Abstract
Abstract. In recent years, the use of radiographic inspection with cosmic-ray muons has spread into multiple research and industrial fields. This technique is based on the high-penetration power of cosmogenic muons. Specifically, it allows the resolution of internal density structures of large-scale geological objects through precise measurements of the muon absorption rate. So far, in many previous works, this muon absorption rate has been considered to depend solely on the density of traversed material (under the assumption of a standard rock) but the variation in chemical composition has not been taken seriously into account. However, from our experience with muon tomography in Alpine environments, we find that this assumption causes a substantial bias in the muon flux calculation, particularly where the target consists of high {Z2∕A} rocks (like basalts and limestones) and where the material thickness exceeds 300 m. In this paper, we derive an energy loss equation for different minerals and we additionally derive a related equation for mineral assemblages that can be used for any rock type on which mineralogical data are available. Thus, for muon tomography experiments in which high {Z2∕A} rock thicknesses can be expected, it is advisable to plan an accompanying geological field campaign to determine a realistic rock model.
Highlights
The discovery of the muon (Neddermeyer and Anderson, 1937) entailed experiments to characterise its propagation through different materials
To better visualise the difference between the fluxes after having passed these 10 rock types and the standard rock, we report the ratio between fluxes calculated after the different materials and that after the standard rock in Fig. 4: f rrock
Our results suggest that it is safe to use the standard rock approximation for all rock types up to thicknesses of ∼ 300 m, as the flux ratio will mainly remain within 2.5 % of the standard rock flux, which generally lies within the cosmic-ray flux model error
Summary
The discovery of the muon (Neddermeyer and Anderson, 1937) entailed experiments to characterise its propagation through different materials. The fact that muons lose energy proportionally to the mass density of the traversed matter (see Olive et al, 2014) inspired the idea of using their attenuation to retrieve information on the traversed material This was first done by George (1955) for the estimation of the overburden upon building of a tunnel, and later by Alvarez et al (1970) to search for hidden chambers in the pyramids in Giza (Egypt). Morishima et al (2017) successfully accomplished quest of Alvarez’s team in the Egyptian pyramids Besides these applications, which have mainly been designed for archaeological and civil engineering purposes, scientists have begun to deploy particle detectors to investigate and map geological structures.
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