Technical note: The impact of storage humidity on the response of reference-class ionization chambers.

Purpose: A number of Farmer-type ionization chambers were tested against storage humidity to confirm whether they satisfied the criteria for the long-term stability of reference-class ionization chambers.

Methods: The ionization chambers were stored for several months in an environment with relative humidity adjustable from 20% to 80%. The ionization chambers were removed from the storage environment at variable intervals ranging from 1 to 70 days and irradiated in a Co-60 radiation beam. The responses for each ionization chamber were evaluated from the measured currents corrected for the atmospheric air density, and were compared with those predicted by the Co-60 half-life.

Results: Certain ionization chambers gave a constant relative response regardless of the storage humidity, while the relative responses of two types of ionization chambers changed as a function of the storage humidity. The difference between the relative responses for the low (20-30%) and high (70-80%) storage relative humidity was 0.7%. The response was larger for the high relative humidity storage. Immediately after the storage humidity changed, the relative response started to change by the day, and it took approximately 2 weeks to 2 months for the relative response to converge. For one type of the ionization chamber, the plastic outer wall and the outer electrode were replaced with those made of solid graphite, and it was confirmed that the remodeled ionization chamber did not exhibit the response change.

Conclusions: The present results and previous reports by other authors indicate that the magnitude of the change depends on the magnitude of the water absorption of the plastic used for the outer wall and/or the electrode of the ionization chamber. Thus, it is important in the selection of the reference-class ionization chamber to note the material and structure of the outer wall and electrode of the ionization chamber. If the ionization chamber has a hygroscopic wall and electrode and it is used as a reference ionization chamber, it is necessary to pay additional attention to the humidity difference for the storage, daily irradiation, and yearly calibration especially in regions with large seasonal humidity fluctuations.