Abstract
Muography is an imaging technique based on the measurement of absorption profiles for muons as they pass through rocks and earth. Muons are produced in the interactions of high-energy cosmic rays in the Earth’s atmosphere. The technique is conceptually similar to usual X-ray radiography, but with extended capabilities of investigating over much larger thicknesses of matter thanks to the penetrating power of high-energy muons. Over the centuries a complex system of cavities has been excavated in the yellow tuff of Mt. Echia, the site of the earliest settlement of the city of Naples in the 8th century BC. A new generation muon detector designed by us, was installed under a total rock overburden of about 40 metres. A 26 days pilot run provided about 14 millions of muon events. A comparison of the measured and expected muon fluxes improved the knowledge of the average rock density. The observation of known cavities proved the validity of the muographic technique. Hints on the existence of a so far unknown cavity was obtained. The success of the investigation reported here demonstrates the substantial progress of muography in underground imaging and is likely to open new avenues for its widespread utilisation.
Highlights
Muons are penetrating particles produced in Nature from the interactions of cosmic rays in the Earth’s atmosphere, with a spectrum extending to very high energies
An expexted muon flux is obtained from a map of the average density and traversed thicknesses as given by a Digital Terrain Model (DTM)
Two MU-RAY half planes are assembled in a single aluminium shell together with the Front-End Electronics, forming a detection plane of about 1 m2 with handles installed that can be transported
Summary
The muon detector is an upgraded version of the MU-RAY detector, developed for volcano muography[17,18,19]. Two MU-RAY half planes are assembled in a single aluminium shell together with the Front-End Electronics, forming a detection plane of about 1 m2 with handles installed that can be transported (see Fig. 5). The light signals produced by muons in the plastic scintillator bars are conveyed to photosensors by the fibres. The passage of a penetrating charged particle through the detector provides a fast digital signal each time a muon crosses a detection plane. In the setup used for the measurement reported here, the distance between the bottom and the top module was about 50 cm In this configuration the muon track angular resolution was 4 mrad in each projection with a maximum observable field of view from the Zenith of about 63°. During the whole data taking, a control system maintained the SiPM temperature at about 18 °C with a stability of about 0.01 °C, using an hydraulic circuit connected to a chiller
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