نشریه سنجش و ایمنی پرتو
سال دوم شماره 3 (پیاپی 7، تابستان 1393)

In this study MCNP-4C code is used to simulate the least shielding requirements for a mammography room. In order to simulate the X-ray tube, a molybdenum target is irradiated by 28 keV electrons. Then, a molybdenum filter with a thickness of 30 micrometers was used to remove the low-energy parts of X-ray spectrum. Finally the dosimetry evaluations were conducted after designing the lead shield of the X-ray tube and considering the default minimum thickness of the gypsum walls, concrete floor and ceiling that more often is expected in most buildings. Accordingly, 8.976 mrems/h was obtained as a result of simulation by the MCNP-4C code at a distance of 5 cm from the lead shield with 0.05 cm thickness. This amount of dose rate is substantially fewer than the maximum safe value of 100 mrems/h reported by the NCRP. Moreover, the dose rates calculated for the walls, floor and ceiling of the mammography room were obtained1.03e-02, 1.463E-02 and 6.476e-03 mrems/h respectively, that are fewer than the maximum safe amount of 5e-02 mrems/h. Therefore the results clearly indicate that extra shielding consideration is not required due to influence of the low X-ray energies used in mammography equipment.

Due to the increasing proliferation of radiotherapy centers in the country to treat cancer patients and raise the quality of treatment¡ it is needed to find the human equivalent phantom for dosimetry¡ calibration and quality control of radiotherapy devices. Generally¡ they are enumerated into two categories: homogeneous and nonhomogeneous phantoms (anthropomorphic). The main purpose of this study is the feasibility of the materials required for construction of nonhomogeneous phantoms of the human body with respect to existing facilities in the country¡ as well as phantom of the entire human body. For this purpose¡ Rando phantom was chosen as a reference to be made like Rando phantom with different materials. So the whole body Rando phantom CT images were available in the country. Using Solid Works software on CT images¡ the design was done. So that part of the bone¡ the hollow organs and lungs were removed in each design¡ then each were filled with the equivalent material. The Soft tissue equivalent was selected from the Plexiglas plastic¡ which has features similar to human tissue. The experiments were carried out on bone tissue equivalent percentage of burnt bone powder¡ epoxy resin and hardener. According to the results¡ the pulping samples of bone density 1.7 gr/cm3 and effective atomic number 13.674 (Zeff) are feasible. The design included a disk of Plexiglas plates that each disc includes the bone¡ the hollow organs and lung¡ filled with material equivalent. After complete drying of the dough bones on each disc¡ it is mounted on anthropomorphic phantoms that make up the human body.

Use of carbon nanotubes (CNTs) in dosimetry is a promising technique for real time in vivo dose measurements with high resolution for nuclear medicine and also dose rate monitoring for radiation protection utilizations. In this research with respect to extraordinary electrical properties of CNTs for several order of magnitudes increasing in electrical conductivity of polymeric matrix, the idea of application of polymer-CNT composite is introduced. One of the effective factors in the response of this kind of dosimeter is electrical conductivity variation due to radiation absorption. In this research, we simulated electrical conductivity of polyethylene-CNT composite with COMSOL Multiphysics software in different weight percentages of CNTs and then percolation threshold for two kinds of geometries random and array was evaluated and compared with Bruggeman Symmetric Equation. The results of simulation showed that the application of COMSOL software for evaluating effective electrical conductivity of PE-CNT composite is suitable for dosimetry purpose.

The use of gaseous ionization to calculate the absorbed dose of radiation beams is attractive because of its relative ease, suitable response, high accuracy and also cost-effectiveness. This work introduces the design and fabrication of a new parallel plate ionization chamber with two sensitive volumes. A series of standard tests was performed with x-rays, gamma radiation units, and beta sources at the Secondary Standard Dosimetry Laboratory (SSDL) in Karaj-Iran, for performance tests and calibration to assess the accuracy and reliability of the design. The stability, low leakage current and high ion collection efficiency are some of this instrument compared with the similar detectors. The result of performance tests showed the usefulness of this new parallel plate ionization chamber as a standard dosimeter in laboratories. Additionally, the Fluka Monte Carlo simulations provide useful information regarding energy deposition in the chamber’s sensitive volumes and the effect of various materials on the efficiency of the device.

Precise measurement of void coefficient in two phase flow is one of important parameters in thermal hydraulic parameters such as average density, two phase flow pressure drop and two phase flow pattern. In this study the void coefficient has been correlated experimentally based on attenuation coefficient gamma ray emitted form 137Cs source passing a rectangular channel. The average void fraction was reasonably measured for a two-phase flow in the rectangular channel

In this research work, the biological effect of local green algae on adsorption of 226Ra concentration in 5 samples of hot spring̕ s water of Ramsar city has been investigated. To increase the cellular adsorption capacity of green algae it was first treated by one molar solution of NaCl for omission of obtrusive ions. The radioactivity of 226Ra in the samples was measured using HPGe spectrometry system. The amount of algae used for each sample was 3 gr/lit. The obtained results show that the maximum adsorption occurs in radium concentration of 30 Bq/lit and equilibrium time of 4 hours. When the amount of radium increases, absorbent being saturated and then the adsorbtion rate of radium in water̕ s samples decreases. Due to the effect of temperature on adsorption, so all of the adsorption processes have been performed in 20° C using magnetic mixer which controls the temperatures of the samples.

In this study, the MCNPX code version 2.6 has been used to simulate a proton therapy in thyroid gland. The neck phantom is cylindrical including skin, adipose, skeletal muscle and thyroid gland. Radiation source is close to the skin surface and single energy protons are collided perpendicular to the skin surface. The results of the simulation shows that the best proton energy interval, to cover completely the thyroid tissue, is from 42 to 74 MeV, assuming that the thyroid gland has 2 cm thickness and is located 1.12 cm under the skin surface. Share of the energy distribution inside thyroid gland from protons and neutrons produced in (p,n) reaction for the best proton energy interval is 52-92 and 82-87 percent, respectively. Proton and neutron average energy deposition raises by increasing incident proton energy in thyroid gland and obey the quadratic polynomial equation.