نشریه سنجش و ایمنی پرتو
سال هفتم شماره 1 (پیاپی 26، زمستان 1397)

This study proposes a method to estimate RBE of fast neutrons using Monte Carlo simulations. This approach is based on the combination of an atomic resolution DNA geometrical model and Monte Carlo simulations for tracking particles. Atomic positions were extracted from the Protein Data Bank. The GEANT4 code was used for tracking the secondary particles generated by fast neutrons during their interaction with liquid water. Since secondary particles spectra were used instead of simulating the interaction of neutrons explicitly, this method reduced the computation time dramatically. Double strand break induction was used as the endpoint for the estimation of fast neutrons relative biological effectiveness (RBE).60Co γ -rays were used as the reference radiation quality. The model succeeded in reproducing the behavior of the RBE and the position of RBEmax as a function of the incident neutron energy between 0.1 MeV to 2 MeV.

The main objective of this research is measurement and assessment of the Radon-222 and Radium-226 radioactive levels in mineral water and drinking water in the Sareyn city. The guidelines used for Radon-222 and Radium-226 radiation sampling and analysis were ASTM D: 5072-09 and ISO 13165-1: 2013, respectively. The concentration of Radon-222 and radium-226 in mineral water and drinking water samples was measured by Liquid Scintillation Counter (LSC). The results of the measurements show that the Radon-222 radionuclide concentration in mineral water samples is 1.14-6.71 Bq/L and 4.25-9.72 Bq/L for drinking water samples. Furthermore, Radium-226 was determined in mineral water samples in the range of 0.048-0.248 Bq/L and in drinking water samples in the range of 0.044-0.078 Bq/L. Consequently, Radon-222 and Radium-226 radioactivity concentration in all samples of drinking water are below the permissible limit. However, Radon-222 radioactivity concentration is higher than the standard level in a number of springs that few people use as a source of drinking water. Nevertheless, in order to ensure more than the non-contamination of drinking water sources in Sareyn, radionuclides of Radon-222 and Radium-226 can be systematically evaluated and considered by the proposed method.

Recently, polymeric nanocomposites have been used as real-time detectors and dosimeters of gamma radiation. In this experimental work, multi-wall carbon nanotubes (MWCNT) were dispersed in polystyrene (PS) martix with a weight percentage of 0.05 wt%. SEM images confirmed the appropriate and uniform distribution of carbon nanotubes in the polymer matrix. The dark current and photocurrent of the nanocomposite were measured under gamma irradiation of 60Co source using an electrometer in the voltage region of 1-500 V, and the dose rate range of 40-134 mGy/min. The results showed that this dosimeter in the array form of 2 pieces in comparison with the 1 and 3 pieces show a better response in the mentioned dose-rate and voltage ranges. Thus, this nanocomposite can be used as an active dosimeter in the diagnostic and therapeutic level.

This research aimed to measure the received photon and thermal neutron doses to contralateral breast (CB) surface in breast cancer radiation therapy for different field sizes in presence of dynamic and physical wedges. The photon and thermal neutron doses were measured by thermo uminescent dosimeter (TLDs) chips for 11 × 13, 11 × 17 and 11 × 21 cm2 field sizes in presence of physical and dynamic wedges. The findings of current study demonstrated that the received doses (both of the photon and thermal neutron) to CB surface in presence of physical wedge for 11 × 13, 11 × 17 and 11 × 21 cm2 field sizes were 12.06, 15.75 and 33.40% of the prescribed dose, respectively, as well as for dynamic wedge were 9.18, 12.92 and 29.26% of the prescribed dose, respectively. The received photon and thermal neutron doses to CB surface increased with increment of field sizes. In addition, the received photon and thermal neutron doses to CB surface in presence of dynamic wedge were less than physical wedge. Similar to recommendations of previous conducted studies, in breast radiation therapy with wedge technique, the using a dynamic wedge is preferable than a physical wedge, especially for medial tangential field.

Understanding the dose distribution of beta-emitting eye plaques respect to the location of tumor is of importance. In this study, the geometry of CCB, COB and CIB plaques (BEBIG manufacturer) for specific eye tumors were simulated and the effects of different coated radionuclides and plaque materials were investigated, by Geant4 simulation toolkit. For validation, a dosimetric investigation was performed for Ru-106 eye plaques, using radiochromic EBT3 films and Plaque Simulator software. Ru-106 plaques can be replaced by Sr-90 or Pr-142 plaques for the tumor apex up to 3 mm. Ho-166 and Re-188 plaques only can be utilized for superficial lesions due to their intense dose fall-off. Gold plaques show more attenuation and can be made thinner. They may be used in COB and CIB models, due to their vicinity of optic nerve and iris, respectively.

In radiation therapy, ions heavier than proton have more biological advantages than a proton beam. Recently, ion helium has been considered due to high linear energy transfer (LET) to the medium and a higher relative biological effect (RBE). To design the spread-out Bragg peak (SOBP) of biological dose for radiation with any type of ion, we need exact values of RBE, which is dependent to dose, LET, and tissue specific parameter, and has spatial variations relative to depth in the tissue. Here, we calculate the exact value of RBE in helium ion irradiating V79 cell line by applying a parametric expression for RBE variations relative to LET, as well as using the Monte Carlo simulation code Geant4 to calculate the LET and the dose profile. The profiles of the Bragg Peak and LETs are calculated for each slice in the tumor region. To generate an appropriate biological SOBP, we compute a set of weighting factors using matrix computations, and by modulating helium ion beams, creation of optimal homogeneity at SOBP for biological doses was done.

In an electron curtain accelerator, when the electron beam passes through the titanium exit window of the accelerator chamber, X-ray photons are produced as a negative by-product of retarding accelerated electrons. Controlling the produced X-ray photons to avoid their detrimental effects or in fact, proper shielding of the accelerator is an important issue that has to be considered in the use of electron accelerators. In this work, based on the proposed geometry for a certain electron curtain accelerator, and using the MCNP4C code, the bremsstrahlung X-ray dose due to the collision of an electron beam with constant current of 50 mA and various energies within 100 ~ 300 keV energy range with the 13 µm-thick titanium foil were simulated. The results indicate a reduction in the bremsstrahlung X-ray dose from 685 Gy to 176 Gy per unit time for an increase in the electron beam energy from 100 keV to 300 keV. Moreover, the simulated values of the simulated radiation stopping power by the MCNP4C for the mentioned increase in beam energy, showed an increasing trend from 0.012 MeV.cm2/g to 0.021 MeV.cm2/g .The maximum and minimum percent deviation of the simulation and theoretical radiation stopping power were 33% and 3%, respectively. The energy loss due to Bremsstrahlung X-rays for an electron passing through a 13-μm thick titanium foil in the energy range of 100 ~ 300 keV, was calculated to be 7.19×10-2 up to 12.44×10-2 keV.  In addition, based on the simulated results obtained for the accelerator shield using the MCNP4C code, the optimum thickness of the lead shield for the maximum electron energy of 300 keV, was found to be 2.5 cm.