Electron Paramagnetic Resonance, and Thermoluminiscence Mechanism In Radiation Shielding Cr2O3 - Ba(La)2SiO6 Glasses


 The research on Cr2O3 doped SiO2 glasses is well known for advanced dielectrics. However, there are many other valuable properties associated with Cr2O3 inclusive various glasses. In this view, the current research aimed to develop the radiation shielding, elastically rich, and the EPR based Cr2O3 doped Ba(La)2SiO6 glass resource. Electron paramagnetic resonance, radiation shielding, and elastic studies have been employed to investigate the advanced characteristics. Structural characterization suggests glassy behavior with the Cr2O3 undoped glass. Whereas the other involved with Cr2O3 mol% shown with the ceramic behavior. The glass transition phenomena and forming abilities are studied with the help of differential thermal analysis techniques. Elastic studies have been done with the limit on the glasses, which suggests the glasses are flexible for elastic use. The electron paramagnetic resonance reports suggest high order of dipole-dipole super-exchange interaction and rhombohedral distortion within the glasses. Furthermore, we have tested the glasses for radiation shielding properties. The values of mass attenuation coefficient, radiation protection efficiency, mean free path, and energy absorption build-up factor of the glasses are measured and compared with values obtained with the help of standard photon shielding and dosimetry software. The studies indicate that the glasses developed are capable of radiation shielding. Upon 50 kGy, γ - irradiation, the thermoluminescence properties of the glasses are reported. The results found to be interesting, and reveal the resource developed are thermoluminescent at low activation energies. Furthermore, we have tested, the glasses for radiation shielding properties. Moreover, to introduce the detailed correlation between electron paramagnetic resonance, and thermoluminescence phenomenon, we have annealed the glasses under 0 to 300 oC temperature and upon the 0 to 50 kGy, γ - irradiation dose level. The electron paramagnetic resonance and thermoluminescence properties obtained for the glasses are highly correlative.


Introduction
Usually, the SiO2 glass substances are translucent, hard, non-corrosive, and thermally stable. Their anticipated structural and dielectric characteristics, such as high dielectric constant, low A.E., and values of density of states, will be used for various dielectric applications. There has been significant investigation on silicate glass substances due to their abnormal dielectric determinations considering few decades to recent years [1,2]. The La2O3 is not a pure glass former, but the joining of La2O3 to the silicate glass substances promotes their elastic characteristics, thermal resistance, non-corrosion features. The joining of La2O3 to the SiO2 glasses enhance sharp melting point and grainy hardness. Generally, La2O3 doped silicate glasses are employed as a dosimeter for radiation healing and protection utilization due to their high radiation shielding ability [3,4]. Incorporating alkali oxides such as Li2O, BaO, KF, and CaF2 into the La2SiO5 glasses improves third-order non-linearity and electro-optical Kerrlike effect. The addition of BaO to the La2SiO5 glasses acts as a refining agent and enhances the polymerization phenomenon [5,6]. Amongst the whole transition metal oxides, the nucleation agent Cr2O3 has been adopted to improve the features of Ba(La)2SiO6 glass substances considering the collaboration inside the glass interface, and the Cr 3+ ions provide fast electron-phonon interaction and promote additional diffusion of thermal radiation [7,8]. Usually, Cr 3+ ions substantially affect the electron paramagnetic resonance, radiation shielding, and dielectric properties of glass materials. Silicate substances, including combined valence states of Cr 3+ ions, are of modern importance as a cathode resource in rechargeable batteries as of their unusual energy density and dielectric capacitance [9,10]. The octahedral coordinated Cr 3+ ions influence polymeric anions within the glassy network, stimulating various other prospects such as chemical durability and volatile nature. The Cr2O3 uses different materials for paramagnetic studies to survey the magnetic results employing electron spin resonance. The thought, Cr2O3 doped Ba(La)2SiO6 glasses will be the most advantageous, academic, and technological subject of research [11,12].
Subsequently, in the existing work, Cr2O3 doped Ba(La)2SiO6 glass substances are developed and typically study for its suitability towards numerous electron paramagnetic, radiation shielding, and dielectric use. Impending credentials such as high density, elastic, thermoluminescent, and radiation shielding etc., are probable over the development of competent solid-state glass substances has fascinated noble recognition.

Methodology
The compounds of (25-x) mol % BaO, (x) mol% Cr2O3, 15 mol% La2O3 and 60 mol% SiO2 uses for sample development; where, x varies with a step size 0.2 mol % from 0 to 1.0 mol %. The melt quenching procedure was to produce the present series of samples. The detailed chemical composition of the present series of glass tests are as follows; Cr-0.0 (25.0 BaO + 15 La2O3 + 60 SiO2 + 0.0 Cr2O3), Cr-0.2 (24.8  have been chosen in powder form. All the essences in proper mol% are well mixed in an agate mortar, and programmed furnace and platinum crucibles are employed to melt the mixed powder to form essential glass melt. The melting took place around at 1440 o C, observed, and quenched in the brass holder; further, it has been annealed around 490 o C as a glass. The dimensionally designed glass samples are used for various characterization. Mass of the glasses recorded with the help of Scale Tech digital weighing balance with a precision of 10 -4 gm/cm 3 . Archimedes' principle was used to calculate density values employing known weights of the glasses. The diffraction patterns of glasses are recorded with the help of a Shimadzu X-Ray (XRD-7000) diffracto-meter with a precision of 0.1 degrees. Chemical analysis was examined with the help of the Hitachi S 3700N instrument. The thermal analyser Hitachi DTG-60 H is used to record the DTA thermograms of glasses with a precision of  1 O C. Ultrasonic velocities, which will be helpful to evaluate the elastic measurements of the glasses, are registered with the help of a WT-311D flaw detector with a precision of  10 m/s. Varian E11Z X-Band spectrometer was used to record the EPR spectra of the glasses. The cobalt radioisotopes irradiate the glasses using the GC-5000 irradiation chamber with 0 -40 kGy dose and a 03 kGy/h rate. Fricke ASTM E 1026 Standard dosimetry method was used to understand the variation of absorbed doses within the irradiation chamber. The UV-Vis Spectrometric technique was used to record an absorbance value.
And the photon-shielding and dosimetry (~ 0.015 to 15 MeV) software is used to obtain the theoretical radiation shielding characteristics. The MAT Lab 2.3, Chem Draw Ultra 12.0, and Mac Office 2013 plus software used to analyse the results. Fig.1(a). illustrates, the X-Ray diffraction pattern of the Cr2O3 free Ba(La)2SiO6 sample suggests glassy behavior, whereas Fig.1(b). reports the X-Ray diffraction pattern of the other samples expressing ceramic behavior. The peak intensities in X-ray diffraction pattern of the crystalline phase such as 1.LaCrO3, 2.Ba4SiO6, 3.BaSi2O6, 4.CrSiO4, 5.La2Ba3O8, 6.CrSiO3, 7.BaSiO3, 8. CrBa2O4 and 9.Ba3SiO7 were found to be increasing with increased Cr2O3 mol%. Parallel to this, the width of the peaks becoming sharper with increased Cr2O3 mol%. This suggests de-clustering in the glass network and crystallinity with increased Cr2O3 mol% [13,14]. Fig.2 reports the chemical analysis of one BaLaSiCr-1.0 glass, which signifies Cr, Si, O, Ba, and La chemicals in weight %. The physical properties of the Cr2O3 -Ba(La)2SiO6 glasses are studied. The values of the glasses' molecular weight and glass density are found to be increased with Cr2O3 weight%. In comparison, the molar volume of glasses decreased with increased Cr2O3 weight%. Similarly, the refractive index and molar refraction of the glasses increased with increased Cr2O3 weight%. The quantities optical basicity and oxygen packing density were found to be improved with increased Cr2O3 weight% [15,16]. Fig.3 reports the DTA thermograms of the Cr2O3 -Ba(La)2SiO6 glasses. From which the glass transition (Tg) and crystallization (Tc) temperatures are reported. The forming abilities of glasses are also computed. Results suggest that the glass with 1.0 Cr2O3 mol% was the lowest informing abilities, which suggests the depolymerization in glassy behavior and upsurges towards crystallinity behavior [17,18]. The high orders of intermolecular force between the Cr 3+ ions to the Ba 2+ , Si 4+ , and La 3+ ions lead to the order of forming abilities of the glasses. Changes in enthalpy values within the glass network result from variation in both endothermic and exothermic peak intensities. SiO2 is a glass former, and its tetrahedrons have a dissimilar structure

Elastic properties
Elastic behavior of the Cr2O3 -Ba(La)2SiO6 glasses are reported. The elastic (bulk, shear, and young) modulus, poison ratio, and microhardness of glasses are tested and evaluated. With the increase of Cr2O3 weight% from 0 to 1 mol %, the density, and elastic (bulk, shear, and young) modulus of the glasses are found to be increased. In comparison, the value of the poison ratio of glasses decreased with increased Cr2O3 weight%. Similarly, the microhardness of glasses increased with increased Cr2O3 weight%. They have computed all the elastic standards of glasses reported in table. The vulnerabilities in the dimensions of the interstitial gaps and variation in the co-ordination of the Ba 2+ , Cr 3+ , Si 4+ , and La 3+ ions are the reason for the interpretation of density, molar volume, refractive index, and optical basicity. For most of the parts, the glass materials are of supreme elastic relevance under any glass formation. Generally, the various elastic modulus of glass materials is interdependent thought of holding intermolecular dominance. In some glass (or) glassceramic materials, elastic modulus improves through a conventional magnitude of the atomic density.
Replacement of the La 3+ ions by divalent Cr 3+ ions within the glass leads to higher orders of the elastic module.
Microhardness of glasses prescribe additional information and supports for a covalently interlinked structure.
An increase in atomic density and variations in interstitial defects could be another reason for elastic characteristics [19,20]. Similarly, intensities observed around at the center of the spectra with a 'g' values of 1.97, 2.38, 4.25, and 5.23 are due to rhombic distortion. The 'g' values of glasses were increased with Cr2O3 concentration, suggesting the increased nature of ionic bonding. At first, in the Cr2O3 -Ba(La)2SiO6 glasses, the Cr 3+ ions predominately occupy (CrO4) sites tetrahedrally, and it is changing with the increase of the Cr2O3 weight% from 0 to 1 mol % with a step size of 0.2 mol% they are occupying octahedral (CrO6) sites. The intensities of resonance signal are increased with an increase of the Cr2O3 weight% from 0 to 1 mol %. The super-exchange interaction between dipoles raised due to Cr 3+ ions within the glasses might be the reason for such a phenomenon. The observed increase in the values of g-factor with an increase of the Cr2O3 weight% from 0 to 1 mol % is due to non-zero orbital angular momentum. The hyperfine splitting factor (g) gives precise information about the predominance of the Cr 3+ ions within the glass network. The E.P.R. spectra of the Cr2O3 -Ba(La)2SiO6 glasses reveal the ionic environment around the Cr 3+ ions. And which is also increasing with increasing concentration of Cr2O3 within the glassy network. The product of the peak height and square of the bandwidth reveal the intensity of the E.P.R. signal. And it is increased with Cr2O3 weight% from 0 to 1 mol %. The Chromium ion has three unpaired electrons with S = 3/2, L = 3, and J = 3/2. Hence these ions' orbital degeneracy is seven with the ground state of 4 F3/2. The system's total spin-Hamiltonian [21][22][23][24]  The high values of g factor (~ 5.23) are responsible for the rhombic symmetry with in the glasses. In contrast, low g values at g (~ 1.97) arise from the Cr 3+ centers in the distorted cubic sites. Due to the simultaneous interaction of Cr 3+ ion pairs, and the isolated Cr 3+ ion centers, additional g values at ~ 2.38. The Fe 3+ impurity ions cause weak signal with g value at ~ 4.25.

Radiation shielding properties
The photon-shielding and dosimetry (~ 0.015 to 15 MeV, Cs-137) software is used to obtain the radiation shielding characteristics of the Cr2O3 -Ba(La)2SiO6 glasses. The shielding characteristics such as MAC (μ/ρ), EAN (Zeff), HVL, MFP, RPE, and EBF are evaluated theoretically to explore the proficiency of the Cr2O3 -Ba(La)2SiO6 glasses. The subsequent equation can be employed to calculate the experimental LAC (μ -linear attenuation coefficient) values [25,26].
Where, tthickness, I0incident photons, and I -transmitted photons Mass attenuation co-efficient = μ / ρ μ -LAC (linear attenuation coefficient), and ρdensity The radiation shielding aspects are intensely interlinked with a density of glasses. The photon absorption and scattering probability can be applied to determine with the help of the MAC (cm 2 /g) = ∑ ( ) .
In this expression, ( ) represents MAC of i th element of the glass matrix.
The variation in the MAC (cm 2 /g) against photon energy (~ 0.015 to 15 MeV) with an increase in the Cr2O3 concentration is presented in Fig.5(a) -Inset (b). The results reveal, the peak at 0.0659 MeV in the MAC traces represents the K-absorption edge and which was due to the presence of Cr 3+ ions. The results also suggest that the variations in the value of the MAC (cm 2 /g) with the Cr2O3 increased weight% and photon energy. The is required to attenuate high-energy radiation [29,30].  With the increase of Cr2O3 weight% from 0 to 1 mol %, the thermoluminescence measurements of glasses found to be increased, up to 01 % of Cr2O3 content found to be best in results. One of the section in Table.1 reports the TL characteristics of the Cr2O3 -Ba(La)2SiO6 glasses. With the increase of Cr2O3 weight% from 0 to 1 mol %, the TL measurements of glasses found to be increased, and the glass with 1.0 % of Cr2O3 content found to be best in results. Thermoluminescence dosimetry is the widely used means of dosimetric Cr2O3 %, and the optimum concentration was determined for each of the glasses. Also, the other dosimetric parameters like reusability, storage stability, light sensitivity, reproducibility in synthesis, etc., are studied.

Thermoluminiscence
Lithium tetraborate is one of the TL phosphors studied for a long time since its first introduction due to its tissue equivalence. But the Cr2O3 -Ba(La)2SiO6 glasses synthesized by using the melt quenching technique have a high-temperature glow peak and hence benefit minor fading. Hence a detailed investigation has been carried out on this material concerning increasing the TL intensity and dose-response. The increase of Cr2O3 weight% from 0 to 1 mol % improves the structural defect centers within the glasses [33,34].

Correlation -Electron paramagnetic resonance, and thermoluminiscence
In this section, the correlation between TL and EPR signals is explained in detail. The phenomenon of irradiated induced defects in the materials is the necessary identification in TL-based many applications. The complete information concerning the charge traps in the TL mechanism cannot be acquired by carrying TL measurements alone. In this view, there is an EPR technique, which can be helpful to find a lot more information about paramagnetic species such as charge traps (trapped hole or an electron). In general, EPR provides a non-destructive way of obtaining the results about charge traps in TL materials. Generally, TL analysis of glasses provides information about the charge trap centers but the nature of traps. But the information concerning the nature of the hole and electron traps cannot be determined by using the TL technique. In comparison, the EPR technique, which describes the splitting of energy levels under the applied magnetic field, will be used to evaluate the number of defects centers up to the 10 -9 molar concentrations, which is not in the order of 10 -15 molar concentrations, where the range, at which TL analysis will typically be taking place. The following assumptions have been introduced to correlate the TL and EPR phenomena [35,36].
(a) TL : all the traps centers need not be paramagnetic radicals (b) EPR : all the paramagnetic radicals of irradiated glass need not be traps centers For accurate correlation, one should study the isothermal decay of the TL analysis, which includes annealing treatment at all the prominent temperatures that could be investigated. Parallelly, the EPR has been done within a range of radiation doses, which has to be close to the TL saturation dose. In comparison, the TL analysis has done within a radiation dose, which has to be lower than the saturation dose. This is obviously due to the EPR-TL correlation provides the information only on trap centers. Similarly, the radiation-induced defects in lanthanum-based silicates will be evaluated by using the EPR technique. The EPR analysis will understand the possible forming defect centers of the glasses with irradiation. NBO's (non-bridging oxygen) at D1 will act as another defect (negatively charged), which can trap a hole by interstitially surrounded cations in the network. The irradiation effect on glasses will induce SEC (silicate electron trap centers) within the network due to differences in electron affinity and similarity in valency of D2 and D1 network formers. In such cases, the electrons within the network will be trapped at the orbital of Si-O bonding. The presence of interstitial ions (charge compensated) nearer to the defect centers will increase defect canters' significance and serve the electron traps themselves. The addition of Cr2O3 will form oxygen vacancies within the network. Those will further help to increase the formation of electron trap centers within the network. The non-irradiated, irradiated (1 kGy), and annealed glasses at various temperatures are taken for the EPR studies to identify the TL responsible defect centers. The results suggest that the glass with 1.0 mol% Cr2O3 concentration was optimal for TL responsible defect centers. Fig.8 Cr2O3 is doped in the barium-based lanthanum silicates, it is understood that Cr 3+ can occupy the La 3+ sites and also to the Si 4+ tetrahedral sites. The charge compensated oxygen vacancies are created within the lattice based on the dopant (Cr2O3) concentration [37,38].
And the created oxygen vacancies are stable at room temperature and act as electron trap centers within the lattice. After the required irradiation, the holes are trapped with bridging oxygen atoms, whereas the electrons are trapped with oxygen vacancies. The EPR studies under the different annealing temperature range from 50 to 300 °C, and with a step size of 50°C have done on irradiated barium-based lanthanum silicate glasses are introduced to understand the thermal stability behavior of traps and to make an analogy for TL results. The results reveal the centers observed are stable until 210 o C; after that, a sudden decrease in order of temperature.
The TL glow curves are also recorded on all the barium-based lanthanum silicate glasses. The EPR measurements are carried out to measure the thermal decay of the TL in the glass with 1.0 mol% of Cr2O3 concentration. Fig. 9 reports the relative variation of TL and EPR signal of the distinguished radicals of irradiated glass with 1.0 mol% of Cr2O3 concentration at different annealing temperatures around one hour.
The observed results from the figure suggest that the order of decrease in the EPR signal relative to the TL signal. The subsequent comparison will be used to calculate the thermal activation energy of the SOHC center within the network, which is the leading cause for these radicals in the TL mechanism [39,40]. In this process, the Si 4+ ions are replaced with two Cr 3+ ions, which leads to weaker Si-O linkages within the network. Upon heating, the holes are unconstrained from the SOHC center and recombine with the electrons trapped at oxygen vacancies. The non-radiative energy liberated through recombination is transferred to the nearby Cr 3+ ions substituted for La 3+ ions. And which also leads to characteristic emission spectra from excited Cr 3+ ions with the orange-red region.

Conclusion
In the synchronous research, we have integrated multifunctional Cr2O3 -Ba(La)2SiO6 glasses. Various structural, elastic, photo-electronic, and thermoluminescent techniques are used to characterize the glasses. increased, which also suggests the progressive nature of ionic bonding within the network. The observed increase in the importance of g-factor with an increase of the Cr2O3 weight% from 0 to 1 mol % is due to the non-zero orbital angular momentum of predominant Cr 3+ ions. Under irradiation, the defect centers formed within the 1.0 mol% Cr2O3 concentration are determined. The thermal decay of the EPR signals is obtained and compared with the thermal decay of TL signals. To introduce the EPR -TL correlation, the necessary TL responsible free radicals, and paramagnetic defect centers are identified. The thermal decay of the EPR signal relative to these defects and the thermal decay of TL are in line with each other. Also, the activation energy of the paramagnetic centers and the trap depth obtained by kinetic analysis are nearly equal. Hence, the defects identified in EPR were responsible for TL in this glass. The electron trap was identified as oxygen vacancies, and the hole traps are the bridging oxygen atoms. When the glass is heated, the electrons are released from the electron trap and recombine with the trapped holes. From the obtained results, a mechanism for TL in these glasses is proposed.

Ethics approval
The manuscript has written as per journal ethical guidelines  Where 'x' varies 0 to 1 mol% with a step size of 0.2 mol%. The diffraction angles are taken up to an accuracy of ± 0.1o.

Figure 2
Chemical analysis of the BLSCr-1.0 glass is recorded at the room temperature.       Upon irradiation, thermal decay of the TL and EPR signal of SOHC and oxygen vacancy O_) radicals formed with in the BLSCr-1.0 glass.

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