###### Abstract  
The study concerns the analysis of \({}^{220}\)Rn (thoron) recorded in the surface soil in two sites of the Campi Flegrei Caldera (Naples, Southern Italy) characterized by phases of volcanic unrest in the seven-year period 1 July 2011-31 December 2017. Thoron comes only from the most surface layer, so the characteristics of its time series are strictly connected to the shallow phenomena, which can also act at a distance from the measuring point in these particular areas. Since we measured \({}^{220}\)Rn in parallel with \({}^{222}\)Rn (radon), we found that by using the same analysis applied to radon, we obtained interesting information. While knowing the limits of this radioisotope well, we highlight only the particular characteristics of the emissions of thoron in the surface soil. Here, we show that it also shows some clear features found in the radon signal, such as anomalies and signal trends. Consequently, we provide good evidence that, in spite of the very short life of \({}^{220}\)Rn compared to \({}^{222}\)Rn, both are related to the carrier effect of CO\({}_{2}\), which has significantly increased in the last few years within the caldera. The hydrothermal alterations, induced by the increase in temperature and pressure of the caldera system, occur in the surface soils and significantly influence thoron's power of exhalation from the surface layer. The effects on the surface thoron are reflected in both sites, but with less intensity, the same behavior of \({}^{222}\)Rn following the increasing movements and fluctuations of the geophysical and geochemical parameters (CO\({}_{2}\) flux, fumarolic tremor, background seismicity, soil deformation). An overall linear correlation was found between the \({}^{222-220}\)Rn signals, indicating the effect of the CO\({}_{2}\) vector. The overall results represent a significant step forward in the use and interpretation of the thoron signal. \({}^{220}\)Rn; volcanic unrest; hydrothermal alteration; CO\({}_{2}\); anomaly and trend; Campi Flegrei 2021  
surface, on the other hand, is produced only locally due to its much shorter half-life, so it cannot provide information on remote processes as its variations are due to local effects. The exhalation of radon/thoron from materials containing the parents is influenced by external factors, which can also act at a distance from the measuring point. In this way, the measure of the thoron activity produced locally can be correlated with the deep source. Radon generation and migration in the environment has been seen to depend on many factors: the porosity and permeability of the bedrock, the nature of the fluid transporting radon (i.e., carrier gases), increase due to weathering and chemical-physical alteration processes [4; 5; 6]. Furthermore, the effectiveness of radon release from minerals or radon emanation is contingent upon the specific surface area of the minerals, their size and shape and the presence of imperfections [7; 8]. Within this framework, many studies have investigated the analysis of radon time series with the aim of identifying anomalies and extracting anomalous seasonal trends. Radon anomalies of non-tectonic origin have been reported to be very similar to those of seismo-tectonic origin, where strong crustal deformative processes and high heat flows are recognized [9; 10; 11]. Other ones reported the physico-chemical phenomena in volcanic geothermal areas as possible mechanisms to explain the anomalous increase in radon signal in soil and groundwater [1; 6; 12]. The common aspect is that the radon measurement can provide useful information if it is continuously recorded at a site, where the half-lives of the radon isotopes play a key role; the half-life of \({}^{220}\)Rn makes the distance that it can travel, from its source site within the rock and filtrate through the ground and through cracks, before undergoing radioactive decay, much shorter than that traveled by \({}^{222}\)Rn in the same medium [2; 13].