SYNTHESIS OF LIF:MG,TI NANOPARTICLES BY CO-PRECIPITATION METHOD AND EVALUATION OF INCREASING THE 5A/5 RATIO IN ALPHA AND GAMMA RADIATION.

The peak 5 in LiF: Mg, Ti includes sub-peaks 5a and 5b, which occur at the temperatures lower and higher than that of peak 5, respectively. Peak 5a in LiF:Mg,Ti occurs due to the localized recombination of trapping/luminescence center (TC/LC), in which the electron is released from the electron trap by obtaining energy from heat and recombines through the tunneling phenomenon with a hole located in the adjacent luminescence center at a distance of 3 nm. Concerning the standard TLD tablets, which are composed of micron-sized particles, the peak 5a either does not occur or appears with very low intensity, which is insignificant in terms of dosimetry. Thus, the present study focuses on synthesizing thermoluminescent nanoparticles by co-precipitation method in several stages by citing models based on the maintenance of linear behavior of thermoluminescence nanopowders up to high doses and its relationship with localized electron-hole recombination. In addition, by changing the concentration of ingredients, altering the temperature of the reaction medium and presence or absence of surfactant, nanoparticles with suitable geometric shapes were achieved. The synthesized nanopowders were irradiated with different doses of alpha and gamma, and after analyzing the glow curves, the increase in peak 5a/5 was reported as the main factor in nanodosimetry. Based on the results, the LiF: Mg, Ti thermoluminescence nanopowders can increase the 5a/5 ratio and can be used as a convenient, inexpensive and practical tool to estimate the amount of energy deposited by the beams in nanoscale.

Synthesis and evaluation of thermoluminescence properties of ZrO2:Mg for radiotherapy dosimetry.

This study aimed to investigate the thermoluminescent properties of ZrO2:Mg irradiated with a 6 MV X-ray beam and its potential application in radiotherapy dosimetry. ZrO2 powder was synthesized using the sol-gel method and Mg was used as a dopant. Irradiations were performed with ZrO2:Mg chips located at the center of a 10 × 10 cm2 radiation field at a source surface distance of 100 cm, below a stack of solid water slabs, at the depth of maximum absorbed dose. The investigated characteristics of the material included linearity with radiation dose, reproducibility, accuracy, sensitivity and fading. Regarding the intrinsic difference of the samples, the glow curves of the investigated ZrO2:Mg chips exposed to 1 Gy of 6 MV X-rays exhibited three or four peaks. The ZrO2:Mg samples showed a 47% fading at 24 h after irradiation, and the reproducibility of the thermoluminescence reading of ZrO2:Mg for equal irradiation conditions was ± 21%. The thermoluminescence response of the investigated ZrO2:Mg samples to various absorbed doses from 0.5 to 2.5 Gy showed a gentle increase of the thermoluminescence intensity with increasing absorbed dose. The obtained results show that ZrO2:Mg is not an appropriate candidate for X-ray photons in radiotherapy, due to low thermoluminescence peak temperature, low reproducibility, low sensitivity to various absorbed doses and significant fading.

A Monte Carlo evaluation of dose distribution of commercial treatment planning systems in heterogeneous media.

INTRODUCTION: Calculations from a treatment planning system (TPS) in heterogeneous regions may present significant inaccuracies due to loss of electronic equilibrium. The purpose of this study is to evaluate and quantify the differences of dose distributions computed by some of the newest dose calculation algorithms, including collapsed cone convolution (CCC), fast Fourier transform (FFT) convolution, and superposition convolution, in heterogeneity of the lung. MATERIALS AND METHODS: A 6-MV Siemens Primus linear accelerator was simulated by MCNPX Monte Carlo (MC) code, and the results of percentage depth dose (PDD) and dose profile values were compared with measured data. The ISOgray TPS was used and PDDs of CCC, FFT, and superposition convolution algorithms were compared with the results obtained by MCNPX code. CT2MCNP software was used to convert the computed tomography images of the lung tissue to MC input files, and dose distributions from the three algorithms were compared to MC method. RESULTS: For PDD curves in buildup region, the maximum underdosage of ISOgray TPS was at the surface (19%) and comes in closer agreement when depth increases (average 7.08%). Dose differences (DD) between different algorithms and MC were typically 4.81% (range: 1.95% to 7.30%), -1.55% (range: -5.14% to 5.26%) and 4.96% (range: 2.00% to 7.4%) in the lung for the CCC, FFT, and superposition algorithms, respectively. The difference between monitor units and maximum dose calculated using the three algorithms were 0.5% and 1.61%, respectively. The maximum DD of 7% was observed between MC and TPS results. CONCLUSION: Significant differences were found when the calculation algorithms were compared with MC method in lung tissue, and this difference is not negligible. It is recommended to use of MC-based TPS for the treatment fields including lung tissue.

Evaluation of the accuracy of various dose calculation algorithms of a commercial treatment planning system in the presence of hip prosthesis and comparison with Monte Carlo.

PURPOSE: High atomic number elements are commonly used in a hip prosthesis which can cause uncertainty in accurate dose calculations in radiation therapy. The aim of this study is to assess the accuracy of the three various algorithms of ISOgray treatment planning system in the presence of hip prosthesis by Monte Carlo (MC). MATERIALS AND METHODS: A MC model of Siemens PRIMUS linear accelerator has been built and verified by the measured data of the different algorithms of ISOgray treatment planning systems (TPS) in 6 and 15 MV photon beam energies. Two types of hip prosthesis have been used: stainless steel and titanium. The accuracy of mentioned dose calculation algorithms in the presence of hip prosthesis was evaluated. RESULTS: There were 24.78%, 27.68%, and 27.72% errors in fast Fourier transform (FFT) Convolution, collapsed cone (CC), and superposition in 6 MV photon beam and 26.45%, 30.45%, and 28.63% in 15 MV photon beam for titanium type, respectively. However, there were 32.84%, 35.89%, and 35.57% in 6 MV photon beam and 38.81%, 47.31%, and 39.91% errors in 15 MV photon beam in steel type, respectively. In addition, the ISOgray TPS algorithms are not able to predict the dose enhancement and reduction at the proximal and distal prosthesis interfaces, respectively. CONCLUSIONS: Hip prosthesis creates a considerable disturbance in dose distribution which cannot be predicted accurately by the FFT convolution, CC, and superposition algorithms. It is recommended to use of MC-based TPS for the treatment fields including the hip prosthesis.

The effect of Mg, Cu and P impurities on dose response of LiF:Mg,Cu,P phosphors: simulation versus experiments.

In this research, the effect of magnesium (Mg), copper (Cu) and phosphorus (P) impurities on dosimetry response of LiF:Mg,Cu,P phosphors is studied experimentally and by the simulation procedure. In the experimental procedure, LiF:Mg,Cu,P phosphors in the powder form were synthesised by chemical co-precipitation method. After annealing at 250°C for 10 min, known amounts of powder were exposed to gamma doses from 0.2 to 1 Gy. The activation energy of the electronic traps for the dosimetric peak at 150°C in LiF:Mg,Cu,P crystalline lattice obtained was 0.69 eV. In the simulation study, the role of stated dopants on electronic and structural properties of LiF crystalline lattice is investigated with the WIEN2 K Code. The activation energies of the electronic and hole traps for the dosimetric peak at the same temperature in LiF:Mg,Cu,P crystalline lattice obtained are 0.75 and 3.1 eV, respectively. It is shown that the experimental results are in agreement with simulation results.

Neutron dose measurements of Varian and Elekta linacs by TLD600 and TLD700 dosimeters and comparison with MCNP calculations.

High-energy linacs produce secondary particles such as neutrons (photoneutron production). The neutrons have the important role during treatment with high energy photons in terms of protection and dose escalation. In this work, neutron dose equivalents of 18 MV Varian and Elekta accelerators are measured by thermoluminescent dosimeter (TLD) 600 and TLD700 detectors and compared with the Monte Carlo calculations. For neutron and photon dose discrimination, first TLDs were calibrated separately by gamma and neutron doses. Gamma calibration was carried out in two procedures; by standard 60Co source and by 18 MV linac photon beam. For neutron calibration by (241)Am-Be source, irradiations were performed in several different time intervals. The Varian and Elekta linac heads and the phantom were simulated by the MCNPX code (v. 2.5). Neutron dose equivalent was calculated in the central axis, on the phantom surface and depths of 1, 2, 3.3, 4, 5, and 6 cm. The maximum photoneutron dose equivalents which calculated by the MCNPX code were 7.06 and 2.37 mSv.Gy(-1) for Varian and Elekta accelerators, respectively, in comparison with 50 and 44 mSv.Gy(-1) achieved by TLDs. All the results showed more photoneutron production in Varian accelerator compared to Elekta. According to the results, it seems that TLD600 and TLD700 pairs are not suitable dosimeters for neutron dosimetry inside the linac field due to high photon flux, while MCNPX code is an appropriate alternative for studying photoneutron production.

The measurement of gamma-emitting radionuclides in beach sand cores of coastal regions of Ramsar, Iran using HPGe detectors.

Radionuclides which present in different beach sands are sources of external exposure that contribute to the total radiation exposure of human. (226)Ra, (235)U, (232)Th, (40)K and (137)Cs analysis has been carried out in sand samples collected at six depth levels, from eight locations of the northern coast of Iran, Ramsar, using high-resolution gamma-ray spectroscopy. The average Specific activities of natural radionuclides viz., (226)Ra, (235)U, (232)Th, (40)K and (137)Cs, in the 0-36 cm depth sand were found as: 19.2±0.04, 2.67±0.17, 17.9±0.06, 337.5±0.61 and 3.35±0.12 Bq kg(-1), respectively. The effects of organic matter content and pH value of sand samples on the natural radionuclide levels were also investigated. Finally, the measured radionuclide concentrations in the Ramsar beach were compared with the world average values, as reported by UNSCEAR (2000). None of the studied beaches were considered as a radiological risk.