Question

A series of whole-body counts indicated that a worker had a body burden of 18 microCi of Na-24 which was cleared from his body with effective half-life of 12 hours. Calculate: a. The biological half-life b. Cumulative activity c. Dose to the whole body

Answer 1

(a) The biological half-life of a biological substance is the time it takes for half to be removed by biological processes. Na-24 was cleared from his body with effective half-life of 12 hours. Hence the biological half-life is approximately 12 hours.

(b) 1 curie = 3.7x10¹⁰ radioactive decays per second, hence 18 microCi = 3.7x10¹⁰x 18 x 10^-6 decays per second = 66.6 x 10⁴ decays per second. We now consider the relationship of activity (A) and the number of nuclei, N, in a specific sample.

Cumulated Activity: The quantity of radioactive nuclei that undergo transitions in a period of time is usually designated the cumulated activity, Ã, and is expressed in the units of microcurie-hours. 1 microCi-hr is equivalent to 133 million (13.3 x 10⁷) transitions. The quality of nuclear radiation images is generally related to activity, whereas patient dose is more dependent on the amount of radioactive material or cumulated activity.

The relationship between cumulated activity, Ã, and the initial activity of a collection of radioactive material, A, is

(c) Dose to the whole body can not be calculated from the data supplied. If you have data like body mass, absorbed dose, quality factor etc., then you can easily get the whole body dose by the definition given by the International Committee for Weights and Measures (CIPM). The quantity dose equivalent H is the product of the absorbed dose D of ionizing radiation and the dimensionless factor Q (quality factor) defined as a function of linear energy transfer

The unit sievert – quantity H - Dose equivalent, 1 Sv = 1 joule/kilogram – a biological effect.