Humidity effects in GafChromicTMEBT4 films for postal audits: characterization, reversibility and exploration of correction methods.

No harmonized methodology exists that enables traceable reference dosimetry with EBT4-film. The influence of humidity on EBT4 films is often overlooked and currently under-documented. This study quantified humidity effects for postal-audit reference dosimetry and explored the feasibility of empirical correction methods.
Approach. EBT4 films mounted in slide-frames were stored in humidity-controlled environments between 0 %RH and 100 %RH and scanned using a slide-scanner. Optical density (OD) was evaluated as a function of dose and humidity. Humidity darkening, changes in dose response and post irradiation growth were quantified. A humidity response factor, HOD, was introduced to describe the humidity dependent changes in dose-response. Two film intrinsic proxies were evaluated as possible proxies for the film humidity state.
Main results. Humidity was shown to affect OD and the humidity response factor, HOD. If not accounted for, this can lead to significant dosimetric errors. Between 12 %RH and 68 %RH, HOD exhibited linear behaviour as did the two film intrinsic proxies. Both humidity darkening and HOD showed to be reversible within the experimental uncertainty after re-acclimatization at 56 %RH. Exposure to 0 %RH and 100 %RH resulted in behaviour outside the practical operating range. The use of 0-Gy control films substantially reduced humidity-dependent offsets. The film-intrinsic humidity proxies showed relationships with HOD within the present single-sheet experiment, but their practical use requires further validation across film-lots and scanners.
Significance. This study demonstrates that humidity is a relevant influence quantity for EBT4-based reference dosimetry and should be controlled during film transport and storage. For the VSL postal audit service, humidity is regulated using BOVEDA-49 packages and monitored during transport as a practical mitigation strategy. The proposed correction formalism provides a method to investigate humidity-response behaviour for a specific film-sheet and scanner combination but does not yet constitute a generally validated correction method.