D. P. Kozlenko, S. E. Kichanov, B. N. Savenko, E. V. Lukin, K. M. Nazarov, I. Yu. Zel, B. A. Bakirov, M. Kenessarin, A. Zhomartova, V. S. Smirnova

Neutron radiography and tomography that allow to obtain images and volumetric 3D reconstructions of the internal structure of research objects with a spatial resolution of about 100 μm are at present widely developed using modern neutron sources. A significant depth of neutron penetration provides a number of advantages to neutron radiography and tomography as compared to X-rays and allows their use for meeting a wide range of interdisciplinary scientific tasks, from studying the structural features of various functional materials, the microstructure of construction and structural materials and their changes during the processes occurring in them to studying the internal structure of unique objects of cultural and natural heritage, objects of extraterrestrial origin. A specialized facility for research using neutron radiography and tomography has been developed on beamline 14 of the IBR-2 high flux pulsed reactor of FLNP JINR and work has been carried out to improve its technical parameters. The advent of the neutron radiography and tomography facility has allowed to develop a new applied research area at FLNP, related to the non-destructive analysis of the internal structure of a wide range of objects, products and materials, including paleontological objects of natural heritage, engineering products, objects of extraterrestrial origin - meteorites, archaeological objects of cultural heritage.

Fig.1. Schematic of the neutron radiography and tomography facility on beamline 14 of the IBR-2 high flux pulsed reactor. The length of the collimator system is 11 m and the linear dimensions of the neutron beam at the system output are 20×20 cm. The goniometer system is used to carry out tomographic experiments. The neutrons are registered using a specially designed detector system based on a high resolution CCD camera

Neutron radiography and tomography is an applied non-destructive testing technique in modern technology. The penetration of neutrons into the thickness of the material provides information on the spatial distribution of internal components, cracks and internal faults, locations and ways of corrosion penetration into industrial and engineering facilities.

Using the neutron tomography technique, two meteorites of different types Seimchan pallasite and Chelyabinsk chondrite have been studied. The peculiarities of the interaction of neutrons with substance have allowed to detect and separate the internal mineral components of these meteorites, to obtain a 3D spatial distribution of nickel in the metal component of these pallasite meteorites, to construct distributions according to the occupied volumes and average sizes of internal composite minerals and to determine their morphological features. It should be noted that the corresponding neutron attenuation factors for iron and nickel are several times greater than the corresponding factors for the mineral olivine; therefore, in neutron radiography experiments, metal inclusions and veins contrast well against the background of olivine grains (Fig. 2a). The difference between the neutron attenuation coefficients of the metallic and olivine components has allowed to clarify a range of petrological and morphological features of meteorites. Neutron tomography has revealed the anisotropy of nickel distribution inside the iron component of the Seimchan meteorite and large grains of the kamacite mineral (Fig. 2b) in the internal volume of the studied fragment of the Chelyabinsk meteorite.

Fig.2. a) Neutron tomography of meteorites: a) 3D model of a fragment of the Seimchan meteorite, the metal areas are marked in red, the olivine mineral is marked light. b) a virtual cut of a fragment of the Chelyabinsk meteorite, light areas correspond to metal kamacite, gray areas correspond to silicate minerals

Investigations of cultural heritage sites using natural scientific techniques are significant for expanding and deepening their source study capabilities. Research allows to determine where the artifacts were created, how they were distributed, to establish their age, manufacturing technology, original appearance and subsequent changes. It also allows to obtain new data about culture, science, crafts, technologies, trading relations, the history of entire countries and peoples. The particular value and often fragility of cultural heritage sites requires the use of modern non-destructive testing techniques for their research, such as neutron tomography and radiography. It should be noted that often a peculiarity of cultural heritage sites is that they can combine materials with different physical and chemical properties: metals, substances of mineral and organic origin, so the use of complementary techniques for their investigation is very important. Neutron imaging data can be supplemented by X-ray imaging, as well as by investigations of the elemental and phase composition of substances.

It is necessary to empasize the great scientific achievements in non-destructive investigations of cultural heritage sites using neutron radiography and tomography: unique research of the internal organization and phase composition of Ancient Greek and Bulgarian coins, ancient Russian cultural heritage sites and in particular, the Tver treasure found in 2014 and dated to XIII century - the period of the invasion of Batu. The high radiographic contrast between silver and copper allowed to successfully study both the spatial distribution of these compnents and to estimate the chemical composition of ancient coins: from the Bosporan staters to the ancient Bulgarian pools. The high penetrating ability of neutrons allowed to restore the 3D internal structure of Old Russian (Fig. 3) and medieval European ornaments, to reveal their hidden decorative elements and to clarify the historical and cultural origin of these archaeological finds. The information obtained is important for the restoration of these cultural heritage sites, as well as for the investigations of ancient technologies for the preparation of coins, jewelry and household items.

Fig. 3. Photograph and a 3D model of an Old Russian ornament from the Tver treasure - a beam collar, restored from neutron tomography data. From the neutron tomography data, both the surface components of the decor and the internal components of the fasteners were restored

Publications:

1. Kichanov S., Saprykina I., Kozlenko D., et al., Studies of ancient russian cultural objects using the neutron tomography method. Journal of Imaging, 4(2), 25 (2018). Doi:10.3390/jimaging4020025
2. Kichanov S.E., Kozlenko D.P., Lukin, E.V., et al., A neutron tomography study of the Seymchan pallasite. Meteoritics & Planetary Science, 53(10), 2155-2164 (2018). Doi:10.1111/maps.13115
3. Kichanov S.E., Kozlenko D.P., Kirillov A.K., et al., A structural insight into the Chelyabinsk meteorite: neutron diffraction, tomography and Raman spectroscopy study. SN Applied Sciences, 1(12), 1-9 (2019). Doi:10.1007/s42452-019-1614-x