Development and calibration of a passive sampler for monitoring airborne ¹³¹I in nuclear medicine facilities

Monitoring volatile iodine-131 (¹³¹I) in nuclear medicine is of great importance for occupational protection. Conventional active monitors require continuous power and generate noise, necessitating reliable passive alternatives. Herein, we report the development, geometric optimization, and dynamic calibration of a passive sampler tailored for airborne organic ¹³¹I (CH₃¹³¹I). To maximize High-Purity Germanium (HPGe) detection efficiency while maintaining an optimal Fickian diffusion length, the sampler's geometry was optimized using LabSOCS Monte Carlo simulations. An optimal diameter-to-height ratio of 4 was established for sorbent volumes <150 mL and validated against traceable multi-gamma standards. A 12-16 mesh coconut shell activated carbon impregnated with 5 wt% KI served as the trapping matrix due to its efficient chemisorption via isotopic exchange. Applying these parameters to a 40 g carbon loading yielded a prototype featuring an 8.4 cm internal diameter, a 2.1 cm carbon depth, and a 2.3 cm effective diffusion length. Dynamic calibration in a micro-reactor under clinical conditions (26°C, 60% relative humidity) revealed a stable sampling rate (SR) of 0.030 m³ h^-1 within the linear kinetic regime (≤25% EA). The SR demonstrated high thermodynamic stability across typical ward temperatures (10-26°C) with a 6.0% coefficient of variation. With a combined expanded relative uncertainty of 12.16% (k = 2), the sampler achieved a Minimum Detectable Concentration (MDC) of 0.74 Bq m^-3. This work opens a new avenue for determining time-weighted average (TWA) airborne radioiodine concentrations, advancing internal dosimetry protocols.