1. Introduction

Rare earth elements (REEs) play an increasingly important role in the transition to a low-carbon economy. In addition, they possess a high quantum efficiency in the visible region [1]. Recovering REEs from secondary sources is remarkably rare due to the scarcity of RE-bearing minerals, which results in a limited supply of REEs on the global market [2]. As a result, waste phosphors have become an urban mining resource from which REEs, such as yttrium (Y), europium (Eu), dysprosium (Dy), terbium (Tb), and cerium (Ce), can be extracted [3]. There are many high-tech applications of REEs. For example, europium-doped barium magnesium aluminate (BaMgAl₁₀O₁₇:Eu²⁺, BAM) is used in various high-resolution devices, including mercury free lamps, field emission displays (FEDs), light emitting diodes (LEDs), and plasma display panels (PDPs). BAM, an attractive blue phosphor, offers excellent chromaticity, chemical stability, and high luminance efficiency under both ultraviolet (UV) and vacuum ultraviolet (VUV) excitations [4–6]. Terbium-doped cerium magnesium aluminate (CeMgAl₁₁O₁₉:Tb, CMAT) has been widely used as the green-emitting component in three-band lamps and has been studied for use in some PDPs [7,8]. In particular, it is used in long-life and high-loading fluorescent lamps due to its high durability under intense UV radiation [9]. Sodium–yttrium–fluoride (NaYF₄), among all RE-doped fluoride host nano-crystals, is preferred for use in medical and biological fields due to the noninvasive deep-tissue penetration of its radiation [10], its low-toxicity [11], and its biocompatibility [12]. Finally, the incorporation of rare-earth ions, i.e., Eu²⁺, Dy³⁺, etc., into the lattice structure of afterglow phosphors both confers upon it the afterglow effect and introduces defects in crystal lattices [13].

Further, gamma radiation shielding features of REEs make them desirable for use in Pb (lead)-free radiation protection aprons due to health, environmental, and economic benefits of using substances that are less toxic than Pb for radiation protection. Moreover, a study of the radiation-shielding features of REEs may aid in the design of new radiation protection materials made of non-toxic and green products. Recently, there has been an increasing demands for non-Pb (or non-toxic) materials that provide radiation protection. Some research has been performed on the high-energy attenuation performance of green and non-toxic materials [14–16]. Basic parameters, such as linear and mass attenuation coefficients, effective atomic number, and effective electron density yield important information about the radiation attenuation properties of these materials when matter and photons interact.

To the best of our knowledge, there are no studies in the literature on the radiation shielding performance of the green materials NaY_0.77Yb_0.20Er_0.03F₄, Ba_0.86Eu_0.14MgAl₁₀O₁₇, Sr_3.84Eu_0.06Dy_0.10Al₁₄O₂₅, Ce_0.63Tb_0.37MgAl₁₁O₁₉, and Sr_0.95Eu_0.02Dy_0.03Al₂O₄. The primary aim of this work was to experimentally evaluate the radiation shielding characteristics of these environmentally friendly products at different energies, i.e., between 81-1333 keV, and compare our results to the theoretical ones obtained using the WinXCOM software.

2. Material and Method

2.1 Experimental Details

The spectroscopic pure materials were in the form of a powder and supplied by Sigma Aldrich (St. Louis, MO, USA). All the materials had a purity of ≥ 99%. The particle size was 37 mm after being sieved to minimize the effects of the powder particle size on the results. To evaluate the gamma radiation shielding effectiveness, pellets with a diameter of 10 mm and thickness varying from 0.08 to 0.128 cm were produced by pressing the powder under a pressure of 10 ton cm^-2 utilizing a manual hydraulic press after precisely determining the mass of the powder using a digital balance with an accuracy of 0.001g. The evaluated samples consisted primarily of O, F, Na, Mg, Al, Sr, Y, Ba, Ce, Eu, Tb, Dy, Er, and Yb. The amounts of these elements in the samples were associated with the coding of NaY_0.77Yb_0.20Er_0.03F₄, Ba_0.86Eu_0.14MgAl₁₀O₁₇, Sr_3.84Eu_0.06Dy_0.10Al₁₄O₂₅, Ce_0.63Tb_0.37MgAl₁₁O₁₉, and Sr_0.95Eu_0.02Dy_0.03Al₂O₄, with densities of 3.46, 2.33, 2.61, 2.15, and 1.96 g/cm³, respectively.

A high purity germanium (HPGe) gamma-ray spectrometric system was used to measure the intensities of high-energy photons from several radioactive point sources: ¹³³Ba (81, 276, 303, 356 and 384 keV), ⁵⁷Co (121 and 136 keV), ¹³⁷Cs (662 keV), ⁶⁰Co (1173 and 1333 keV), ⁵⁴Mn (835 keV), and ²²Na (1275 keV). The diameter and thickness of the HPGe detector were 70 and 25 mm, respectively. The resolutions were 0.380, 0.585, and 1.8 keV at full width and 5.9, 122, and 1330 keV at full width half maximum (FWHM). During the measurements, the detector was kept at a temperature of -196 ^oC by liquid nitrogen (N₂). The system’s energy calibration was performed using the calibration sources ²²Na, ⁵⁴Mn, ^57,60Co, ¹³⁷Cs, ¹³³Ba, ²⁰³Hg, and ²⁴¹Am. A schematic of the experimental setup used in this study is given in Fig. 1. An absorbent material was placed between the radioactive point source and the detector. The data were analyzed with an Ortec Maestro software package program [17,18]. Using the Origin 7.5 program (demo), the net area counts were determined via the least-squares fitting method. Furthermore, the experimental uncertainties in the measurements were calculated according to the following equation [19, 20]:

Fig 1.Full-screen View

Schematic of the experimental system (revised from [38]), where the distances between the gamma ray source and the sample, and the window the of HPGe detector and the sample are 1 cm and 14 cm, respectively.

$\Delta {\mu }_{\text{m}}=\frac{1}{\rho x}\sqrt{{\left(\frac{\Delta I}{I}\right)}^2+{\left(\frac{\Delta I_0}{I_0}\right)}^2+\ln {\left(\frac{\Delta I}{I}\right)}^2{\left(\frac{\Delta \rho x}{\rho x}\right)}^2}$ (1)

where ρ and x represent the density and thickness of the samples, respectively, Δρx represents the uncertainty in mass per unit area, ΔI₀ and ΔI denote the uncertainties for I₀ and I, respectively.

3. Results and Discussion

The experimental values of µ/ρ for NaY_0.77Yb_0.20Er_0.03F₄, Ba_0.86Eu_0.14MgAl₁₀O₁₇, Sr_3.84Eu_0.06Dy_0.10Al₁₄O₂₅, Ce_0.63Tb_0.37MgAl₁₁O₁₉, and Sr_0.95Eu_0.02Dy_0.03Al₂O₄ at several photon energies (ranging from 81-1333 keV) using several radioactive point sources were measured and are listed in Table 1. The experimental µ/ρ value was compared with the theoretical one calculated using the WinXCOM computer software [22] to confirm the accuracy of the experimental results of this study. The experimental and theoretical results were plotted and the graphs are shown in Fig. 2. We see from Table 1 and Fig. 2 that the experimental and theoretical µ/ρ values decrease as the photon energy increases. The trend of µ/ρ with energy can be understood photon interaction mechanisms, namely, the photoelectric effect (PE), Compton scattering (CS), and pair production (PP), as discussed in previous studies [26, 30, 31]. At lower energies, the changes in the attenuation coefficients are due changes in the PE as the energy varies, whereas at medium-energies, they are dependent on changes in the CS effect. Furthermore, NaY_0.77Yb_0.20Er_0.03F_4, which includes two elements with a high Z value (Yb, Z=70 and Eu, Z=68), has the highest µ/ρ value among all of the studied materials. However, NaY_0.77Yb_0.20Er_0.03F₄ is twice as expensive as any of the other materials. Although the mass attenuation coefficient of Sr_3.84Eu_0.06Dy_0.10Al₁₄O₂₅ is lower than those of the other samples, it is the least expensive among the samples. It is seen in Table 1 and Fig. 2 that the experimental and theoretical µ/ρ values are in good agreement. However, the small differences between the experimental and theoretical data are due to uncertainties in the system, thickness of the sample, original intensity (I₀), and attenuated photon intensity (I) [25]. The uncertainties in attenuation coefficient values are less than ± 3.97%.

Fig. 2Full-screen View

(Color online) Mass attenuation coefficients for the investigated materials

Table 2 presents the mass attenuation coefficient values for Al, Fe, and ordinary concrete [32] calculated by WinXCOM for comparison with the experimental values. From Table 1 and Table 2, it can be seen that the µ/ρ values for all studied samples are higher than that of Al in the energy range of 81-511 keV. Moreover, Fe has a higher µ/ρ value than all the experimental samples in the energy range of 81–303 keV. We note that all the samples have higher µ/ρ values than ordinary concrete at low energies (E<356 keV), while the µ/ρ value of ordinary concrete is higher than those of the samples at high energies (E> 384 keV).

To test the effectiveness of any absorber in the attenuation of gamma photons, the RPE (%) was calculated using the measured original (I₀) and attenuated photon intensities (I); our experimental values are listed in Table 3. The graph of RPE versus energy is as shown in Fig. 3. As seen in Table 3 and Fig. 3, the RPE values decrease with increasing photon energy. The RPE values of NaY_0.77Yb_0.20Er_0.03F₄, Ba_0.86Eu_0.14MgAl₁₀O₁₇, and Sr_3.84Eu_0.06Dy_0.10Al₁₄O₂₅ are quite close to each other at high energies (E>200 keV), while NaY_0.77Yb_0.20Er_0.03F₄ has maximum RPE values of 14.75%, 7.45%, 6.55%, and 5.27% at energies of 81, 122, 136, and 161 keV, respectively.

Fig. 3.Full-screen View

The radiation protection efficiencies of the samples versus energy

Fig. 4 contains a plot of the Z_eff of the five samples versus photon energy in the photon energy range of 0.015-10 MeV, obtained using the Auto-Z_eff program [33]. The size of Z_eff values plotted in the figure can be explained by considering the three types of photon scattering in matter. In the lower energy region, the values of Z_eff for all samples are highest, primarily due to the photoelectric interaction mechanism, as seen in Fig. 4, and the effective cross-section for this process depends directly upon the atomic number as Z⁴ and the photon energy as E^−3.5 for any absorber. Apparently, Z_eff reduces exponentially with increasing photon energy up to about 800 keV, and then Z_eff values are almost constant. This is because the cross section for Compton scattering is related to the atomic number and the energy as Z and E⁻¹, respectively. In this high energy region, Compton scattering is the most important interaction process. From Fig. 4, we see that the Z_eff of NaY_0.77Yb_0.20Er_0.03F₄ > Z_eff of Sr_3.84Eu_0.06Dy_0.10Al₁₄O₂₅ > Z_eff of Ce_0.63Tb_0.37MgAl₁₁O₁₉ > Z_eff of Ba_0.86Eu_0.14MgAl₁₀O₁₇ > Z_eff of Sr_0.95Eu_0.02Dy_0.03Al₂O₄. The high Z_eff values of NaY_0.77Yb_0.20Er_0.03F₄ are due to the presence of elements with high atomic number. We note that the graphs of Z_eff values for all samples have discontinuities due to photoelectric absorption near the K-, L- and M-absorption edges of some elements at low energies (see Table 4). These results are in good agreement with the results for various glass systems obtained by El-Mallawany et al. [34], Kaur et al. [35], and Chanthima and Kaewkhao [36] and with the results for several smart polymer materials reported by Sayyed [37].

Fig. 4.Full-screen View

The effective atomic number of the five samples

4. Conclusion

The shielding performances of five compounds containing REEs, namely NaY_0.77Yb_0.20Er_0.03F₄, Ba_0.86Eu_0.14MgAl₁₀O₁₇, Sr_3.84Eu_0.06Dy_0.10Al₁₄O₂₅, Ce_0.63Tb_0.37MgAl₁₁O₁₉, and Sr_0.95Eu_0.02Dy_0.03Al₂O_4, were studied at photon energies between 81-1333 keV. The values of µ/ρ, RPE, and Z_eff were experimentally and theoretically evaluated. The obtained values indicate that Z_eff is dependent on gamma ray energy intensity as well as the interaction mechanism. The Z_eff values peak when the photon energy values are low because the photoelectric effect around the K-, L-, and M- absorption edges of the samples dominates. Results for RPE and Z_eff also revealed that, among the five materials studied, NaY_0.77Yb_0.20Er_0.03F₄ offers the best gamma ray radiation protection. The obtained data may be helpful in various radiation shielding applications.