Influence of increasing SnO2 content on the mechanical, optical, and gamma-ray shielding characteristics of a lithium zinc borate glass system

A series of six samples were prepared based on the chemical composition of 65B2O3 + 20ZnO + (15-x)LiF + xSnO2 (where x = 0, 0.25, 0.5, 0.75, 1, and 1.25 mol%) to study the role of SnO2 on enhancing the optical and radiation attenuation capacity of the prepared glasses. The preparation of the glass series was performed using the melt quenching method at 1100 °C for 60 min. The density of the fabricated samples was measured using an MH-300A densimeter. The optical parameters of the fabricated glasses were calculated based on the spectrum recorded by a Cary 5000 UV–Vis–NIR double beam spectrophotometer in a wavelength range of 200 to 3000 nm. Furthermore, Monte Carlo simulation code and the XCOM online database were used to estimate the gamma-ray shielding capacity of the fabricated samples from 0.244 to 2.506 MeV. The results show enhanced gamma-ray shielding capacity due to the replacement of LiF by SnO2. The linear attenuation coefficient at 0.244 MeV was enhanced from 0.352 to 0.389 cm−1. The half-value thickness of the investigated glasses decreased from 1.967 to 1.784 cm when the increasing addition of SnO2 from 0 to 1.25 mol%.

www.nature.com/scientificreports/ types of radiation at different doses and varying energies; thus, they may experience health problems, such as cancers 6 . As a result, there is an urgent need to deploy protective radiation shields to safeguard nuclear power plant workers, medical professionals, and patients in radiotherapy facilities from the dangers of radiation [7][8][9][10] .
Concrete and lead have historically been the most extensively utilized barriers for radiation protection since they have high photon attenuation ability. As a result, they are widely utilized as a protective barrier for gamma radiation and X-rays in a variety of forms, such as plates, tubes and bricks. However, concrete and lead are opaque to visible light; thus, they prevent light from passing through [11][12][13] .
To overcome the opaqueness problem of these two materials, researchers have resorted to developing new types of radiation-protective materials using different types of glass because glass has the ability to transmit light [14][15][16] . Recently, several research groups have developed multiple glass systems and studied the ability of these glass systems to attenuate radiation and thus reduce its harmful effects on humans and the environment [17][18][19] .
When studying radiation attenuation qualities, researchers identify fundamental quantities, such as the mass attenuation coefficient and some associated numbers. This amount may be calculated in a variety of ways, the most significant of which is a practical approach that involves exposing the sample to photons. A detector can be used to determine the number of photons that have penetrated the sample.
The Monte Carlo simulation approach is another option 20 . This approach involves creating a virtual reality for the practical experiment taking place in the laboratory. This approach differs from the practical way in that it saves time and effort while also lowering the likelihood of individuals becoming exposed to photons. Its significance is heightened by the shortage of equipment and facilities, forcing more researchers to rely on simulation. In the last few years, several researchers have adopted Monte Carlo simulations to investigate the radiation attenuation performance of borate glasses with different additives. For example, Mahmoud et al. 21 used MCNP5 to examine the influence of CdO on the attenuation shielding performance of alkali borate glasses. They found that the addition of more CdO enhanced the radiation attenuation performance of the selected glasses. Acikgoz et al. 22 prepared new alumina borate glasses, and when using MCNP5, they determined the role of CeO 2 and Er 2 O 3 on the radiation shielding performance of the prepared glasses. The authors concluded that glasses that contained Er 2 O 3 had a higher mass attenuation coefficient than glasses with CeO 2 . Rammah et al. 23  The authors concluded that a 3 cm-thick glass sample containing Cr 2 O 3 was the best attenuator and could reduce the lifetime risk to cancer by five times.
In continuation of the previous efforts made by researchers to study the radioactive attenuation properties of borate glasses using Monte Carlo simulation, enrich the scientific community with more studies on the radiation attenuation properties of this kind of glass, and check the possibility of developing new shielding glasses based on borate glass, the Monte Carlo simulation code is utilized to estimate the gamma-ray shielding capacity of the novel lithium-zinc borate glass system. Moreover, to understand the optical properties of the developed glasses, the optical absorption band was measured in the wavelength range of 200 to 3000 nm. Additionally, the Makishima-Makenzie model was applied to predict the elastic moduli and microhardness of the fabricated glasses.

Experimental techniques
Glass fabrication, characterization, and optical properties. Tin-doped lithium zinc borate glass samples with nominal compositions of 65B 2 O 3 + 20ZnO + (15-x)LiF + xSnO 2 (where x = 0, 0.25, 0.5, 0.75, 1 and 1.25 mol%) were prepared. The glass composition and chemicals used to prepare samples along with glass code are shown in Table 1. Approximately 20 g of the batch composition of these chemicals was mixed well in an agate www.nature.com/scientificreports/ mortar for 60 min and then preheated at 350 °C for 60 min. This homogenous mixture was then heated in a porcelain crucible at 1100 °C for 30 min before being cast onto a stainless steel mold to generate glass discs. After quenching, the samples were immediately transported to a muffle furnace set at 300 °C for annealing. The densities of the glasses were measured using the MH-300A densimeter according to Archimedes' rule, as shown in Eq. (1). Toluene was used at room temperature as an immersion liquid during the density measurements.
where W a and W b are the weights of the glass sample in air and in liquid toluene, respectively. The measured density, as well as the calculated molecular weight and molar volume, are listed in Table 1 for the fabricated BZLSn glasses.
The optical absorption data of the glass samples were obtained for the polished glass samples using a Cary 5000 UV-Vis-NIR double beam spectrophotometer.
Gamma-ray shielding capacity evaluations. Both Monte Carlo simulation and the XCOM theoretical program were used to predict and affirm the photon shielding capacity of current BZLSn glasses. The calculation using the XCOM program depended only on the chemical composition of the fabricated glass system. After that, the calculation for the attenuation coefficient depended on the mixture, as presented in Eq. (2).
where ω i is the fractional abundance of the ith element in the shielding material.
On the other hand, the Monte Carlo simulation code is based on many parameters, such as the sample composition, sample density, problem geometry, and ENDF/B-VIII library, from which the cross-section of interaction is extracted for various elements. The Monte Carlo simulation geometry consists of many cards that describe all components required for the simulation process, as illustrated in Fig. 1. The first important card is the cell card, which introduces the cell type, density, and boundaries. The mentioned figure shows many cells inside the prepared geometry, such as the collimators of lead with a density of 11.34 g/cm 3 , glass samples with their densities listed in Table 1, a radioactive source, and F4 detector tally to estimate the photons average track length per unit cell of the detector. The second is the surface card, which usually describes the shape and dimensions of the cells introduced in the first step. The dimensions of the various geometric components are illustrated in the figure. The third is the material card that introduces chemical composition of each cell in the arranged geometry. The source card also describes the source position, type, energy, decay probability, and emission distribution. The cutoff card is one of the physical cards used in the prepared geometry, and it is set to stop interaction after 106 historical cards.

Results and discussion
Physical and optical properties. The density and molar volume of the 65B 2 O 3 + 20ZnO + (15-x) LiF + xSnO 2 glass system with various SnO 2 concentrations are shown in Fig. 2. The density of the sample improves as the SnO 2 content (x) is increased; on the other hand, the molar volume decreases. The density of the fabricated glasses slightly increase from 3.025 ± 0.151 to 3.265 + 0.163 g/cm 3 , increasing the SnO 2 insertion ratio www.nature.com/scientificreports/ from 0 to 1.25 mol%. The substitution of Li 2+ ions (ρ = 0.534 g/cm 3 ) with Sn 2+ ions (ρ = 7.31 g/cm 3 ) causes a slight increase in the density of the fabricated glass. Additionally, the increase in the fabricated glass density may be related to a significant decrease in the interatomic distance due to the substitution of LiF by TeO 2 . Figure 3 shows the UV-Vis absorption spectra of the produced SnO 2 -doped lithium zinc borate glass samples, and the inset in Fig. 3 shows the absorption spectra from 350 to 800 nm. These measurements were carried out on samples with a thickness of 2.2-2.5 mm. The spectrum shows that the produced samples have significant UV absorption and that the strength of the absorption decreases as the concentration of SnO 2 is increased 27 . As the SnO 2 substitution ratio is increased, distinctive bands of SnO 2 are observable in these glass samples: the peak at 445 nm (*) is attributable to a ligand in the metal charge assigning transition (Sn 4+ ).
Tauc's rule, which Mott and Davis 28,29 have modified, is shown in Eq. (3). This rule is used to estimate the optical energy band gaps for the investigated SnO 2 -doped lithium zinc borate glass samples in this study:      www.nature.com/scientificreports/ of undoped and doped SnO 2 glass samples. The E g optical values decrease during the direct and indirect transitions when the SnO 2 concentration in the produced glasses is increased.
The difference between the Eg of SnO 2 (3.6 eV) 30 and the large band gap of LiF (14 eV) 31 may be related to the decreasing optical energy values E g with increasing the content of SnO 2 .
Equation (4) is used to compute the refractive index values (n) of the manufactured samples during the direct and indirect allowed transitions 32 : Table 2 lists the values of (n) for all synthesized samples. Clearly, the optical energy band gaps are inversely related to the refractive index (n) of all the manufactured glasses. The refractive index increases from 2.376 to 2.395, indicating that the proposed glasses may be used as a promising material for optical filters and photoelectronic applications.
Th absorption coefficient (α) can be estimated using Urbach's empirical formula 33,34 : where α 0 is a constant and E U is the Urbach energy. Equation (5) can be written as: As a consequence, the slope of the straight line obtained by plotting ln(α) vs. (hv) may be used to calculate Urbach's energy (E u ). The change in ln(α) with photon energy ( hυ ) is exhibited in Fig. 6. As listed in Table 2, the Urbach energies decrease as the SnO 2 concentration is increased, indicating an increase in stability and decrease in disorder in the glass samples.
Mechanical properties of the fabricated samples. The effect of replacing LiF with SnO 2 on the dissociation energy (G t ) and packing factor (V t ) of the fabricated glasses was studied. Figure 7 depicts a linear decrease in G t as well as an increase in V t with an increasing replacement ratio of LiF by SnO 2 . The G t values decrease slightly from 69.620 to 69.613 kJ/cm 3 , while the V t values increase from 0.732 to 0.777 when the SnO 2 insertion ratio is increased from 0.0 to 1.25 mol%, respectively. The mentioned decrease in the G t values is due to the replacement of the LiF compounds with G t = 38.801 kJ/cm 3 by SnO 2 with a comparable G t value where (G t ) SnO2 = 38.264 kJ/cm 3 . The increase observed in the Vt values is due to the replacement of a low packing factor (V i ) compound with a compound having a higher one; for instance, the V i of LiF is 4.894 cm 3 /mol while it is 13.862 cm 3 /mol for SnO 2 . Figure 8 shows the effect of replacing LiF with Sn 2 O on the elastic moduli of the fabricated glasses. It is known that the elastic moduli are related and proven from the predicted values of both G t and V t presented in the previous section. According to the results presented in Fig. 8 A, the Young modulus increases from 101.949 GPa to 108.226 GPa when the Sn2O insertion ratio is increased from 0.0 to 1.25 mol%, respectively. This increase in the Young modulus is directly related to the increase achieved for the V t of the fabricated glasses when LiF is replaced by SnO 2 , where Y = 2V t G t . Additionally, Fig. 8B,C,D show that the bulk (B), shear (S), and longitudinal     www.nature.com/scientificreports/ matrix is 0, 1, 2, 3, 4, and 5 mol%, respectively. This increase in the LACs is caused by the replacement of LiF (with a lower density and absorption cross-section (ρ LiF = 2.64 g/cm 3 )) with SnO 2 (with a higher density and photon absorption cross-section(ρ SnO2 = 6.95 g/cm 3 )). As a result of the replacement of a light compound (LiF) with a dense compound, the probability of the interaction between the incident photons and the distributed atoms and electrons increases due to the increase in the interaction cross-section. This increase in the interaction cross-section is reflected in the LACs and MACs, where both the LAC and MAC increase with increasing SnO 2 ratio in the fabricated samples. The enhancement of the LACs and MACs is positively reflected in other shielding parameters, as illustrated in Fig. 11. The thickness required to decrease the source radioactivity to half (half-value layer, HVL) is enhanced by adding SnO 2 into the glass matrix. The HVL regularly decreases with increasing LiF substituted by SnO 2 . Among the current study, thinner HVLs are achieved at 0.223 MeV. The HVL decreases from 1.967 to 1.784 cm when the SnO 2 concentration is increased from 0 and 5 mol%. On the other hand, the thicker layers achieved at 2.506 MeV decrease in the following order: 5.975, 5.923, 5.863, 5.773, 5.652, and 5.535 cm for the prepared BZLSn0, BZLSn1, BZLSn2, BZLSn3, BZLSn4, and BZLSn5 samples, respectively.
Additionally, Fig. 11 shows that, at a gamma-ray energy of 0.662 MeV, the HVL decreases from 3.069 to 2.845 cm as the SnO 2 ratio is increased from 0 to 5 mol%. For all fabricated BZLS glasses, the HVL reduction is due to the LAC being enhanced with increasing SnO 2 addition, where HVL = 0.693/LAC. Figure 12 illustrates the gamma photon energy effects on the HVL values. Figure 12 shows an increase in the HVL associated with raising the incident gamma photon energy from 0.223 to 2.506 MeV. For all samples, the HVL linearly increases with the incident energy. For example, the HVL increases from 1.967 to 5.975 cm for BZLSn0, while it increases from 1.784 to 5.535 cm for BZLSn5 when the gamma-ray energy is increased from 0.223 to 2.506 MeV, respectively. The average HVL in the investigated energy range is approximately 3.551, 3.519, 3.483, 3.428, 3.355, and 3.286 cm for the BZLSn0, BZLSn1, BZLSn2, BZLSn3, BZLSn4, and BZLSn5 glasses, respectively. The increase in the HVL with energy is related to the penetration power of the incident gamma ray, where the gamma-ray penetration power increases with increasing energy. Thus, the transmission factor (Io/I) increases with the penetration power, and the thickness required to attenuate half of the incident photons will increase as a result.
The transmission factor is a measure of the number of photons that penetrate the shielding thickness after a collision or without collisions 38 . It is mainly dependent on three parameters: the source energy, chemical composition of the shielding material, and thickness. Figure 13a shows the dependence of the gamma photon transmission factor (TF) on the source energy, while Fig. 13a illustrates the effect of material thickness on the transmission ability of gamma photons. According to Fig. 13a, the TF regularly increases with increasing source energy. The lowest TFs are approximately 43.74, 34.36, 33.73, 33.00, 32.06, and 31.17% for BZLSn0, BZLSn1, BZLSn2, BZLSn3, BZLSn4, and BZLSn5, respectively. On the other hand, the highest TF is achieved at 2.506 MeV equaling 70.61, 70.39, 70.14, 69.75, 69.22, 68.68 for the previously mentioned glasses. As mentioned in the HVL part, the TF is related to the photon penetration power. Thus, the resistance and attenuation of the shielding materials decrease with increasing source energy, so the number of photons that can escape and penetrate the shielding material thickness increase as a result. In contrast, Fig. 13b shows a significant decrease in the TFs of all prepared glass samples with increasing glass thickness. Among the selected thicknesses, the highest TFs are from glass with a thickness of 1.5 cm. The TFs at 1.173 MeV vary, with values of 77.39, 77.22, 77.03, 76.72, 76.29, and 75.87% for glasses with SnO 2 concentrations of 0, 1, 2, 3, 4, and 5 mol%, respectively. The lowest TF is achieved for materials with a thickness of 10 cm. The TFs vary between 18.11 and 15.86%. Increasing the glass thickness forces the incident photons to undergo more collisions before passing the glass layer. Thus, the attenuation of photons increases, and the TF decreases significantly. Additionally, the replacement of light compounds, such as LiF, with a denser compound, such as SnO 2 , increases the net density of the glass, which affects the attenuation The radiation protection efficiency (RPE, %) is used to describe the energy deposited inside the fabricated BZLSn glasses. Figures 14a and b depict the RPE variation with respect to the incident gamma photon energy and glass thickness. In the case of Fig. 14a, the RPE shows a linearly decreasing trend with increasing energy from 0.244 to 2.506 MeV. Among the studied energies, the best RPE is recorded at 0.244 MeV. Notably, the RPE varies, with values of 65.261, 65.743, 66.999, 67.936, 68.833% when the content of SnO 2 is 0, 0.25, 0.5, 0.75, 1.0, and 1.25 mol%, respectively (representing BZLSn0, BZLSn1, BZLSn2, BZLSn3, BZLSn4, and BZLSn5). These results indicate that at a gamma-ray energy of 0.244 MeV, the photons undergo many collisions with the glass atoms and lose a very large amount of their energy inside the glass layer. Thus, only a small number of photons can penetrate the glass and reach exposed workers. Then, with increasing gamma photon energy, the photon penetration power increases, the number of photon collisions decreases, and the energy transferred from photons to the glass layer decreases with increasing number of photons that can penetrate the glass layer. Hence, the number of photons reaching workers increases with a decrease in the glass thickness (i.e., RPE). Among the   The glass thickness also greatly affects the protection capacity of the shielding material. As illustrated in Fig. 14b, the RPE increases linearly with increasing BZLSn glass thickness from 1.5 to 10 cm. Among the studied glass thicknesses, the lowest RPE is obtained at a thickness of 1. 5  , and BZLSn5, respectively. It is clear that increasing the glass thickness causes an increase in the path length of the gamma photons inside the investigated samples, which forces the incident photons to undergo additional collisions with glass atoms and electrons. Thus, the energy deposited inside the glass layer increases, and the number of photons that can escape from the glass thickness decreases. Therefore, the RPE ratio increases.
To select the best fabricated glass that possesses suitable shielding and mechanical properties, as well as being inexpensive to fabricate, the variations of the LAC, HVL, microhardness, and fabrication cost versus the SnO 2 concentrations were studied; the results are shown in Fig. 15. As mentioned in the last paragraphs, the insertion of SnO 2 causes a significant increase in the µ values due to the significant increase in the packing factor V t and bulk density associated with the replacement of LiF by SnO 2 . The significant increase in the packing factor causes an increase in the Poisson ratio associated with a decrease in the microhardness of the material. According to the mentioned figure, the best sample in the present study is has a SnO 2 concentration of 0.75 mol% (i.e., BZLSn3). The µ value of the mentioned sample is 0.2338 cm −1 , and the HVL is 2.9638 cm at 0.662 MeV. Additionally, the mentioned sample contains a microhardness value of 4.9022 GPa. Furthermore, the fabrication cost for a sheet of BZLSn3 glass with dimensions of 100 cm × 100 cm, which can attenuate half of the incident photons when the energy is 0.662 MeV, is approximately $3151.46. This relatively high cost is due to LiF being expensive, which costs $0.384/g of fabricated sheet. Therefore, the replacement of LiF by SnO 2 causes a significant decrease in the fabrication cost due to SnO 2 having a relatively low price compared to LiF (SnO 2 costs approximately $0.05/g).

Conclusion
In the present study, the effect of increasing SnO 2 in a glass system based on zinc lithium borate was studied. The fabricated glass density increased from 3.025 and 3.265 g/cm 3 , while the molar volume decreased from 21.628 to 20.516 cm 3 /mol for the fabricated BZLSn glasses when the content of SnO 2 was increased from 0 to 1.25 mol%. According to the optical property calculations, the energy gap during direct and indirect transitions decreased from 3.257 to 3.197 eV and from 2.902 to 2.815 eV, respectively. The Monte Carlo simulation showed that BZLSn5 with a SnO2 composition of 1.25 mol% had the highest shielding capacity among the fabricated samples. The linear attenuation coefficient of the previously mentioned sample decreased from 0.389 to 0.125 cm −1 , and the half-value layer increased from 1.784 to 5.535 cm as the photon energy was increased from 0.244 to 2.506 MeV. The previously illustrated results showed that the replacement of LiF by 1.25 mol% SnO 2 resulted in an enhanced shielding capacity, reaching 9.3% at 0.244 MeV.    Figure 15. Variation of the linear attenuation coefficient, half-value layer, manufacturing cost, and microhardness versus the SnO 2 insertion ratio.