Newly developed CeO2 and Gd2O3-reinforced borosilicate glasses from municipal waste ash and their optical, structural, and gamma-ray shielding properties (2024)

Table 1 revealed XRF analysis of the burned MSW ash that was used in preparing glasses where it was the source of SiO₂ in such borosilicate glass systems because of containing about 38.65% of silica content. Table 2 revealed the whole chemical compositions of the prepared glass systems in wt%.

Figure1 pattern revealed the XRD spectra of the three fabricated RE-borosilicate glasses. As noticeable, broad hump peaks and the whole lack of crystalline peaks, indicate the amorphous and disordered structures of the prepared samples and then their glassy natures.

Optical UV–visible measurements

The glass structure is highly favored as a host medium for rare earth ions because of their heterogeneously widened line widths of optical conversions and the simple compositional change. Lanthanides or RE ions are characterized by their electronic configuration [Xe] 4f¹²6s²; they have the outer-shell configuration of 4fⁿ5s²5p⁶6s², with partially filled 4f shell¹¹. So their optical transitions in glasses are resemble predominantly to intra f^N transitions of the primarily electric dipole oddity and their noticeable absorption peaks are detected from infrared to ultraviolet regions. While visible and near IR absorption fit to the transitions between f–f energy levels however, UV absorption is produced by the movement from the low energy level 4f to the high energy level 5d¹⁹.

Figure2a–c presents UV-near infrared absorption spectra of the fabricated RE (Ce, Gd and Ce + Gd) doped borosilicate glasses before and after gamma irradiation. As offered in Fig.2a, the absorption spectrum of Ce doped glass mainly contains absorption bands as follows; a main peak at 351nm (UV-regions), cutoff peak at 430nm (Vis-regions) and wide broad band at 950nm (NIR-regions). The main UV peak observed at 351nm originates from the absorption of Ce³⁺ with 4f → 5d transitions²⁰, where, the absorption of Ce³⁺ occurs in the higher energy region, while the charges transfer absorption of Ce⁴⁺ occurs in the lower energy region.

Also, from Fig.2b, the absorption spectrum of Gd doped glass revealed obvious peaks at about 230, 270 and 348 nm allocated to the transition of 4f → 5d in Gd³⁺ ion²⁰. As known there are two steady oxidation statuses for gadolinium; Gd³⁺ and Gd⁴⁺, having f1 and f⁰ electronic configurations, respectively^20,21. The absorption of Gd³⁺ ions has the allowed transition (f¹ → d¹) that performs as broad and exhaustive absorption bands in several solid environments and depends intensely on the host composition and appears usually in UV region in most oxides. While, absorption bands correlated to Gd⁴⁺, are caused by the charge transfer nature O₂ → Gd⁴⁺, that typically arises at the longer wavelength comparing to that of Gd^3+21,22.

Moreover, Fig.2c shows also the absorption of a mixed RE Ce and Gd doped borosilicate glass, where the absorption spectrum contains weaker absorption bands than the glass doped only Ce or Gd ions, separately. Additionally, the detected absorption peaks for Ce + Gd doped glass are at ~ 370nm in UV-region can be ascribed to the presence of Ce within the glass composition rather than Gd. The small red shift of UV peaks detected at 351 and 348nm in Ce and Gd doped glasses, respectively, is directly correlated to the small difference in the concentrations of both Ce and Gd ions and the expected electric dipole allowed 4f → 5d transitions for Ce and Gd ions¹⁹.

Figure2 depicts also the UV–VIS-NIR absorption spectra of the same specimens after being exposed to 100kGy of gamma irradiation. From Fig.2a–c, all the glass samples displayed a minor reduction in their UV absorption bands. This reduction in optical absorbance was accompanied with a red shift in the fundamental absorption edges. The impact of ionizing radiation such as gamma rays on materials might occur some photochemical reactions e.g., redox reactions accompanying with variations in oxidation states of doped ions^23,24.The obvious decrease in absorbance intensity indicates the optical stability of the glasses towards irradiation, while the clearness of secondly peaks at 480, 430 and 500nm for Ce, Gd and Ce + Gd doped glasses, respectively can be attributable to a possible crystal field splitting of 5d (2p) state in Ce and Gd ions²⁵. Some authors^19,26 stated that the coexistence of several peaks in Ce doped glass is recognized to the non-equivalence of Ce ions, such as the change in coordination number with the adjacent ligands from one situate to another leading to an inhom*ogeneous broadening of optical peaks.

To quantify the impact of γ-radiation on the optical features of the investigated glasses, the optical gap (\({E}_{g}\)) can be considered from Mot and Davis model²⁷ according to the following relation;

Herein, \(h\nu\) is photon energy and Fig.3a–c shows Tauc’s plots (at which we draw (αhν)^1/2 versus hν)²⁷) for (Ce, Gd and Ce + Gd) doped glasses before and after 100kGy of gamma irradiation and obtained values of E_g are displayed in Fig.4. From Fig.4 we can easily observe the slight reduction of E_g values for the three glass samples after being irradiated with 100kGy of gamma irradiation. Furthermore, the percent of E_g changes (\(\text{\% }{E}_{g}\)) was employed here to assess the RE-glass stabilities under gamma radiations according to the relations;

where \({E}_{g0}\) and \({{E}_{g}}_{100}\) represent the band gap for the un irradiated glass samples and 100kGy irradiated glass samples. The \(\text{\% }{E}_{g}\) values for all glasses are exhibited in Fig.5. From the figure it is noticeable that \(\text{\% }{E}_{g}\) is the smallest for Ce doped glass sample (20.2%) and the largest for the Gd glass sample (29.6) and in between for Ce + Gd doped glass sample (21%). From these values we can deduce that, the Ce-doped glass sample has the highest optical stability against gamma irradiation.

Photoluminescence measurements (PL)

The results of photoluminescence measurements on CeO₂ and/or Gd₂O₃ doped glasses are shown in Fig.6. As illustrated in Fig.6, the excitation spectra of all samples are located in the UVA spectral range, approximately at about 332–385 nm. An observable small sharp excitation peak at about 410 was characteristic of Gd₂O₃-doped glassy sample. The fluctuation in excitation energy is caused by the nature and concentration of the co-doped rare earth ions. Glasses can contain cerium in the polyvalent states of Ce³⁺ and Ce⁴⁺, whose balance or proportion varies depending on the host glass's type and composition, temperature of melting and atmosphere²⁸. For the first sample shown in Fig.6a, the excitation spectrum may be due to the occurrence of mutually two cerium valences within the investigated glasses, and an asymmetric broad excitation at about 332–360 nm identifies the trivalent transition (Ce³⁺ ion) 4f → 5d of Ce³⁺ ion agreed with the optical spectra shown in Fig.2a for Ce doped glass.

Cerium is a fascinating element that often exhibits remarkable luminescent properties when incorporated into host materials. The emission spectra of cerium-doped materials can provide valuable insights into its electronic structure and the intricate interactions it has within the host lattice. Cerium can exist in different oxidation states, and the electronic configuration of Ce⁴⁺ is particularly interesting. Unlike Ce³⁺, which has a 4f¹ configuration and displays strong 5d-4f transitions, Ce⁴⁺ has a 4f⁰ electronic configuration, meaning it lacks any 4f electrons. This absence of 4f electrons significantly influences the luminescence properties of Ce⁴⁺. In this case, the luminescence observed in Ce⁴⁺doped materials arises from charge transfer transitions, typically from the valence band or ligand orbitals to the empty 4f orbital of Ce⁴⁺. These transitions are often referred to as ligand-to-metal charge transfer (LMCT) transitions. The study of these LMCT transitions in Ce⁴⁺doped materials can provide valuable insights into the electronic structure and interactions within the host lattice, offering a deeper understanding of the fundamental properties of this fascinating element.

According to Fig.6b, the excitation spectra of Gd³⁺ doped glass reveal two characteristic excitation bands that can be allied to transitions from the ground state of Gd³⁺ ion to the higher ⁸S_7/2–⁶P_7/2, ⁶I_J, and ⁶D_J excitation states²⁹. For the mixed rare-earth doped glass, the spectral excitation bands are located sharply at 335 nm with an extended band at 363 nm as shown in Fig.6c. The emission spectra of the three samples depicted an intense bands centered at 412, 434, and 417 nm with a fixed extended emission band centered at about 360 nm for CeO₂, Gd₂O₃, and (CeO₂ + Gd₂O₃) doped glasses, respectively. The emission of pure Gd₂O₃- doped sample shows a characteristic emission band centered at about 562 nm and when mixed with CeO₂ the emission band shows a minor shifting to smaller wavelength at 550 nm. Consequently electronic transitions from the excited (5d) to (4f) ground statuses, trivalent cerium ions produce an emission spectrum that is split into two energy sublevels such as ²F_7/2 and ²F_5/2, and to identify the sub-peaks originating from 5d → ²F_7/2 and ²F_5/2 transitions. It is determined that the amount of Ce³⁺ ions present affects the relative emission intensity of the glass^30,31. Kumar et al.³² proposed that, when gadolinium is utilized as a dopant, the energetic levels of Gd³⁺ ion contain several transitions detected as a simultaneous fluorescence analogous to transitions ⁴F_(4)J → ⁶G_J, ⁶G_J → ⁶P_J, and ⁶P_J → ⁸S_7/2 centered nearby 826, 601 and 313 nm, respectively³². The proposed hypothesis by Mal’chukova et al.²⁹ assumed that the matrix's energy may migrate to the Gd³⁺ ion's ⁶D_J levels and then non-radiatively relax to the ⁶P_7/2 level. With a subsequent emission, the energy of the Gd³⁺ ion may likewise shift from the ⁶D_J (or ⁶P_J) levels to the levels of defects²⁹. The photoluminescence spectra appearing in Fig.6c are strong indication that efficient energy transfer from Gd³⁺ to Ce³⁺ should occur. Depending on the emission spectrum of the third rare earth mixed glassy sample that can be attributable to charge transfer bands, ⁸S_7/2 → ⁶I transitions of Gd³⁺ ions, and 4f → 5d transitions of Ce³⁺ ions. The Gd³⁺ ions can be excited to ⁶I, and then quickly return to ⁶P_7/2 which can transported to the 5d levels of Ce³⁺ by resonant energy transfer, improving the luminescence of Ce³⁺³³. The energy transfer process in the glass co-doped Gd³⁺ and Ce³⁺ ions, is illustrated by additional important energy levels of Gd³⁺ and Ce³⁺ ions that Wang et al.³³ detected. According to this diagram of energy levels, energy can be transferred from the ⁶P, ⁶I, and ⁶D states of Gd³⁺ to the 5d state of Ce^3+31,33.

Upon gamma irradiation, the intensity of the photoluminescence spectra is slightly reduced. The effect of gamma irradiation can be initiated by a number of different processes or mechanisms, including charge trapping, radiolysis, photochemical reaction, rupture, atomic displacement, or reformation of some connecting bonds^28,30. The effects of one or more of the previously mentioned processes may be related to the alterations created by gamma radiation, but only to a limited extent due to specific shielding behavior resulting from the glass composition^28,30. Although, the lower intensity of the PL spectra may be correlated to coloration or formation of induced defects that occur in the glassy shape that may lead to variations in the bond angles and/or lengths of specific structural groups and affect both intensities and positions of the emission peak, the reduction in absorbance intensity shown before in Fig.2 agreed with PL spectra of the glasses indicating the possible relaxation in the glasses’ optical properties after irradiation.

Figure7 displays the CIE chromaticity diagram of the prepared CeO₂ and/or Gd₂O₃ doped glasses denoted by S1, S2, and S3. A complete set of CIE color coordinate values can be found in Table 3. The color coordinates are equally distributed and centered in the blue region. The interpretation of the observed colors is based on the presence of rare-earth dopants that can emit blue hues, and the protracted appearance of various blue degrees is related to the type of dopant in the host glass matrix as a whole. For CeO₂–doped glass, the deep blue coloration shared the areas (a) and (b) of Fig.7. The rest of the areas (c,d,e, and f) are located in the brightest blue area with the addition of Gd³⁺ ions. This suggests that Gd³⁺ ions increase Ce³⁺ ions luminescences in certain materials and more effectively transmission energy to Ce³⁺³³.The optical absorption spectra with other doped nano-CeO₂ doped silicate glasses revealed a characteristic peak of Ce³⁺ ions that generates a yellowish appearance³⁴, while a mix of blue, yellow, and red bright yellow emissions were produced for Gd/Dy (at 576nm) in Gd₂O₃-B₂O₃ glasses³⁵.

FTIR absorption spectra

The various building units of the present borosilicate structure was identified by FTIR spectroscopy that involves predominantly the trigonal building units of borate BO₃, and tetrahedral building groups of silicate, SiO₄ and borate BO₄. Figures8, 9 and 10 displayed FTIR absorption spectra of the three deliberated borosilicate glass systems before and after 100 kGy of gamma irradiation. The three figures revealed similar spectra before irradiation showing an intersecting between silicate and borate vibrational bands. The spectral features and their assignments in the three spectra before and after irradiation can be discussed as follows;

(a) Before irradiation.

1. The small bands appeared at 468 or 475 cm⁻¹ can be ascribed to the vibrational signals of the present modifier cations into residing locations shown in Tables 1 and 2 such as the transition metal (TM) ions e.g., Cr³⁺, Ti⁴⁺, Fe³⁺, Cr³⁺ and Mn²⁺ as well as the vibration of alkali ions such as Na⁺, Mg²⁺, Ca²⁺ and Sr²⁺ions.

2. The band appeared at 620 or 615 cm⁻¹ interrelated to the silicate and borate bending mode vibrations, in addition the vibration of compacted units of aluminum ions as AlO₄ or AlO₆^6,36 as well as a possible combined vibrations of the tetrahedral units; AlO₄, SiO₄ and BO₄ units^5,6.

3. The band appeared at 715, 720 or 730 cm⁻¹ recognized to silicate bending mode vibration (Si–O–Si or O–Si–O), or bridging oxygens (BO) bending vibrations of borate amid two trigonal boron atoms (B–O–B linkages) and a possible Si–O–B link bending vibrations^5,6,37.

4. A main band appeared at 920 cm⁻¹ correlated either to Si–O–NBO stretching vibration, or vibrations of numerous borate groups (tri, tetra or penta borate) and B–O–M linkages’ stretching vibrations (M signifies metal ions or diborate group)³⁸.

5. A main larger band appeared at 1010 cm⁻¹ assigned to mutual Si–O–Si and B–O–B stretching vibrations in tetrahedral BO₄ building units.

6. A small band appeared at 1200 attributed to tri, penta, or tetraborate boroxal groups’ stretching vibrations³⁹.

7. Moderate bands appeared at 1348 or 1360 and 1420 cm⁻¹ can be attributed to the asymmetric stretching vibrations of B–O bonds in the trigonal borate groups [BO₃], [BO₂O]⁻, BO₃ with non-bridging oxygen (NBO)^36,38.

8. Bands appeared from 1600 cm⁻¹ up to the spectrum end at 3000 cm⁻¹ recognized to H-bonding, molecular water, OH⁻, Si–OH and B–OH stretching vibrations that are not dominant glass forming units^5,6,40.

As obviously shown the three spectra are very similar before irradiation because of the similar composition and the small doping contents of the added rare earth ions 0.7 Ce, 0.25 Gd or 0.5 Ce + 0.3 Gd ions.

(b) After 100 kGy of gamma radiation.

Figures8, 9 and 10 revealed two core observations after samples’ irradiation with 100 kGy of γ-rays. The first is the obvious decrease in absorption intensity of the spectra after irradiation specially the two main bands that appeared before irradiation at 920 and 1010 cm⁻¹, and the second is shifting the band at 920 cm⁻¹ to high wavenumber at 960 cm⁻¹. The two observations are directly interrelated to more stable structures where the lower absorption intensity refers to the release of extra energy and possible formation of relaxed structures. However, the shift of the band from 920 to 960 cm⁻¹ and the slight increase in absorption intensity of the non-essential groups ranged from 1600 to 3000 cm⁻¹ can be assigned to a quite adjustment in the polymerization degree^37,41. Mostly, the three spectra revealed the same effect towards irradiation as they give approximately more relaxed structures, indicating the negligible irradiation influence on their rigid structures. This manner is governed by the valuable compositions of the fabricated glasses, as shown in Tables 1 and 2, where the elemental rich composition of the MSW ash that utilized chiefly in glasses’ preparation—as listed in Table 1—is the main reason for their stability against the influence of ionizing radiation. The following arguments elucidate in detail the helpful effect of glass composition on its structural stability against irradiation;

(c) The dual behavior of the high compacted silicate tetrahedral (SiO₄), trigonal (BO₃) and tetrahedral borate (BO₄) building units, strengthens the glassy network to resist the consequence of radiation photons.

(d) Presence of different transition metal ions in the used MSW ash as listed in Table 1 (e.g., Mn²⁺, Cr³⁺, Ti⁴⁺, and Fe³⁺) plays an important role in absorbing possible defects that may be caused by irradiation, due to their ability to change their outermost configurations and coexistence in different oxidation states.

(e) The effect of the doping rare earth ions such as CeO₂ and Gd₂O₃ support borosilicate glass resistance to irradiation ascribed to their remarkable mass attenuation owing to their large atomic number, especially cerium ions that participate in the glassy system in both valence states of ;Ce⁴⁺ and Ce^3+42,43.

Electron spin resonance ESR

ESR technique was used to recognize chemical species that contain one or more free electrons and identify changes in paramagnetic centers formed because of irradiation processes. In the amorphous glassy network, these defect centers may be positive holes and/or electron defect centers^2,6. Figure11 reveals ESR spectra of three prepared RE borosilicate glasses before and after 100kGy of γ-radiation. Figure11 shows the high similarity between the three studied glasses either in positions or intensity. The three ESR spectra revealed the same spectra before and after irradiation with 100kGy besides the absence of real sharp peaks, signifying the trivial effect of irradiation on the glass structures and no formation of free radicals². Consequently, the high bonding connectivity of the glasses because no breaking in bonds was taken place and thus no free radical was formed under the effect of irradiation. The stable ESR spectra can be correlated also to stability in optical spectra and the relative stability in FTIR spectra after irradiation. This large stability in glasses properties are owing to the rigid structure of borosilicate network that consists of the tetrahedral SiO₄, trigonal BO₃ and the tetrahedral BO₄ building units, so that they work together to block the path of radiation photons⁴⁴. Subsequently, the high resistance of the considered glasses to ionizing gamma radiation and formerly their potential use for radiation protection applications.

Physical and mechanical measurements

Table 4 displays the microhardness values for 0.7 CeO₂, 0.25 Gd₂O₃, and (0.5 CeO₂ + 0.3Gd₂O₃) doped borosilicate glasses, respectively, having values of 473, 492, and 510kg/mm². The observed progressive increment in the hardness values are interrelated to the change in the main glassy structure as a result of rare-earth addition. It can be associated to the changes in lengths or strengths of bonds of the glassy network due to the additional effects of CeO₂ and Gd₂O₃. These data demonstrate that in the current glasses, CeO₂ and/or Gd₂O₃ act as a glass modifier allowing the excess NBO formation that primes an increase in the glassy network firmness giving better and resilient bonding, so the glass exhibits higher hardness⁴⁵.

Figure12 depicts the thermal expansion curve of the CeO₂ and/or Gd₂O₃-doped glasses before irradiation. Glass transformation temperature (T_g), dilatometric softening temperature (T_d), and thermal expansion coefficient (CTE) data are also itemized in Table 4. It is perceived that CeO₂-doped glass reveal higher dilatometric softening temperature and thermal expansion coefficient compared with the Gd₂O₃- doped glass sample. A few of the factors influencing CTE, T_g, and T_d of glasses such as the type of chemicals constituting the glass, bond strength, and the field strength of the cations^46,47. According to the structures of the current glasses, adding CeO₂ causes a drop in the thermal expansion coefficients while a rise in the glass transformation temperature (T_g), and dilatometric softening temperature (T_d). These occurrences are thought to be related to the creation of more NBO, which is what causes cerium ions to disrupt the network structures of glasses and reduce their connection^48,49.

It is generally accepted by many previous studies^49,50,51 that the ionic radius and cation field strength of rare-earth ions are often the two factors that have the most influence on the action of rare-earth oxides in produced glass. Due to the strong field and excellent coordination of Ce⁴⁺ ions, CeO₂ plays an important role in the mechanical and thermal behavior of the glasses^49,50,51. On the other side Fig.13 reveals the thermal expansion curve of CeO₂ and/or Gd₂O₃-doped glasses after 100kGy of gamma irradiation, revealing a large thermal stability of the glasses towards ionizing radiation.

Radiation shielding characteristics

The three RE borosilicate glasses' potential for ionizing radiation protection has been reached by examination and regulation of the mass (MAC = µ/ρ), and linear (LAC = µ) attenuation coefficients. MAC and LAC mostly influenced by the properties of materials (in this case, the glass compositions) and the incident photon energy (E), rather than materials’ thickness. Photoelectric effect (PE), Compton Scattering (CS), and Pair Production (PP) processes are three examples of photon interaction mechanisms that are taken into account while calculating the MAC physically^52,53,54,55.

Using the user-friendly online Phy-X/PSD tool, the MAC of the currently constructed glasses was assessed as a function of photon energy (E) in the range of 0.015 to 15MeV¹². The weight fraction of the elemental composition and the density of glasses were used in these computations. Figure14 depicts the variance in MAC values for the inspected glasses as a function of photon energy (E) in MeV. Figure14 illustrates how the inclusion of CeO₂, Gd₂O₃, or combined CeO₂ and Gd₂O₃ has a direct helpful impact on the MAC values. For all photon energies, it was found that the S3 sample doped with CeO₂ and Gd₂O₃ had the highest MAC values compared to S1 sample doped with CeO₂ alone and the S2 sample doped with Gd₂O₃. The highest MAC values for the S1, S2, and S3 samples were 9.239, 9.166, and 10.016 cm²/g at 0.015MeV respectively. The lowest MAC values for S1, S2, and S3 were, respectively, 0.024 cm²/g, 0.024 cm²/g and 0.025 cm²/g at 15MeV. The behavior of the MAC typically follows the hierarchy (S3)_MAC > (S1)_MAC > (S2)_MAC. The following are some explanations for the MAC of all glasses trend: The photoelectric effect (PE) cross section varies directly with Z⁴ E⁻³ (Z is the atomic number of the absorbing material) at the lowest photon energy (E), as shown by the MAC's trend. The Compton Scattering (CS) cross section varies straightly with (Z/A) and oppositely with E in the intermediate zone of the photon energy. The cross section of pair production (PP) interaction varies with Z² in the highest photon energy zone. In conclusion, the greater (Z) and denser the absorber, the higher the MACs have values. As a result, S2 glass with the lowest density (3.0078g/cm³) and S3 glass with the highest density (3.0100g/cm³) had the lowest and the highest MAC values, respectively.

The fluctuation of the created glasses' linear attenuation coefficient (LAC) in cm⁻¹ as a function of photon energy (E) is displayed in Fig.15. The curves in Fig.15 show that the LAC for all glasses follows a similar pattern, with S2 glass having the lowest values and S3 glass having the maximum values. Consequently, the LAC inclination moves in the following order: (S3)_LAC > (S1)_LAC > (S2)_LAC.

In relation to the half-value layer (HVL), Fig.16 shows how the HVL changes in cm with photon energy (E) of the produced glasses. The variance of HVL is minimal, and their values incline to be near together, in accordance with the dominance of (PE) cross sections in the low photon energy area. Attributable to the prevalence of both (CS) and (PP) interactions processes, the HVL raised and the values varied more between samples in the middle photon energy zone. While S3 glass, which has a high density of 3.0100g/cm³, performed the deepest HVL values and changed from 0.023cm at 0.015MeV to 9.384cm at 15MeV, S2 glass, which has a low density of 3.0078g/cm³, performed the highest HVL values changes from 0.025cm at 0.015MeV to 9.478cm at 15MeV. As a result, the HVL of the produced glasses behaves differently from the MAC and LAC. Consequently, (HVL)_S2 > (HVL)_S1 > (HVL)_S3. The HVL results show that among all the examined glasses, S3 glass provides the best radiation protection. Figure17 compares three samples of marketable concrete materials (Ordinary concrete (OC), Hematite-Serpentine concrete (HSC), and ILmenite-Limonite concrete (ILC)) with the investigated S1-S3 glasses to show how the HVL of each differs⁵⁶. Figure17 shows that the currently available glasses outperform OC, HSC, and ILC materials as radiation shielding materials.

To explain the shielding characteristics of materials, the effective atomic numbers (Z_eff) parameter is frequently utilized^{13,14,15,16,17}. Figure18 illustrates the difference of the (Z_eff) as a function of photon energy (E) for all created rare earth glasses. The Z_eff trend indicates that adding rare earth oxides to modern glasses has a beneficial effect on raising Z_eff values by the order (S3)_Zeff > (S1)_Zeff > (S2)_Zeff. This could be explained by the Z_eff, which is highly reliant on the glass density. As a result, mixed Ce and Gd doped glass displayed the highest Z_eff factor.