Low-level deterrmination of skeletal 228Ra and 228Th in the presence of gross amounts of 226Ra

A method is described for determining 228Th (and hence 228Ra) in small samples of human bone when accompanied by many times as much 226Ra. Such determinations are needed for the radiation dosimetry of skeletally-deposited radium 30-50 years after intake of mixtures in unknown proportions of the two isotopes of radium. The only preparation of the bone sample is ashing and dissolution in nitric acid. The short-lived radioactive gas, 220Rn (54.5 seconds), is de-emanated from the solution with nitrogen, which carries it into a 3.8-litre collection chamber. Positively charged ions of 216Po and 212Pb produced by the decay of the 220Rn during its stay in the chamber are collected on a negatively charged electrode in the form of a copper disk, to which is cemented a zinc sulphide phosphor-coated Mylar disk. De-emanation and ion collection continue for up to 24 hours. The electrode is then removed and presented to a second phosphor disk on the window of a photo-multiplier tube. Alpha particles from the collected radioactivity are thus counted with almost 4π geometry, and an initial efficiency is obtained of about 0.3 count per disintegration of 228Th in the solution. Positively charged ions of 218Po and 214Pb from the decay of some 222Rn in the chamber are also collected. The overall efficiency for this is much lower than for the collection of the daughters of 220Rn but, because of the much higher activity of the 226Ra, the initial counting rate is primarily due to its daughters. To make maximum use of all the counting data, they are analysed by a computer method of least squares. In routine use, de-emanation for 1400 minutes, followed by counting for 1400 minutes, permits the measurement of a few tens of fCi of 228Th. The need for correct weighting of very low counting rates is demonstrated. (author)