Long-Duration Microdosimetry at the LNGS External Laboratory

Environmental radiation fields are characterized by very low dose rates and sparse energy-deposition events, conditions under which absorbed dose alone provides an incomplete description of irradiation at cellular and subcellular scales. This study assessed the feasibility of environmental microdosimetry in the external laboratory of the Laboratori Nazionali del Gran Sasso (LNGS) using a refurbished 5 cm spherical tissue-equivalent proportional counter (EuTEPC). The detector was converted into a stand-alone system, equipped for remote control and timed spectral acquisition, and installed at LNGS for long-duration measurements. Initial LNGS measurements were dominated by electromagnetic interference and did not yield physically interpretable spectra. Practical troubleshooting indicated that the dominant disturbance was associated with sporadic electromagnetic pickup, although the coupling mechanism was not investigated systematically. Extending the shielding to the detector and the complete cabling chain substantially reduced the noise and enabled the acquisition of stable lineal-energy distributions. Spectra acquired several months apart preserved the same overall shape, indicating consistent measurement performance under very low event-rate conditions. Comparison with measurements at Legnaro National Laboratories (LNLs) and with published environmental microdosimetric data from Ottawa and the high-altitude site of Gulmarg showed physically consistent spectral features, while also highlighting differences consistent with variations in radiation-field composition, detector characteristics and acquisition conditions. These results establish an operational platform for environmental microdosimetry at LNGS and identify electromagnetic compatibility as a critical requirement for measurements in very-low-background environments.