Development of a conceptual scheme for the creation of environmentally friendly Gd-containing neutron-absorbing nanocomposites

The nature and properties of neutron radiation impose specific requirements on the creation and operation of such materials. The presence of accompanying and induced radiation, weather conditions, as well as the peculiarities of the location of functional layers require a hierarchical approach to the development of such neutron-absorbing materials. It was shown that the conceptual scheme of creating Gd-containing protective materials consists in taking into account all factors of their operation. In particular, it is shown that the presence of neutron-absorbing components is a necessary but not sufficient condition for their successful application in environmental conditions. In this work, Gd-containing chitosan films were prepared as an example of the proposed conceptual scheme for the creation of environmentally friendly neutron-absorbing nanocomposites. The prepared Gd films were characterized and their neutron permeability was estimated. The approach presented in this work contributes to the development of sustainable and responsible production practices, supporting SDGs 9 and 12.


Introduction
The need for protection against neutron radiation is due to the increasing level of radiation load.The growing level of radiation danger is associated with the rapid progress of science and technology and the expansion of the spectrum of the use of radiation technologies in the industry.Examples of such applications are non-destructive testing systems (cargo and car scanners), high-energy particle detectors (CERN), creation of new radiation sources (subcritical assembly "Neutron Source", Kharkiv, Ukraine) and compact nuclear power plants.
The need for protection against neutron radiation is also urgent in space, where protection against cosmic radiation (high-energy heavy particles) [1] during travel and equipment operation 1254 (2023) 012100 IOP Publishing doi:10.1088/1755-1315/1254/1/012100 2 is important.The main task of neutron-absorbing materials is to cut off the neutron part of the spectrum during the collision with high-energy particles, which reduces the radiation load.

Analysis of research and publications
Nowadays, nuclear technologies are widely used all over the world, in particular for environmental protection [2], scientific research [3,4], medical imaging and therapy [5], neutrons in the field of nuclear energy [6], agriculture [7], etc. Neutron radiation is one of the most dangerous types of radiation and poses a real danger to radiation workers and the public.Its impact on biological tissue leads to ionization of the material and, as a result, significant changes in the functionality of the cell or loss of the ability to recover.Other chemical elements can also be formed, including radionuclides that create induced radioactivity in the body.This problem is also relevant for nuclear installations because, to neutron irradiation, the equipment becomes radioactive and is unusable.
To solve the problem of exposure to neutron radiation, it is necessary to limit the duration of exposure, maximize the distance from the radiation source, and use shielding material [8].Over the past few years, several studies have been conducted on various protective materials against the dangers of ionizing radiation [9].According to scientific studies, shielding materials usually consist of a matrix and a filler.Frequency the matrices used are hydrogen-enriched polymers, cement and glass [10][11][12][13].Examples of elements and their compounds that can be used as fillers for neutron shielding are boron, cadmium, and selenium [14][15][16].However, the price of these fillers is relatively high.Concrete is not optimal because it is heavy, cracks easily, and is prone to voids.Glass has a limited range of potential as a protective material [10].Hydrogen-enriched polymers are widely used as matrix materials for nuclear shields due to their lightweight, good manufacturability, and strong ability to slow down neutrons [13,17].However, cost-effective composite shields with hydrogen-rich polymer matrices should be further investigated as shielding materials against the dangers of neutron radiation [8].
It is worth noting that epoxy resin is often used as a matrix in radiation shielding materials due to its high hydrogen content, excellent processability, strong radiation resistance, and suitability for adding a lot of inexpensive shielding fillers [13,17].The paper [13] presents the use of some secondary metallurgical resources and epoxy resin for the preparation of composites for various types of radiation.However, the shielding mechanism in these multiphase composites was never been elucidated.
Natural isotopes of Gd have a high ability to capture neutrons.Two of the seven stable isotopes 155 Gd and 157 Gd have very high neutron capture cross sections (55,000 and 255,000 barn, respectively). 155Gd has the highest neutron capture cross section among all stable elements of the periodic table.The neutron capture cross section of 155 Gd is 65 times higher than that of the widely known 10 B [18,19].In figure 1 shows the cross-section of neutron capture by boron and gadolinium isotopes depending on the neutron energy.When a neutron is absorbed in the Gd nucleus, complex transformations occur, as a result of which γ-quants are generated, which displace electrons from the inner levels.This leads to the emission of internal conversion electrons, Auger-Koster-Kronig electrons, as well as photons and γ-quants.Computer modeling of the Gd neutron capture reaction predicts the output of 1.83 γ-photons, 0.84 γ-quant, and 0.69 internal conversion electrons [20].
The latest achievements in the development of nano-and micro-composite materials for the protection of workers and equipment from neutron irradiation are covered in the book [22].In particular, the use of powder fillers as neutron absorbers is considered.In particular, it has been shown that the use of samarium (Sm) and Gd as fillers is effective in terms of the intensity of their interaction with neutrons [23], but at high concentrations it can cause the release of a significant amount of secondary gamma radiation.In this case, additional measures should be taken to protect against gamma radiation.As an optimal solution, the use of mixed boron- gadolinium-containing fillers is proposed, which makes it possible to achieve optimal absorption of neutrons for different energy spectra and, accordingly, the release of an "optimized" amount of secondary radiation.
It is known from other literary sources that wood is a good adsorber of ionizing radiation [24].In this regard, it can be assumed that the use of wood in combination with Gd-containing materials is promising, since wood has good absorbing properties relative to secondary gamma radiation and, in addition, good structural properties.
The use of thermoplastic composite materials for protection against neutrons in outer space is covered in [25].Also, to protect workers and equipment from neutron radiation, it was proposed to use thermoset polymers filled with inorganic fillers.Methods of their production, properties and neutron shielding characteristics were analyzed [26].
The goal of this scientific article is to develop a conceptual scheme for the creation of environmentally friendly Gd-containing neutron-absorbing nanocomposites, and to validate this scheme through the preparation and characterization of Gd-containing chitosan films as a potential neutron shielding material.
This work, we believe, is contributing to sustainable development goals 9 (industry and innovation) since it offers an innovative approach for neutron shielding and 12 (responsible production) due to the possibility to develop environmentally friendly neutron-shielding materials.

Results and discussion
Neutron-shielding composite materials act as part of certain objects as components of protective structures.In particular, neutron-shielding composites can act as components of coatings, insulating, protective and structural materials, etc., to fulfill their functions.Accordingly, during the operation of materials containing neutron shielding composites, it is important to take into account the influence of various factors on them, including the environment [27].Among the factors that can have a direct (negative) impact, it is worth highlighting: • weather conditions; • accompanying radiation (gamma radiation, beta particles); • induced radiation (caused by the interaction of neutrons with matter); • the factor of material properties change due to aging and mechanical stress, etc.; • others.
In this context, the use of Gd-containing composites as part of protective materials seems most likely in two scenarios: indoor and in the open air.
Obviously, the use of neutron shields outdoors includes a set of parameters that are suitable for the indoor applications, plus a protective coating against weather conditions (humidity, solar radiation, temperature, etc.).
The basics of technologies and materials that are suitable for protection against weather conditions are well studied and worked out, so their detailed coverage here is unnecessary.
In particular, in figure 2, a four-component scheme is proposed, which consists of a protective coating (layer 1), an intermediate layer 2, a neutron protective layer 3, and a structural or main layer 4.An example of the arrangement of protective layers is shown in figure 2   In addition to weather conditions, the impact of accompanying radiation is important, the presence of which should be expected at facilities with increased nuclear danger.It is important that the protection against accompanying radiation is at the front of the composite, for example as a protective paint, coating, varnish, etc.The radiation-resistant coating is applied to the base, which should ensure its reliable fixation and, accordingly, resistance to weather conditions.The basics of such technologies are well studied and worked out, so there is no need to cover them in detail in this project.
The base can be any material that meets the technical requirements, such as wood.Wood has good structural properties and is a sufficiently effective material for protection against gamma radiation.
The next layer can be directly Gd-containing neutron-absorbing composite material.The neutron protective layer should be located closest to the front of interaction with potential radiation sources.The work resource must exceed the maximum one-time load by several times.The thickness of the layer and the amount of neutron-shielding composite in it may vary depending on the specific purpose.
Taking into account the accompanying factors present during the operation of such composites in real conditions, it is also important to take into account other components of the interaction of neutrons with matter.It is worth noting that it is important to provide the neutron-absorbing composite material with a radiation-resistant shell several microns thick, made of a material that prevents the propagation of secondary electrons and neutralizes free radicals.It is worth noting that the formation of secondary electrons due to the interaction of neutrons with boron and gadolinium can cause the breaking of chemical bonds on a micrometer scale, and that is why it is important to "neutralize" such secondary electrons.Examples of such electron-and radicalabsorbing materials can be cross-linked polymer hydrogels (for example, based on lignin) and surface-modified inorganic oxides (silica, magnetite, titanium oxide, etc.).The effective mileage (thickness of the substance layer that stops almost all particles) is given in table 1.The features of the structure of layer 3 and its interaction with neutron radiation are shown in the figure 3.
Beta particles (electrons and positrons) interact with electrons and nuclei in matter to a complete stop.The distance of beta particles depends on their energy.
When considering the use of neutron-shielding composite materials in protective structures, it is important to take into account various factors that can negatively impact the materials, such as weather conditions and accompanying radiation.
In this study, as chitosan films with and without Gd were prepared by a solution casting method.The films were cast from a chitosan solution containing the appropriate concentration of Gd salt, and then dried to obtain a thin film.The resulting films were characterized using scanning electron microscopy and their stability to electron beam was evaluated at the first stage.
The SEM micrograph in figure 4 shows the cross-sections of the chitosan film with and without the Gd, as a model of the Gd-containing neutron-absorbing composite material.The thickness of the film with Gd-containing composite material is 15 micrometers, while the thickness of the  The results of this study suggest that chitosan, a natural polymer, can be used as a base material for the development of Gd-containing neutron shielding composites.The addition of Gd to the chitosan film increases its thickness and neutron shielding properties, making it a promising material for further development and application in neutron shielding.The micrographs reveal the non-stability of the "just" chitosan film when exposed to the electron beam, as evident from the irregularities and deformations observed in the image (figure 5).In contrast, the Gd-containing chitosan film exhibits a more stable morphology under the same conditions, highlighting the potential of Gd as a stabilizing agent for natural polymer-based neutron shielding composites.The table 2 below compares the thickness, Gd content, and neutron shielding efficiency of the samples.The control sample (Chitosan) has a thickness of 9 µm and no Gd content.The Chitosan-Gd samples have a thickness of 15 µm and varying Gd content ranging from 3 wt% to 8 wt%.The neutron shielding efficiency of the samples is also provided, with the Chitosan-Gd samples showing increasing efficiency with increasing Gd content.This table helps to provide a concise overview of the properties of the different samples and facilitates easy comparison between them.This table helps to provide a overview of the properties of the different samples and facilitates easy comparison between them.The values for neutron permeability vere calculated according to [28] and are in good agreement with the data obtained in [29].Further studies can be conducted to optimize the concentration of Gd within the film and to explore the feasibility of scaling up the production of such composites for practical use.It should be noted that the work described in this research paper represents only the beginning of the development and preliminary testing of the Gd-containing neutron shielding composites based on natural polymers such as chitosan and cellulose.Further research and testing are needed to optimize the composition and manufacturing process of these composites and to evaluate their performance under various environmental conditions and neutron radiation sources.

Conclusions
The nature and properties of neutron radiation impose specific requirements on the creation and operation of such materials.In particular, it is shown that the presence of neutronabsorbing components is a necessary but not sufficient condition for their successful application in environmental conditions.The presence of accompanying and induced radiation, weather conditions, as well as the peculiarities of the location of functional layers require a hierarchical approach to the development of such neutron-absorbing materials.As a result of literary data analysis, such an approach was proposed.It was shown that the principle scheme of creating Gdcontaining protective materials consists in taking into account all factors of their operation.The approach, developed in this work is contributing to sustainable development goals 9 (industry and innovation) due to offering a more advanced and innovative approach for neutron shielding.Also, the development of environmentally friendly neutron-shielding materials contributes to SDG 12 (responsible production) due to decreased risk of the contamination of nature by toxic materials.The current study provides valuable insights into the potential of using chitosan as a matrix for Gd-containing composites and demonstrates the successful incorporation of Gd into the chitosan film as shown in the SEM images and EDS analysis.However, more extensive testing and evaluation of the material properties and performance are necessary to determine the feasibility and practical applications of these composites in real-world scenarios.Ie V Pylypchuk https://orcid.org/0000-0001-5467-2839Y B Krasnov https://orcid.org/0009-0009-7971-0761V N Bliznyuk https://orcid.org/0000-0002-3883-6941T M Budnyak https://orcid.org/0000-0003-2112-9308

Figure 2 .
Figure 2.An example of proposed scheme of the arrangement of protective layers for a neutronshielding composite material.

Figure 3 .
Figure 3. Radiation factors to be taken into account when developing a neutron shielding shell.

Figure 4 .
Figure 4. SEM micrograph of cross-sections of chitosan film with and without Gd-containing neutron-absorbing composite material.

Figure 5 .
Figure 5. SEM micrographs of the cross-section of the chitosan film with and without Gd under electron beam.

Table 1 .
Thickness of the layer of substance that stops most of β−particles.

Table 2 .
Thickness comparison, Gd content, and neutron shielding efficiency of the samples.