Revisiting the humidity correction - A challenge to the current consensus

The humidity correction factor, $k_h$ , for ionization chambers in radiation dosimetry has relied on a consensus value established in ICRU Report 31 nearly half a century ago.

This study aims to rigorously re-evaluate this long-standing consensus by employing modern instrumentation.

First, the equilibration time for the cavity air was examined by implanting a miniaturized humidity sensor directly into the cavity of a broken commercial ionization chamber to address uncertainties regarding stabilization in recent reports. Second, the $k_h$ was measured using a vacuum-assisted gas exchange method, in which the air inside the cavity was exchanged actively. Finally, Monte Carlo simulations were performed to analyze energy deposition within the cavity, enabling the conversion of the measured $k_h$ to the relative $W$ -value.

The direct monitoring of humidity inside the cavity revealed that the equilibration time scale extended to several days, which was much longer than the time scales assumed in the recent reports. The measured $k_h$ for the Co-60 beam exhibited slight but apparent differences from the long-standing consensus: the measured $k_h$ extrapolated to 0 % relative humidity (RH) was 0.9962-0.9970, and the sudden response increase started from 0 % RH to 40 % RH (or possibly 30 % RH). The derived relative $W$ -value showed a roughly linear dependence on RH, which contrasted with the non-linear behavior of the current consensus curve.

Measurements using modern instrumentation indicate that the humidity dependence of $k_h$ and the corresponding relative $W$ -value for Co-60 beam quality differ slightly from the long-standing consensus established in ICRU Report 31. Although these differences are of minor importance for practical dosimetry, they may be relevant to primary air-kerma standards. The present results suggest that primary standard laboratories may re-evaluate their uncertainty budgets in light of these findings.