The influence of air humidity on gamma-ray calibration and neutron absorbed-dose measurements with different types of A-150 plastic tissue-equivalent ionisation chambers

Small humidity changes in air-conditioned environments normally employed, coupled with the slow rate of change of water loss or gain, does not lead to major problems using A-150 plastic chambers in photon or neutron dosimetry. Only when chambers are kept for several days in air with a high relative humidity, can variations in weight and size up to several % be caused by water absorption. Observed changes are then about twice as high for the FWT chamber compared with the Exradin, both for changes in weight and photon calibration factor. Because almost identical changes are observed (even in high humidity) in photon calibration and neutron beam measurement, no significant changes in total absorbed dose determinations in the neutron beam could be demonstrated. The different hygroscopic properties of A-150 plastic used for the construction of FWT and Exradin chambers and build-up caps, cannot therefore explain differences in absolute absorbed dose values observed between chamber types. Both effects might be caused by different methods of removing plasticiser from the nylon used for the fabrication of the A-150 plastic. A nylon capped Baldwin-Farmer chamber exhibited a much larger effect, thus reinforcing this explanation. (U.K.)