نشریه سنجش و ایمنی پرتو
سال ششم شماره 2 (پیاپی 22، بهار 1397)

In this work, lithium fluoride doped with impurities of Sr and Ce, was produced using melting method and its thermoluminescence (TL) properties was investigated. The optimized values of 0.5 mol% of Sr and 0.1 mol% of Ce were obtained for the most TL sensitivity. Reusability of the prepared samples has been investigated for 5 cycles of annealing, irradiation and reading and no considerable change in TL response was found. For the storage time of 30 days, low temperature peak was faded while the main and high temperature peaks remained almost unchanged. A linear dose- response was identified in the absorbed dose range of 1 mGy to 10 Gy. Three overlapped peaks at temperatures of 416, 472 and 506 K were determined using TL glow curve de-convolution results. Kinetic parameters such as activation energy and order of kinetic were calculated by general order kinetic model.

Recently, polymeric nanocomposites have been used in dosimetry and detection of gamma rays, but due to their low photon absorption cross section, they exhibit a small sensitivity to gamma rays. In order to overcome this problem, metal oxide particles with a high atomic number are added to the polymer matrix. For this purpose, in this work, tungsten oxide particles (WO3) were distributed in polyvinyl alcohol (PVA) matrix via two scales of nano and micro sizes of the fillers with a weight percentage of 20 wt%. A solvent solution was used in fabricating WO3-PVA composite (20 wt%). The FESEM test was performed to verify the composite rheology and to confirm the distribution of tungsten oxide micro/nanoparticles in the polymeric matrix. The quantum efficiency of gamma rays for 60Co was calculated for the aforementioned composite using the MCNP code, which showed an acceptable agreement with the XCOM results. One of the effective factors in detection response and dosimetry of this composite group is the change in the electrical current of the composite due to beam absorption. The dark current and photocurrent of tungsten oxide-polyvinyl alcohol nano/micro composite were measured in gamma and neutron radiation fields respectively by 60Co and 241Am-Be sources using two-probe electrometer in 100 V and 400 V voltages. The results showed that the sensitivity of nanocomposite to gamma rays is better than microcomposite. Also, the signal-to-noise ratio of nanocomposite for gamma-ray in the dose rate of 90-138 mGy/min increased linearly from 5 to 40, while the composite did not show a remarkable response to neutron beams.

In the extremely low frequency range (3 to 300 Hz), the intensities of electric and magnetic fields increase with increasing voltage and current. If the intensities of electric and magnetic fields are high compared with standard exposure limits, they can have harmful effects on human health. In the vicinity of power lines and high voltage power stations, the intensities of these fields are usually high. Therefore, in order to protect the workers and the general public against electric and magnetic fields, measurement of the intensities of the electric and magnetic fields with a suitable and calibrated device is necessary. Then the measurement results should be compared with the reference levels proposed in the national standard of Iran. In this paper, a constructed measuring device (as a survey meter) is discussed. Then the calibration system and its method are argued. Finally, the calibrated survey meter was used for measurement of electric and magnetic fields near to equipment installed in power stations. The measurement results are compared with the reference levels given in national standard. Accordingly, suggestions are made to optimize the conditions.

The exposure buildup factors are important for determining the shield thickness of gamma and X-ray. Without taking this factor into account, the estimated thickness of the shield is less than what is expected which leads to an excessive received radiation exposure of staffs or patients. Given that the exposure buildup factor for the monoenergetic homogenous point sources have been calculated by others, the calculation of these factors for continuous energy spectrum source through the Monte Carlo MCNPX code has not been studied so far. In this study, we attempted to investigate the difference in the exposure buildup factor due to a continuous energy spectrum source compared to a monoenergetic source. Therefore, the exposure buildup factor of a continuous energy spectrum source with 1 MeV endpoint energy (a typical energy spectrum) and a 1 MeV monoenergetic source for the shield of water, iron and lead up to depths 10 mean free path is calculated. Significant difference was observed in the obtained values. So that, the relative difference between the exposure buildup factor of radiation from monoenergetic sources and a source with the continuous energy spectrum for 1 to 10 mean free paths of water, iron, and lead are 38% to 39%, 17% to 30%, and 8% to 50%, respectively. Accordingly, in order to calculate the shield thickness for a continuous energy spectrum source in comparison with a monoenergetic source, a significant distinction should be made.

The precise and timely manner modeling of received photon counts from gamma-ray sources has an important role in providing afore information for Airborne Gamma Ray Spectrometry (AGRS). In this manuscript, the Auto-Regressive Integrated Moving Average (ARIMA) model has been used to model AGRS. The proposed method provides gamma source and environmental disturbances ARIMA model, using known radioactive sources, to arrange the afore information for AGRS process experts to analyze the spectrometry data. The model extraction process and training will be done offline using different sizes and types of radioactive sources. The extracted models then being validated by evaluation functions to determine the type and amount of radionuclides during online AGRS. In order to evaluate the implemented modeling, the proposed ARIMA method is compared with other process modeling methods, including the Auto Regressive Moving Average ARMA in three bias, median absolute deviation (MAD) and the mean square error (MSE) criteria. The results show that the proposed method models the received photon counts much more accurate than other common methods.

TL dosimetry is known as one of the effective dose estimation methods in diagnostic radiology. Cephalometry is used by dentists, oral and maxillofacial surgeons to choose the appropriate treatment. The purpose of this study is to measure the entrance skin dose, and dose to different organs of the patients in cephalometry using TLD-100. The entrance skin, and organ dose were obtained by putting TLDs on the surface, and inside the Rando phantom. The dose to different organs like thyroid, eyes, maxilla, parotid, sub-lingual, and submandibular salivary glands were measured. According to the results obtained in this study, the entrance skin dose to neck, cheek, and temporal are 75.8, 58.1, 113.3 µGy, while minimum, and maximum organ doses were 2.5, and 107.8 µGy for right temporal, and right parotid salivary gland.

In radiation therapy of lung tumors, respiratory motion causes target moving, so a larger margin is needed to cover the CTV. With the margin increasing, a larger volume of normal tissue will be exposed to high-dose. In this study, dosimetric parameters of normal lung tissue were compared between gated and conventional radiotherapy (RT), using the NCAT digital phantom.
Different series of CT images of sample patients were created, using the NCAT digital phantom and MATLAB software. Lesions with diameters of 3, 4 and 5cm were located in different positions of lung (left upper and lower lobes, right upper and lower lobes) in the CT images. Appropriate margins were applied to the CTVs depending on the radiation therapy technique and respiratory cycles. To evaluate the effect of diaphragm extension on the margins to CTV, different diaphragm motions were considered from 2 to 3.5 cm with 0.5 cm intervals. PCRT treatment planning system with superposition computational algorithm was used to deliver a dose of 60Gy in 30 fractions to the PTVs. Totally, 36 treatment plans were evaluated for each RT technique and mean lung dose (MLD) and V20 were calculated for both conventional and gated RT techniques.
In all cases, gated RT has a superior advantage compared to conventional RT. A significant impact on MLD and V20 reduction was seen in lung normal tissue in gated RT up to 5Gy and 37%, respectively.