**8. Conclusions**

of the detector as a function of the water content inside the tower. Post-injection monitoring

Since cosmic rays originate from any direction and permeate all the known Universe, the question whether the same concepts discussed so far apply also to other places outside our terrestrial environment is intriguing. Limiting ourselves to the nearest environment outside the Earth, namely the Solar System, the interaction of the primary cosmics with other planets and celestial bodies produces different results depending on the presence of an atmosphere around the solid structure of the body. For planets or satellites where no atmosphere at all exists, such for instance the Moon, energetic primary particles interact with the surface without producing an extensive air shower. In other situations, where a massive atmosphere exists, much deeper and dense than the Earth atmosphere, as for instance on Venus or Jupiter, air showers may be created but most of the secondary particles are subsequently absorbed by the atmosphere itself, so that only a very small fraction of particles is able to arrive to the surface. Monte Carlo calculations of the interaction of energetic particles with the detailed structure of these bodies should be carried out for any specific situation in order to understand their peculiarities. In case of Mars, where the atmospheric pressure near the surface is only 1/100 with respect to Earth, the development of air showers has been studied in some detail by Tanaka [61], allowing to understand how the proportion between primary protons and secondary pions or muons is very much different than on Earth. In particular, due to the reduced thickness of the Martian atmosphere, the vertical flux of muons is much smaller with respect to the values obtained on the Earth; however, for inclined muons, close to the horizontal, the situation is reversed, and a larger flux of muons would be observed. In any case, contamination from the primary protons is a challenge, and some way to discriminate between the two species should be devised. It is interesting that such concepts have been discussed with relation to realistic Martian exploration missions, trying to understand even the practical aspects and problems which would be

required to solve to carry out tomographic measurements on the Red Planet [62, 63].

Muon tomography has offered over the last years also a good environment for the development of mathematical and statistical algorithms, numerical simulation and procedures, to

**7. Imaging and simulation methods and algorithms**

 stored in subsurface locations was investigated by several authors [54–56] by means of muon tomography. Industrial applications have also seen contribution from muon tomography [57, 58]. Finally, another application for the monitoring of civil buildings and structures which employs cosmic muons as a useful probe is the study of the angular distribution of muons detected in coincidence between a tracking detector on the ground and a set of additional detectors mechanically linked to the structure being monitored. Any movement of the structure with respect to the ground will result in a small modification of the distribution of the orientations of cosmic muons detected in coincidence, provided a good reconstruction of tracks and stable working conditions are achieved during long measurements [59, 60].

of CO2

52 Cosmic Rays

**6. Muon tomography outside the Earth?**

The list of possible applications provided by this short review is not at all exhaustive, and many other examples of the use of cosmic-ray muons to explore various aspects of our environment are available in the literature [67]. Since the first quantitative investigations at the end of 1990s, in about 20 years, the use of cosmic-ray muons for imaging has grown in interest and this technique is now enough consolidated to be proposed even for commercial use. The variety of possible applications in the field has promoted interdisciplinary studies, with the contribution of experts from different areas, and has already given interesting results in many practical situations. The number of papers, articles, technical reports and conference contributions is more and more large, and specialized conferences and workshop have been organized in the last few years to promote exchange of opinions and results, new collaborations and efforts. Even though many promising results have been reported in various fields of interest, there is still a wide territory where to improve the existing techniques. One important aspect is the detection technology, which has been object of several possible choices, and it is still a point of discussion in terms of the optimization of the performance (especially efficiency and resolution) and cost. Also the existing algorithms and methods may be largely improved to arrive to a better reconstruction and imaging processing of experimental data. The years to come are then a promising period for the development of all such aspects, in view of new applications, only limited by the creativity of interested people, or of large improvements in the existing ones.

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