نشریه سنجش و ایمنی پرتو
سال ششم شماره 5 (پیاپی 25، پاییز 1397)

Polymer nano-composites are a group of materials that represent proper mechanical, chemical, thermal and optical properties due to the presence of certain percentages of a filler in a polymeric matrix. In this study lead oxide nanoparticles were prepared by direct precipitation method. Then the lead oxide-epoxy resin nanocomposite was prepared by mixing (without solvent) with different weight percentages of filler. Gamma-ray attenuation of nanocomposites was studied using a 137Cs source which produces gamma rays with energy of 662 keV. The results of the measurements show that nano-composite shields have a good performance compared to lead. The linear attenuation coefficient of samples increases as the filler concentration increases. Also composites contain the nano-sized particles as filler are more efficient than the one has macro-sized particles. Therefore, these types of shields are more relevant due to lightness, affordability and less environmental hazards.

Precise calibration of energy and efficiency of gamma spectrometry systems for different geometries is very important. In most cases, preparation of these sources for educational and research centers  is very difficult due to high costs and lack of access to the reactor. The purpose of this paper is to explain the procedure for production of calibration sources 65Zn and 60Co using instrumental neutron activation analysis (INAA) method. These sources are used for calibration of gamma spectrometry systems. To do so, 65Zn (3.83 and 9.38 µCi) and 60Co (3.65 and 3.85 µCi) were produced. The quality control of the sources indicates the acceptable quality of the produced sources.

Some areas in Ramsar, a northern city of IRAN, have the most natural radiation level in the world which is due to high concentration of 226Ra and its daughter products specially 222Rn and therefore these regions have been interested for radiation related investigations. The aim of this study also has been to evaluate public exposure in such areas in Ramsar, in order to obtain up to date information of the region. In this regard, 24 houses in the Ramsar HBNRAs were selected and indoor annual effective dose for some of the inhabitants (24 persons) were measured by passive methods using thermoluminescence dosimeters (TLDs) and radon diffusion chamber. Gamma dose rates were determined using survey meter, RDS-110.
The measured annual effective dose due to 226Ra and 222Rn were in the range of: 2.11 up to greater than 68 mSv and indoor Rn concentrations varied between50 to greater than 2280 Bq.m-3.  The total annual effective dose due to indoor gamma radiation (due to 226Ra and its daughter products, 214Pb and 214Bi) and also 222Rn was measured in the range of 2.11 to greater than 68 mSv.

The aim of this study was to estimate the delivered doses to tissues and organs around the liver tumor during radiotherapy and to determine organs with a high probability of secondary induction cancers. This study was carried out using Monte Carlo method.
The used Phantom, the ORNL phantom is for an adult female. The volume of the tumor is as much as 30% of the liver tissue.In the initial step of this study, we will focus on simulating the linear accelerator of Varian 2100 6MV and by comparing the output results with the calculated values;​​ we will make sure that the results of MCNPX code are sufficiently accurate. In the next step, the corrections are considered in phantom and Monte Carlo technique was used for radiotherapy. The dose calculations was done for 38 organs. According to the obtained results in this simulation, the reached doses to other tissues surrounding tumor was in the range of 0.01-13.07%. Among the examined organs, the healthy tissue of the liver, kidney, the left part of bone_rib cage, the right part of bone_rib cage, adrenals pancreas and lung have received 88, 24.5, 24.2, 18.8, 15.9, 7, 5 cGy, respectively. Due to the linear relationship between the dose received in each organ and the risk of cancer of the organ in subsequent years, in the BERT model, the kidneys have a higher risk of secondary cancers than other organs. In calculating of the risk by Monte Carlo method at a given time, the healthy part of the liver has the highest risk of inducing secondary cancers, and the risk of inducing secondary cancers for kidney, the right part of cage, the left part of cage and adrenal glands were 81, 48, 36 and 18 percent, respectively. Secondary cancer induction is more likely for patients who require more therapy for effective radiotherapy than other patients.

Bipolar junction transistors (BJTs) are active semiconductor devices commonly used for amplification and switching applications. In this study, transistors have been biased to operate in active region and by measuring the electrical characteristics of BJTs, the effect of gamma irradiation on each of these characteristics was investigated before and after the gamma irradiation by 60Co source. In order to measure the electrical characteristics, for each of the transistors under consideration, the appropriate circuit was designed. The experimental results show that by increasing the dose received by each transistor, the collector current decreases and collector-emitter voltage increases. So that the highest change in electrical characteristics of BD911 and 2N3420 transistors is observed after receiving the amount of 20kGy dose of gamma irradiation. On the other hand, doses of gamma radiation in the range of less than 1kGy does not have considerable effect on the electrical characteristics of BJT transistors. So, these transistors have the highest structure resistance against the gamma rays and can be used in designing electronic circuits used in radiant environments.

In this study, the energy dependency for dosimeters of air ionization chambers, lithium fluoride, silicon and plastic scintillator has been studied using the MCNPX Monte Carlo simulation code and simulated for gamma energy in the range of radiotherapy energy. The simulation results show that the response of each of the dosimeters for gamma photon beams in the energy range of 0.2 to 20 MeV varies. Dependence of dose to energy for the air ionization chamber dosimeter is higher than other dosimeters. Among the dosimeters under study, the ratio of absorbed dose of plastic scintillator to the absorbed dose of water in the energy range indicates the lowest dependence to energy. The results of the absorbed dose of the dosimeters to the absorbent dose of water show that the plastic scintillator has the highest and the silicon has the lowest equivalence with water. In the analytical studies, the ratio of absorbed dose of the dosimeters to the absorbed dose of water was calculated using the Berlin theory and compared to Monte Carlo results. Comparisons show good agreement between analytical and simulation results.

One of the methods of studying the physiology and metabolism of important tissues such as the heart, brain and cancer tumors is the use of PET/CT System. A small number of patients are pregnant women who undergo a PET/CT scan due to lack of knowledge about pregnancy or due to dire clinical need. The final dose received by the fetus is based on three factors: the absorbed dose of the fetal tissue due to radiopharmaceuticals absorption, the maternal tissue doses resulting from the absorption of radiopharmaceuticals in the mother's body, and the dose received from the CT scan. Overall, it is necessary to evaluate doses in order to minimize damage to the fetal nervous system. In this paper, using the pregnant woman's Body Builder Computed Phantom and the MCNPX computational code, the absorbed doses received from the CT component of Biograph 6(20-345mA, 80,110, 130 kV), used for attenuation correction of PET images, was calculated to be 4.5, 9.2, 13 (mGy/100mAs) for the first trimester, second trimester and third trimester of pregnancy, respectively, and according to ICRP recommendations No. 103, this range of doses should be considered for the fetus. Using the results of this study, feasibility of assessing doses to embryos in pregnant women in nuclear medicine centers is possible. The results also showed that using the Builder Body Phantom, it is possible to evaluate the fetal doses during pregnancy during CT imaging.