ИСТИНА

Analysis of the vertical distribution of 137Cs in accumulated sediments is a powerful tool for evaluating the rates of modern sedimentation, which is widely used in fluvial geomorphology. If source of radionuclides is reliable detected, 137Cs may be argued as time marker. Determination of the distribution of 137Cs acivity in the soil can be carried out in two ways: depth increment sampling followed by laboratory processing or via measurements of gamma activity directly in the field. The first method is more labor-intensive, but also more accurate since it involves control over the measurement conditions, which makes it possible to produce a quantitatively reliable result. Special laboratory conditions, transportation and processing significantly increases costs and time required to obtain desired data. In addition, such a scheme does not involve obtaining information directly in the field, which could help to quickly adjust the network of sampling points if necessary. An alternative way is to carry out measurements directly in the field. The inability to control all possible external conditions of measurements as much as possible certainly reduces the quality and accuracy of measurements carried out in situ, compared with laboratory studies. However, field gamma-spectrometry has the main advantage associated with the ability to speed up the process of obtaining a result and obtaining preliminary conclusions already at the very place of examination. In the case of high concentrations of the desired radionuclides in the soil, the sensitivity requirements of gamma detectors to register a sufficient amount in the desired range of gamma quanta energy can be reduced. This makes it possible to use more compact and mobile technology solutions. Ideally, the measurement accuracy in each sample should be as high as possible, but technical, economic and time capabilities greatly limit the research. At the same time, there may be situations when it is advisable to sacrifice the accuracy of measurements to obtain a larger volume of spatial data. The justification for allowing distortions is the ability to productively interpret the data obtained. For example, it may be conducted in the case when the level of radioactive contamination is high enough to use detectors with a relatively low detection efficiency, and the distribution of radionuclides itself has the expected pattern with a clearly defined maximum. The Chernobyl accident resulted into intensive and uneven 137Cs fallout over vast areas of Europe. Chernobyl pollution has a high spatial heterogeneity. The most affected areas were those where, due to the synoptic conditions of the period of intense emissions after the accident, precipitation occurred, and pollution is represented as a series of hot spots. If the accumulation of material is carried out naturally due to the activity of water flows, the concentration of 137Cs has a steady tendency to decrease over time. As a result, the layer with the "Chernobyl Peak" is visible when analyzing the vertical distribution of radionuclides. To assess the rate of sedimentation, it is important to determine the position of "Chernobyl peak" corresponded to 1986. The results of field measurements with a portable gamma detector inside the soil column without the use of any external protection is presented. The results was compared with the actual distribution of 137Cs in the soil, established based on depth incremental sampling and laboratory studies. Collecting of material was carried out in 2019-2021 in different locations in the centre of intense radioactive contamination - the "Plavsk radioactive hot spot" (Tula region), formed in the Central part of Russia after the Chernobyl accident. The information obtained as a result of field measurements agrees quite well with laboratory studies. The main distortion is the smoothing of the "Chernobyl peak" caused by the absence of a collimator. The applied technique has great prospects of application in areas with a high level of radioactive contamination to assess the rates of sedimentation. However, field spectrometry can only be considered as an additional technique method to the usual procedure of depth increment sampling. The study is conducted under support of Russian Scientific Fund, project 21-55-00050.