First muography of Stromboli volcano

Abstract

Muography consists in observing the differential absorption of muons - elementary particles produced through cosmic-ray interactions in the Earth atmosphere - going through the volcano and can attain a spatial resolution of tens of meters. We present here the first experiment of nuclear emulsion muography at the Stromboli volcano. Muons have been recorded during a period of five months by a detector of 0.96 m² area. The emulsion films were prepared at the Gran Sasso underground laboratory and were analyzed at Napoli, Salerno and Tokyo scanning laboratories. Our results highlight a significant low-density zone at the summit of the volcano with density contrast of 30-40% with respect to bedrock. The structural setting of this part of the volcanic edifice controls the eruptive dynamics and the stability of the "Sciara del Fuoco" slope, which is affected by recurrent tsunamigenic landslides. Periodical imaging of the summit of the Stromboli volcano such as that provided by muography can become a useful method for studying the evolution of the internal structure of the volcanic edifice.

Figure 1

Stromboli island. Stromboli and Ginostra are the two villages that are located on the north-east and south-west coast respectively. SdF indicates the north-western side of the island called “Sciara del Fuoco”. The black spot marks the detector installation site (Le Roccette) and the yellow area indicates the crater region (graben-like collapsed zone).

Figure 2

(a) The upper part of the Sciara del Fuoco and the crater region seen from the detector location. (b) Detector location and crater in a map from the Digital Elevation Model, with muon trajectories for a tangent of the elevation angle equal to 0.28 and azimuthal angle acceptance of ±0.6 rad. The color scale indicates the elevation in meters.

Figure 3

(a) Angular resolution of the tracking system as a function of track inclination; (b) Single plate basetrack efficiency.

Figure 4

Track reconstruction probability as a function of muon momentum for two simulated configurations: 4 emulsions without absorber (green line) and with 5 mm absorber (red line). In our detector (red line) all muons below 0.1 GeV are rejected, 50% of muons at 0.5 GeV are rejected, most of the muons above 1 GeV are reconstructed.

Figure 5

Structure of one detector module: (a) OPERA emulsion film cross-section and track reconstruction procedure; (b) cross-section of an emulsion stack where two couples of films are positioned on both sides of a 5 mm thick stainless steel plate; (c) Photo of the insides of one module with ten emulsion stacks positioned.

Figure 6

Two-dimensional histograms of the tangents of horizontal (θ_x) and elevation (θ_y) angles of the muon tracks, over the full angular acceptance of the detector: (a) data, (b) Monte-Carlo simulation, with total entries normalized to data. The color scale represents the number of entries in the bins.

Figure 7

(a) Rock thickness and mountain profile as seen by the detector, given with 10 × 10 mrad² binning. The color scale is the rock thickness in meters. The white profile gives the statistical sensitivity limit, as defined in the text. (b) Difference between the observed muon flux and the one expected from Monte Carlo simulation over an angular range centered at crater region. Color scale represent muons counts. The average density ranges in between 1.4 and 2.2 g/cm³ above the sensitivity limit.

Figure 8

For each angular bin in the free sky region the relative spread between expectation and data is calculated.