Environmental Fluxes of Thermal Neutrons and Geophysics

Environmental fluxes of thermal neutrons originate from two sources: cosmic rays and natural radioactivity. Owing to a long lifetime of free neutrons, they are able to propagate over rather long distances in surrounding media prior to undergoing absorption, provided that these media do not contain elements that have large cross sections for neutron capture. The real lifetime of free neutrons and distances that they travel are determined by the properties of the medium with which they are in a dynamical equilibrium. At rather large depths under the ground, natural radioactivity associated with \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$(\alpha,n)$$\end{document} reactions on light nuclei of the Earth’s crust is the main source of neutrons. Radioactive gaseous radon, especially its long-lived isotope \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}^{222}\textrm{Rn}$$\end{document}, which is able to migrate over significant distances (several tens of meters in soil and several kilometers in the Earth’s atmosphere) plays a great role in this process. This means that the changes in the medium that are caused by various geophysical processes or by Moon–Solar–Earth phenomena should also be reflected in the neutron flux escaping from the Earth’s crust. The present article gives a brief survey of studies devoted to this subject and a discussion on them.