نشریه سنجش و ایمنی پرتو
سال ششم شماره 4 (پیاپی 24، تابستان 1397)

Due to the increasing of closed ventilation systems used in buildings, the risk of indoor radon is increased day by day. In this study, we selected two sample rooms to calculate the radon emission coefficient from different levels, radon concentration measured when the natural ventilation was completely closed and compared with the analytical results. The results clearly show the difference in the amount of radon concentration at different levels; then, by overcoming the common gaps in the entrance of the rooms, the natural ventilation caused by the temperature difference between the room and the outside is obtained. By substituting them in the analytical equations, the radon concentration is calculated with ventilation. The calculated results are in good agreement with the experimental results and show the important role of natural ventilation to reduce indoor radon even to over 90%.

In this work, calculations of leaked dose from vehicle inspection area, the absorbed dose by driver and operator, and the required concrete thickness for shielding the inspection area are presented using the Monte Carlo code MCNPX2.6. The X-ray source of the present vehicle inspection system is a LinatronMi6 accelerator, end of which has only been simulated in this study (the electron beam colliding with the anode, anode, filters, and the paralyzers in the path of the generated photons). In this study, first the optimum electron beam current with the energy of 6 MeV in the LinatronMi6 is calculated in such a way that a dose rate of 8 Gy/min in 1 meter away from the accelerator is obtained. Then, the absorbed dose rate by driver is calculated in different vehicle speeds: 0.2m/s, 1m/s, 2m/s, 3m/s and 4m/s. Also, three different materials - air, water and iron for cargo have been considered and for each material, the absorbed dose by driver is calculated separately. Simulation results show that the absorbed dose by the driver for the lower speeds of vehicle and higher atomic numbers and densities of material is higher. Therefore, by assuming 2 m/s as the vehicle speed and iron as the cargo material (worst case scenario), the required lead and steal thickness for shielding the operator room is calculated to be 3mm steal and 10.5 mm lead. At the end of this study, the required thickness of concrete for the inspection area walls is calculated. According to the simulation results, 52 cm concrete in the entrance gate and 40 cm in other parts are sufficient to keep the leaked dose from the wall lower than 5μSv/h.

Proton therapy is a common form of external radiation therapy based on the manipulation of Bragg peak of this beam, it can treat the tumor by delivering high levels of doses to it, while protecting surrounding healthy tissues against radiation. In this work, the dose distribution of proton and secondary particles such as neutrons, photons, electrons and positrons in gastric cancer proton therapy have been studied. For this purpose, using a single-energy proton beam, the suitable energy range has been calculated for treating a tumor in the gastric tissue of a male adult male MRD-UF phantom. Then, the SOBP of the matrix method has been constructed using the dosimetry results. The primary and secondary doses were also examined in 12 surrounding healthy organs. The results show that the primary dose in healthy organs is much smaller than the dose received by the tumor. Although the secondary dose is small, but its amount in the stomach, spleen, pancreas and left kidney is higher than those in other organs. The results of the study show that the proton therapy of gastric cancer can represent an almost ideal radiation therapy by measuring the risk levels of secondary particle dose, in particular, the equivalent dose of neutrons.

In this work, the growth of CsI(Tl) crystals as gamma detector was performed using vertical Bridgman method with Ca and Tm co-dopants. For evaluation of the growth crystals, X-Ray Diffraction (XRD), photoluminescence and thermoluminescence spectra as well as recorded gamma spectra were employed. In addition, the scintillation properties including energy resolution, scintillation decay time, absolute light yield, and afterglow were studied and measured. Scintillation properties of CsI(Tl) as the standard sample (Amcrys Company) and the fabricated detectors were compared. Based on the results, the light yield, energy resolutio, and decay time for CsI with Tl dopant for 661 keV are 97%, 11% and 1054 ns, respectively. The change in observed scintillation characteristics is due to creation of luminescence level and reduction of hole-trap centers by Tm, Ca codopant in CsI(Tl) structure. By Tm2+ codopant in CsI(Tl) structure, the light yield, energy resolution for 661 keV, and decay time are 103%, 9.7%, and 1373 ns, respectively. This crystal has smaller scintillation decay time and slower afterglow in comparison with CsI(Tl). Ca2+ codopant has undesirable effects on the light yield and energy resolution, while its scintillation decay time is faster and leads to higher afterglow than CsI:Tl. The light yield, energy resolution for 661 keV, and decay time are 98%, 12%, and 537 ns, correspondingly.

Monte Carlo method is the most accurate method for simulation of radiation therapy equipment. The linear accelerators (linac) are currently the most widely used machines in radiation therapy centers. Monte Carlo modeling of the Siemens Primus linear accelerator in 6 MeV beams was used. Square field size of 10 × 10 cm2 produced by the jaws was compared with TLD. Head simulation of Siemens accelerator and dose calculation were performed with BEAMnrc and dosxyznrc respectively. The results of simulation were validated by measurements in water by TLD. At 6 MeV and 0.35 cm FWHM, the agreement between dose calculated by Monte Carlo modeling and direct measurement was obtained. The results were accurate and the EGSnrc simulation can be used for dosimetry. After validating this code, it was used to confirm the function of the optical computed tomography system built in the Nuclear Physics Department of the University of Mazandaran. The system used to perform the gel dosimetry independent of other methods such as MRI.

Imaging and identifying materials with high atomic numbers and densities, especially radioactive materials, is one of the issues that have been especially considered in recent years. Due to some limitations in conventional and old imaging techniques, finding an alternative method is very important. The cosmic muons with an infinite source are one of the sources that have been recently studied for heavy objects imaging, and the major efforts in this field have focused on increasing the accuracy and efficiency of muon radiography systems. Use of plastic scintillator improves the accuracy and reduces the cost of muon radiography. In recent decades, different methods have been proposed for the use of these scintillators in development of muon radiography systems. Since the basis of the work of radiography systems is the spatial resolution of the detector, any study that can increase the accuracy of a muon detection system will ultimately improve the quality of muon radiography system. In this study, using a special configuration of plastic scintillators and photomultiplier tubes, the resolution of the muon location identification on the detector plate is optimized. The simulations performed by Geant4 code for different states, show that the final resolution of the detector plates is about 1 cm and, in terms of the resolution of detection, the best scintillator available for the proposed system is BC-408. In addition, among the examined photomultiplier tube layouts, the most accurate configuration is square alignment with uniform distances and the most suitable angle of positioning of the plates relative to the horizons is 10 degrees.

This paper presents the study on the effect of detector type in estimating gamma rays buildup factor. In this regard, the flux buildup factors of gamma rays emitted by an isotropic point 60Co source in mean energy 1.253 MeV was evaluated after passing through the Pb, W, Zn, and Ti sample in thickness of 1-5 cm by MCNPX. Four common detector types in simulation of measurement was defined to estimate the gamma rays flux: an empty cylinder, Geiger-Muller counter, NaI(Tl) scintillator, and point detector. Due to the deep penetration condition, weight windows varience reduction technique is applied to achieve more accuracy and less computer run time simultaneously. The present work confirm the buildup factor dependence on atomic number of materials, and detector type. The results show that the value of buildup factors was varied from 1 to 4 in different states. However, the variation of buildup factors follow almost similar trend in different detector types. According to the results, applying point detector in simulation is suggested which lead to increased accuracy of simulation and decreased computer run time.