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 Validation of gamma scanning method for optimizing NaI(Tl) detector model in Monte Carlo simulation

Huynh Dinh Chuong, Nguyen Quoc Hung, Nguyen Thi My Le,  Vo Hoang Nguyen, Tran Thien Thanh

Abtract:

The aim of this study is the validation of gamma scanning method for optimizing NaI(Tl) detector model in Monte Carlo simulation. The experimental procedure involved: scanning on front and lateral surfaces of the detector with collimated low-energy photon beam; calibrating the efficiency with energies between 31-1408 keV for point sources at distances of 0 cm and 30 cm from source to the detector. The Monte Carlo code used for the simulations was MCNP6. The diameter and the length of crystal were determined according to the measured results of gamma scanning with a collimated 241Am radioactive source. The distance from window to crystal was estimated using transmission measurement recorded on a second detector. The density of reflector was adjusted to obtain the match between measured and simulated values of efficiency ratio of 81 and 31 keV from a 133Ba radioactive source. The optimized model was applied in Monte Carlo simulations to determine the efficiency and energy spectrum response function of NaI(Tl) detector for point source measurements in two configurations. Good agreement was obtained between measured and simulated results.

 

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 Natural radioactivity and radon emanation coefficient in the soil of Ninh Son region, Vietnam

Huynh Nguyen Phong Thu,  Nguyen VanThang, Truong Thi Hong Loan, Nguyen Van Dong,  Le Cong Hao

Abstract

The natural radioactivity (238U, 226Ra, 232Th and 40K) and radon emanation coefficient for 57 soil samples belonging to alluvial, red, forest surface, slip-debris, metamorphic and sandy soil of the Ninh Son region in Ninh Thuan province have been determined. The soil gas radon was measured by in-situ with RAD7 radon monitor coupled with a soil gas probe while activity concentrations of 238U, 226Ra, 232Th, and 40K were measured by an HPGe gamma-ray spectrometry system. The 226Ra/238U disequilibrium occurred in the soil samples and a great majority of the 226Ra/238U values lie above 1. Average activity concentrations of 226Ra, 232Th, and 40K are significantly higher than the worldwide average concentrations in soils published by UNSCEAR, 2008. The gamma dose rate ranged from 55 ± 2 to 248 ± 7 nGy h−1 with an average of 130 ± 4 nGy h−1 which is greater than the world value. Strong positive correlations were recorded between 238U and 226Ra, 232Th and 226Ra, 232Th and 238U, and 226Ra and 222Rn. The results of weathering and alteration processes were proposed to be dominated reasons for the 226Ra/238U disequilibrium occurred in the soil samples. Most of the radon in soil gas samples are considered “normal risk” or low radon index. The mean values of the emanation coefficient for alluvial, red, forest surface, slip-debris, metamorphic and sandy soil were found to be 0.51 ± 0.03, 0.40 ± 0.02, 0.36 ± 0.02, 0.30 ± 0.02, 0.26 ± 0.02 and 0.15 ± 0.01, respectively. Radon emanation was found to be an inverse function of grain size for grain sizes larger than 0.1 mm in diameter and independent on the radium content of the soil sample.

 

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‘Time to crop’ for 137Cs in the surface soil and its long-term effects to population based on model assessment

Van Thang Nguyen, Ngoc Ba Vu, Nguyen Phong Thu Huynh, Cong Hao Le

 

Abstract:

Long-term behavior of artificial radionuclides in surface soils is very important to assess the radiological effects to population. Among artificial radionuclides, 137Cs is most important because of its long half-life and its biggest abundance in the environment. In this study, the fate of 137Cs in the surface soil layers was assessed by the Canadian Centre for Environmental Modelling and Chemistry (CEMC) soil model which is well known as a simple assessment of the relative potential for degrading reaction, evaporation, and leaching of a pesticide applied to a surface soil. The total decrease rate of 137Cs activity concentration in the surface soil (Te1/2) was 10.4 years found in the top 0–5 cm of the soil layer. The activity decrease of 137Cs and the corresponding Te1/2 values under the different depths of surface soil layer were investigated. The influences of soil organic material, soil water content and soil porosity to the losing rate of 137Cs were considerable. Long-term effects of 137Cs to population were assessed through activity concentrations of 137Cs in any parts of the food chain. Soil-to-plant transfer factor (TF), transfer coefficients Fm (transfer to animal milk) and Ff (transfer to animal meat) collected from many literatures were used for activity calculation. The effective doses to population due to ingestion of edible parts of plants, milk and meat was evaluated. The incorporation of four terms: radiological doses, soil-to-plant transfer factors (TF), plant-to-animal coefficients (Fm and Ff) and the total decrease rate of 137Cs is the new approach. The new concept ‘Time to crop’ (TC) based on the effective doses to population was explored and first used for agricultural proposals in the topsoils of the 137Cs exclusion zone. TC was calculated for many scenarios of radioactive exposures, many type of plants, animals and plant groups. TC values were found various in order fruits < leafy vegetables < tubers < cereals < grasses. The highest TC was found for grasses group as a result of the long-term accumulation of radionuclides in animals.

 

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