## 5 Conclusions  
This paper focused on the study of surface soil gas \({}^{220}\)Rn (thoron) monitored during a long data series from 2011 to 2017 at two sites of the Campi Flegrei caldera, in the district of Naples, Southern Italy. Over the last two decades, the Campi Flegrei caldera has been in an unrest phase characterized by ground uplift, seismicity, fumaradic degassing and a general increase in the emission of volcanic-hydrothermal fluids. Based on a previous similar work centered on \({}^{222}\)Rn, the major radon isotope, which highlights a remarkable correlation with geo-indicators used to characterize the caldera unrest, here, the main objective was to verify whether and how this volcanically induced crisis might also affect the \({}^{220}\)Rn activity concentration in the surface soils, considering that its surface exhalation rate is influenced by external factors, which can also act at a distance from the measuring point in these particular areas. The results of the study show that \({}^{220}\)Rn presents an increasing pattern very similar to that of the more classic geophysical and geochemical parameters routinely monitored during the evolution of volcanic systems, as well as the \({}^{222}\)Rn. The strong increase in temperature and pressure alterations of the hydrothermal system of Campi Flegrei acts as a perturbation significantly affecting the surface thoron emanation power, favored by the high permeability and porosity of the rocks. The carrier effects, due to CO\({}_{2}\) gas fluxes, may further enrich the gas emission phenomenon. In fact, \({}^{222}\)Rn and \({}^{220}\)Rn highlight a general good linear correlation due to the CO\({}_{2}\) carrier action, although small effects of different Rn gas sources, likely from a quiet period of hydrothermal and seismic activity, are present. These results represent an absolute novelty in the study of the \({}^{220}\)Rn isotope, as it is very rarely used in geophysical studies, due to its very short half-life of 55.6 s. The long-term observation of \({}^{220}\)Rn at multiple sites in surface soils of a seismic-volcanic area can improve the understanding and use of the short-lived isotope, at the same level of the most mentioned in the literature and long-lived \({}^{222}\)Rn. A further simple positive advantage in using thoron instead of or together with radon measurements could be in the context of low-cost large-area monitoring; since the \({}^{212}\)Po progeny of \({}^{220}\)Rn bears the highest naturally occurring alpha particle energy (almost 8.8 MeV, with a more than 1 MeV-large gap to the next \({}^{214}\)Po), it could be more easily distinguished from other alpha decays. Conceptualization, F.A. and C.S. ; methodology, F.A. ; software, F.A. ; validation, F.A., C.S. and V.R. ; formal analysis, F.A. ; investigation, F.A. and C.S. ; resources, F.A. and C.S. ; data curation, V.R., F.G., W.D.C. and M.P. ; writing--original draft preparation, F.A. and C.S. ; writing--review and editing, F.A., C.S., V.R., F.G., W.D.C. and M.P. ; visualization, F.A., C.S., V.R. and F.G.; supervision, F.A., C.S. and V.R. ; project administration, F.A., and C.S. ; funding acquisition, C.S., V.R. and F.G. All authors have read and agreed to the published version of the manuscript. Figure 6: Activity concentration of \({}^{222}\)Rn (radon) vs. \({}^{220}\)Rn (thoron) at OLB (**a**) and MSA (**b**) sites during the monitoring period of 1 July 2011–31 December 2017. The red lines indicate a linear correlation between the two isotopes of the gas; the green and yellow lines indicate the different correlations for the parts of recorded data that should come from shallow and deep sources, respectively [62]. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. References  
* [PERSON] et al.