The influence of γ-irradiation on molecular structure and mass attenuation coefficients of γ-Al₂O₃ nanoparticles

In recent years, nuclear technology has been widely used in electricity generation, medical care,¹ and various industries,² which increases the risk of radiation exposure. The commonly used topics in nuclear technology are laser radiation, γ-rays, and electron beams.³ γ-rays are widely employed in industries due to their high permeability. Excessive and unnecessary exposure to x rays and γ-rays is harmful and dangerous as these rays stimulate the atoms and affect the cells and tissues that survive and increase the risk of cancer.⁴ Therefore, radiation protection is an essential measure.⁵ The most important factor to reduce the γ-irradiation effect is using shields against the rays.⁶ γ-ray protection is used in nuclear power plant buildings,⁷ hospitals, etc.⁸ Since γ-shielding usually utilizes heavy materials that are difficult to carry, the need for finding new, lighter, and portable ones is strongly felt for a variety of applications, such as movable nuclear power plants, nuclear material containers, and other related technologies.⁹ 

Nowadays, with the advancement of nanotechnology, it is possible to change and improve material properties.¹⁰ Al₂O₃ nanoparticles (NPs) are among the most important nanostructured materials.¹¹ They provide unique properties, such as high hardness,¹² phase stability, high strength, and thermal,¹³ mechanical, and chemical¹² properties.

Al₂O₃ NPs are employed in ceramic technologies, which are being widely used in various industries,¹⁴ such as electronics,¹¹ medicine,¹ artificial joints,¹⁵ thigh bones,¹³ dental implants, and restorations.¹⁶ Voevodin et al. indicated that Al₂O₃ NPs can be used to make ceramics for aircraft and aerospace applications¹⁷ and improve the durability and mechanical strength at high temperatures.¹⁸ According to Hincapie et al., Al₂O₃ NP coating is one of the best NPs facade coatings, which is highly protective and predicted to be a major candidate in the global market due to its simple washability property and heat and corrosion resistance against UV-irradiated coatings.¹⁹ The effect of Al₂O₃ NPs on the modulus of elasticity and compressive strength of composite cement was studied by Li et al.²⁰ Among the different crystalline phases of Al₂O₃, γ-Al₂O₃ has a more interested application prospect in most catalytic reactions as a catalyst or catalyst substrate in the automotive and petroleum industries,^21,22 solar-blind UV signals,²³ and solar energy and solar collector heaters²⁴ because of its high stability and large specific surface area.

Due to the extensive use of Al₂O₃ NPs and the possibility of being exposed to γ-irradiation during the manufacturing process of alloys, research on the nuclear properties of these NPs is of high prominence. The linear attenuation coefficient (μ_l) denotes a part of the X- and γ-irradiation that is absorbed or scattered per unit thickness of the absorber.²⁵ However, μ_m is the power of the protective matter to reduce the effect of radiation intensity.⁵ The shielding parameters such as effective atomic number, electron density, atomic and electronic cross sections, and shielding efficiency¹ were extracted after computing the μ_m factor. Nuclear engineering researchers have focused on investigating μ_m and photon interaction parameters with the matter.²⁶ 

There have been a great number of experimental and theoretical investigations into the effects of γ-irradiation on the physical properties of different materials and on determining μ_m values in various elements and compounds/mixtures.⁵ For example, Singh et al. measured the values of μ_m for some polymers with potential applications in dosimetry and medical physics using the Monte Carlo simulation code (MCNP-4C) at different γ-ray energies.¹ Dehghani et al. investigated the effect of different doses of γ-irradiation on the third-order nonlinear optical properties, molecular structure, and mass attenuation coefficients of synthesized colloidal silver NPs and gold NPs.^3,27 Limkitjaroenporn et al. investigated μ_m and the effective atomic number of Ag/Cu/Zn alloy at different photon energies using the Compton scattering technique.²⁸ Elmahroug et al. calculated μ_m in the photon energy range of 1 keV–1 GeV using WinXCom software.⁶ The effect of γ-irradiation on the structural, electrical, and gas sensing properties of tungsten oxide NPs was examined by Lavanya et al.²⁹ Faraji Alamouti et al. used Kramers–Kronig relations and the Z-scan technique and investigated the optical coefficients of Al₂O₃ NPs.³⁰ Raut et al. studied the effect of 100 kGy γ-irradiation on the structural, electrical, and magnetic properties of CoFe₂O₄ NPs.³¹ 

Today, manufacturing companies around the world are making attempts to build and design new ways to reduce weight, increase flexibility, and use more effective materials for γ-shielding. Although a lot of work has been carried out to investigate the effect of ion irradiation on different nanostructures, there is little concentration imposed on the study of irradiation induced modification in the molecular structure and mass attenuation coefficients of γ-Al₂O₃ NPs. Knowing the value of μ_m is very important in γ-shielding and dosimeters as well. The present study aims to evaluate the μ_m values of γ-Al₂O₃ NPs uncertainly and investigate the difference between the γ attenuation properties of γ-Al₂O₃ NPs and the bulk material. Furthermore, the variation in the structure of γ-Al₂O₃ NPs under different doses was investigated in detail via UV–Vis spectroscopy, x-ray diffraction (XRD) patterns, and scanning electron microscopy (SEM) images.

There are various available methods for making Al₂O₃ NPs, such as hydrothermal,¹² sedimentation,¹² emulsion,³² electrolysis,³³ flame oxidation,³⁴ and sol–gel methods.¹² In the present study, the sol–gel method has been employed to synthesize Al₂O₃ NPs based on the desired morphology, size, particle size distribution, and available facilities.¹² 

For the synthesis of Al₂O₃ NPs, aluminum tri-chloride (AlCl₃) was used as the primary metal precursor, and a stirred alcoholic solution of 0.1 M AlCl₃ and ammonia 28% was added dropwise to prepare the sol.³⁵ The gel was kept at room temperature for 30 h. The metal precursor (aluminum chloride) was hydrolyzed at ambient temperature and condensed in the form of sol. The result of this reaction is the production of aluminum hydroxide [Al(OH)₃]. Ultimately, aluminum hydroxide was converted to aluminum oxide and water. Then, the material was filtered in vacuum, dried in a furnace at 100 °C for 24 hours, and then slowly cooled.³⁶ The reactions during this process are as shown in the following equations:

The synthesized NPs were divided into five samples: one non-irradiated sample and four samples irradiated with 0.5, 1, 10, and 20 kGy doses of γ-irradiation emitted by a ⁶⁰Co source with an activity of 8302 Ci (about 3 × 10⁵ GBq) at a dose rate of 2 Gy/s.

According to the Beer–Lambert law, when γ-rays pass through the shielding material, an exponential reduction in the intensity of the primary beam is observed, and the linear and mass attenuation coefficients are given by the following equations:³⁷ 

Rewriting the above-mentioned equation, the logarithmic relationship can be given as

where I₀ is the intensity of the incident photons with a specific energy and I stands for that of photons transmitted through the sample without interaction. Moreover, x (cm) denotes the sample thickness, and $μlcm−1$⁠) is the linear attenuation coefficient. μ_l is used to calculate the mass attenuation coefficient, (⁠$cm2/g$ as

in which $ρ(gcm3)$ is the density of the samples. The schematic arrangement of the experimental setup used for the measurement μ_m is shown in Fig. 1.

The setup was composed of lead bricks, a detector collimator, a source collimator, and a gamma spectroscopy system with a 2″*2″ CsI(Tl) scintillation detector. The γ-spectroscopy system has been calibrated using γ-rays with energies of 26.3 and 59.6 keV of $Am95241$ and 662 keV of $Cs55137$ sources. The γ-Al₂O₃ NPs were turned into pills with a density of 1.18 g/cm³ and different thicknesses.

For each sample of γ-Al₂O₃ NP pills, two gamma spectroscopy experiments were performed. At the first evaluation, the spectroscopy was conducted without a specimen, and the intensity of incident γ-rays (I₀) was measured for 2000 s. In the second evaluation, the spectroscopy was performed with the specimen placed between the source and detector, and the counts of γ-rays transmitted through the sample without any interaction (I) were measured for 2000 s. Then, by means of Eq. (5), the values of the linear attenuation coefficient (⁠$μl$ were calculated. In addition, by using Eq. (6), µ_m of γ-Al₂O₃ NPs was calculated by dividing μ_l by the density of the pills. In addition, the same experiments were carried out on γ-Al₂O₃ NPs subjected to 20 kGy dose of γ-irradiation, and the values of μ_l and μ_m were obtained as well.

The theoretical values of the mass attenuation coefficients for Al₂O₃ micro-particles were obtained using the WinXCom computer program. This software uses cross section tables and the mixture rule to calculate the mass attenuation coefficients at different energies for micro-particle elements, mixtures, and compounds.³⁸ 

The UV–Vis absorption spectrum of γ-Al₂O₃ NPs before and after irradiation has been studied by using a UV–Vis spectrophotometer (model: Photonix Ar 2015). Initially, a solution of 0.01 M concentration with de-ionized water was prepared to investigate the UV–Vis absorption spectrum. Then, from γ-Al₂O₃ NPs with and without γ-irradiation, the absorption spectrum of UV–Vis was measured in the wavelength range of 200–800 nm.

A D8-Advance Bruker advanced diffractometer was used with Cu-Ka1 radiation (λ = 0.15406 nm) in the range of 2θ = 10–80°, and the phase purity and crystal structure of the prepared NPs were determined by XRD. These XRD patterns of Al₂O₃ NPs were recorded and compared before and after γ-irradiation by X-Pro software. Then, the position of the peaks and Miller plates corresponding to these peaks were determined. The average crystalline size was obtained by the Debye–Scherrer formula, given by

where k is a constant (0.9), λ is the x-ray wavelength of the Cu target (λ = 0.15406 nm), θ is the Bragg diffraction angle (in degrees), and B is the full width at half maximum (FWHM) of x-ray diffraction peaks (in radians).³⁹ 

To study the morphology and surface of un-irradiated and irradiated γ-Al₂O₃ NPs, first, the γ-Al₂O₃ NPs are aligned with the layer of gold. Then SEM analysis was carried out with a TESCAN Vega Model electron microscope, and the NP size was estimated by analyzing the statistical data and plotting the particle size distribution chart.

In this research, μ_m of γ-Al₂O₃ NPs before and after γ-irradiation has been measured. Moreover, the effects of γ-irradiation on the molecular structure of γ-Al₂O₃ NPs were investigated.

The experimental values of μ_l for NPs with a density of 1.18 g/cm³ before irradiation and after receiving a radiation dose of 20 kGy are presented in Table I. It is observed that μ_l of γ-Al₂O₃ NPs decreases as the photon energy increases.

Figure 2 shows the values of μ_m for γ-Al₂O₃ NPs before and after γ-irradiation. It is seen that the μ_m value of γ-Al₂O₃ NPs decreases after γ-irradiation, which, as shown in the SEM analysis, is due to increasing the particle size after irradiation.

Table II lists the theoretical values of μ_m for γ-Al₂O₃ micro-particles and the experimental values of μ_m for γ-Al₂O₃ NPs before and after receiving 20 kGy dose of γ-irradiation. The results of comparing the mass attenuation coefficients of γ-Al₂O₃ NPs with those of micro-particles show that the photon attenuation property of NPs is slightly greater than that of micro-particles, indicating that reducing the size to the nanoscale can enhance the attenuation properties of γ-Al₂O₃, which is in agreement with the results reported for PbO by Elsafi et al.⁴⁰ The enhancement of gamma attenuation properties is of great importance in the nuclear industry; it shows the effect of the shielding material on the attenuation of primary gamma rays and can be used in manufacturing light gamma shields in industry and medicine.

For investigation of the molecular structure of γ-Al₂O₃ NPs, UV–Vis spectroscopy, XRD patterns, and SEM images were applied.

The UV–Vis absorption spectrum shows that the absorption peak of un-irradiated γ-Al₂O₃ NPs was obtained at 212.5 nm (Fig. 3).

After γ-irradiation with different doses, the absorption peak of γ-Al₂O₃ NPs is eliminated (Fig. 4).

When determining the UV–Vis absorption spectrum, the matter absorbs the energy of electromagnetic radiation in the ultraviolet and visible regions, and the initial radiation beam will be weakened.

There are many mechanisms by which photons interact with materials, including the photoelectric effect, the Compton effect, the production of pairs, elastic scattering, and photonuclear interactions. Photons interact with electrons, not nuclei, during the photoelectric effect and Compton effect, which imparts energy to matter by transferring the photon’s energy to the electrons.⁴¹ In addition, γ-irradiation can transfer some small ions to positions in the crystal lattice, fill the holes, and change or eliminate the absorption peak.⁴² 

The XRD pattern of γ-Al₂O₃ NPs before and after γ-irradiation is illustrated in Fig. 5. It was determined that all of the Bragg reflection peaks were due to the structure of γ-Al₂O₃ (ICSD 66559).^43,44 The γ-Al₂O₃ structure is typically described as a defect spinel structure with varied degrees of tetragonal distortion. Due to disordering of aluminum ions, especially at the octahedral sites, as well as the smaller particle size, the Bragg peak broadens. A face-centered cubic (fcc) close packing of oxide ions results in a dominant XRD pattern with (400) and (440) reflections. Both reflections (400) and (440) of the spinel shown in Fig. 5 do not exhibit two distinct peaks, nor are they asymmetric, indicating that tetragonal deformation was not strong. A possible candidate for the structure is γ-Al₂O₃, with the so-called “three spinel block structure,” which develops more extensively with greater tetragonal distortion. Due to its superstructure formation, it provides additional Bragg reflections,⁴³ which can be caused by higher doses of γ-irradiation.

The average crystalline size was obtained at 3.647 nm by the Debye–Scherrer equation. In addition, after γ-irradiation with 0.5, 1, 10, and 20 kGy radiation doses, the x-ray diffraction patterns were reviewed again. The calculated average crystalline size, peak position, Miller plates corresponding to these peaks, and FWHM before and after γ-irradiation are shown in Table III.

In this study, the ⁶⁰Co source (1.173 and 1.332 MeV) was used for irradiation, and the Compton effect dominated the photoelectric effect. The Compton effect occurs when a photon collides with an electron and transfers energy and momentum to the electrons, which in turn recoil. Despite this, the energy and momentum of this elastic collision stay conserved. Depending on how much energy is lost due to the recoiling electrons, photons scatter with less energy and momentum at different angles at the instant of the collision. The recoil electron does not receive sufficient energy from photons in most interactions to knock the atom out of its position. A minimum energy is required to displace an atom from its lattice site, according to binary collision cascade theory. An atom that has been displaced by a lattice site is recombined with a vacancy when it rests inside an instability volume. Therefore, for stable defects, the recoil atom should receive an energy that exceeds the threshold displacement energy of the material. As a result, the atomic positions in the lattice would be unchanged,⁴¹ as shown by the XRD spectra before and after γ-irradiation. A peak at about 20° is only created at the highest radiation dose, which shows an increase in the crystalline size and makes the structure closer to the structure of γ-Al₂O₃.⁴⁶ However, the grain size increased as the dose of γ-irradiation was enhanced, as shown in Table III.

Siddhartha et al. also achieved similar results for the polyethylene terephthalate (PET) polymer.⁴⁵ They observed an increase in the crystallinity and crystalline size with an increasing dose of γ-irradiation. In addition, Xiong et al. showed that the increase in γ-irradiation increases the crystallinity of the Al₂O₃ NP ultra-high molecular polyethylene (UHMWPE) composites.⁴⁶ Baena et al. indicated that after γ-sterilization, the crystallinity of UHMWPE increases, too.⁴⁷ 

The SEM of un-irradiated and irradiated γ-Al₂O₃ NPs samples is depicted in Fig. 6. The SEM images showed the spherical shape of the γ-Al₂O₃ NPs.

By analyzing the statistical data and plotting the particle size distribution chart (Fig. 7), the NP size was estimated.

The results of the estimation of the NP size are recorded in Table IV. The results indicate an increase in particle size with increasing γ-irradiation intensity.

The radiation shielding properties of γ-Al₂O₃ NPs before and after γ-irradiation were studied in detail. It is seen that irradiation to γ-Al₂O₃ NPs causes the mass attenuation coefficients to decrease. This is due to increasing the size of particles after irradiation, as shown in SEM analysis, and it is in agreement with other studies. In addition, the comparison between the mass attenuation coefficients of γ-Al₂O₃ NPs and micro-particles shows that the values of μ_m associated with γ-Al₂O₃ NPs are slightly greater than those of micro-particles. This confirms the previous result that μ_m decreases by increasing the size of γ-Al₂O₃ NPs.

The XRD results confirm the γ-Al₂O₃ NP phase, and the pattern study shows that the number of peaks increased when the γ-irradiation increased to 20 kGy. The average crystalline size of γ-Al₂O₃ NPs was enhanced by increasing the γ-irradiation dose. The results of the microstructural properties of SEM show that γ-Al₂O₃ NPs were spherical and also show an increase in particle size after γ-irradiation. Based on the XRD results and SEM data analysis, it can be seen that the microstructure of γ-Al₂O₃ NPs changes before and after γ-irradiation.

By summarizing all the results obtained, it can be concluded that γ-irradiation increases the crystalline size of the γ-Al₂O₃ NPs and makes them less absorbent to γ-irradiation.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.