pp. 40–47, https://doi.org/10.1016/j.phpro.2015.07.005.arrow_upward