The tungsten oxide within phosphate glasses to investigate the structural, optical, and shielding properties variations

We prepared a series of sodium phosphate glasses by changing WO3/P2O5 content and investigated structure optical and radiation shielding features as a function of the glass composition. The average density (ρexp\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\rho_{{{\text{exp}}}}$$\end{document})was found to increase gradually from 2.49 to 3.07 g/cm3, while the average molar volume values reduced from 47.37 to 44.28 cm3/mol with WO3 addition. Also, the average field strength was also computed and found to increase with increasing WO3. The study of optical absorption spectra reveals that the absorption peaks in the visible region become higher compared to the peaks In the Ultraviolet region. This observation is accompanied by a color transformation of glasses from light to dark blue color, with more WO3 adding. The existence of a pentavalent tungsten state (W5+) is identified by this blue color; with WO3 addition an absorption band at around 350–390 nm is appeared. Moreover, this band is overlapped with the Urbach edge, which regularly produces an artificial edge-like feature at ~ 400 nm. A detailed deconvolution protocol is required for an appropriate understanding of these spectra and unraveling the hidden Urbach edge. Our analysis shows that, with increasing WO3/P2O5 content, the optical band gap decreases. This behavior can be elucidated in terms of a lower band gap of WO3 (2.7 eV) than that of P2O5 (8.5 eV) and the high polarizing power tungsten. Further, the radiation shielding parameters were investigated for the prepared glasses. WO3 addition improves these shielding parameters against radiation. Besides, upon the increase of WO3 concentration, the linear attenuation coefficient of glass material increases, which leads to decrease in have value layer values. Then it is deducible that the amount of WO3 in this glass sample has an important impact on the shielding capability at lower energy values and has a slight impact at higher energy values.


Introduction
The fabrication of novel glass compositions is a very dynamic field of research due to their motivating applications in our daily life, and the capability of fabrication and flexibility of shape and size at a quite low cost [1]. A conventional glassy matrix former, for example, silicate (SiO 4 based), borate (B 2 O 3 based), and phosphate (P 2 O 5 based), with varied formulas of modifiers and intermediates were studied for numerous technological appliances. From literature, it is well known that P 2 O 5 -based glasses with their excellent thermal properties (low melting temperature and high thermal expansion coefficient) are promising sealing materials [1], excellent UV-Vis transparent windows and a wide range of solubility for glass modifiers, for example, alkali, rare-earth (RE), and transition metals [2], appropriate candidates for lenses fabrication that diminish color distortion and acceptable in numerous optical applications [3][4][5], and appropriate candidates for optical memories and optical data storage [6,7]. The structure of P 2 O 5 -based glasses deeply relies on the type and quantity of glass modifier in the glassy matrix. Thus, the glassy state of P 2 O 5 glass formers can transform from Q 3 sets, containing three bridging oxygens (BO), to Q 2 , Q 1 , and Q 0 sets with two, three, and four non-bridging oxygens (NBO), respectively [1].
From literature, it is well known that transition metal cations as dopants in glasses can change the mechanisms and processes for the creation of radiation-induced color centers, depending on the type and concentration of transition metal cations as well as the formulation of host glass [8]. The existence of W 5? cations may create W 5? O 3 molecular orbital states and possibly help in the depolymerization of the glassy matrix by break into P-O-P links. Moreover, the insertion of WO 3 in phosphate glasses within high amounts ([ 10 mol%) reduces the maximum phonon energy of the phosphate matrix by the development of highly polarizable WO 6 clusters, resulting in interesting features, such as photochromism and non-linear optical absorption [9].
The addition of WO 3 is well known to enhance mechanical, chemical, and thermal stabilities [10]. Previous investigations on WO 3 -containing phosphate glasses [3,8] have reported that these glasses show bluish color and various characteristic optical absorption bands representing the existence W 5? (pentavalent) cations.
Further, the usage of shielding glasses in radiation protection applications is increasing day after day, because of their motivating characteristics, such as excellent transparency, the capability of fabrication, low fabrication cost, and good thermal and structural features. Along with that, the radiation shielding parameters of glass systems can be altered by using different contents of oxides in the glass formula [11][12][13][14]. Tungsten oxide was found to increase the shielding performance of oxide glasses against gamma irradiation [15,16].
In this Article, we prepared WO 3 -doped tungsten phosphate glasses and characterized them to study the microstructural-properties relationships, in view of their structure, optical, and radiation shielding applications.

Experiment section
Studied ternary sodium phosphate glasses with the formula [30 mol% Na 2 O-(70-x) mol% P 2 O 5x mol% WO 3 ], (x = 0, 5, 10, 15, and 20 mol%), synthesized using melt quenching method, where analytical grade sodium dihydrogen orthophosphate, tungsten oxide, and sodium oxide supplied by Sigma Aldrich Co. Precalculated weights of chemicals needed for glass fabrication in mol% were put in a 50 ml porcelain container in an electric furnace regulated at about 1100°C for 90 min to assure homogenous and bubble-free melts. The liquid was poured onto brass plates at room temperature. All glass samples were kept in a desiccator to avoid humidity absorption from air.
Density data were calculated from the Archimedean principle by obtaining the weight of each specimen before and after immersion in stable liquid as xylene. Measurements of specimens were repeated four times (N = 4) to obtain the error in the average density and average molar volume. The error in the average density is calculated from the formula and the uncertainty in the average molar volume is Absorbance (A) of all glass specimens was performed using a double-beam spectrophotometer, model JASCO within 200 to 1100 nm with 2 nm resolution and accuracy 1% A, utilizing UV-Vis spectrophotometer (SPECTROUV-VIS DUAL BEAM8 AUTO CELL UVS 2800). Further, the glass samples used in the measurements were larger than (10 Â 10) mm 2 and have thickness of * 1 mm. All measurements were carried out at room ambient conditions. The study of the optical absorption spectrum is a suitable method to give information regarding the optical transitions in the glass. Besides, this study provides information for the electronic band structure in glassy material. The absorbance spectra of NaPW-glasses were considered in wavelength region (300-1050 nm) and are presented in Fig. 1a. It was noticed from Fig. 1a that with increasing WO 3 in glasses the absorption peaks in the visible region become higher compared to the peaks in the UV region. This observation is accompanied by a color transformation of NaPW-glasses from light blue color to dark blue, with more WO 3 adding. The existence of a pentavalent tungsten state (W 5? ) is identified experimentally from producing a distinct bluish color in the glass [17]. In order to discuss this color transformation, and the absorption peaks in the visible region, the succeeding arguments are considered [3, 7- For deep analyses for optical spectra a detailed deconvolution protocol is required. The deconvolution of these spectra and the related details shows visible bands positioned at about 380-420, 580-600 nm, and 830-860 nm as shown in Fig. 1b. These latter three broad bands signify to the existence of (W 3? , d 3 ), (W 4? , d 2 ), and (W 5? , d 1 ), respectively. Also, the optical spectra display that the heights of the broad bands referring to W 3? , W 4? , and W 5? increase compared to W 6? with further WO 3 doping. Similar results were previously reported for sodium metaphosphate glass [3] and cadmium zinc phosphate glasses doped with WO 3 [22]. This increment in intensities is accompanied by a redshift toward the longer wavelength with further WO 3 doping. This shift toward the longer wavelength might be attributed to the increment in the average bond length of W-O with the regular increase of WO 3 from 5 to 20 mol%. The increase in the absorption band intensities (in the visible region) with further WO 3 doping is in an excellent consistency with Beer-Lambert-Bouguer's Law [23,24]. Furthermore, such increase in the absorption bands is the reason for the color transformation of NaPW-glasses from light to dark blue color.

The UV absorption bands and optical band gap
The investigation of optical absorption in the UV range is a suitable tool to recognize the band structures of electrons in glasses. In these optical spectra of glasses, a fast increase in the absorbance toward smaller wavelengths (k) is observed. The fast increase of absorption coefficient (a) is described as the fundamental absorption edge (UV ''cut off''). Further than this UV ''cutoff'', glasses are opaque for electromagnetic radiation when the photon energy becomes larger than the optical band gap (i.e., k B 200 nm) that presents between the valence and conduction bands. In glasses, estimating this band gap can give information about the structural modifications and the chemical bonds inside the glass network, in addition to determining semiconducting features of materials used for optoelectronic and photocatalyst devices. The width of the localized states in the band gap created as a result of the disorder inside the material can also be estimated from the study of the optical absorption spectrum for such material [25].
Essentially, two forms of transitions can happen near the fundamental absorption edge of both crystalline and amorphous materials-direct and indirect transitions. In the case of indirect transition, simultaneous interactions among the electron and lattice vibrations (phonons) are happened. In this case, the electron's wave vector can be changed due to the phonon's absorption/emission. Further, for indirect transition, the lowest point of the conduction band and the highest point of the valence band lie in a dissimilar k-space. On the other hand, for the direct optical transitions, it is necessary for the wave vector of an electron to be the same as it absorbs a photon [26,27].
For the present glasses, in the ultraviolet region, one can observe a strong UV absorption band (at 300 nm) is ascribed to the charge transfer mechanism. Consequently, the strong UV absorption band displayed in Fig. 1a for the W-free sample is attributed to the presence of ferric cations traces (Fe 3? ) as impurities inside the chemicals intended for the preparation process of the NaPW-glasses. With further WO 3 doping, UV absorption band at 350-390 nm is appeared. This confirms the existence of all the tungsten cations in their W 6? state. Moreover, this band is overlapped with the fundamental absorption edge (UV ''cut off''), which regularly produces an  Fig. 1 a Optical absorption spectra as a function of wavelength for the prepared glass samples. b The deconvoluted optical absorption spectra for the sample that contains 5% WO 3 as a representative figure for low WO 3 content. c The deconvoluted optical absorption spectra for the sample that contains 15% WO 3 as a representative figure for high WO 3 content artificial edge-like feature. This overlapping is observed as broadening in the UV band (marked * in Fig. 1a). A detailed deconvolution protocol is required to overcome this overlapping and extract appropriate details (to unraveling the hidden absorption edge). Figures 1b shows an example for such a deconvolution method applied to the NaPWglass sample that contains 5% WO 3 as a representative figure for low WO 3 content and Figs. 1c shows an example for such a deconvolution method applied to the NaPW-glass sample that contains 15% WO 3 as a representative figure for high WO 3 content. This deconvolution was done for all glasses except the Wfree sample. Thus, the optical band gap (E opt ) value can be calculated using Tauc's plot by the following relation [28]: where hm is the photon energy, C is constant, and furthermore, a is the absorption coefficient and calculated from absorbance (A) and thickness (d): The E opt can be determined by extrapolating of a straight section to the value hm = 0 in x-axis in the plot of hv versus ahm 1=2 ( Fig. 2a and b). It is significant to observe that with increasing dopants content (WO 3 ), the E opt of NaPW-glasses get decreased. Figure 3 displays the estimated values of E opt for NaPW-glass system. In comparison, our results are consistent with those obtained in Refs. [8,29]. According to Fajan and Kreidle [30], the cations' polarizability is directly proportional to cations' positive charge and inversely proportional to cations' radius. Compared to P 2 O 5 , W 6? has a higher positive charge and a medium cation radius (0.6 Å ), so W 6? has the highest polarizability. Moreover, the WO 3 has a much smaller band gap (2.7 eV) [29] than that of P 2 O 5 (8.5 eV). So, the high polarizability and small band gap of WO 3 produced the reduction in the glass band gap. Further, the absorption edge in numerous materials applies the Urbach rule [28]: where a 0 is a constant and (E U ) is Urbach energy and refers to the width of localized states tail in the forbidden band gap of the material. The exponential relation between a and hm may result from the electronic transitions between the localized states, which already present in the forbidden band gap [28].
Further, this exponential relation between a and hm might arise from the random fluctuations of the internal fields related to the structural disorder in various materials. E U is determined from the inverse slopes of ln (a) versus h m graph. As demonstrated from Fig. 3, Urbach energy (E U ) get increased by increment dopants content (WO 3 ). The shift toward smaller energy is associated with the creation of NBO which binds excited electrons less tightly than BO. Therefore, it might be proposed that the NBO content rises with increment WO 3 content, leading to a

Macro-and micro-structural parameters
The density is a macroscopic tool to recognize the microscopic variations in the material structure. It provides important information about physical and optical features. The density measurements were repeated four times, and the average value is listed in Table 1. It is clear that the average density (q exp ) increases gradually from 2.49 to 3.07 g/cm 3 with WO 3 adding. The observed increment in density is attributed to the difference between the composition content's molar weight; where the addition of a higher molecular weight rather than a lower one; (WO 3 = 231.84 g/mol.), (P 2 O 5 = 141.94 g/mol.). Table 1 offers the alteration of q exp and V m with WO 3 adding. It can be observed that the V m values diminished regularly from 47.37 to 44.28 cm 3 /mol, with the gradual substitute of P 2 O 5 by WO 3 . Such decrement may be ascribed to the succeeding points: (I) The reduction in the number of oxygen ions in the glassy matrix, since five oxygen ions (P 2 O 5 ) are substituted by three ions (WO 3 ). (II) The reduction of the positive ions also, since two P ions are substituted by a W ion. (III) The differences between the covalent radii of phosphorus (1.06 Å ) and tungsten (1.3 Å ) (with two phosphorus ions together of approximately 2.12 Å ). All these reasons act to decrease the V m of NaPW-glasses with further WO 3 addition. Another suggestion of the diminution in V m is the simultaneous decrement of average interatomic separation in the glass matrix; as recorded in Table 1.
Indeed, a number of significant physical parameters can be estimated from the density data, such as W cation concentration, N W (number per unit volume), Average interatomic separation in the glass   Fig. 3 The change of E Opt and E U versus WO 3 content for the prepared glass samples  Table 1, where z i is the mole fraction of WO 3 , N A is Avogadro's number, and M W is the average molecular weight of such specimen. The W cations concentration was found to increase with WO 3 . In contrast, the polaron radius and W-W Interatomic separation were found to decrease. This trend consistent with that obtained for the V m . The average field strength was also computed and found to increase with increasing WO 3 . This increment is attributed to the augmentation in electrostatic interaction due to increasing W concentration accompanied with decreasing Average interatomic separation in the glass matrix.

Radiation shielding properties
For the five prepared glasses, the linear attenuation coefficient (LAC) was obtained and exhibited versus photon energy in Fig. 4. The Phy-X software was used for the determination of the LAC between 0.284 and 2.506 meV [33]. By analyzing the figure, two main trends were noticed. First, at all photon energies, LAC decreases as energy increases for all five NaPW# glasses. This trend can be subdivided into two sections. The first energy region, region a, between 0.284 and 0.826 meV, is the energy range where the photoelectric effect is dominant. A sharp decrease in the LAC values can be observed in region a. For instance, at this low energy region, the LAC of the NaPW0 glass decreases from 0.271 to 0.172 cm -1 as the energy increases from 0.284 to 0.826 meV. LAC relatively decreases at a small rate at the range 1.173 meV to 2.506 meV. At 1.173 meV NaPW0, NaPW5, NaPW10, NaPW15, and NaPW20 LAC become 0.145, 0.151, 0.162, 0.168, and 0.180 cm -1 respectively. This zone, (say zone b), is dominated by Compton scattering, which is behind the slower decrease in values. The significance of WO 3 percentage in the glass can be observed in the values of LAC in various samples of glasses and at any energy range. For example, for NaPW20, which has the richest quantity of WO 3 , LAC is taking the highest value while for NaPW0, which has the least quantity of WO 3 (0 mol%), LAC is taking the lowest value. For example, LAC takes the value of 0.250 cm -1 and 0.432 cm -1 for NaPW0 and NaPW20 at 0.347 meV, respectively. As observed in Fig. 4 and at higher energies, the difference between the values largely decreases though it is maintained at all energy values. At 2.506 meV, NaPW0 has a LAC of 0.098 cm -1 , while NaPW5 has a LAC of 0.120 cm -1 . It is clear that the obtained results at higher energies look much closer to each other. Since a greater value of LAC indicates a better shielding capability, NaPW20 exhibits the better promising shielding properties through the NaPW# specimens at lower energy values. At higher energies, the shielding capabilities of the samples look close to each other. Then it is deducible that the amount of WO 3 in this glass sample has an important impact on the shielding capability at lower energy values and has a slight impact at higher energy values. The (HVL) is the half value layer of the material. It has been studied versus energy values for different WO 3 content and presented in Fig. 5. HVL begins with a tiny value (varied between 1.26 and 2.56 at 0.284 meV). The HVL then experiences a rapid increase. From Fig. 5, it is noticed the HVL for NaPW0 (as an example) changes from 2.56 cm (at 0.284 meV) to 3.25 cm (at 0.511 meV), to 4.03 cm (at 0.826 meV), and to 7.08 cm (at 2.506 meV). The same trend in the HVL values with the energy values is documented for the other glass samples. This behavior happens upon the increase of photons energy. Then a considerable number of photons can penetrate inside the sample material. One can deduce Photon energy (MeV) NaPW0 NaPW5 NaPW10 NaPW15 NaPW20 Fig. 4 The linear attenuation coefficient as a function of the energy for the prepared glasses that reducing the intensity of the incident photons in half requires a greater thickness of the material. According to the same figure, HVL gets lower values as the WO 3 percentages increases. This phenomenon is referred to the inverse relationship, which connected density and HVL; upon the increase of WO 3 concentration, the density of the glass material increases, which leads to a decrease in HVL value. Once a lower HVL indicates a more effective protector against radiation, the figure signifies that when WO 3 getting grater content wise, the glass sample will be better as a shield against radiation. The influence of the density of the glass samples on the shielding competence can be determined by plotting the tenth value layer (TVL) as a function of the density (see Fig. 6). The TVL was determined using the following formula: Figure 6 shows that the TVL for the glass samples declined with the increase in density, indicating that the shielding competence of the glass system can be enhanced with higher density. In addition, it was also observed that the shielding competence of the glass system was better at low energy radiation (TVL is small for 0.284 meV). From the data provided in Fig. 6, the glass samples' shielding competence was significantly improved when density was increased from 2.5 to 3.

MFP (cm)
Photon energy (MeV) NaPW0 NaPW5 NaPW10 NaPW15 NaPW20 Fig. 7 The mean free path as a function of the energy for the prepared glasses Also, we reported the mean free path (MFP = 1/ LAC) for the fabricated glasses in Fig. 7. Samples with a low MFP have been found to be more useful for applications in radiation-related fields as a low MFP means a high LAC (HVL = 1/LAC) [34]. Hence, more attenuation is expected for a sample with low MFP. The results given in Fig. 7 showed that the addition of WO 3 led to an increase in density that was responsible for the decrease in the MFP. The addition of WO 3 resulted in decreased MFP from 3.693 to 1.821 cm at 0.284 meV, from 3.999 to 2.314 cm at 0.347 meV, from 5.820 to 4.521 cm at 0.826 meV, and from 10.210 to 8.084 cm at 2.506 meV. These results further emphasized the importance of density of the prepared samples in determining the sample thickness that was able to attenuate certain desired radiation levels. Additionally, we can observe from Fig. 7 that the energy of the photon is also affected the MFP values. The lowest values of MFP were at 0.284 meV within the range of 1.821 to 3.693 cm. The results also revealed that at 2.506 meV, samples had the greatest MFP compared to the other energy levels. The glasses attenuated the smallest amount of radiation at the highest energy level. This indicated that the increase in energy positively correlated with an increase in MFP and that a thicker NaPW-glass is preferred for applications.
The ratio between photons that passed through the NaPW-glasses (I) to the total incident radiations (I 0 ) is called transmission factor (TF = I/I 0 ). Lower TF means more ability for the sample to serve as shielding material against radiation. The TF values of NaPW0 and NaPW20 samples at four various thicknesses are plotted in Figs. 8 and 9, respectively. The selected thicknesses are 0.2 cm, 0.5 cm, 0.75 cm, and 1 cm, representing thinner and thicker specimens. The two figures show a decreasing behavior as the thickness increases. Also, the TF increases with increasing energy. TF will be increased because of the ratio increase, which occurs due to the increasing number of photons that penetrate through the sample at higher energies. For the sample NaPW0 with a thickness of 0.2 cm, the minimum TF value, which happens at 0.284 meV, is equal to 94.7%, and this becomes 98.1% at 2.506 meV. The same trend was observed for the same glass sample at a thickness of 1 cm. The TF for this thickness at 0.284 meV is 76.3%, and this is increased to 90.7% for a photon with an energy of 2.506 meV. For NaPW20 glass, this is also correct, namely, the TF at a specific thickness increases with increasing energy. For instance, from Fig. s9, the TF for a sample with x = 0.75 cm at 0.284, 0.511, 0.826, and 2.506 meV is, respectively, 66.2%, 79.5%, 84.7%, and 91.1%. The TF results also proved that the greater the thickness of the sample, the lower the TF, and the better the shielding properties. In addition, a thicker shielding material is desirable as many collisions can happen between the sample and the incident photons, attenuating the radiation. At low energy values, a big difference between thick and thin material samples can be easily noticed.  Fig. 9 The transmission factor as a function of the energy for the NaPW20 glass Comparison between the effect of the WO 3 in each glass sample on the TF values and accordingly on the shielding properties of the prepared glasses can be examined from both Figs. 8 and 9. It is found that TF experiences a decrease with WO 3 in the sample, indicating a higher percentage of WO 3 has more influence in radiation attenuation purposes. At the same energy, the TF for NaPW0 (with the lowest percentage of WO 3 ) is lower than the TF for NaPW20 (with the highest percentage WO 3 ) for all thicknesses. These findings show that the richer the WO 3 percentage in the glass samples, the more influence the glass will be as a protection material against radiation. Figure 10 exhibits the effective atomic number (Z eff ) of the glass samples at the examined energies (i.e., between 0.284 and 2.506 meV). Effective radiation shielding capability match up with a greater Z eff since more radiations are absorbed by the medium with high Z eff values. The results show a similar trend when compared to LAC values. The values are in a direct proportionality with the WO 3 content in the glass. Instantaneously, the Z eff for NaPW0 and NaPW20 glasses with 0 mol% and 20 mol% of WO 3, respectively, increases from 10.02 to 18.84 at 0.284 meV, from 10.01 to 13.56 at 0.662 meV, and from 10.00 to 12.72 at 1.33 meV. In other words, at all energies, Z eff goes with the trend of NaPW0 \ NaPW5 \ NaPW10 \ NaPW15 \ NaPW20. This trend in Z eff emphasizes again that the greater the WO 3 percentage in the glass material, the better the shielding characteristics against radiation the glass. According to the Z eff values, NaPW20 will be the most capable shielding sample among the studied glasses. Further, the radiation shielding parameters were investigated for the prepared glasses. WO 3 addition improves these shielding parameters against radiation. Besides, upon the increase of WO 3 concentration, the LAC of glass material increases, which leads to a decrease in HVL value. Then it is deducible that the amount of WO 3 in this glass sample has an important impact on the shielding capability at lower energy values and has a slight impact at higher energy values.

Conclusion
We prepared a series of sodium phosphate glasses by varying WO 3 /P 2 O 5 content and studied structure, optical, and radiation shielding properties as a function of the glass composition. The average density (q exp )was found to increase gradually from 2.49 to 3.07 g/cm 3 , while the average molar volume values decreased gradually from 47.37 to 44.28 cm 3 /mol with WO 3 addition. Also, the average field strength was also computed and found to increase with increasing WO 3 . The study of optical absorption spectra reveals that the absorption peaks in the visible region become higher compared to the peaks in the UV region. This observation is accompanied by a color transformation of glasses from light to dark blue color, with more WO 3 adding. The existence of a pentavalent tungsten state (W 5? ) is identified by this blue color. With WO 3 addition an absorption band at around 350-390 nm is appeared. Moreover, this band is overlapped with Urbach edge, which regularly produces an artificial edge-like feature at * 400 nm. A detailed deconvolution protocol is required for an appropriate understanding of these spectra and unraveling the hidden Urbach edge. The optical band gaps were determined by analyzing the optical absorption edge using Tauc's model. Our analysis shows that, with increasing WO 3 /P 2 O 5 content, the absorption edge shifts toward longer wavelengths and the optical band gap decreases. This behavior can be explained in terms of the lower band gap of W O 3 (2.7 eV) than that of P 2 O 5 (8.5 eV) and the high polarizing power W. Further, the radiation shielding Photon energy (MeV) NaPW0 NaPW5 NaPW10 NaPW15 NaPW20 Fig. 10 The effective atomic number as a function of the energy for the prepared glasses parameters were investigated for the prepared glasses. WO 3 addition improves these shielding parameters against radiation. Besides, upon the increase of WO 3 concentration, the LAC of glass material increases, which leads to a decrease in HVL value.
Then it is deducible that the amount of WO 3 in this glass sample has an important impact on the shielding capability at lower energy values and has a slight impact at higher energy values.