نشریه سنجش و ایمنی پرتو
سال هشتم شماره 5 (پیاپی 34، تابستان 1399)

Damage of tungsten due to helium and argon ions of a PF device was studied. Tungsten samples were irradiated by 20 shots of the plasma focus device with argon and helium as working gases, separately. The tungsten surface was analyzed by SEM, before and after irradiation. SEM revealed dense blisters with diameters of a few hundred nanometers, on the samples which were irradiated by helium ions, while on the samples irradiated by argon, cracks were developed on the surface. XRD analysis was used for crystallography of tungsten, before and after irradiation. Irradiation by helium and argon affects peak location, peak intensity, FWHM, and spacing of planes, which shows that the crystalline structure of tungsten is affected by irradiation.Characteristics of ion beam of the plasma focus device were calculated by the Lee code. The depth profiles of dpa/shot and ion concentration were calculated using the SRIM code. Using the Lee code, the average fluencies of helium and argon ion beams of the PF device were calculated about 7.9×1014 cm-2 and 0.25×1014 cm2 per shot, respectively. SRIM revealed that the maximum DPA was about 1.7 and 0.17 and occur in the depth of 7 nm and 30 nm for argon and helium, respectively. Also, the maximum concentration occurs in the depth of 20 nm and 40 nm for argon and helium, respectively.

Neutron is one of the products of the interaction of high energy protons with light nuclei. Using computational Monte Carlo codes, it is possible to calculate and predict the neutron spectrum, neutron flux, and doses by simulating the system. But the use of different hadronic models in this process can influence the results of these calculations. In this research, the response of the four hadronic models in the Geant4 Monte Carlo tool is compared with each other in the interaction of proton with light nuclei in terms of the characteristics of the produced neutrons. These models are referred to as the binary cascade model, the Precompound model, the Bertini intranuclear cascade model, and the Liege Intranuclear Cascade model. This comparison has been done in the area of clinical energy in proton therapy. Also, the results of this simulation are compared with experimental data and the adaptation or incompatibility of these models with empirical results has been reviewed and interpreted. The results show that none of the studied models fully reproduces empirical results, but in a certain range of energy, the Bertini model has the best fit with experimental results.

In this study, the MAGIC- f polymer gel was used to determine the absorbed dose by external radiation therapy. The MAGIC- f gel was made under normal pressure and temperature conditions and irradiated with a Siemens 6MV linear accelerator at 10 × 10 cm2 and at a depth of 10 cm. While simulating the head of the accelerator unit by the Monte Carlo method with BEAMnrc code, the percentage depth dose (PDD) and dose profile is calculated in different fields of 4 × 4, 6 × 6 and 10×10 cm2 with DOSXYZnrc code. Also after gel irradiation, the gel readings were performed by CT optical apparatus and made at the Nuclear Laboratory of Mazandaran University. The results of the experimental and simulation results show that the PDD diagram and dose profile are in good agreement with the maximum error rate of 2% and these results confirm the accuracy and efficiency of the OCT device designed for dose reading in radiation therapy.

In the present work, dose equivalents of radiosensitive organs in head and neck region have been calculated during nasopharynx proton therapy. For this purpose, a middle-size tumor was designed with the use of clinical information, and was then incorporated into the adult male ICRP phantom. According to the medical data, eight radiation paths around the neck were selected. After that, appropriate proton beams for each radiation paths were designed considering the type and thickness of tissues in the beam path. Since both of soft tissue and bone are existed in pathways of radiation, firstly, the effect of the existence of bone tissue in the path of protons on the position of the Bragg peak was studied. Results showed that when the thickness of bone tissue within the phantom increased by 1 cm, the Bragg peak was pulled back about 0.6 to 0.8 cm. It was also found that the displacement of the Bragg peak by increasing the bone thickness follows a polynomial function for each proton energy. Considering the thickness of the tumor, optimized SOBP were designed for each of eight directions. Finally, doses to different sensitive organs of head and neck region were computed in terms of the therapeutic dose of the tumor. Results indicated that thyroid and brain received higher doses in comparison with other organs.

In this research, water equivalent ratios (WER) at helium ion beam energy ranging 25-250 MeV/u for four potential plastic dosimetric materials: polycarbonate (PC), polypropylene (PP), polymethyl methacrylate (PMMA), and paraffin have been calculated using MCNPX Monte Carlo code. Among studied materials, PC and PP with 0.979 and 1.177 show minimum and maximum differences with water respectively. In mentioned materials by increasing mass density, WER values increase and by decreasing mass density they decrease. Calculated values are in good agreement with the results published in literatures.

In this study, the detection of alpha particles in the high current mode is investigated in the presence of two gaseous electron multipliers in the micro-pattern structures. Thick Gas Electron Multiplier (TGEM) and an Electron Multiplier Assemblies (EMA) were designed and constructed as amplification gap in the detector chamber. An ion chamber is employed to measure the current of interaction between alpha particle and quenching gas. The appearance of visible light columns in the space between the multiplier electrodes at the location of input particles is one of the advantages of using this optical method to detect ionizing radiations.

Radiation therapy, is one of the treatments for cancer. Brachytherapy is a radiotherapeutic technique that a sealed radiation source is placed inside or next to the tissue and has become a mainstream treatment option for cancer. Achieving maximum dose to the gland as well as minimum injury to the adjacent tissues is a basic principle in radiation therapy. Sources configuration must be designed in such a way that the dose distribution agrees with treatment planning and the prescribed dose. Exact evaluation is necessary to avoid excessive dose to other organs especially organs at risk. Optimization algorithms are applied in preplanning treatment. The aim is to deliver a desire dose in the border of tumor while all of the points inside the tumor receive the absorbed dose more than the dose border values. The accuracy of optimization and finding the best position of seeds is so important in treatment planning. We used PSO algorithm to optimize the places of 103Pd seeds which are applied in the prostate brachytherapy. The algorithm was performed for desired tumor shape in various conditions. In circle and ellipsoid tumor with 5 and 40 desire seeds, more than 90% of points of border receive the prescribed dose. Moreover, the optimized characteristics of seeds and their best positions were determined. The results show that the PSO algorithm can be used for optimizing the position of interstitial implantation brachytherapy that so many seeds used. Distribution dose and isodose curve were studied to assurance delivering adequate dose to target volume while keeping healthy tissues. Few parameters, easy implementation and fast convergence around the global answer are advantages of this algorithm.

The present study deals with a new method for measuring the power of a reactor. This method uses gamma and neutron radiation resulted from the entire reactor structure, without changing its structure (online). In terms of functionality, this method can measure the reactor power in real-time and report it instantly. In order to obtain the relationship between reactor power and gamma and neutron leakage radiation by simulation, the values of the F5 tally are calculated at different distances from the reactor wall, using the MCNP5 code (Monte Carlo N-Particles). Then, we plot the diagram to observe the variation trend. However, an increase in the distance from the reactor reduces both the amount of radiation and the energy of the radiation particles until the neutron leakage radiation reaches zero within five meters of the body, due to its good insulation. However, this number is only about four meters for gamma irradiation. The reactor power can be calculated by measuring the flux at the given point and any time by determining the point for a range of leakage radiation reduction to background and the linear relationship between power and leakage flux.