Further comments on the effective wall material of Baldwin-Farmer chamber

The author comments on the interpretation of the results of an experimental determination of the effective wall material of a Baldwin-Farmer chamber (Henry, W.H., 1979, Phys. Med. Biol., vol. 24, 37). Henry concluded that a Baldwin-Farmer chamber with a Tufnol inner wall behaves as though the effective wall were halfway between water and air for 60Co radiation, yet the conflicting assumption that Tufnol is air-equivalent can be shown to be a very good approximation, and in agreement with Henry's own results. His results also provide convincing experimental support for an alternative approach based on calculations of modified stopping power ratios and taking into account the composite nature of the wall for particular radiation qualities. The Baldwin-Farmer chamber has a thin inner coating of graphite, and the presence of a graphite lining in an air cavity irradiated with 60Co is known to have unexplained effects on the effective wall material. The analyses may therefore all be invalid. (U.K.)