نشریه سنجش و ایمنی پرتو
سال هفتم شماره 4 (پیاپی 29، پاییز 1398)

Generally, doses greater than 2Gy can produce acute biological effects in humans. For this reason, to measure and record the cumulative dose, especially in radiation users, the need for a personal dosimeter has been strongly felt by the researchers. The purpose of this study is to introduce retrospective dosimetric techniques, in particular the use of nails as a natural tool for determining cumulative absorbed dose in the body. The traditional methods of dosimetry are based on the effects of radiation on inorganic materials. Contrary to conventional methods, organic compounds are also used for dosimetry, which the effect of radiation on them is creation of free radical. EPR spectroscopy is used to determine the concentration of these radicals. So far, various solids such as sugars, polymers, quartz and bone have been used for a wide range of absorbed doses (10-108 Gy). In order to develop measurable lower doses (1-5 Gy), materials that are more sensitive to radiation are required. EPR spectroscopy is usually performed in the X-band region at a frequency of 9.5 GHz. The sugar dose-response curve for gamma radiation is linear in the range of 0.5-100 Gy. The dose-response curve for most plastics is nonlinear with low stability of EPR signal. The flax which composed of a polysaccharide chain has a linear dose-response curve in 10-104 Gy, but it is not easy to interpret the EPR signal because of the presence of detergent in it. The intensity of the EPR signal of wool is weak and the recombination of its radical occurs rapidly. Tooth enamel is a good tissue for EPR dosimetry and its dose-response curve is within the range of 0.02-0.2 Gy. The detection limit for the bone is also about a few kGy and not precise at low doses. The background signal in hair is high and its signal stability is low. Also, the EPR signal of fingernail is stable for at least a few days. Generally, materials used in EPR dosimetry should have features such as, being everywhere, non-invasive sampling, signal stability, and accurate and fast dose evaluation. Due to having all of these features compared to other materials, the fingernail is very much considered in the EPR dosimetry method.

Melatonin has important roles in many growth and stress-related responsive processes in plants and algae. In this study, the mediating role of 100 and 500 µM melatonin on photosynthetic pigments and non-enzymatic antioxidant responses of microalga, Chlorella vulgaris to gamma irradiation stress has been studied. The result showed that the alga treated with 100 µM had a stable amount of chlorophyll and a more stability of cell membrane under gamma stress. It was concluded that although melatonin could increase resistance of alga to gamma by strengthen non-enzymatic antioxidant system like more production of proline, however this resistance probably further be related to enzymatic antioxidant system.

Finding accurate methods to be employed for the treatment of esophageal cancers is of especial interest for researchers, due to the sensitivity of this tissue. Recently radioactive stents loaded with I-125 brachytherapy seeds have been widely investigated for the treatment of advanced esophageal cancer. It is necessary to investigate the dose distribution of any radioactive esophageal stents before the clinical use. This study performs a dosimetric comparison between four kinds of radioactive esophageal stents to be used in the treatment of advanced esophageal cancers by Monte Carlo simulation (MCNPX2.6 and Geant4), based on the arrangement of the seeds. According to results of this study, esophageal stents loaded with I-125 seeds seems to be better than iodine-eluting stents, with the distance of 20 mm or less between the centers of two adjacent seeds. This arrangement could be an appropriate candidate to be utilized for treatment of advanced esophageal cancer.

The images of dental CBCT imaging systems used in conic shaped beams, stored in the DICOM format, have various applications in the dentistry, including bone density estimation to select the location of the orthodontic implant, bone loss detection and etc. In these systems, unlike CT imaging systems, the resulting images exhibit gray-scale non-uniformity in each of the different axis in FOV. This specification along with the inability of the CBCT to display the CT numbers accurately, makes it difficult to obtain bone density information, so diagnosing of dental complications using a quantitative information from the images cannot obtained accurately. In this regard, in order to be able to use the quantitative information of the images of these systems to assess more precisely the bone density, the gray scale changes in the images of several different CBCT systems were studied. The results showed that the process of gray scale changes for each system in each direction is unique for the each system, and it can be concluded that in each system, changes in the gray scale of images in each direction have a roughly constant trend. One of the most important results is that the gray scale changes of the images for all systems along the axis of the body (Z axis) have a steady trend and It has the lowest standard deviation. One of the most important results is that the gray scale changes of the images for the Scanora 3D and  New Tom GIANO systems in the axis of the body (Z axis) have a stable trend and the lowest standard deviation (0.54 ± 0.009) And (0.18 ± 0.004) respectively.

Head computed tomography is a common diagnostic examination, which may cause lenticular opacity and cataracts. Cataract induction is one of the non-stochastic effects of radiation, that happens at threshold dose of 0.5 Gy. Recent studies illustrate that only irradiation to the sensitive (germinative) zone of the lens is a prerequisite to cataract development. Recently, the dose values absorbed in substructures of a detailed eye model at head CT scan has been evaluated. In the present work, the effect of shielding on flux reduction and dose to different eye substructures was investigated. For this purpose, after placing eye model in the head of ICRP adult male phantom, CT exposure was simulated at tube voltage of 80, 100 and 120 kVp by MCNPX Monte Carlo code. Dose values as well as photon flux were estimated without and with different bismuth-polyurethane composite shields. Given its advantages, polyurethane composite with bismuth concentration of 20% was selected as an appropriate protective shield. Results show that applying this composite shield reduces dose almost 70%, 65% and 60% at tube voltages of 80, 100 and 120 kVp, respectively.

Neutron radiography is an unique, advanced and useful non-destructive test method in various industries and researches. Nuclear reactors are powerful and stable neutron sources for the neutron radiography system. In this research, the MNSR research reactor has been used as a neutron source for a neutron radiography system, and its neutron beam parameters have been evaluated. Also, using the direct conversion method and the gadolinium converter, the neutron image was obtained in its neutron beam and the image quality of the neutron was evaluated using the standard image quality standard ASTM-E545. The results of the measurements show that the neutron flux of the system in the maximum reactor power is equal to 3×105 n.cm-2.s-1 and the collimation ratio is 100. Image quality evaluation also showed that the obtained image has a V quality category based on the ASTM-E545 standard.