3D Muography for the Search of Hidden Cavities

Abstract

Muography (or muon radiography) is a technique that exploits the penetration capability of muons, elementary particles similar to electrons but with a mass about 200 times larger. High energy muons are naturally produced in the interactions of cosmic rays with the Earth atmosphere. The measurement of their absorption in matter allows the imaging of the inner structure of large bodies. The technological developments in the detection of elementary particles have opened the way to its application in various fields, such as archaeology, studies of geological structures, civil engineering and security issues. We have developed a new approach to the three-dimensional muography of underground structures, capable of directly localising hidden cavities and of reconstructing their shape in space. Our measurements at Mt. Echia, the site of the earliest settlement of the city of Naples in the 8th century BC, have led us to the discovery of a hidden underground cavity, whose existence was not evident with the usual two-dimensional muography graphs. We demonstrate here that our original approach definitely enhances muography discovery potential, especially in case of complex underground systems.

Figure 1

A half module of the MU-RAY muon tracker with its 32 triangular scintillator bars and the 32 wavelength shifting optical fibres that transmit the light to the photosensors.

Figure 2

The MU-RAY (left) and MIMA (right) muon trackers.

Figure 3

The system of known cavities and the three locations A, B and C of the muon trackers.

Figure 4

a–c The relative transmission R(α, ϕ, ρ) observed at the locations A, B and C, respectively, in the reference systems of the corresponding muon trackers. The angular regions associated to the hidden cavity are indicated with rectangles. The plot was obtained using the software ROOT and the smoothing tool Contour4 was applied.

Figure 5

Regions in the map of the relative transmission R(α, ϕ, ρ) selected by the clustering algorithm as corresponding to a cavity. The angular regions associated to the hidden cavity are indicated with rectangles. The plot was obtained using the software ROOT and the smoothing tool Contour2 was applied.

Figure 6

The simulated spherical cavity with a 6 m diameter (blue dots, on the left) and two views of its 3D reconstruction (red dots, on the right), in a coordinate system with origin at the centre of the MIMA muon tracker at location C.

Figure 7

Two views of the 3D reconstruction of the hidden cavity, in a coordinate system with origin at the centre of the MIMA muon tracker at location C.

Figure 8

The 3D reconstruction of the hidden cavity (in yellow) inserted in the CAD model. The ellipse indicates the cavity where debris were found, providing a hint for a hidden cavity above it.

Figure 9

Fits of the cumulative distributions of the relative transmission R of the muography taken with the MU-RAY muon tracker at the location B (a) and with the MIMA muon tracker (b). The distributions are fitted by two Gaussian components, one corresponding to transmission through rock without voids (red) and another corresponding to trasmission through rock with voids (green).