Influence of Rare Earth Elements on the Radiation-Shielding Behavior of Serpentinite-Based Materials

Abstract

In this study, the neutron and gamma radiation-shielding properties of serpentinites from the Guleman ophiolite complex were investigated, and results were evaluated in comparison with rare earth element (REE) content. The linear and mass attenuation coefficients (LAC and MAC), half-value layer (HVL), mean free path (MFP), and effective atomic numbers (Z_eff) of serpentinite samples were experimentally measured in the energy range of 80.99–383.85 keV. Theoretical MAC values were calculated. Additionally, fast neutron removal cross-sections, as well as thermal and fast neutron macroscopic cross-sections, were theoretically determined. The absorbed equivalent dose rates of serpentinite samples were also measured. The radiation protection efficiency (RPE) for gamma rays and neutrons were determined. It was observed that the presence of rare earth elements within serpentinite structure has a significant impact on thermal neutron cross-sections, while crystalline water content (LOI) plays an influential role in fast neutron cross-sections. Moreover, it has been observed that the concentration of gadolinium exerts a more substantial influence on the macroscopic cross-sections of thermal neutrons than on those of fast neutrons. The research results reveal the mineralogical, geochemical, morphological and radiation-shielding properties of serpentinite rocks contribute significantly to new visions for the use of this naturally occurring rock as a geological repository for nuclear waste or as a wall-covering material in radiotherapy centers and nuclear facilities instead of concrete.

1. Introduction

In recent years, geological materials and applied mineralogy have become increasingly important. In addition to industrial raw materials, many rocks and minerals are used in many different areas, from space technologies to medical and industrial waste management, which are suitable for technological developments. The radiation-shielding performance of geological materials—such as minerals, concrete, and rocks—depends on three main factors: the material’s density, the proportion of structural water (H₂O) within its crystal structure, and the suitability of any added components. Density is the most effective property for attenuating gamma (γ) rays, as demonstrated in heavy-weight minerals, rocks, and dense concrete samples. Shi et al. [1] found that graphene quantum dots and different curing methods improved the mechanical and microstructural properties of serpentine mortar. Abdalla et al. [2] demonstrated that local granites have a high capacity for radiation shielding. Abrefah et al. [3] showed that serpentine concrete is an effective and reliable material for neutron shielding. And finally, Akkurt et al. [4] identified that certain igneous rocks exhibit high performance in gamma ray shielding. Structural H₂O plays a significant role in the attenuation of fast neutrons and is prevalent in hydrous minerals such as amphibole, muscovite, or serpentine [5], as well as in certain types of concrete. Carbon additives are effective for fast neutron attenuation, and this property can be observed in concrete mixtures containing boron or colemanite minerals [6,7,8,9]. The quantitative analysis of rare earth elements and radioactive elements present in natural minerals or rocks provides critical data regarding toxic element content and the potential usability of these materials [10]. In this context, the development of new materials is of great importance for addressing current challenges. These new materials should be environmentally friendly, possess high mechanical strength, and offer optimized radiation shielding. Therefore, determining key radiation-attenuation parameters—such as LAC, MAC, HVL, TVL, MFP, Z_eff, and N_eff—is essential to evaluate their interaction with X-rays or gamma rays. Since these radiation-attenuation parameters define the interaction between a substance and X- or gamma rays, they are frequently used in radiation-protection applications. Knowing the mass attenuation coefficient of a substance is crucial, as it (μ/ρ) provides a measure of the average number of photon interactions occurring when the photon encounters the material. In recent years, there has been increasing interest in the use of natural minerals as raw materials for concrete and cement production. Durable natural minerals such as serpentine, which can be used as radiation-shielding materials, are abundant in nature. These minerals offer significant advantages, including reduced cement consumption in concrete, lower costs, and decreased CO₂ emissions. Moreover, the potential to construct thinner shielding walls with such materials contributes to reduced concrete usage, energy savings, and conservation of raw materials. Mymrin et al. [11] conducted XRD, XRF, flexural strength, and water-absorption tests on serpentine aggregates and stated that serpentine waste can be beneficial in production of building materials and may also offer environmental advantages.

Several studies have investigated the radiation-shielding properties of various igneous and ultramafic rocks. Libeesh et al. [12] conducted advanced nuclear radiation shielding studies on mafic and ultramafic complexes, integrating lithological mapping to better understand their shielding potential. In a subsequent study, Libeesh et al. [13] evaluated the radiation-shielding competencies of ultramafic–alkaline–carbonatite complexes using both experimental methods and Monte Carlo simulations, supported by detailed lithological characterization. Similarly, Çelen et al. [14] estimated the radiation-shielding capacity of different rock types through combined experimental and simulation approaches. Obaid et al. [15] focused on calculating the gamma ray attenuation coefficients and exposure buildup factors for select rocks to assess their effectiveness in gamma radiation-shielding applications. Furthermore, the researchers provided insights into the petrogenesis of high-K calc-alkaline granodiorites from the SE Lhasa block, Tibet, linking their geological formation processes to recycled subducted sediments, which may influence their mineralogical composition relevant to shielding properties. Collectively, these studies contribute valuable data toward identifying natural rocks with potential applications in radiation shielding [16]. Günoğlu et al. [17] investigated the radiation-shielding properties of various igneous rocks from the Isparta province across different gamma energy levels, combining experimental and theoretical approaches to provide comprehensive data on their effectiveness as natural shielding materials. Similarly, Tekin et al. [18] examined the gamma radiation-shielding performance of hematite-serpentine concrete composites enhanced with WO₃ and Bi₂O₃ micro- and nanoparticles, demonstrating that the addition of these heavy metal oxides significantly improves the concrete’s shielding efficiency, as validated through MCNPX Monte Carlo simulations. These studies contribute valuable insights into developing advanced radiation-shielding materials by leveraging both natural rocks and engineered composites.

In another study, Baalamurugan et al. [19] examined the use of steel slag as an aggregate in concrete for gamma ray protection and confirmed that incorporating 50% slag significantly improves gamma-shielding effectiveness. Çullu and Bakırhan [20] studied heavyweight concretes containing lead–zinc mine waste serpentine rock within the energy range of approximately 662–1460 keV and showed that the strength of the concrete influences its radiation-absorption capability.

In geology, Saeed et al. [21] evaluated neutron and charged particle-attenuation properties in volcanic rocks using the Monte Carlo method for neutrons and the SRIM program for charged particles. Mahmoud et al. [22] investigated the radiation-absorption properties of sandstone, one of the sedimentary rocks, while other researchers [4] have evaluated the radiation-shielding performance of limestone. Radiation-absorption characteristics have also been studied in some metamorphic rocks such as metagabbro, marble, and gneiss [23]. Among these rock types, serpentinites are generally considered suitable for use as aggregates in radiation-shielding concrete. The high radiation-absorption capacity of serpentinites is attributed to their high density and content of heavy elements (e.g., Fe, Mg). Studies have shown that concretes containing serpentinite exhibit gamma-absorption performance comparable to that of barite- or hematite-based concretes in experiments involving gamma sources such as ¹³⁷Cs (662 keV) and ⁶⁰Co (1.17 and 1.33 MeV).

The radiation-absorption property of a material depends on its density, high H₂O content in the crystalline structure, and suitable additives (such as colemanite and boron minerals) [24,25]. Serpentinite, composed of serpentine minerals (lizardite, chrysotile, antigorite), is a metamorphic rock. It is usually associated with small amounts of dolomite, magnetite, chlorite, chromite, talc, tremolite, and brucite minerals. It has a density of less than 2.7 g/cm³ [26]. It is the magnetite content that gives the color difference to this rock which can be seen in gray, gray-oily black, and green colors. Serpentine rocks are used as aggregates in normal concrete.

Several studies have examined the use of geological materials in concrete structures as radiation-shielding materials. Dąbrowski et al. [27] examined the suitability of serpentinite aggregates for radiation-shielding concrete and demonstrated that serpentinite exhibited neutron-absorption performance in the range of 12–33%. Oto et al. [28] conducted an experimental study on the radiation-shielding parameters of magnetite- and serpentine-doped ceramics. Zayed et al. [29] reported that the addition of boric acid to serpentine concrete enhanced its thermal and fast neutron-attenuation performance while decreasing its gamma-attenuation capability. Sayyadi et al. [30] compared coarse serpentine aggregates (SCAg) with fine serpentine aggregates (SFAg), concluding that the shielding, mechanical, and physical properties of the concrete were significantly affected by the natural porosity of the materials. Utilizing modern technologies, the shielding performance of iron–magnetite–serpentine cement concrete (IMSCC) for gamma and neutron radiation in nuclear reactor shielding was evaluated [31]. Despite its low hydrogen concentration, high-neutron-absorption cross-sections were observed. In the application of serpentine concrete as a neutron-shielding material, researchers used the Monte Carlo code and determined that the average gamma dose was 4.395  ±  0.122 μSv/h, which is lower than the dose measured for ordinary concrete (10.399  ±  0.083 μSv/h) [3].

Rare earth elements (REEs) are currently incorporated into composite materials to provide effective neutron shielding. Among the heavy rare earth elements, including Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y, gadolinium (Gd), samarium (Sm), and europium (Eu) are known in the literature for their high neutron-absorption cross-sections (for 2200 m/s neutrons) of 49,700 barns, 5922 barns, and 4530 barns, respectively. These elements are commonly used in shielding, control, and structural materials to ensure the safe operation of nuclear reactors and play an active role in processes such as slowing down and scattering fast neutrons, absorbing thermal neutrons, and capturing secondary particles [32,33,34]. A review of the literature reveals a lack of experimental studies investigating the radiation-shielding properties of REEs in combination with natural rocks, as well as the testing of such materials for radiation-protection purposes. Therefore, this study is significant in terms of comparing the radiation-shielding properties of serpentinite rocks, particularly in relation to their rare earth element content, within the Guleman ophiolite complex, which presents extensive surface exposure.

2. Materials and Methods

2.1. High-Energy Spectroscopy Experiments

The radiation-shielding parameters of selected serpentinite samples were evaluated in the photon energy range of 80.99–383.85 keV, based on the Beer–Lambert law under narrow-beam transmission geometry. The arrangement of the radioactive source, semi-conductor detector, and sample for the experimental geometry is shown in Figure 1a. The neutron equivalent dose rates (Figure 1b) were also measured by using average 4.5 MeV energy ²⁴¹Am-⁹Be fast neutron source and a BF₃ gas proportional neutron detector. Measurements were performed with five repetitions, resulting in an overall expanded combined uncertainty estimated at ±7%.

Serpentine rocks were cut about 15 mm thick to form samples that could be analyzed in gamma spectrometry (Figure 2). Serpentine samples were exposed to gamma photons with energy of 80.99 keV, 276.39 keV, 302.85 keV, 356.61 keV, and 383.85 keV emitted from a ¹³³Ba radioactive source. The ULEGe detector (Mirion Technologies (formerly Canberra), Meriden, CT, USA) was used to measure incident and transmitted photon intensities. The samples’ gamma ray pulse height spectra were collected for 1500 s. The gamma ray spectra of transmitted photons for the Sr-1 and Sr-5 samples are shown in Figure 3. All of the equations used to calculate the physical parameters in the present study are given in Table 1.

2.2. Elemental and Structural Analysis

Samplings of serpentinite rocks were selected from areas around Guleman, Alacakaya, and Sivrice within the study area. Fresh serpentinite samples were examined using a polarizing microscope, and selected samples were further analyzed by LA-ICP-MS (ACME Analytical Laboratories, Vancouver, BC, Canada). The polarizing microscope analyses were conducted to determine mineralogical composition and textural characteristics of samples.

Major oxides, trace, and rare earth element (REE) analyses of samples were performed using LA-ICP-MS. Major and rare earth elements were measured by the ICP-MS technique from glass pellets prepared by fusing powdered samples with lithium tetraborate (Li₂B₄O₇) in a 1:5 ratio at 1150 °C in a platinum-gold crucible. Scanning electron microscopy (SEM) (MERLAB, Avcılar/İstanbul, Turkey) analyses of selected samples were carried out in the Physics Laboratory of Erzincan Binali Yıldırım University.

3. Results

3.1. Mineralogy–Geochemistry

When the geochemical enrichment of heavy metals in serpentinites is examined, ionic substitutions are observed during exchange of ferromagnesian minerals. In particular, the concentrations of Cr and Ni vary according to the occupancy rates of siderophile elements such as Mg, Fe, Cr, Mn, and Ni within the crystal structure of serpentine minerals [38]. The macroscopic view of serpentinite rocks is given in Figure 4. When the enrichment of heavy metals in serpentinites is examined geochemically, ionic substitution is observed during the exchange of ferromagnesium minerals. In particular, siderophile elements such as chromium and nickel vary according to the replacement rate of siderophile elements such as Mg, Fe, Cr, Mn, and Ni in the crystal structure of serpentine minerals [38]. The chemical composition of five serpentine samples collected from six outcrops from the Guleman ophiolite (Elazığ) serpentinites is summarized in Table 2 [39]. The amount of SiO₂ in all samples is 35.1 to 41.9 wt%. Moreover, the amounts of MgO and Fe₂O₃ range from 22.8 to 38.8 and 5.0 to 7.4 wt%, respectively. The MnO composition is less than 1% (0.1–0.3 wt). These values are consistent with mineralogical studies showing that serpentinite minerals are ferromagnesian. The average amounts of Na₂O, K₂O CaO and P₂O₅ in rock samples were determined as 0.1–0.3, 0.1–1.1, 0.1–4.9, and 0.1 wt%, respectively. The nickel (Ni) content of the samples ranges from 101 to 1819 ppm, while the cobalt (Co) content varies between 18 and 91 ppm. The Ni (101–1819 ppm) and Co composition of the samples varies between 18 and 91 ppm. According to total chemical analysis data shown in Table 2, nickel is the most abundant heavy metal analyzed in serpentinites (Ni commercial use limit = 500 ppm) [40].

3.2. Elemental and Structural Analysis Results

Rare earth element (ppm) analysis results are listed in Table 3. The microstructural analyses of Sr-1, Sr-2, Sr-3, Sr-4, and Sr-5 samples were carried out using high-resolution scanning electron microcopy (SEM analysis). A QUANTA FEG 450 model electron microscope was used for the analysis. Figure 4 shows a macroscopic image of a serpentinite rock from the Guleman ophiolite, exhibiting different textures (mesh, nodular, massive) and mineral proportions (antigorite, lisardite). EDS results and SEM images are given in Figure 5 and Figure 6, respectively.

3.3. Mineralogy–Geochemistry and Elemental and Structural Analysis

The geochemical composition of serpentinite samples, as summarized in Table 2, reflects their ferromagnesian nature with significant amounts of SiO₂ (35.1–41.9 wt%), MgO (22.8–38.8 wt%), and Fe₂O₃ (5.0–7.4 wt%). These heavy-metal oxides, along with trace elements such as MnO and minor oxides (Na₂O, K₂O, CaO, and P₂O₅), contribute to the intrinsic density and elemental makeup of serpentinites. Importantly, as the thickness of serpentinite samples increases, their capacity to attenuate ionizing radiation also improves. This is due to the longer path length within the material, allowing greater interaction probability between incident radiation and heavy elements present in the rock matrix. Consequently, thicker serpentinite slabs exhibit enhanced radiation-shielding performance, making them promising candidates for applications requiring effective gamma and neutron attenuation.

The SEM images of the serpentinites are provided in Figure 5. In Figure 5B, the Mg content is higher compared to Figure 5A. The elevated atomic percentages of both O and Mg relative to other elements in the samples are indicative of the serpentine type and suggest the presence of antigorite-composed serpentinites. As seen from Figure 6, the color distribution ranging from light gray to gray is related to the chemical composition of the serpentinites. The morphology of serpentine crystals, with sizes ranging from 0.1 to 0.3 µm, is shown as antigorite in Figure 6A,B, and as lizardite in Figure 6C,D. Lizardite exhibits a smooth, platy surface, while antigorite displays a wavy surface morphology.

3.4. High-Energy Spectroscopy Experiment Results

As presented in Table 4, the calculated MAC values demonstrate close agreement with the experimental results for Sr-2 and Sr-4 across the 80.99–383.85 keV range, with discrepancies limited to about 1.3–4.4%, confirming the reliability of the theoretical approach. Figure 7b illustrates the pronounced energy dependence of the MAC values, revealing a sharp decline from approximately 0.227 cm²/g at 80.99 keV, followed by a gradual decrease to nearly 0.113 cm²/g at energies above 279.39 keV for the Sr-2 sample. This behavior primarily stems from the varying dominance of photon-interaction mechanisms: the photoelectric effect (∝ E⁻³) prevails at lower energies, progressively giving way to Compton scattering (∝ E⁻¹) as energy increases. The enhanced MgO and Fe₂O₃ content in the serpentinite matrix contributes to higher electron density, thus amplifying photon interactions, especially under photoelectric conditions. The ranking of the MAC values among all samples was as follows, Sr-2 > Sr-5 > Sr-3 > Sr-4 > Sr-1, with Sr-2 consistently exhibiting the highest MAC values.

Complementary trends were observed for MFP and HVL (Figure 8a,b), which rise with photon energy due to the declining probability of photoelectric events. Notably, Sr-2 achieved an MFP of 3.97 cm at 383.85 keV and 3.85 cm at 356 keV—outperforming standard concrete (4.30 cm) and hematite–serpentine concrete (3.92 cm) reported in the literature [41]. The ranking of the MFP and HVL values among all samples was as follows, Sr-2 < Sr-5 < Sr-3 < Sr-4 < Sr-1, with Sr-2 consistently exhibiting the lowest MFP and HVL data.

Similar to how the atomic number characterizes individual elements, the effective atomic number (Z_eff) serves as a fundamental parameter for assessing composite materials, directly reflecting their overall photon-interaction capability. Consequently, Z_eff is widely used to evaluate the radiation-shielding performance of such materials. In this study, Z_eff values for all serpentinite-based samples were experimentally determined over a range of photon energies and are presented in Figure 9. A pronounced decline in Z_eff was observed at lower energies (80.99–276.39 keV), attributable to the reduced probability of photon interactions as energy increases. At higher energies, Z_eff values continued to decrease, reaching approximately 9 e⁻/atom near 383.85 keV. It was also evident that higher MgO and Fe₂O₃ contents contributed to elevated Z_eff values, particularly at lower photon energies, thereby enhancing the shielding capability of the glass composites. Among the examined samples, Sr-1 consistently exhibited the lowest Z_eff.

From the EpiXS algorithm, the energy exposure buildup factors (EBFs) for serpentinite samples were computed within the 15–15,000 keV energy range. Figure 10 shows that the low energy range has the lowest EBF values because most of the photons in the sample are absorbed. In 40 mfp and 200 keV, EBF values are 938, 672, 823, 844, and 767 for Sr-1, Sr-2, Sr-3, Sr-4, and Sr-5 samples. These results obtained for EBFs are parallel to the results obtained for absorption coefficients.

Figure 11 presents the thermal neutron macroscopic cross-sections alongside the corresponding radiation-protection efficiencies (RPE, attenuation rate) for serpentinite samples of 16 mm thickness. As detailed in Table 2 and Table 3, variations in the concentrations of major oxides and rare earth elements (REEs) among the five serpentinite compositions significantly influence these neutron-interaction parameters. Notably, the elevated macroscopic cross-sections are primarily attributed to the incorporation of REEs—especially gadolinium (Gd), which possesses an exceptionally high thermal neutron cross-section of 49,700 barns [41]. This is evident in Sr-2, which not only exhibits the highest REE/HREE ratio (Table 2) but also greater Gd content compared to the other samples. Even slight fluctuations in Gd concentration lead to pronounced changes in thermal neutron behavior, as previously demonstrated [42,43]. Specifically, a reduction in Gd from 0.24 ppm to 0.18 ppm corresponds to a notable decrease in RPE, from 72.87% down to 61.75%. This underscores how closely the macroscopic cross-section governs the composite’s neutron-shielding performance, with Sr-2 emerging as the most effective barrier against thermal neutrons in this study, in line with observations reported by Zhou et al. [44].

When neutrons pass through a material that can scatter or absorb them, they interact in ways that depend on both their energy and the atomic makeup of the medium. These interactions generally fall into a few main categories, like elastic or inelastic scattering, absorption, and, though less common, nuclear fission. The microscopic cross-section gives a measure of how likely it is for a neutron of a certain energy to interact with the nuclei through any of these processes, adding up the probabilities for scattering and absorption separately [29,42,43,44,45]. On a larger scale, the macroscopic fast neutron removal cross-section is used to describe the probability that a high-energy neutron will be effectively taken out of the uncollided flux by some kind of interaction. It is worth noting that this does not represent the chance of any single, specific type of neutron–nucleus event. Because fast neutrons usually lose energy by bumping into atoms multiple times, the removal cross-section tends to be close to, but a bit less than, the total macroscopic cross-section. Although this concept is most commonly applied to materials that contain hydrogen—since hydrogen is particularly good at slowing down fast neutrons—it can also be extended to other materials that include hydrogen in their structure. This approach generally works well for neutrons in the 2–12 MeV energy range [7,44]. Figure 12 and Figure 13 display the calculated fast neutron macroscopic cross-sections and RPEs, as well as the fast neutron removal cross-sections and corresponding RPEs for serpentinite samples of 16 mm thickness. According to these results, the fast neutron macroscopic cross-section reaches its highest value in the Sr-2 sample (0.3820 cm⁻¹), followed by Sr-5, Sr-3, Sr-4, and finally Sr-1, which shows the lowest value at 0.3405 cm⁻¹.

This ordering correlates well with the samples’ loss on ignition (LOI) percentages listed in Table 2, where Sr-2 exhibits the highest LOI% and Sr-1 the lowest. The LOI parameter serves as an indirect indicator of volatile and light element content—often associated with bound water or hydroxyl groups—which contributes significantly to neutron moderation due to elastic scattering by hydrogen and other light nuclei [37]. This moderation effect enhances the probability of neutron interactions, thereby increasing the macroscopic cross-section. As evidenced by Table 2, Sr-2’s higher LOI content supports this behavior, resulting in superior fast neutron-attenuation characteristics. Turning to the removal cross-sections depicted in Figure 13, a similar trend is observed. The fast neutron removal cross-section of Sr-2 was determined to be 0.0875 cm⁻¹. When the LOI decreases by approximately 11.3%—as seen when comparing Sr-2 to Sr-1—the removal cross-section drops to 0.0819 cm⁻¹. This outcome underlines the pivotal role of light elements, particularly hydrogen, in facilitating energy reduction of fast neutrons through successive collisions, which is consistent with the theoretical framework described by standard neutron-transport models. Overall, the sequence Sr-2 > Sr-5 > Sr-3 > Sr-4 > Sr-1, consistently observed across both macroscopic and removal cross-sections, reinforces the notion that compositional attributes like LOI and the associated hydrogen content markedly influence the neutron-attenuation efficiency of serpentinite-based materials. These findings align well with broader studies on hydrogenous-shielding matrices, which emphasize the effectiveness of light elements in moderating and ultimately removing fast neutrons from the uncollided flux.

The slight differences in the fast neutron removal cross-section values between the Sr-2 and Sr-5 samples are attributed to minor variations in their LOI percentages, which indicate the amount of structural water (including OH groups). This structural water contributes to the hydrogen content within the mineral lattice. Due to their low atomic number and mass, hydrogen atoms have a high probability of undergoing elastic scattering with fast neutrons. As a result, successive collisions effectively reduce neutron energy, enhancing attenuation. Thus, even small differences in hydrogen content play a significant role in moderating fast neutrons and directly influence the macroscopic cross-section values.

Figure 14 presents the LAC and RPE values for serpentinite samples at a photon energy of 0.302 MeV. The LAC value for Sr-1 was initially 0.2585 cm⁻¹. For Sr-2, the LAC value showed a noticeable increase, reaching up to 0.2773 cm⁻¹. The reason for this is that the Sr-2 sample contains a greater amount of REE than Sr-1, which increases the density of the sample. According to Figure 14, the Sr-2 sample has the highest RPE. The density reinforcement significantly improves the composite’s ability to attenuate photons, which is especially beneficial for applications requiring enhanced radiation-shielding properties.

The radiation-shielding parameters described in Figure 11, Figure 12, Figure 13 and Figure 14 are summarized in Table 5. Research on radiation shielding of REE-containing serpentinite rocks has shown that the Sr-2 sample has the best gamma and neutron-shielding ability.

The fast neutron-absorption equivalent dose rates (μSv/h) of the serpentinite samples were measured experimentally. The fast neutron-absorption equivalent dose rates are given in Figure 15. As seen from Figure 15, the dose of 1.2039 μSv/h emitted by the fast neutron source was absorbed 36.905% by Sr-1, 25.543% by Sr-2, 29.656% by Sr-3, 33.459% by Sr-4, and 27.431% by Sr-5. Since the fast neutron cross-section decreases as the neutron absorption increases, the results obtained in Figure 15 and Table 5 are consistent with each other.

4. Conclusions

In this study, the radiation-shielding capabilities of five varieties of serpentinite rocks were investigated and the main findings can be compiled as follows:

It is found that serpentinite minerals are ferromagnesian. Nickel is the most abundant heavy metal analyzed in serpentinites. Serpentinites, formed by the hydration of silica-poor olivine at low temperatures, may consist of antigorite, chrysotile, or lizardite. The optical properties and chemical composition of the selected samples indicate that they are predominantly composed of antigorite.

The presence of REE elements in the serpentinite structure influences the thermal neutron cross-sections, while the LOI (crystalline H₂O) content affects the fast neutron cross-sections.

It was found that the radiation-shielding behavior followed the following order, Sr-2> Sr-5 > Sr-3> Sr-4> Sr-1, for thermal and fast neutrons and γ-rays. The fast neutron-absorption equivalent dose rates (μSv/h) of the serpentinite samples’ behavior followed the following order: Sr-2< Sr-5 < Sr-3< Sr-4< Sr-1.

Sr-2 exhibits an HVL of 1.8143 cm for thermal neutrons, which is approximately 58% lower than the typical HVL of standard concrete (~4.30 cm). The Sr-2 samples are more convenient and competent for radiation shielding compared to the other serpentinite samples.

Due to their specific mineralogical composition and geochemical characteristics, serpentinite formations show strong potential as effective shielding materials against fast neutrons and gamma radiation in nuclear environments. In the serpentinite sample with antigorite composition, the gamma ray test revealed the significant impact of pores on radiation-shielding properties; it was observed that when the pores are empty, radiation can pass through directly without any deviation.

Author Contributions

Conceptualization, A.D.K. and D.Y.; methodology, A.D.K. and D.Y.; software, A.D.K. and D.Y.; validation, A.D.K. and D.Y.; formal analysis, A.D.K. and D.Y.; investigation, A.D.K. and D.Y.; resources, A.D.K. and D.Y.; data curation, A.D.K. and D.Y.; writing—original draft preparation, A.D.K. and D.Y.; writing—review and editing, A.D.K. and D.Y.; visualization, A.D.K. and D.Y.; supervision, A.D.K. and D.Y.; project administration A.D.K.; funding acquisition, A.D.K. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that the data used to support the findings of this study are available within the article.

Acknowledgments

The authors would like to thank Sedanur Kalecik for laboratory support.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1 Experimental geometries for (a) radiation absorption, (b) neutron equivalent dose rates.

Figure 2 The investigated serpentine samples.

Figure 3 The gamma ray spectra of Sr-1 and Sr-5 samples.

Figure 4 Macroscopic view of serpentinite rocks: (A) Sr-1, (B) Sr-2, (C) Sr-3, (D) Sr-4, (E) Sr-5.

Figure 5 EDS results of serpentinite rock samples: (A) Sr-1, (B) Sr-2.

Figure 6 SEM images of serpentinite rocks: (A) Sr-1, (B) Sr-2, (C) Sr-3, (D) Sr-4.

Figure 7 (a) LAC and (b) MAC values of the serpentinite samples.

Figure 8 (a) MFP and (b) HVL values of the serpentinite samples.

Figure 9 Z_eff values of the serpentinite samples.

Figure 10 EBF values of the serpentinite samples.

Figure 11 Thermal neutron macroscopic cross-section and RPE graph of serpentinite samples.

Figure 12 Fast neutron macroscopic cross-section and RPE graph of serpentinite samples.

Figure 13 Fast neutron removal cross-section and RPE graph of serpentinite samples.

Figure 14 Gamma ray linear attenuation coefficient and RPE graph of serpentinite samples.

Figure 15 Absorbed equivalent dose rates (μSv/h) of the serpentinite samples.

Table 1

The used equations and definitions for the present study.

Parameters	Equations	Descriptions
Linear attenuation coefficients, (LAC, cm−1) 1	$\mu =\ln \left(I_0/I\right)/t$	$I_0$ and $I$ are the intensities of the incident photons and those passing through the absorber, respectively.
Mass attenuation coefficients, (MAC, cm2/g) 1	${\mu }_m=\ln \left(I_0/I\right)/\rho t$	$\rho$ is the density and $t$ is thickness.
Mean free path, (MFP, cm) 1	$\begin{array}{l}MFP=1/\mu \\ MFP=1/{\Sigma }_R\end{array}$	MFP is the part traveled by radiation between two collisions. $\mu$ and ${\Sigma }_R$ are the linear attenuation coefficient and neutron macroscopic cross-section, respectively.
Half value thickness, (HVL, cm) 1	$\begin{array}{l}HVL=0.693/\mu \\ HVL=0.693/{\Sigma }_R\end{array}$	HVL is the thickness of the material that reduces incoming radiation by half.
Effective atomic number 1	$Z_{eff}={\sigma }_{t,a}/{\sigma }_{t,e}$	The total electronic cross-section is ${\sigma }_{t,e}$ and the total atomic cross-section is ${\sigma }_{t,a}$.
Radiation-protection efficiency, RPE	$(1-\mathrm{I}/{\mathrm{I}}_0)\times 100$	$I_0$ and $I$ are the intensities of the incident radiation and those passing through the absorber, respectively.
Buildup factor 1	$I=B{I}_0{e}^{-{\mu }_m\rho t}$	$B$ is the buildup factor.
Fast neutron removal cross-section 2	${\sum }_R={\displaystyle \sum _i}{W}_i{\left({\sum }_{R/\rho }\right)}_i$	Fast neutron removal cross-section of the ith element is denoted by ${\sum }_{R/\rho }$, while $W_i$ represents its partial density.
Thermal and fast neutron macroscopic cross-section 3	${\Sigma }_R=\ln \left(I_0/I\right)/t$	$I_0$ and $I$ are incident and transmitted intensities for thermal and fast neutrons, respectively, within the energy range of 25.4 meV and 0.8–11 MeV, respectively, and $t$ is absorber thickness.
The absorbed equivalent dose percentage	${(\mathrm{I}}_0-\mathrm{I})/{\mathrm{I}}_0\times 100$	$I_0$ and $I$ are the intensities of incident neutrons and those passing through the absorber, respectively.

¹ The EpiXS software program is used to calculate theoretical values [35] (accessed on March 2025 from https://www.pnri.dost.gov.ph/index.php/downloads/software/epixs). ² Theoretical values are calculated using the Phy-X software package [36] (accessed on March 2025 from https://phy-x.net/PSD). ³ Theoretical values are calculated using the NGcal software package [37] (accessed on March 2025 from http://ngcal.com/).

Table 2

Results of major oxide (wt%) and trace element (ppm) analyses by ICP-MS and densities of serpentinite samples.

Major Oxides
Sample	Na2O	MgO	Al2O3	SiO2	P2O5	K2O	CaO	TiO2	MnO	Fe2O3	LOI	Sum.	ρ (g/cm3)	Ni	Co	t (cm)
Sr-1	0.3	22.8	6.9	37.3	0.1	1.1	4.5	0.2	0.3	5	11.3	89.8	2.585	101	18	1.6
Sr-2	0.1	37.7	0.5	40.4	0.1	0.1	0.1	0.1	0.1	7.4	11.8	98.4	2.412	965	87	1.6
Sr-3	0.1	34.9	0.6	35.1	0.1	0.1	4.9	0.1	0.1	7.2	11.4	94.6	2.504	972	91	1.6
Sr-4	0.1	38.2	0.3	41.9	0.1	0.1	0.2	0.2	0.1	6.6	11.6	99.4	2.542	1545	75	1.6
Sr-5	0.1	38.8	0.8	41.5	0.1	0.1	0.1	0.1	0.1	6.4	11.3	99.4	2.455	1819	73	1.6

Table 3

Rare earth element (ppm) analysis results of serpentinite samples.

Sample	LREE						HREE
	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	∑REE	∑LREE	∑HREE
Sr-1	0.90	0.48	0.16	0.34	0.22	0.11	0.18	0.18	0.18	0.08	0.19	0.05	0.18	0.04	3.29	2.10	1.19
Sr-2	0.35	0.27	0.12	0.38	0.18	0.14	0.24	0.22	0.16	0.09	0.18	0.04	0.17	0.05	2.59	1.30	1.29
Sr-3	0.44	0.42	0.10	0.36	0.21	0.11	0.20	0.18	0.19	0.10	0.18	0.04	0.17	0.04	2.74	1.53	1.21
Sr-4	0.90	0.34	0.14	0.37	0.15	0.13	0.19	0.21	0.17	0.09	0.20	0.05	0.19	0.04	3.10	1.90	1.20
Sr-5	0.66	0.44	0.13	0.42	0.16	0.14	0.21	0.16	0.18	0.08	0.19	0.04	0.18	0.05	3.04	1.81	1.23

Table 4

MAC values (cm²/g) of some serpentinite samples.

Energy (keV)	Sr-2Exp.	Theo.	Sr-4Exp.	Theo.
80.99	0.227 ± 0.010	0.221	0.203 ± 0.007	0.195
276.39	0.119 ± 0.005	0.115	0.113 ± 0.004	0.117
302.85	0.111 ± 0.005	0.113	0.105 ± 0.004	0.108
356.01	0.102 ± 0.004	0.103	0.097 ± 0.003	0.100
383.85	0.103 ± 0.004	0.101	0.097 ± 0.003	0.094

Table 5

The radiation-shielding parameters of serpentinite samples.

The thermal neutron-attenuation parameters of serpentinite samples (25.4 meV)
Parameter	Sr-1	Sr-2	Sr-3	Sr-4	Sr-5
Σ (cm−1)	0.6007	0.8152	0.7132	0.6513	0.7508
MFP (cm)	1.6647	1.2266	1.4021	1.5354	1.3318
HVL (cm)	1.1536	0.8501	0.9717	1.0640	0.9230
RPE (%)	61.7548	72.8662	68.0544	64.7275	69.9211
The fast neutron-attenuation parameters of serpentinite samples (4 MeV)
Parameter	Sr-1	Sr-2	Sr-3	Sr-4	Sr-5
Σ (cm−1)	0.3405	0.3820	0.3564	0.3565	0.3599
MFP (cm)	2.9367	2.6181	2.8061	2.8051	2.7788
HVL (cm)	2.0351	1.8143	1.9447	1.9440	1.9257
RPE (%)	42.0061	45.7264	43.4575	43.4692	43.7744
The fast neutron removal-attenuation parameters of serpentinite samples
Parameter	Sr-1	Sr-2	Sr-3	Sr-4	Sr-5
Σ (cm−1)	0.0819	0.0876	0.0842	0.0835	0.0864
MFP (cm)	12.2091	11.4222	11.8831	11.9761	11.5690
HVL (cm)	8.4611	7.9154	8.2347	8.2996	8.0172
RPE (%)	12.2824	13.0713	12.5978	12.5058	12.9165
The gamma ray-attenuation parameters of the serpentinite samples (302 keV)
Parameter	Sr-1	Sr-2	Sr-3	Sr-4	Sr-5
Σ (cm−1)	0.2585	0.2773	0.2689	0.2632	0.2726
MFP (cm)	3.8682	3.6066	3.7189	3.7988	3.6682
HVL (cm)	2.6806	2.4994	2.5772	2.6326	2.5421
RPE (%)	33.8756	35.8295	34.9641	34.3731	35.3499

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