Impact of Ag2O on the mechanical and shielding features of ZnO- Er2O3-TeO2 glasses

Jamila S. Alzahrani Princess Nourah bint Abdulrahman University NAZIRUL NAZRIN SHAHROL NIDZAM (  nazirulnazrin@ymail.com ) Universiti Putra Malaysia M. K. Halimah Universiti Putra Malaysia K. Mahmoud Ural Federal University M. I. Sayyed Isra University M.Y. Han Ural Federal University M. K. Izzaty Imperial College London Z.A. Alrowaili Jouf University M. S. Al-Buriahi Sakarya University Imed Boukhris King Khalid University Imen Kebaili King Khalid University


Introduction
Gamma-ray shielding is a very important topic across a wide range of technologies in industries ranging from medical imaging and healthcare to radiotherapy, research facilities and nuclear energy. Several principles have been developed to control the exposure to radiation doses, and one of the most popular principles is ALARA which strives to reduce the exposure of the radiation to the environment and humans [1,2]. Considering the social and economic factors, consulting with a protection materials expert is extremely important to develop e cient solutions to the shielding needs. The choice of shielding material depends on the kind of radiation that is present. The atoms of the shielding materials interact differently with different kinds of radiation. For instance, lead is capable to shield gamma photons e ciently, but it is quite ineffective for shielding from neutrons. In general, dense materials such as lead, tungsten, heavy metal oxide glasses and certain types of alloys are the strongest in abating the gamma photons. Some types of concretes and polymers are also good materials for blocking neutron radiation [3][4][5][6][7][8][9]. Practically, selecting the suitable material allow the manufacturer to utilize a smaller amount of material which can save space. For example, a glass shield with relatively high density would only occupy a quarter or half of the space occupied by a commercial glass with similar attenuation ability. Transparency of a medium is also an important factor when choosing the shielding material for eyeglasses or window glasses used in the X-ray rooms. Other important factors to be considered when selecting the shielding materials are the environmental conditions, the cost of the materials, ease of shipping, handling, and installation. When planning to protect people and workers in the medical and industrial elds from radiation, the attenuation properties of different materials need to be known and the most appropriate material that provides the appropriate radiation protection should be chosen. The aforementioned factors must be considered when selecting the right medium as accurate determination of the shielding factors for different materials is an important part of any radiation shielding plan [10][11][12]. Cheap and eco-friendly glasses, with interesting optical and physical features, are a good choice to manufacture novel radiation shielding products. Different researchers are trying to develop a variety of thicknesses and sizes of several glass systems that is outstanding in providing radiation protection [13][14][15][16][17][18][19]. Radiation shielding glasses are used extensively in dental clinics, radiation therapy rooms, operating theatres, laboratories, veterinary clinics and different materials testing.
TeO 2 glasses, one of the most common glasses, hold technological and scienti c importance because of their distinctive features which include good chemical durability, high refractive indices, interesting optical properties, good semiconducting properties and high dielectric constant. Also, it is reported that TeO 2 glasses have relatively high density and thus possess a good attenuation ability against gamma radiation [20][21][22]. During the preparation process of the TeO 2 -based glasses, some glasses modi ers such as ZnO and Ag 2 O are used to enhance their optical features and radiation attenuation competences [23,24]. Introducing the Er 2 O 3 ions enhances the chemical durability of glass as well as the radiation shielding features. Therefore, the chance for the TeO 2 glasses with Er 2 O 3 to be used in several applications elds such as glass bers optoelectronics and medical applications is promising [25,26].
One of the most extensively used techniques for the evaluation of radiation shielding glasses performance is Monte Carlo simulation (MC) [27][28][29]. MC is an excellent method to study the physical and radiation shielding parameters in case the experimental preparations are di cult or limited to reach. Accordingly, this investigation is aimed to detect the mechanical, and radiation shielding features of a glass system containing TeO 2, ZnO, Er 2 O 3 and Ag 2 O compounds. The Makishima-Mackenzie (M-M) model is utilized to examine the mechanical features. Moreover, the Monte Carlo N-particle transport code (MCNP-5) was applied to determine the properties of radiation shielding for the studied glass system.

Mechanical properties
Studying the hardness and elastic properties of the glasses is vital to achieving proper implementation of shielding materials. Therefore, the mechanical features and elastic moduli were theoretically determined for the glass system containing the TeO 2 , ZnO, Er 2 O 3 and Ag 2 O compounds with different ratios using the M-M model. The chemical composition (mol %), density (ρ, g/cm 3 ), and molar volume (V M , cm 3 /mol) of the synthetic glass samples were tabulated in Table 1 [30]. Beginning from the constating compounds dissociation energy Gi, the total dissociation energies (Gt, kJ/cm 3 ) were computed for the examined glass samples. The packing factor (Vi, cm 3 /mol) was also estimated for the studied TZEAg glasses. The packing density (V t ) depends on The molecular weight (M W , g/mol), the molar fraction (x i ), and the predicted values for V i where V t = (ρ/Mw) ∑v i x i . Then, the elastic moduli Young (Y, GPa), bulk (B, GPa), shear (S, GPa), and longitudinal (L, GPa) were computed by utilizing the expected values of Vt and Gt, where Y=2V t G t , B=1.2 Vt Y, S=(3EB)/(9B-E), and L=B+0.75S. Furthermore, the Poisson ratio (σ) estimated from σ = 0.5-0.1388V t and the micro hardness (H, GPa) based on the calculated σ values, where H= (1-2 σ)/ (6(1 + σ)). The softening temperature (Ts), ultrasonic velocities (V l and V s ), and fractal bond connectivity (d) were estimated as well [31].

Shielding capacity
The MCNP-5 code was employed to detect the protection factors of the TZEAg glass samples. The released photons average track length (ATL) was simulated in the energy interval between 0.240 and 1.408 MeV. An input le was built to reach the essential target. An input le was created to achieve the required target. Figure 1 exhibited the 3D geometry that represents the generated input le. The 3D geometry demonstrates a large lead cylinder with a maximum of 500 mm and a diameter of 200 mm. This cylinder is applied to block the photons from leaving outside the geometry and shield geometry from the surrounding background radiations. Inside this big cylinder, the source of radioactive was installed in the center of this cylinder at point (0, 0, 0). The type of source, radioactivity distribution, dimensions, and emission direction were added to the source card (SDEF). The photons released by the radioactive source were directed to the TZEAg glass samples utilizing a cylindrical collimator of lead with the highest of 70 mm and diameter of 10 mm. The collimator includes a vertical slit with a diameter of 10 cm to collimate the emitted photons. The chemical composition and density of the examined glass samples were noted in Table 1. The glass samples were presented to the input le as a small cylinder with a diameter of 15 mm and various thicknesses. The detector applied in the immediate simulation has a type of F4 which is tally to predict the number of photons incident per unit detector cell. The NPS was set up to prevent the interaction after 106 historical. The MCNP-5 code employs the primary cross-section data sources ENDF, ACTI, ENDL, ACTI, and T-16 les [32].
The simulated ATL was transferred to the linear attenuation coe cient (LAC, µ). Based on the LAC for the synthesized TZEAg glasses, the transmission rate (TR) was calculated to describe the ratio of photons penetrated the glass thickness, where TR= (1-(I/I o )). Io and I represent the gamma-ray intensities' values before and after passing the glass thickness [33,34].

Mechanical and physical properties
The fabricated glass of ρ (cm 2 /g) and V m (cm 3 /mol) were plotted against the Ag 2 O insertion content, as illustrated in The measured values of ρ, molecular weight (M, g/mol), and heat of formation (enthalpy) for the glass constating compounds were used to evaluate the constating compounds dissociation energy (G i , kJ/cm 3 ). After that, the total dissociation energy (G t , kJ/cm 3 ) was calculated, where G t =∑x i G i , x i represents the constituting compound's molar fraction. The packing factor (V i , cm 3 /mol) was also calculated based on each constituting element's ionic radius. The change in V i and G t with the Ag 2 O insertion ratio was presented in Figure 3.  Table 2. The data listed in Table 1 showed agreement between the experimental and M-M model results. This con rms the ability of the M-M model to predict the mechanical properties of such glass systems. The values of σ and H are calculated based on the elastic moduli and displayed in Figure 5.  closed to 3. Thus, the fabricated glasses have a 3D layer structure, as reported in ref [37]. In this regard, the evaluated d values were compared to those based on experimental measurements. There is an agreement between the theoretical and experimental d values for glass samples TZEAg1, TZEAg3, and TZEAg4. Still, there is a disagreement for glasses TZEAg2 and TZEAg5, where the experimental measurement showed that these glasses are 2D layer structure glasses.

Shielding features of the studied glass
In recent years the different materials are developed for radiation protection. Among these materials, the researchers are fabricated and modi ed several glasses forms to enhancement in their ability to attenuate the gamma and neutron radiation. The ability is measured and detected via many shielding factors such as linear attenuation coe cient (LAC), transmission rate (TR), half-value layer (HVL), lead equivalent thickness and the effective removal cross-section (ERSCFN).   (Fig. 8). The linear attenuation coe cient (LAC) values for TZEAg1 and TZEAg5 were found to be higher than all commercial glasses except RS 520. Thus, the glasses under study are a candidate for applications in different radiation protection elds.
The half value layer (HVL) is the shielding parameter that was computed to detect the ability of the studied TZEAg glasses to reduce the E γ in half. The HVL values depend on the E γ and the density of the investigated glass. Therefore, the materials with the minimum values of HVL are signi cant and can be employed in the different shielding applications. It is obvious in Fig. 9 that the increment on HVL rates is directly proportional to the elevation of E ϒ from 0.015 MeV up to 5 MeV. For instance, at the low E ϒ range (0.015-0.08 MeV), the HVL values of TZEAg glasses increase from 0.003 to 0.045 cm for TZEAg1 and from 0.002 to 0.046 cm for TZEAg5. This displays the HVL values also impacted the addition of Ag 2 O content in the zinc erbium tellurite glasses.
As can be seen in Fig. 9, the HVL values diminish in all E γ values with the insertion of Ag 2 O content increases from 1 to 5 mol %. The HVL rates dropped from 0.003 to 0.002 cm for TZEAg1 (density-4.47 g/cm 3 ) and TZEAg5 (density-4.97 g/cm 3 ), respectively. This means that the increase in density of TZEAg glasses leads to the HVL values being reduced.
Consequently, TZEAg5 is the best-studied glass material that can be used in shielding applications where the incoming photon will travel for a shorter distance inside the TZEAg5 glass material.
The lead equivalent is a shielding factor describing the ratio of radiation attenuation which fabricated material offered compared to the pure lead element. The lead equivalent was calculated for the fabricated glass samples and presented in Figure 10 as a function of photon energy. Figure 10 showed that the lead equivalent increased with increasing the photon energy between 240 and 1408 keV. This means that the fabricated TZEAg glasses are more shielding effective at the end of the studied range (i.e., 1000 to 1408 keV). The lead equivalent varied between 0.136-0.349, 0.240-0.358, 0.143-0.368, 0.146-0.377, and 0.150 -0.389 for glass samples TZEAg1, TZEAg2, TZEAg3, TZEAg4, and TZEAg5, respectively. The mentioned results also showed that the equivalent lead ratio was enhanced with the addition of Ag 2 O content.
The transmission rate (TR) was calculated for the fabricated TZEAg glasses. Figure 11 displays the TR changes versus the Ag 2 O content for the TZEAg glasses at different photon energy and glass thickness. Figure 11 depicts that TR values are affected by the gamma photons energy, glass thickness, and modi er type. Gamma photon energy has the highest effect on the transmission rate, where it is increased with an increase the photon energy. The increase in the TR is related to the incoming photons' penetration power, where the penetration power also increased with an increase in the photon energy. Thus, a thicker thickness is required to stop the gamma photons with high penetration powers. The TR increases between 0.0791, 0.427, 0.564, and 0.584 with increasing in photon energy between 240, 662, 1250, and 1406 keV, respectively, for 2.5 cm thickness of the glass TZEAg1. The TR's increase is linearly in the studied energy region due to the Compton Scattering interaction, which is the primary interaction in the investigated energy interval.
The second important factor affecting the TR is the glass thickness, where thicker glasses are better in stopping the incident radiations. The thicker glass layer strongly forced the gamma photons to interact with glass electrons and atoms; producing a high resistance for the passing photons. Thus, the LAC of the studied glass increase, and the photon TR decreases. At gamma-ray energy 662 keV, Figure 9b showed that the TR decreased from 0.698 to 0.166 with the increase of the glass TZEAg3 thickness from 1 to 5 cm.
Also, the modi er type plays a role in reducing or increasing the TR. Figure 11a Structural properties X-ray diffraction X-ray diffraction or also called XRD is one of the characterizations that is used to study the structural properties of the glass. In this work, the XRD was employed to identify whether the fabricated glass sample is amorphous or crystalline.
The XRD spectra of the glass series are illustrated in Figure 12. The spectra were recorded at room temperature in the range of 20° ≤ θ ≤ 80°.
The XRD spectra as depicted in Figure 12 display a broad diffusion hump at the lower scattering angles proposing the presence of a lack of long-range structural order in the glass samples. The existence of a broad hump around 2θ = 30° or in other words, the absence of a sharp peak rati es the non-existence of the crystalline phases in the material and a rms that both glass series are completely amorphous [39]. The increasing concentration of the dopant leads to the narrowing of the broad humps. It can also be further studied that the shift of the hump towards a high angle might be due to the lower values of d spacing between atomic levels that decrease the bond length. This statement had been supported by Cai et al. (2016) [40] had also reported that the non-existence of long-range atomic arrangement in the glass system proves the amorphous nature of the glass samples.

Fourier transform infrared spectroscopy
Fourier transform infrared (FTIR) is one of the non-destructive methods which provides information regarding the structure and vibrational properties of the elements that exist in the glass system. Nanda et al., (2015) had also reported that FTIR is one of the methods that can be used in determining the functional group of the elements as well as providing information about the vibrational modes of the molecules which exist in the disordered amorphous materials. The observable transmission bands in FTIR spectra attained for both glass series are in the range of 611-616 cm −1 which is presented in Figure 14. The assignments of the transmission spectra for silver-doped zinc tellurite glasses are tabulated in Table 3. The positions of the valley are related to the tellurite network as mentioned by Azlan et al., (2015) [41]. Table 3 Assignments of the peaks observed in the FTIR spectra of silver-doped erbium zinc tellurite glasses  [42]. This single band is considered to be broadened with the presence TeO 2 structural unit [43]. On the other hand, the variation in the composition of the glass network might also affect the shifting of the functional group in the disordered amorphous material.
Meanwhile, structural units of zinc oxide, erbium oxide ad silver oxide are also not found in the transmission band of the glass system. The zinc lattice in the glass system is said to be broken down which causes the absence structural unit of zinc oxide as an additional functional group. However, at an early stage, the structural unit of zinc oxide will break down the Te-O-Te bonds that subsequently will form coordination effects known as dangling bond (non-bridging oxygen) forming (Te-O − …Zn 2+ …O-Te) bonds. Consequently, this will increase the formation structural unit of the trigonal pyramid but in return will reduce more structural units of trigonal bipyramid in the glass system [44]. The presence of both structural units of erbium oxide and silver oxide are also not evidenced which can be associated with low concentration.

Deconvolution technique
The transmittance results are then converted to absorbance data. The absorbance spectra results will be deconvoluted immediately. The deconvolute data will determine the area of every band which corresponds to each element that exists in the glass system. The deconvolution is implemented by using Origin 6.0 software and the result for deconvolution spectra is illustrated in Figure 14.
The FTIR spectra have been deconvoluted to identify the exact peak positions of the structural units which exist in the prepared glass. The peak position (x c ) and amplitude (A) for all the peaks are observed after the deconvolution process.
Then, the data is tabulated in Table 4. The assignments for each functional group in every element are attained from the deconvoluted FTIR spectra which are based on the information obtained from the literature done by previous researchers.  presents in the range around 480 cm −1 and 560 cm −1 respectively. The existence structural unit of erbium oxide in the materials modi es the structure of the tellurite glass network [47] and there is also an existing structural unit of Ag 2 O in the glass system which falls in the range of 550-650 cm −1 . According to Coelho et al., (2012) [48], the occurrence of shifting bands is due to the presence structural unit of silver oxide which breaks some of the bonds and modi es the structure of the glass system . The increment of both dopants concentration signi es the change of Te-O bond from TeO 4 to TeO 3 which indicates the creation of NBO's that is followed by the shift of the primary structural unit of TeO 2 to a higher wavenumber. The presence of the TeO 4 structural unit acts as evidence towards the formation of BO's at the same time.
The concentration of the structural units present in both glass systems can be attained using the following equation [49]: where A 3   The density and molar volume of the fabricated Figure 3 Page 15/17 Variation of the packing factor (V i , cm 3 /mol) and dissociation energy (G t , kJ/cm 3 ) versus the Ag 2 O insertion ratio.   The variance between the LAC simulated using MCNP-5 simulation code and calculated theoretically using the XCOM database.

Figure 8
Comparison between studied glasses on their values of LAC with the commercial SCHOTT market glasses at the photon energy of cesium source (0.662 MeV) Figure 9 The alteration of HVL values against the photon energy.

Figure 10
The equivalent lead ratio as a function of the gamma photon energy for the TZEAg glass samples.

Figure 11
The gamma-ray transmission ratio as a function of Ag 2 O content at various glass thickness Figure 12 Page 17/17 The fast neutron shielding properties [a] The mass removal cross-section ∑ R (cm 2 /g), [b] the removal cross section ∑ R (cm -1 ), [c] Relaxation length λ (cm), and [d] the HVL (cm).

Figure 13
XRD patterns of silver-doped erbium zinc tellurite glasses Figure 14 FTIR spectra of silver-doped erbium zinc tellurite glasses Figure 15 Deconvolution of FTIR spectra of silver-doped erbium zinc tellurite glasses Figure 16 Concentration of TeO 4 and TeO 3 structural units of silver-doped erbium zinc tellurite glasses