Geant4 Simulation of Precipitated Activity-to-γ-Dose Rate Conversion Factors for Radon and Thoron Decay Products

Valentina Yakovleva,Sergei Smirnov,Grigorii Yakovlev,Roman Parovik,Aleksey Kobzev

doi:10.3390/math10030293

Abstract

The results of modeling the conversion factor from rainfall-deposited unit activity of gamma-emitting radon and thoron daughter decay products to their created gamma-radiation dose rate as a function of height above the Earth’s surface using the Geant4 toolkit are presented in this paper. Thin layers of water, soil, and air, with the height of 0.1–10 mm, are considered as the source in order to examine whether the composition of the radiation source environment affects the simulation result. Cases with different absorber-atmosphere densities are simulated. The contribution of each radionuclide 212Bi, 214Bi, 212Pb, 214Pb and 208Tl to the total gamma background was determined. The dependence of dose rate growth during the precipitation period on the detector position in relation to the area covered by precipitation was investigated numerically. The obtained conversion factors are universal values, because do not depend on soil type (material) on which radionuclides are deposited by precipitation. These coefficients can be used for solving both direct tasks of radiation background recovery during precipitation and inverse tasks of determining the intensity and amount of precipitation by the known gamma background, as well as tasks to decipher the gamma background by the shape of the response to various phenomena. Also in this work, it is shown how thoron decay products can affect the response shape of gamma background on atmospheric precipitation.

Highlights

An increase of radiation gamma background in the form of bursts of different forms and duration during periods of atmospheric precipitation is registered practically in any corner of the globe [1,2,3,4,5,6,7,8]
It is generally believed that 214Pb and 214Bi make the greatest contribution to the total gamma and beta background of the near-ground atmosphere because their activity in the atmosphere is almost two orders of magnitude greater than the activity of short-lived thoron decay products (DPs), such as 212Pb, 212Bi and
In order to study how the thickness of a source layer of soil or water, including air, with radionuclides distributed uniformly over the entire volume of this layer, affects the simulation results, calculations were performed for different thicknesses

Summary

Introduction

An increase of radiation gamma background in the form of bursts of different forms and duration during periods of atmospheric precipitation is registered practically in any corner of the globe [1,2,3,4,5,6,7,8]. The main task of radiation monitoring is the precise identification of the nature of registered bursts. The solution to this problem cannot avoid gammabackground modeling and comparison with measured data. Quite a number of works have been devoted to the study of gamma-background growth due to deposited radionuclides- short-lived gamma-emitting decay products of natural radon isotopes. Atmospheric precipitation washes out radioactive aerosols, most of which are short-lived daughter decay products (DPs) of radon and thoron. It is generally believed that 214Pb and 214Bi make the greatest contribution to the total gamma and beta background of the near-ground atmosphere because their activity in the atmosphere is almost two orders of magnitude greater than the activity of short-lived thoron DPs, such as 212Pb, 212Bi and

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