نشریه سنجش و ایمنی پرتو
سال چهارم شماره 3 (پیاپی 15، تابستان 1395)

The purpose of this study is to select the best place for the detector towards shield and the gamma source which have the least flux buildup factor of the photon. In this research, the flux buildup factor of gamma rays was calculated by using of MCNP-4C code for cylindrical sources 137Cs, 60Co, 16N coaxial shields made up of aluminum, iron and lead with a constant thickness 1.55 cm in detector CsI (Tl). For this purpose, detector was respectively placed at different distances from 9 to 18 cm and different angles from 30 to 150 degrees with respect to the axis of the source. In this study, we tried to assess the dependency of the buildup factor with material atomic number, gamma-ray energy, relative distance of detector-material-source and angle of detector to source. The obtained results showed that the buildup factor values are variable in a wide range from 1 to 3.5. According to the results of simulation, if the aim is reduction of the buildup factor source, shield and detector should be designed parallel and coaxial. Otherwise, whatever deviation of detector from the central axis is increased, the gamma-rays buildup factors are greater in detector.

In this research, a method for considering the spatial variation of deal layer (DL) thickness at the lateral and top surfaces of HPGe detectors is presented to achieve a more precise simulation model for efficiency calculation especially in marinelli beaker geometries. As a good approximation, the lateral surface of the detector is divided into 12 segments, assuming each segment covers 30 degrees and the top surface of detector is divided to 25 sections. The detector efficiencies are experimentally measured in each lateral segment and also in 25 points at the top of detector, using Am-241 source at each mentioned positions. The nearest DL thickness for each segment is selected through MCNP calculations of Full Energy Peak Efficiency (FEPE) with different DL thicknesses. Then a proposed detector model with a lateral DL thickness equal to average of selected DL thicknesses on lateral surface and a top DL thickness equal to average of selected DL thicknesses on top surface is used for FEPE calculation of bulk sample geometries such as Marinelli beaker containers. The resulting efficiencies are compared with the experimentally measured efficiencies of a Marinelli beaker containing a set of standard radiation source solution with specified activities. The experimental and simulation results of FEPE show a very good agreement.

Ionization radiation induced decomposition of dyes is a concept frequently exploited to develop devices to measure the radiation dose. In this study, using this concept, preparation of polyvinyl chloride films doped with different amounts of quinacrine dihydrochloride were done and their capabilities for dosimetry of gamma radiation doses in the range of 0 to 60 kGy were evaluated. The results showed that films containing 2%w quinacrine dihydrochloride have the best dose response in the investigated dose range and the lowest fading within 60 days. Therefore, these films are suitable for high dose dosimetry in radiation processing industry.

The Space Environment consists of various radiations sources which emit different types of particle. Due to their high energy and high speed, these space particles can affect materials which they impact, in different ways. One of the most important damages which affect electronic instruments is Total Ionizing Dose (TID).In this article, modeling of these effects is performed by GEANT4 toolkit and Monte Carlo method. This toolkit performs simulations by using C programming language. Total Ionizing Dose effects are simulated for an operational satellite. Regards to considered conditions, failure or success probability in worst case has been analyzed. Based on ECSS, results satisfy standard and mission requirements.

Radon concentration was measured in ground floor of two buildings with moisture controlled sub slab construction and slab on the ground. Results show that radon concentration in ground floor of the building with moisture controlled sub construction is more than 11 times smaller than that of the building with slab on the ground. Also indoor radon concentration in this building is about %41 higher than that of free air around the building. The obtained results of radon emanation on the ground surface and soil gas radon in depth of 1 meter show that soil gas radon of building 2 is about 5 times higher than that of building 1. Whereas the radon emanation rat from the ground surface of building 2 is only about 30 percent higher than that of building 1. It is concluded the there is no direc correlation between them. So different parameters such as compressibility of the soil matrix is highly affected on radon emanation from the ground surface.

An alpha-gamma coincidence method, using multi-parameter analyzer (MPA) system in list mode was used for measuring parameters of transuranium sources. The benefit of using the list method is preventing duplication of experiments a lot and thus saving the cost and time taken for this type of testing that due to very small sample rate, spend a lot of time. The multi-parameter analyzer stores the coincidence alpha and gamma-ray detector events without the use of specific timing modules, in the form of a list file and the software of MPA construct one and two parameter spectra through the list file. The alpha-gamma coincidence standard test by 241Am source that is often used to demonstrate the effectiveness of these systems was carried out and obtained spectra is reported. As well as demonstrating system performance, the testing of unknown source activities, using alpha-gamma coincidence techniques, is carried out without the need for detection efficiency calibration and the activity of the source is determined to equal 25%.

The object of this research is the investigation of carbon nanotube (CNT) distribution effect on electrical conductivity of polymer-CNT composite as dosimetry response of this system. Thus, electrical conductivity of this composite was simulated for aligned, homogenous, and agglomerated distributions in 2D, and 3D situations. The results of simulation were compared and validated by experimental data in the literature. For 3D distribution of CNTs in polymer matrix, Monte Carlo method and excluded volume approach were utilized. The results of 3D simulation showed that electrical conductivity of polymer-CNT composite was depended on dispersion angle of CNTs in polymer matrix. The results of 2D simulation showed that more agglomeration of CNTs in polymer-CNT composite was leading to sever reduction of electrical conductivity. In the other hand, with transition of agglomeration to homogeneous dispersion of CNTs into polymer matrix, increasing of electrical conductivity of the composite was observed.