###### Abstract
This fact can provide information on soil gas sources: deep soil layers for \({}^{222}\)Rn and shallow soil layers for \({}^{220}\)Rn [2; 6]. Thus, the contribution of \({}^{220}\)Rn is usually negligible compared to that of \({}^{222}\)Rn. However, in some cases high levels of thoron activity concentration can be found in weakly ventilated areas where advective transport of soil gas is particularly high and/or rich in rocks containing significant amounts of \({}^{232}\)Th-rich mineral phases [13]. For these reasons, the literature is replete with \({}^{222}\)Rn measurement in soil and surface water as a tool to monitor geophysical events (earthquakes, seismotectonic activity and volcanic activity), while only few studies have fully reported thoron measurement for the same application. In general, the investigation on \({}^{220}\)Rn in the literature is lacking [14; 15]. The main studies deal with laboratory experiments that aimed to develop a thoron reference for in air measurement systems [2; 16]. The remaining few studies focus on soil gas \({}^{222}\)Rn-\({}^{220}\)Rn isotope pair measurements in underground environments (caves, mines), where significant concentration levels of thoron can be detected [3; 13; 17]. There is a lack of papers concerned with thoron alone for geophysical purposes, despite the fact that interest in thoron monitoring has grown considerably in recent years and is still increasing. The present study focuses on the analysis of soil gas \({}^{220}\)Rn activity concentration time series at two different sites of the Campi Flegrei caldera (Naples, Italy), over a period of about seven years, from 1 st July 2011 until 31 st December 2017. The gas was continuously monitored with systems based on the electrostatic collection, and the subsequent spectrometry analysis, of the \(\alpha\)-particles of \({}^{220}\)Rn daughters. The two sites were located in: _i._ Monte Olibano (OLB), inside a disused railway tunnel into the central area of the caldera; and _ii._ Monte Sant'Angelo (MSA), outside the Department of Physics \"[PERSON]\" of the University of Naples \"Federico II\" [18] (Figure 1). This work retraces the analyses performed in [1] on the soil gas \({}^{222}\)Rn activity concentration time series at the same two sites, which concluded that the \({}^{222}\)Rn follows the evolution of the Campi Flegrei caldera, in agreement with the more classic geophysical and geochemical parameters routinely used [19; 20]. Campi Flegrei is a caldera located on a NE-SW-trending structure in the Campanian Plain, occupying an area of about 120 km\({}^{2}\). This area consists of an interesting geological system characterized by interaction between tectonics and volcano-tectonic activity, with NE-SW- and NW-SE-trending faults that control the distribution of volcanic centers [1,20]. In the last two decades, the caldera has been subject to a long-term volcanically induced crisis, still ongoing, characterized by numerous episodes of ground uplift with seismic activity, an increase in the acceleration of the deformation of the soil due to magma intrusion at shallow depth and large emission of hydrothermal-volcanic and geothermal fluids through soil diffuse degassing and fumarolic vents [21,22]. The complete description of the structural geological setting of the Campi Flegrei caldera, and its current unrest phase from a geophysical and geochemical point of view, can be found in the literature [20,23]. The analysis of the \({}^{220}\)Rn time series at the OLB and MSA sites is performed using a well-proven hybrid method, successfully applied for the trends and residuals (anomalies) extraction [1,24,25,26]. The hybrid method optimally combines standard techniques for the signal decomposition to filter the known and unknown seasonal/periodical components out, and a technique for the forecasting and reconstruction of the time series in order to obtain the anomalies as differences with the raw signal [11,25].