Dose-rate tables for clinical 137Cs sources sheathed in platinum

Dose tables and isodose curves for several commercially available platinum filtered caesium-137 tubes and needles are presented and shown to be more closely related to those of radium sources than to stainless steel filtered 137Cs sources. Alternative definitions of the concept of exposure equivalent milligrams of radium as a method of specifying the activity of 137Cs sources are discussed. An activity correction factor characteristic of each source is introduced as a function of the active length and the source filter material and thickness. Application of this factor facilitates conversion to millicuries from units of milligrams of radium equivalent when the latter unit is based on the equivalence of the exposure dose rates, in air at 25 cm distance, of the caesium source and a point source of radium screened by 0·5 mm platinum.