A new method to account for the depth distribution of 137Cs in soils in the calculation of external radiation dose-rate

Vertical distribution of 137Cs in soddy-podzolic sandy soil in grasslands and forests of the Bryansk region in 2015–2016

Vertical distribution of natural and man-made radionuclides in the soil profile is a decisive parameter when calculating the dose rate of gamma radiation in the air above the ground and the effective dose of external human exposure. The main purpose of this work was to determine vertical distribution of137Cs in soddy-podzolic sandy and sandy-loam soils in forests and grasslands in the south-western districts of the Bryansk region in the remote period after the Chernobyl accident. In 2015–2016, soil cores were sampled in 7 virgin meadows and 13 forested areas to a depth of 20 cm. The cores were cut into horizontal layers 2 cm thick. The137Cs activity in the samples was determined using a semiconductor gamma spectrometer. The activity concentration in samples of dry soil (n = 200) ranged from 6.35 Bq/kg to 83300 Bq/kg with an average of 4550 Bq/kg. In the meadows in three cases, the maximum activity concentration was determined in the uppermost layer. With increasing depth, the activity concentration decreased and reached a minimum in the deepest layers. A difference between the surface layer and the deepest layer was two to three orders of magnitude. The three other meadows showed a relatively uniform distribution of137Cs in the upper 4–6 cm, followed by a decrease in activity concentration with an increase in depth. In one meadow area, a peak of the137Cs activity was found at a depth of 4–6 cm. In the forest, the most typical (in 10 cases) was the presence of a pronounced maximum activity concentration of137Cs in the uppermost layer. The experimentally obtained values of the137Cs inventory in the upper 20 cm of soil at the surveyed sites ranged from 42 to 1940 kBq/m2. The values of137Cs inventory positively and statistically significantly correlated with officially established levels of137Cs surface ground contamination for the territory of nearby settlements. Vertical migration of137Cs in the soil in the surveyed areas was mainly limited to the upper 10 centimeters. The layer on average contained 94% of the total137Cs inventory. Less than 1% of the total137Cs inventory was found in the deepest soil layer sampled (18–20 cm). The obtained137Сs activity distributions were used to calculate kerma rate in the air at a height of 1 m above the ground at the surveyed sites. The air kerma rate ranged from 52 to 2240 nGy/h (on average, 807 nGy/h). The caesium-137 deposit in the upper 6 cm of soil determined about 95% and 90% of the air kerma rate in the forests and in meadows, respectively. Radioactive caesium, which migrated into the soil to a depth of more than 10–12 cm, gave a negligible contribution (less than 1%) to the gamma-radiation dose rate in the air. In practical terms, it indicates that the depth of soil sampling equal to 20 cm is currently quite sufficient to estimate the dose rate of gamma radiation in the air in virgin grasslands and in forests.

ASSESSMENT OF TERRESTRIAL RADIATION BY DIRECT MEASUREMENT OF AMBIENT DOSE EQUIVALENT RATE OF BACKGROUND RADIATION

Estimation of terrestrial external radiation is essential for assessment of public exposure to natural radiation. During national survey of natural radionuclide in soil in Iran, 979 soil samples were collected from different locations, in the same time ambient dose equivalent rate was measured by a scintillator detector. In this work, terrestrial radiation was estimated by direct measurement of ambient dose equivalent rate of background radiation. The response of dose measuring instrument to cosmic radiation at ground level was measured and other components were discussed and estimated. For verification, terrestrial radiation derived from this method was compared with those calculated from activity concentration of natural radionuclides in soil. The averages of ambient dose equivalent rate derived from activity concentration of by natural radionuclide in soil and from direct measurement are 55.07 and 62.57 nSv/h, respectively. The source of statistical and systematic uncertainties are introduced and discussed.

Modifications in the gamma dose rate in air due to downward and lateral mobility of 137Cs in the soil

¹³⁷Cs can be an important environmental contaminant due to fallout from nuclear reactor accidents and atomic weapons testing. Its contribution to the air gamma dose rate at 1 m height above contaminated ground depends on the soil inventory, the migration processes along the soil profile and possible modifications in the spatial distribution. In this paper the diffusion-convection equation is used to approach the ¹³⁷Cs soil vertical migration transport and standard dose rate factors are jointly applied for estimating the air dose rate. In order to calculate the temporal reduction of the external radiation, an empirical attenuation factor is used and its coefficients are analyzed in terms of the effective diffusion coefficient and downward migration rates. Additionally, it is analyze the corrections that should be introduced in dose rate values attributable to soil redistribution processes. If these processes, natural or as a consequence of human activities, are not taken into account, both the air dose rate values and the attenuation period can be noticeably under or overestimated.

Terrestrial outdoor exposures in the south-west Caspian region

Natural radionuclides (40)K, (232)Th and (238)U present in soil and anthropogenic (137)Cs a fission product released to the environment mainly through atmospheric testing of nuclear arms and accidents in nuclear installations are the major contributors to terrestrial outdoor exposures. As part of a comprehensive study of environmental radioactivity in the south Caspian region, soil inventories of these radionuclides have been measured in more than 50 locations throughout Guilan, a northern province of Iran with the highest population density. In the case of natural radionuclides, outdoor absorbed dose rates in air have been inferred from concentrations of radionuclides in soil on the basis of which annual effective dose values have been estimated using the method described in UNSCEAR 2000. Calculation of (137)Cs contribution to dose has been performed by considering its soil inventory profiles and associated numerically estimated photon fluxes at 1 m above the ground. Maps representing the contribution of natural and anthropogenic radionuclides to annual effective dose in the study area have been produced using relevant geostatistical technique. Combined annual effective dose equivalent ranges from 33.1 ± 4.9 to 91.9 ± 16.5 μSv · y(-1) with a mean value of 64.7 ± 15.2 μSv · y(-1).

Determination of the soil-to-grass transfer of 137Cs and its relation to several soil properties at various locations in Serbia

Transfer coefficients of (137)Cs from soil to grass were determined for the terrain around the city of Kragujevac in central Serbia. Mass activity concentrations of (137)Cs in soil and grass samples were determined with a high-purity Ge-detector (HPGe). The activity concentration at the depth of 20 cm was found to be in the range of 14.92-124.05 Bq kg(-1), whereas the activity in grass for the same location was in the range of 4.60-84.95 Bq kg(-1). Transfer factors (TFs) were in the range of 0.07 up to 1.94. Dependences of TFs on different soil characteristics were presented graphically. Weak dependences were determined between them. Absalom's model was used to predict TFs based on soil characteristics: pH value, contents of clay, exchangeable potassium and humus. A comparison of measured and predicted values from Absalom's model is shown graphically. It has been found that Absalom's model might be carefully used for the prediction of (137)Cs in grass for specific regions.

External doses to humans from 137Cs in soil

Calculations of absorbed doses in organs of the human body and the total effective dose due to Cs in soil as a source of external exposure are presented in this work. Calculations were done using the MCNP-4B software package. The assumption was made that photons with an energy of 662 keV are emitted in a cylindrical volumetric source in soil up to the depth of 20 cm. Depth distributions of Cs at 19 locations around Kragujevac (a city in central Serbia) were measured by a HPGe detector. An ORNL phantom of an adult human standing on the soil above the center of a cylindrical radioactive source was used to calculate the conversion coefficients, i.e., absorbed doses in an organ per unit specific activity. The conversion coefficients in organs are given as a function of the source depth in soil. The largest absorbed dose was found in skin. The annual effective dose in humans was estimated from these calculations and the measured activity depth profile of Cs in soil. The average effective dose was found to be 3.17 microSv y. This value was rather small in comparison with other sources of natural ionizing radiation. One may conclude that Cs was a negligible source of external exposure in the area around the city.

In situ measurements of the sub-surface gamma dose from Chernobyl fallout

Methods of estimating external radiation exposure of soil-dwelling organisms are currently of much research and regulatory interest. In this paper, we report the first in situ measurements of the sub-surface gamma dose rate for 137Cs contaminated land that quantify variation in dose rate with depth. Two contrasting sites have been investigated. The first site comprised a mineral type soil with a low percentage of organic matter and the second site chosen was in a peat-bog. The different soil compositions afford different 137Cs mobility and this results in variations in the measured gamma dose-rate with soil depth. For each site the paper reports the measured dose rates, the 137Cs activity depth profile, the 137Cs inventory and a description of the soil-characteristics. It is suggested that these data can be used to produce estimates of the sub-surface gamma dose rate in other sites of 137Cs contamination.