Energy-based evaluation of airborne thoron progeny interference in the radon progeny sensor using LR-115 sensors with aluminium absorbers

Radon (222Rn) and thoron (220Rn) progeny are the principal contributors to inhalation dose due to their attachment to aerosols and subsequent deposition in the respiratory tract. Conventional Direct Radon Progeny Sensors (DRPS) and Direct Thoron Progeny Sensors (DTPS) assume that the tracks registered in LR-115 detectors originate exclusively from alpha particles emitted by progeny deposited on the absorber surface. Under mixed exposure conditions, where both deposited and airborne progeny coexist, this assumption may bias progeny-specific measurements if airborne progeny also contribute to track density. However, comprehensive studies on this effect remain limited. This study aims to assess the contribution of airborne thoron progeny to the DRPS response through an energy-based framework introducing the concepts of sensor energy-response window, equivalent sensors, and discrimination factor. The framework enables the design of aluminium-absorber, energy-equivalent conventional sensor configurations. Stopping and Range of Ions in Matter (SRIM) simulations were used to model alpha-particle energy loss in air and absorbers, determine equivalent aluminium thicknesses, and estimate detectable air-layer ranges. Experimental validation was performed using alpha spectrometry and controlled exposures to deposited and mixed deposited–airborne progeny. Aluminium absorbers of approximately 27 and 36 µm reproduced the DRPS and DTPS energy-response windows, respectively. Thin DRPS-type absorbers registered contributions from airborne 216Po and energy-degraded 212Po, increasing discrimination factors for both airborne and deposited progeny were more than three times the values obtained for deposited progeny alone. Increasing the absorber thickness to approximately 30 µm suppressed this contribution while preserving sensitivity to deposited progeny. These findings provide a physically grounded basis for refining radon and thoron progeny monitoring and improving radiological risk assessment.

Radon (222Rn) and thoron (220Rn) progeny are the principal contributors to inhalation dose due to their attachment to aerosols and subsequent deposition in the respiratory tract. Conventional direct radon progeny sensors (DRPSs) and direct thoron progeny sensors (DTPSs) assume that the tracks registered in LR-115 detectors originate exclusively from alpha particles emitted by progeny deposited on the absorber surface. Under mixed exposure conditions, where both deposited and airborne progeny coexist, this assumption may bias progeny-specific measurements if airborne progeny also contribute to track density. However, comprehensive studies on this effect remain limited. This study aims to assess the contribution of airborne thoron progeny to the DRPS response through an energy-based framework introducing the concepts of sensor energy-response window, equivalent sensors, and discrimination factor (DF). The framework enables the design of aluminium-absorber, energy-equivalent conventional sensor configurations. Stopping and range of ions in matter simulations were used to model alpha-particle energy loss in air and absorbers, determine equivalent aluminium thicknesses, and estimate detectable air-layer ranges. Experimental validation was performed using alpha spectrometry and controlled exposures to deposited and mixed deposited–airborne progeny. Aluminium absorbers of approximately 27 and 36 µm reproduced the DRPS and DTPS energy-response windows, respectively. Thin DRPS-type absorbers registered contributions from airborne 216Po and energy-degraded 212Po, resulting in DFs for airborne and deposited progeny that were more than three times higher than those for deposited progeny alone. Increasing the absorber thickness to approximately 30 µm suppressed this contribution while preserving sensitivity to deposited progeny. These findings provide a physically grounded basis for refining radon and thoron progeny monitoring and improving radiological risk assessment.