Airborne gamma-ray spectrometry for investigating radon vertical profile

222Rn gas has always been recognized as a sizable source of systematic uncertainty for the estimation of terrestrial 238U concentration by means of Airborne Gamma-Ray Spectroscopy (AGRS) measurements. 238U ground abundance is conventionally retrieved by monitoring the 1765 keV Energy Window (BEW) associated to the decay of 214Bi, a daughter isotope that occurs after 222Rn in the 238U decay series. This prevents to distinguish the gamma signal generated by 214Bi in the ground from the one emitted by 214Bi attached to airborne aerosols and produced after the decay of 222Rn exhaled into the atmosphere. A deep interest exists in understating the 222Rn distribution as it has implications in tracing air vertical mixing processes, studying the dynamics of the atmospheric boundary layer and investigating health impacts of human exposure to low-level ionizing radiation. We present the results of a dedicated off-shore AGRS campaign which led to the acquisition of 14688 1-second radiometric measurements performed in the (70 – 3000) m altitude range with a 16L NaI(Tl) detector. Experimental data were tested against a theoretical model describing the overall count rate recorded in the BEW (nBEW) as a superposition of a constant component due to the radioactivity of the aircraft plus a height dependent contribution due to cosmic radiation and atmospheric 222Rn. The altitude profile of the 222Rn component of the nBEW outlines the combination of a detector field of view effect, reflecting the 1765 keV photon mean free path in air (~ 175 m), and of the vertical distribution of 222Rn itself. The latter has been modeled as a single air layer extending up to a cutoff altitude s and having uniform 222Rn concentration aRn located at the bottom of a radon-free layer. Thanks to the large flight altitude range covered during the data taking and to the adoption of a refined χ2 based statistical analysis we obtained not only a conclusive evidence of AGRS 222Rn detection but also a 222Rn concentration aRn = (0.96 ± 0.07) Bq/m^3 and an atmospheric layer depth s = (1318 ± 22) m fully compatible with literature data.