Optical Properties Characterization of Gamma Irradiated CeO2 Nanoparticles Solution

Optical properties of CeO2 nanoparticles solution have been studied to explore the possibility of CeO2 nanoparticles as a liquid chemical dosimeter to measure gamma radiation in radiotherapy. CeO2 nanoparticles were synthesized by ultrasound irradiation of cerium nitrate solution. The CeO2 nanoparticles were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). CeO2 nanoparticles solution with concentration of 2.5 x 10-4 g/ml was irradiated by 1.3 MV 60Co gamma rays with a dose of 1.0 to 2.5 Gy. The UV absorbance spectrum of unirradiated and irradiated CeO2 nanoparticles solution was measured by using UV-Vis spectrophotometer. The UV absorbance of CeO2 nanoparticles solution increased with increasing radiation dose and caused a change in the band gap. The change in optical properties of CeO2 nanoparticles solution is correlated to chemical reactions induced by gamma radiation. The obtained chemical yield was in the order of 103 mol/J indicates the sensitivity of CeO2 nanoparticles to gamma radiation. The results show the possibility of CeO2 nanoparticles for use as gamma radiation dosimeter.


Introductions
Measurement of gamma rays is important in radiotherapy, medical diagnosis, and environmental monitoring. Gamma rays interact with materials through ionization and excitation. Ionization is the key process to detect gamma rays, where part or all of the gamma rays energy give up to an electron in materials. The materials used to detect gamma rays are gas, solid or liquid. Irradiation of gamma rays to the system of an aqueous solution of the solid materials generates free radicals by radiolysis process. The free radicals recombine or interact with other species present in the solution produce the chemical changes and induce optical property changes of the solution. That phenomenon is used as a basis for developing of the chemical dosimeter to determine the amount of energy absorbed from ionizing radiation. The degree of chemical change associated with the absorbed dose is called as Gvalue and determines the sensitivity of chemical dosimeter.
Dosimeters based aqueous chemical system (such as ferrous sulfate and iodide/iodate solution) have been used in radiation dosimetry. The most widely use of chemical dosimeter is ferrous sulfate or Fricke dosimeter due to high sensitivity and high accuracy to X-ray and -ray irradiation for high absorbed dose level [1,2]. This level of detection is not sufficient to be used in radiotherapy. Consequently, some efforts have been made to improve sensitivity and develop standard Fricke dosimeter by adding various organic solutes including benzoic acid, xylenol and sorbitol [2][3][4][5] efforts successfully improved the sensitivity of Fricke dosimeter. However, its capability to measure Co-60 -ray with the precision of 0,1% is limited for doses range of 5 -25 Gy [6]. Meanwhile, the daily dose of -ray used in radiotherapy is in the range of 1.8 -2.0 Gy. Therefore, there is interest to find dosimeter for dose level radiotherapy application. Nanoparticles can be used to detect the high radiation energy like as: X-rays, gamma rays, and other particles (-particles, -particles) with high sensitivity because of large surface area. CeO 2 nanoparticles have attracted attention due to two oxidation states of Ce (Ce +4 and Ce +3 ) and potential to be developed for medical application [7][8][9][10]. Both oxidation states are associated with optical properties and have two characteristics spectrophotometric UV absorbance peaks. In the 230-260 nm range corresponding to Ce +3 absorbance and in the 300-400 nm range corresponding to Ce +4 absorbance. Similar to ferric ion in Fricke dosimeter, the property of those CeO 2 nanoparticles can be explored as sensitive materials for chemical dosimeter.
In this paper, the optical property of CeO 2 nanoparticles solution was studied to explore the potential CeO 2 nanoparticles for chemical dosimeter. CeO 2 nanoparticles were used in this study produced by ultrasonic radiation as we have reported, previously [11]. The absorbance spectrum of CeO 2 nanoparticles solution unirradiated and irradiated by gamma rays with doses range 1 to 2.5 Gy was measured by using UV-Vis spectrophotometer.

Synthesize of CeO 2 nanoparticles and preparation of CeO 2 solution
To synthesize CeO 2 nanoparticles, Cerium (III) nitrate (Sigma Aldrich, Germany) was dissolved in a mixed solvent of aqua demineralized and isopropanol with a volume ratio of 1:6. Ammonium hydroxide (3M) was added drop by drop to solution until pH 10. The solution was irradiated by ultrasound wave with a frequency of 40 kHz for 60 minutes. The product of irradiated solution was then washed and calcined at 100 o C for 3 hours. CeO 2 nanoparticles solution was prepared by dispersing CeO 2 nanoparticles in distilled water with a concentration of 0.25 x 10 -4 g/ml and then placed into the glass tube.

2.2.
Material characterization X-ray Diffraction (Phillips PW 1710) measurement was used to analyze the structure of nanoparticles. Morphology of the synthesized nanoparticles was observed by scanning electron microscope (JEOL JSM-6360LA). Optical properties of nanoparticles were measured by using ultraviolet-visible (UV-Vis) spectrophotometer (Shimadzu 1420).

Irradiation of CeO 2 nanoparticles solution
The CeO 2 nanoparticles solution in the glass tube was irradiated by Gamma rays by using Co-60 teletherapy air FCC8000F Series with the energy of 1.30 MV. The irradiation dose was varied at 1, 1.5, 2 Gy, and 2.5 Gy. The absorbance of unirradiated and irradiated CeO 2 nanoparticles solutions were measured by using UV-Vis spectrophotometer over the wavelength in the range of 200 nm -400 nm to observe absorption peak. The absorption peak was further used to analyze response and sensitivity of CeO 2 nanoparticles solution.

Structure and morphology of CeO 2 nanoparticles
The diffraction pattern of CeO 2 nanoparticle is shown in Figure 1 Rietveld refinement yielded the agreement factors are wRp = 13. 95% and R p = 10.82% with the goodness of fit (χ 2 ) of 1.02 and the refined cell parameter is 5.4218 Å. Figure 1(b) shows the Rietveld refinement plot of XRD pattern of CeO 2 nanoparticles.
(a) (b) Figure 1. (a) XRD pattern and (b) Rietveld refinement plot of CeO 2 nanoparticles. Experimental data is indicated by red tick marks, while calculated curve obtained after refinement is indicated by green solid line. The rose-colored of continuous curve represents the difference between the experimental data and the calculated curve.
The morphology and particles size of CeO 2 nanoparticles were analyzed by using SEM image as depicted in Figure 2. The surface morphology of CeO 2 appears to be rough and generally spherical in shape with estimated average particle size of 37 nm.

Optical properties of CeO 2 nanoparticles solution
The absorbance spectrum of un-irradiated and gamma irradiated CeO 2 nanoparticles solution under 1, 1.5, 2, and 2.5 Gy dose of gamma rays are shown in Figure 3. When CeO 2 nanoparticles solution was exposed to gamma radiation, the absorbance increased with increasing the gamma radiation dose and the optical absorption edge shifted to a shorter wavelength. Gamma irradiation with a dose of 1 to 2 Gy increased the absorbance slightly and the absorbance increased sharply at gamma irradiation dose of 2.5 Gy. The maximum absorbance was observed at a wavelength of 304 nm and became strong by increasing the dose. The wavelength of 304 nm was then used as a sensitive wavelength reference for further analysis of UV absorbance spectrum.   Figure 4 shows the absorbance at the wavelength of maximum absorption ( = 304 nm) of irradiated CeO 2 nanoparticles solution as function of radiation dose obtained from absorption spectra in Figure 3. The absorbance increased linearly with increasing radiation dose to 2 Gy and then the absorbance increased sharply at a dose of 2.5 Gy. It indicates that CeO 2 nanoparticles solution is capable of detecting gamma radiation. Capability of CeO 2 nanoparticles solution to detect gamma radiation can be explained through basic mechanism in Fricke solution dosimeter [12][13]. When the CeO 2 nanoparticles solution irradiated, the dominant reaction is water radiolysis. Radiolysis produces 5
The changes in absorbance of CeO 2 nanoparticles solution due to gamma radiation and the peak absorbance at 304 nm corresponded to the CeO 2 band gap. An increase in radiation dose causes an increase in number of radical and Ce 3+ ion formation at CeO 2 nanoparticles surface, and then change the band gap. The band gap of unirradiated and irradiated CeO 2 nanoparticles solution were determined by plotting (αhν) 2 vs (hν) for direct transition as described by Equation (1).
α is the optical absorption coefficient, hʋ is a photon energy (eV), E g is a band gap, A is a constant, and n = ½ for an indirect transition or n = 2 for direct transition. The calculated band gap is tabulated in Table 1. An increase in band gap with gamma radiation doses is mainly caused by changes in valence state and point defect formation due to radiation. The point defect may lead blueshift in the band gap [14][15][16][17][18].

Sensitivity of CeO 2 nanoparticles
The sensitivity of CeO 2 nanoparticles solution to gamma radiation can be determined from the change in absorbance (ΔA) and the radiation chemical yield (G-value) as described in Equation (2) [18].

( )
is the change in absorbance at  max , D is a dose (Gy),  is the linear molar extinction coefficient (L.mol -1 .cm -1 ), is a density (gr.cm -3 ), and b is the optical path length (1 cm). The change in The absorbance of irradiated CeO 2 nanoparticles solution as function of irradiation dose is shown in Figure  5. The slope of plot of ΔA to dose in the range 0 -2 Gy is used to calculate the G-value and the result was found to be 3.72 x 10 3 mol/J. The G-value obtained in this study is greater than G-value of other chemical dosimeter that is ~ 1.6 x 10 -6 mol/J for Fricke dosimeter [6] and ~14 mol/J for iodide dosimeter [19]. The result shows that CeO 2 nanoparticles solution possess a high sensitivity to gamma radiation.

Conclusion
The optical properties of gamma irradiated CeO 2 nanoparticles have been characterized by using UVvis spectrophotometer. The UV absorbance spectrum shows the peak absorbance at 304 nm was corresponding to the band gap of CeO 2 nanoparticles. Gamma radiation causes an increase in UV absorbance and the band gap of CeO 2 nanoparticles. High sensitivity and linear response in the range of 0 -2 Gy indicate the possibility of CeO 2 nanoparticles to be developed as a chemical dosimeter for radiotherapy application.