The effect of rock composition on muon tomography measurements

Abstract

Abstract.In recent years, the use of radiographic inspection with cosmic-ray muons has spread into multiple research and 10 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 measurement 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 15 that this assumption causes a substantial bias on the muon flux calculation, particularly where the target consists of high { 2 ⁄ } (like basalts) or low { 2 ⁄ } (e.g. dolomites) rocks and where the material thickness exceeds 300 metres. 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 is available. Thus, for muon tomography experiments in which high/low { 2 ⁄ } rock thicknesses can be expected, it is advisable to plan an accompanying geological 20 field campaign to determine a realistic rock model.

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“…Thereafter, the average densities were given by the average density values that resulted in minimal differences between the calculated and measured fluxes. The systematics errors caused by the uncertainty of spectra model and stochastic energy loss processes (Lechmann et al, ) were estimated to Δ F syst. / F ≈ 15% and Δ ρ syst.…”

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“…The probability of a muon surviving the crossing of a certain amount of rock is approximately determined by the opacity of the latter (defined as the integrated matter density ρ over the path length, i.e., ω ≡ exit entry ρ(x)dx) and the initial energy of the muon. It also depends on the chemical composition of the rocks [17], but these effects are expected to be small, in the Vesuvius case, with respect to the precision expected to be achievable with MURAVES. The measurement of the transmitted flux of atmospheric muons through a target in different directions from a given point of view (the position of the muon detector) provides the two-dimensional projective measurement of the target, and the average density ( ρ) of the material of this target can be calculated by knowing the opacity using the equation ω = L ρ, where L is the total muon path length within the material.…”

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“…In this case, the modified equation takes an equivalent form to Eq. ( 8) when replacing ρ, a, b with their mixture counterparts {ρ} rock , {a} rock , {b} rock (Lechmann et al, 2018), thus yielding…”