Enhanced Emission of Tellurite Glass Doped with Pr³⁺/Ho³⁺ and Their Applications

Abstract

The shielding and spectroscopic properties of Pr⁺³ and Pr³⁺/Ho³⁺-codoped tellurite glass were investigated. The intensity parameters (Ω2 = 3.24-, Ω4 = 1.64-, Ω6 = 1.10 × 10⁻²⁰ cm²) as well as the radiative lifetimes of ³F₄ + ⁵S₂ and ⁵I₆ excited states of Ho³⁺ ions were equal to 301 μs and 3.0 μs, respectively. The former value appears to be much higher than that obtained from the lifetime measurement, indicating the presence of various energy transfer processes. The NIR spectrum of Pr³⁺/Ho³⁺-co-doped tellurite glass is dominated by strong Ho³⁺: ⁵I₆ emission at around 1200 nm, being the result of the energy transfer from Pr³⁺ to Ho³⁺ ions. The shielding effectiveness of the prepared glasses showed good performance against high-energy photons. These findings suggest that the prepared glasses could be used in laser technology such as photodynamic therapy (PDT) treatment procedures and as shielding for radiation protection.

1. Introduction

Tellurite glasses with rare earth (RE) doping are promising materials for photonic applications such as solid-state lasers and lighting, optical amplifiers, and medical and sensing technologies. The applications of tellurite glasses are connected with their high transmittance from the visible to mid-infrared ranges (0.4–6 μm), high refractive index (n > 2), and low maximal phonon energy (~750 cm⁻¹) [1,2]. One of the most frequently used dopants for tellurite glasses is the Ho³⁺ ion, which exhibits both visible and infrared emissions due to its unique energy level structure [3]. In particular, these materials are expected to show an emission in the IR region at 1.2 μm, corresponding to the ⁵I₆ → ⁵I₈ transition. In addition, the emissions are also observed at 1.38 and 1.46 μm, for the ⁵F₄+⁵S₂→⁵I₆ and ⁵F₅→⁵I₆ transitions, respectively. Emission from levels ⁵I₆ → ⁵I₈ and ⁵F₄+⁵S₂ → ⁵I₅ of Ho³⁺ ions may be promising for obtaining a signal band gain of 1.2–1.4 μm [4,5,6].

Ho³⁺ions can induce interesting emission and luminescence decays. Therefore, glasses doped with Ho³⁺ions exhibit visible green and red emissions at exited level (⁵F₄, ⁵S₂)→⁵I₈, and ⁵F₅→⁵I₈, respectively [6,7,8,9,10]. The relationship between green and red emission intensities strongly depends on the base matrix and the concentration of Ho³⁺ ions. Host materials containing Pr³⁺ ions have a broader luminescent range than other trivalent rare-earth ions [10,11]. Glasses that are co-doped with holmium and praseodymium have been studied as sources of applications of lasers in the mid-infrared and white light radiation. Double doping with Pr³⁺ and Ho³⁺ ions is mainly used to increase mid-infrared emissions. As for NIR emission, which is usually weak for Ho³⁺-only doped materials, a significant increase in PL intensity can be achieved by co-doping with Pr³⁺ ions, as signaled in Ref. [12]

In addition to the enhanced luminescence properties, the tellurite glasses have been widely investigated for different medical radiation applications such as shielding material and photodynamic therapy (PDT) treatment procedures [13,14,15,16,17,18,19,20,21,22]. For use in photodynamic therapy (PDT) treatment procedures and medical radiation technologies, laser glasses doped with rare earth elements such (Pr³⁺, Sm³⁺, Ho³⁺, and Tm³⁺) have been developed [20,21,22]. Light sources for PDT include incandescent lamps and xenon lamps with appropriate filters. However, their progress and use are limited by the relatively low light intensity of these sources, making lasers, which can be precisely directed and produce strong light, the preferred illumination method of choice. For application as a safety replacement device in medical workplaces such as PDT, X-ray, and atomic projects in the area of innovations, V-ray equipment, gamma camera rooms, and computed tomography (CT) examination workplaces, researchers analyzed the lasing and shielding qualities of such glasses [21,22]. According to J. Yang et al. [21], the radiant flux and quantum yield for the red fluorescence of Pr³⁺ is calculated to be 219W and 11.80%, respectively, under the commercial blue LED illumination. The excitation band of the majority of photosensitizers (PS) matches the wavelength range of 85.24% of the fluorescence photons in the visible region, showing significant potential for photodynamic therapy (PDT) treatment and clinical studies [21].

The aim of this study is to investigate spectroscopic analyses of novel, multicomponent tellurite glasses co-doped with Pr³⁺/Ho³⁺, to be employed as a laser material in radiology rooms for photodynamic therapy surgery (PDT). Moreover, the shielding effectiveness of the prepared glasses has been investigated due to the advantages of tellurite glasses doped rare earth ions as shielding material.

2. Experimental Work

The core six tellurite glass samples were prepared by melting 25 g batches of highly pure (99.99%) chemicals in gold crucibles at 850 °C in an ambient atmosphere. The resulting material has the molar composition 78TeO₂- 10Nb₂O₅- 5PbO- 1PbF₂- 5Li₂O-1La₂O₃. To prevent vaporization losses, a platinum plate was placed on top of the crucibles. While melting, the melts were periodically agitated to prevent inhomogeneity. The melts were then placed onto plates that had been warmed to 400 °C, generating layers that were a few mm thick, and these layers were then annealed between 320 and 340 °C. The concentrations of lanthanide ions (N_Ln) of Pr₂O₃ and Ho₂O₃ incorporated into the host matrix have been calculated by the equation reported in Ref [23]. The density of the prepared glasses was determined using Archimedes’ law [23]. The glass samples were sliced and polished to a size of around 5’5’2 mm³ for spectroscopic measurements. The M-2000 Woollam ellipsometer was used to collect the ellipsometric data, and the Perkin Elmer Lambda 900 spectrophotometer was used to capture the transmittance and reflectance spectra. The luminescence decay curves were obtained after a short pulse stimulation delivered by an optical parametric oscillator powered by a third harmonic of a Nd:YAG laser [24]. The photoluminescence spectra were measured using an Optron Dong Woo fluorometer system.

Table 1. The sample codes, compositions, molar weight, experimental densities, and the calculated concentrations of lanthanide ions of the investigated samples.

Sample Code	Glass Composition in Mol%	Molar Mass (g/mol)	Density (g/cm3)	NLn (1020 cm−3)
T0	78TeO2-10Nb2O5-5PbO-5Li2O-1La2O3-1PbF2	169.43	5.80	-
TPr	78TeO2-10Nb2O5-5PbO-5Li2O-1La2O3-1PbF23-Pr2O3	171.85	5.93	NPr = 6.30
TPrHo	78TeO2-10Nb2O5-5PbO-5Li2O-1La2O3-1PbF2-1.5Pr2O3-1.5Ho2O3	172.04	5.94	NPr = 3.16 NHo = 2.76

shows the sample code, composition, molar mass, density, and concentrations of lanthanide ions. The shielding parameters of the prepared samples such mass and linear attenuation coefficients (MAC and LAC), half-value layer (HVL), and mean free path (MFP), were investigated using MIKE software [25].

Shielding Properties

The possibility of photons interacting with a barrier is characterized as the mass attenuation coefficient (MAC) of a shielding material, and it is expressed as follows for a combination of elements or any chemical molecule [26].

$$\mathrm{MAC}={ \mathsf{\mu }}_{\mathrm{m}}={\displaystyle \sum }_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{w}}_{\mathrm{i}}{(\frac{\mathsf{\mu }}{\mathsf{\rho }})}_{\mathrm{i}}$$

(1)

where w_i represents the fractional weight of individual components in each compound, and $\rho$ indicate the density of material, µ/$\mathsf{\rho }$ is mass attenuation of the individual components. The LAC can then be estimated by using the following relationship:

$$\mathrm{LAC}=\mathrm{MAC} \times \mathsf{\rho }$$

(2)

The average distance a photon can go through the barrier without interacting is known as the mean free path (MFP), and it is determined by the reciprocal of the linear attenuation [27]:

$$\mathrm{MFP}=\frac{1}{\mathrm{LAC}\left(\mathsf{\mu }\right)}$$

(3)

The thickness of the interaction target at which the attenuated intensities account for 50% of the narrow photon beam intensity is defined by the half-value layer (HVL). The necessary thickness of shielding material is inversely proportional to the HVL value. The HVL can be calculated using the following equation [27]:

$$\mathrm{HVL}=\frac{\ln 2}{\mathrm{LAC}\left(\mathsf{\mu }\right)}=\frac{0.693}{\mathrm{LAC}\left(\mathsf{\mu }\right)}$$

(4)

3. Results and Discussion

3.1. Absorption Spectra and Judd–Ofelt Analysis

Figure 1 shows the net absorption bands of Pr³⁺ and Ho³⁺ ions, obtained by subtraction of the monotonic background absorption of the glass matrix (shown for sample T0 in [28]) from the total absorption determined for samples TPr and TPrHo.

As seen in Figure 1, the spectra of samples TPr and TPrHo show the existence of several single absorption bands attributed to transitions from the ground states of both ions, Pr^3+:3H₄ and Ho^3+:5I₈, to subsequent excited states, as indicated elsewhere [29].

In addition, the spectra of sample TPrHo show overlapping absorption bands associated with transitions to excited levels of Ho^3+:5I₇ and Pr^3+:3F₂ in the NIR region as well as transitions to levels of Ho^3+:5G₆ and Pr^3+:3P₂ in the visible region.

One can see in Figure 1 five Ho³⁺ absorption bands, that do not overlap with Pr³⁺ bands allowing one to perform the standard J-O analysis for the former ion. However, it appears that such an analysis results in a negative, unphysical value of the Ω₂ J-O intensity parameter, due to the neglect of the strong Ho³⁺:⁵G₆ absorption band. In order to include this band, we have subtracted from the sample TPrHo absorption a contribution from the Pr³⁺ absorption, which is equal to half of that measured for sample TPr, considering the difference in Pr³⁺ ion concentrations between the two samples as shown in Table 1. For completeness’ sake, we have also extracted the Ho^3+:5I₇ absorption band, and both bands are shown in Figure 2.

It appears that the extracted absorption bands from Figure 2 are very similar to those determined for a single Ho³⁺-doped tellurite glass [30], justifying the extraction procedure.

Finally, we have applied the standard Judd–Ofelt theory to six electric dipole transitions specified in

Table 2. The experimental and calculated oscillator strengths and Judd–Ofelt intensity parameters for Ho³⁺ ions in sample TPrHo.

Transition from 5I8 to:	Energy (cm−1)	Oscillator Strength f (10−6)
		fexp	fcal
5I6	8564	0.74	0.95
5I5	11,178	0.18	0.18
5F5	15,427	3.02	2.94
5S2+5F4	18,501	4.13	3.73
5G6	22,051	22.76	22.76
5G5	23,887	3.43	3.56
Ω2 = 3.24-, Ω4 = 1.64-, Ω6 = 1.10 × 10−20 cm2; δrms = 0.19 × 10−6 Ω2 = 6.72-, Ω4 = 2.21-, Ω6 = 8.74 × 10−20 cm2 [24] Ω2 = 1.24-, Ω4 = 7.00-, Ω6 = 2.80 × 10−20 cm2 [28]

, calculating their oscillator strengths by numerical integration of the corresponding absorption bands; in order to minimize the root-mean-square deviation, the transition ⁵I₈→⁵I₇ has been omitted in the analysis due to a significant, magnetic dipole contribution to the oscillator strength and uncertainty connected with the extraction procedure [31]. The oscillator strength values of the prepared glass were calculated using Equation reported in Ref [23].

The refractive index dispersion of samples T0 and TPr, obtained from ellipsometric data, is presented in our previous work [28], as being very well described by the Sellmeier model of the form n(λ) = [A + Bλ²/(λ² − C²) − Dλ²]^1/2, where A, B, C, and D are the fitting parameters. Adopting the same procedure for sample TPrHo, the values of the fitting parameters have been found as A = 2.729, B = 1.764, C = 244.16 nm, and D = 2.52 × 10⁻⁹ nm⁻². As for the reduced matrix elements for the electric dipole transitions, we have used the data from Ref. [32]. Table 2 shows the calculated Judd–Ofelt parameter values along with the root-mean square deviation (δ_rms).

The obtained values of the Ω_i (I = 2,4,6) parameters follow the same pattern as those observed for different Ho³⁺ -doped tellurite glasses and follow the same sequence, namely Ω₂ > Ω₄ > Ω₆ [29]. On the other hand, the sequence is different with Tellurite glasses doped with Pr⁺³ [24,28].

The J-O parameters have been used to estimate the transition probability W_r of ⁵I₆ (323 s⁻¹), ⁵S₂ (4549 s⁻¹) and ⁵F₄ (2060 s⁻¹) excited states of Ho³⁺ ions and subsequently the total radiative lifetimes τ_rad = 1/W_r. The levels ⁵S₂ (lower) and ⁵F₄ (higher) are very close to each other, leading to one absorption band, as shown in Figure 1. The occupation of these levels is governed by the Boltzmann factor, exp(-ΔE/k_BT), where ΔE is the energy separation between the levels, k_B is the Boltzmann constant, and T is temperature. To calculate ΔE, we have fitted two Gaussians to the (⁵S₂ + ⁵F₄) absorption band, which yields ΔE = 128 cm⁻¹. The common transition probability for these levels is given by [33]

$${\mathrm{W}}_{\mathrm{r}}\left( {\mathrm{J}}^{\prime }+\mathrm{J}″\right)=\frac{\left(2{\mathrm{J}}^{\prime }+1\right){\mathrm{W}}_{\mathrm{r}}\left( {\mathrm{J}}^{\prime }\right)+\left(2 \mathrm{J}″+1\right){\mathrm{W}}_{\mathrm{r}}\left( \mathrm{J}″\right)\exp \left(-\Delta \mathrm{E}/{\mathrm{k}}_{\mathrm{B}}\mathrm{T}\right)}{\left(2{\mathrm{J}}^{\prime }+1\right)+\left(2 \mathrm{J}″+1\right)\exp \left(-\Delta \mathrm{E}/{\mathrm{k}}_{\mathrm{B}}\mathrm{T}\right)},$$

(5)

where J′ = 2 and J″ = 4 are the total momenta of ⁵S₂ and ⁵F₄ excited levels, respectively.

3.2. Emission Spectra

In addition, the PL signal of both samples (in arbitrary units) has been divided by N_Pr; to illustrate the impact of Pr³⁺ ion concentration (N_Pr) on the emission spectra as shown in Table 1. Emission spectra were obtained under identical conditions, permitting comparison of their relative intensities.

A significant number of excited states of Pr³⁺ and Ho³⁺ ions and their relatively high concentrations in the investigated samples create many possible radiative and non-radiative energy transfer (ET) processes between the Pr-Pr, Ho-Ho, and Pr-Ho pairs as discussed below. Figure 3 shows the visible PL spectrum for samples TPr and TPrHo under 445 nm (22,472 cm⁻¹) excitation, corresponding to Pr³⁺: ³H₄→³P₂ and Ho³⁺: ⁵I₈→⁵G₆ absorption bands, as shown in Figure 1.

When comparing the emission spectra from Figure 3, it is clear that the spectrum of sample TPrHo consists of many Pr³⁺ emission bands that are completed with two Ho³⁺ bands, namely with a relatively strong (⁵F₄+⁵S₂)→⁵I₈ emission at 18,290 cm⁻¹ and a weak (⁵F₄+⁵S₂)→⁵I₇ emission at 13,250 cm⁻¹. With the exception of the ³P₀→³H₆ and ³P₀→³H₄ bands, many Pr³⁺ emission bands are generally proportional to NPr, indicating that the concentration has been quenched by cross-relaxation and energy migration processes [30,34]. Similar cross-relaxation pathways result in numerous transitions between the energy levels of this ion in glasses doped exclusively with Ho³⁺ ions [30,35,36,37,38]. The energy level diagram of Ho⁺³/Pr⁺³ ion codoped in the present host matrix were shown in Figure 4.

Figure 5 displays the NIR emission spectra of the samples TPr and TPrHo, normalized as scale from the visible spectra in Figure 3.

For the sample TPrHo, the weak overlapping emission bands of prepared glasses doped with Pr³⁺ and Pr³⁺/Ho³⁺ ions in the 6000–8000 cm⁻¹ region were attributed. Furthermore, beyond 9000 cm⁻¹, there is another overlapping emission band of present glasses that appeared at 9648 cm⁻¹. The sample TPrHo spectra is characterized by strong emission at 8386 cm⁻¹ attributed to Ho³⁺: ⁵I₆ transition. When comparing the emission of sample TPrHo to the transition Pr³⁺:³P₀→¹G₄ emission from sample TPr at 10,735 cm⁻¹, the energy transfer (ET) from Ho³⁺ to Pr³⁺ ions is observed. After 445 nm, the photons excite the Pr³⁺:³P₂ level, and there is a non-radiative transition to the ³P₀ level, followed by cross-relaxation and the ¹G₄ level as an intermediate state. This mechanism populates the Ho³⁺:⁵I₆ level by the pathways Pr³⁺(³P₀→¹G₄)→Ho³⁺(⁵I₈→⁵I₆) and Pr³⁺(¹G₄→³H₄)→ Ho³⁺(⁵I₈→⁵I₆). The Ho³⁺:⁵I₆→⁵I₈ emission, which is preceded by the non-radiative and radiative transitions ⁵G₆→(⁵F₄+⁵S₂) and (⁵F₄+⁵S₂)→⁵I₆, is amplified by this down-conversion mechanism [35].

3.3. Lifetime

Table 3. Lifetimes of Pr³⁺ and Ho³⁺ emission bands obtained experimentally (τ_eff) as well as the calculated values using the J-O approach (τ_rad).

Sample Code	Pr3+		Ho3+ and Pr3+
	3P0 Lifetime		(3F4+5S2) Lifetime		5I6 Lifetime
	τeff (μs)	τrad (μs)	τeff (μs)	τrad (μs)	τrad (μs)
TPr	2.47 [28]	9.43 [28]	-	-	-
TPrHo	2.75	10.01	5.49	301	3.09

includes the lifetimes of the ³P₀ excited state of the Pr³⁺ ions as well as the predicted lifetime values and experimental data in order to compute the ET efficiency from these ions to Ho³⁺ ions. The ³P₀ Lifetimes of TPr are taken from our previous work [28]. The (³F₄+⁵S₂) lifetime of TPrHo was measured and calculated as shown in Table 3. The ⁵I₆ lifetime is only calculated in the present study.

Figure 6 and Figure 7, respectively, show the excited levels of Ho³⁺:(⁵F₄ + ⁵S₂) and Pr³⁺:³P₀’s PL decays. Since the decay data are non-exponential, we have established that the experimental lifespan is equal to the effective lifetime, which is defined as follows:

$${\mathsf{\tau }}_{\mathrm{eff}}=\frac{\int \mathrm{t} \times \mathrm{I}\left(\mathrm{t}\right)\mathrm{dt}}{\int \mathrm{tI}\left(\mathrm{t}\right)\mathrm{dt}}.$$

(6)

There are numerous potential energy transfer mechanisms between the ions when Pr³⁺ and Ho³⁺ ions are present in sample TPrHo at relatively high and comparable concentrations. As a result, the measured lifetime of Ho³⁺:(⁵F₄ + ⁵S₂) emission is significantly shorter (5.49 μs) than that predicted from the J-O analysis (301μs; Table 3).

As was mentioned before while describing the sample’s emission spectra, sample TPr has a high concentration of Pr³⁺ ions, which results in an effective cross-relaxation (CR). When compared to the J-O radiative lifetime (9.43 μs [28]), the Pr³⁺:³P₀ effective lifetime is significantly reduced by the CR mechanism, going down to τ_eff = 2.47 μs. The concentration of Pr³⁺ ions in sample TPrHo is two times lower than in sample TPr, so one would expect that τ_eff would be significantly higher and equal to 3.92 μs [28]. However, when the former sample is codoped with Ho³⁺ ions, an additional energy transfer (ET) occurs between the two ions, which reduces the Pr³⁺:³P₀ lifetime to 2.75 μs.

The ET efficiency ($\eta$) from Pr³⁺ to Ho³⁺ ions can be estimated using the following equation, calculated as [29]

$$\eta =1-\frac{{\tau }_{Pr-\mathrm{Ho}}}{{\tau }_{Pr}}$$

(7)

where τ_Pr-Ho and τ_Pr are the effective lifetimes with and without Ho³⁺ co-doping, respectively, for the same concentration of Pr³⁺ ions. Using the value τ_Pr = 3.92 μs, corresponding to N_Pr =3.16×10²⁰ cm⁻³, we have obtained the ET efficiency $\eta$ = 29% that can be compared with the values 19 and 32.3% found for tellurite glasses doped with 1 mol% of Pr³⁺ ions, and codoped with 0.5 and 1 mol% of Ho³⁺ ions, respectively [28]. The observed increase of ET efficiency with the increasing N_Ho/N_Pr ratio is in accordance with the theoretical modeling of quantum cutting via a two-step ET [39].

3.4. ⁵I₈→⁵I₆ Transition

The absorption cross-section (ACS) and emission cross-section (ECS) of the ⁵I₈↔ ⁵I₆ transition are the key characteristics for use in optoelectronics, thus we will focus on them.

The absorption cross-section (ACS; σ_abs) of the ⁵I₈→⁵I₆ transition was determined in the range of 1100–1250 nm by dividing the absorption coefficient by the Ho³⁺ ion concentration. This makes it possible to use the McCumber formula to determine the emission cross-section (ECS; τ_em) of the ⁵I₆→⁵I₈ transition [40].

$${\mathsf{\sigma }}_{\mathrm{em}}^{\mathrm{MC}}\left(\mathsf{\lambda }\right)=\exp \left(\frac{{\mathrm{E}}_{\mathrm{mt}}}{{\mathrm{k}}_{\mathrm{B}}\mathrm{T}}\right){\mathsf{\sigma }}_{\mathrm{abs}}\left(\mathsf{\lambda }\right)\exp \left(-\frac{\mathrm{hc}}{{\mathsf{\lambda }\mathrm{k}}_{\mathrm{B}}\mathrm{T}}\right),$$

(8)

where h is the Planck constant., E_mt is the mean transition energy between the ⁵I₈ and ⁵I₆ levels, which can be calculated by averaging the barycenter energies of the absorption and emission spectra. This results in E_mt = 8519 cm⁻¹.

As for the measured ⁵I₆→⁵I₈ emission, it is given in arbitrary units in Figure 5. To scale this emission, we have used the equation for ECS following the Füchtbauer–Ladenburg (F-L) approach and given by (see Ref [9])

$${\mathsf{\sigma }}_{\mathrm{em}}^{\mathrm{F}-\mathrm{L}}\left(\mathsf{\lambda }\right)=\frac{{\mathsf{\lambda }}^5{\mathrm{W}}_{\mathrm{r}}\mathsf{\beta }}{8{\mathsf{\pi }\mathrm{cn}}^2}·\frac{\mathrm{I}\left(\mathsf{\lambda }\right)}{\int \mathsf{\lambda }\mathrm{I}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }},$$

(9)

with the branching ratio β = 0.91 and W_r = 323 s⁻¹ that were calculated for ⁵I₆ → ⁵I₈ emission from the J-O analysis.

The ACS and ECS spectrums were calculated using Equations (8) and (9). The calculated spectrum is shown in Figure 8.

As shown in Figure 8, there is a relatively small shift (107 cm⁻¹) between the peak value of ACS at 1167 nm (10,246 cm⁻¹) and that of ECS at 1181 nm (10,163 cm⁻¹). The McCumber and Füchtbauer–Ladenburg (F-L) curves are quite similar in shape but very different in magnitude; the peak value of ${\sigma }_{em}^{F-L}$(4.63 × 10⁻²¹ cm²) is about two times greater than that of ${\sigma }_{em}^{MC}$ ($2.18$× 10⁻²¹ cm²), suggesting effective ET from Pr³⁺ to Ho³⁺ ions, since the MC curve reflects emission of Ho³⁺ ions in the absence of Pr³⁺ ions, while the F-L curve represents the emission enhanced by codoping with the latter ions [39,40].

3.5. Shielding Properties

Over a broad energy range, ranging from 0.5 to 15 MeV, the shielding effectiveness of the produced glasses was examined. Using MIKE software, the radiation parameters of the investigated glasses were estimated. The mass attenuation coefficient and linear attenuation coefficients were investigated in the intermediate and high photon energy ranges, ranging between 500 keV and 15 MeV. The results of the mass attenuation coefficient and linear attenuation coefficients of the prepared glasses were compared to those of commercially available standard materials coded RS-253 G18, RS-520, and RS-360 [41], as shown in Figure 9a,b. As illustrated in Figure 9, the values of MAC and LAC decrease slowly as the photon energy increases. This trend is in fact due to the dominance of Compton scattering, which is directly proportional to the atomic number and inversely proportional to the photon energy [42]. As the photon energy increases above 5 MeV, a slight increase in the MAC and LAC is observed. This trend is mainly due to the contribution of the pair production process, which is directly proportional to the square of the atomic number and directly proportional to the photon energy [43]. For example, as shown in

Table 4. The recoded MAC, LAC, HVL, and MFP values of the prepared glasses at 6 MeV.

Shielding Parameters	T0	TPr	TPrHo	RS253G18	RS-360	RS-520
MAC (cm2/g−1)	0.03377	0.03394	0.03400	0.02741	0.04080	0.04326
LAC (cm−1)	0.19588	0.20125	0.20195	0.06907	0.14689	0.22493
HVL (cm)	3.53793	3.44344	3.43155	10.03314	4.71772	3.08094
MFP (cm)	5.10524	4.96889	4.95173	14.47783	6.80768	4.44580

, the prepared glasses’ recoded LAC values at 6 MeV are 0.19588, 0.20125, and 0.20195 for T0, TPr, and TPrHo, respectively. As recoded, there is a slight increase in MAC and LAC values using different dopants; sample TPrHo recorded the highest values among the other prepared samples. In comparing the LAC values of the prepared glasses with some of the standard materials, namely RS-253 G18, RS-360, and RS-520, significant shielding performance was observed for the prepared glasses over the standard materials RS-253 G18 and RS-360 (0.069071 and 0.146893 at 6 MeV). On the other hand, the prepared glasses show a slightly lower performance compared with RS-520, this is obviously due to the higher lead oxide content in the RS-520 (70%) compared to 5% lead oxide in sample TPrHo (5%) in Table 4. The recoded MAC and LAC values of the prepared glasses at different energies range between 500 keV and 15 MeV. The good performance of the prepared glasses in terms of good optical, physical, and shielding properties such as good thermal stability, chemical durability, high values of linear and nonlinear refractive index, and shielding effectiveness are all due to the fact that the tellurite-based glass doped with suitable metal oxides and rare earths can form a high-efficiency glass material that can be used in different applications. [2,12,14].

The HVL represents the absorbance thickness necessary to halve the photon intensity. The mean free path (MFP) is the average distance a photon travels before colliding with a particle in a medium. The HVL and MFP values show how well the gamma radiation is slowed down by the shielding glass material. The HVL and MFP are inversely proportional to the shielding material’s linear attenuation coefficient (LAC); the lower the value, the more effective the material as a shield. The HVL and MFP of the prepared glasses are illustrated in Figure 10a,b. As shown in Figure 10a, there are two fundamental features of the HVL: First, the HVL of all prepared glasses increases with the increasing energy until reaching 6 MeV, it then decreases slowly with the increasing photon energy. For instance, as shown in Table 4, the recorded values at 6 MeV are equal to 3.54, 3.44, 3.43, 10.0, 4.71, and 3.08 cm for T0, TPr, TPrHo, RS254G18, RS360, and RS520, respectively. The recorded increase in HVL is caused by the decrease in the probability of interaction with high-energy photons, which increases the probability of penetrating the samples and necessitates a thicker sample to absorb the same amount of radiation. The second property of the HVL is that it decreases as the absorber density increases; at 6 MeV, the order of the HVL results is as follows: RS520 < TPrHo < TPr < T0 < RS360 < RS254G18. As discussed before, the prepared glasses have LAC values higher than standard materials RS360 and RS254G18, which explain the lower values recorded for the prepared glasses compared with the standard materials. Due to the toxicity of lead oxide, the prepared glasses have a better likelihood of being utilized as an alternative shielding material in medical applications such shielding glass windows and shielding materials used directly on patients undergoing X-ray examinations.

The shielding effectiveness of the prepared glasses can also be investigated in terms of radiation protection efficiency (RPE) [44].

$$\mathrm{RPE}\%=1-{\mathrm{e}}^{-\mathsf{\mu }\mathrm{x}}$$

(10)

Figure 11 shows the RPE percentage of the prepared glasses with a thickness of 10 cm at photon energies ranging between 0.1 and 10 MeV. As shown in Figure 11, the RPE decreases with the increasing energy. For instance, the RPE percent decreased from 100 to 87.2% for T0, 100 to 87.9% for TPr, 100 to 88% for TPrHo, from 100 to 45.4% for RS-253G18, from 100 to 79.3% for RS-360, and from 100 to 91.5% for RS-520. As shown in Figure 11, the prepared glasses have good shielding efficiency compared to RS-254G18 and RS-360 and are slightly lower than RS-520. The 10 cm thickness of the prepared glasses has a shielding efficiency above 90% for energies up to 10 MeV.

4. Conclusions

The spectroscopic properties of Pr³⁺-doped and Pr³⁺/Ho³⁺-co-doped multicomponent tellurite glass were investigated. Analysis of experimental data shows that there is energy transfer (ET) from Pr³⁺ to Ho³⁺ ions, which increases the emission of the latter ions by a factor of two at 1200 nm. The Pr³⁺−Ho³⁺ down-conversion ET, realized by two successive processes by cross-relaxation (CR), with the Pr³⁺:¹G₄ level as an intermediate state, is the cause of the extra emission, which was seen under the 445 nm illumination. This mechanism populates the Ho³⁺:⁵I₆ level by Pr³⁺(³P₀→¹G₄)→Ho³⁺(⁵I₈→⁵I₆) and Pr³⁺(¹G₄→³H₄)→ Ho³⁺(⁵I₈→⁵I₆) pathways, with the excess energies dissipated in the glass matrix. A relatively low ET efficiency (29%) obtained for our Pr^3+/Ho³⁺ co-doped tellurite glass can be significantly increased by optimizing the concentrations of both ions. The results of luminesces properties of the prepared glasses showed good performance as a laser source for photodynamic therapy (PDT) treatment procedures. In addition to that, the prepared glasses can also be considered as shielding material.