Environmental ${\hat{I}}^3$-Ray Flux in Hall C at LNGS and Its Correlation with Radon Activity

Abstract

We report a comprehensive measurement of the environmental ${\hat{I}}^3$-ray flux in Hall C of the Gran Sasso National Laboratory. A spatial mapping of the radiation was carried out using a high-purity germanium detector mounted on a movable cart and deployed at eight locations within the hall. The detector response function and full-energy-peak efficiencies were determined through Geant4 simulations validated with calibrated ${\hat{I}}^3$-ray sources, with particular attention devoted to the efficiency modeling and associated systematic uncertainties. In the energy range of 57-2800 keV, the average ${\hat{I}}^3$-ray flux is measured to be $(0.46\pm {0.06}_{stat}\pm {0.03}_{syst})$ ${\mathrm{cm}}^{-2}$ ${\mathrm{s}}^{-1}$. The radon level was monitored for about a month using a radon detector mounted on the same cart, and a clear correlation is observed between the environmental ${\hat{I}}^3$-ray rate and the ambient radon concentration, consistent with the short-lived daughters of ${}^{222}\mathrm{Rn}$. This result represents the first high-precision and efficiency-corrected mapping of the ${\hat{I}}^3$-ray flux in Hall C, substantially improving its radiological characterization and providing key input for future rare-event experiments operating in this hall.

20 pages, 12 figures

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