Answer to Question #4423 Submitted to "Ask the Experts"

Q

What is the specific activity of promethium-147 (¹⁴⁷Pm) ? I am searching for how many curies are in 7 drops of ¹⁴⁷Pm? I have an unknown source that answered 28 microcuries.

A

We are not able to answer your question completely without additional information.

The specific activity, in curies per gram (Ci/g) of pure ¹⁴⁷Pm, a man-made isotope of a man-made element in the rare earth series, is readily obtained by noting that one gram of ¹⁴⁷Pm contains (1/147) (6.023 x 10²³) = 4.097 x 10²¹ atoms and that this number of atoms of ¹⁴⁷Pm represents an activity given by the product of the decay constant, ln2/T_1/2, where T_1/2 is the radioactive half-life of ¹⁴⁷Pm (2.623 years = 8.272 x 10⁷ seconds) and the number of atoms. Thus the activity represented by one gram of pure ¹⁴⁷Pm is A = (0.693/8.272 x 10⁷ s)(4.097 x 10²¹) = 3.432 x 10¹³ atoms decaying per second; since 1 Ci = 3.7 x 10¹⁰ atoms decaying per second, the ¹⁴⁷Pm activity represents 928 Ci, and the specific activity of pure Pm-147 is 928 Ci/g. The fact that you are trying to determine the activity of seven drops implies that you are dealing with a solution of a radioactive salt of ¹⁴⁷Pm, and the above result would not be useful to you unless you know the mass concentration of the ¹⁴⁷Pm in the solution, which is unlikely.

In order to answer your question you need to know the specific activity of the solution. Normally the manufacturer/provider of the radioactive solution would provide this information—for example, Ci/g of solution mass or Ci/ml of solution volume; alternatively, the total activity and total volume of the solution might be provided. Once you have this information it is a simple matter to calculate the activity in one drop, assuming you have a reasonable estimate of the mass or volume of the drop of solution.

You might want to look at some related questions and their answers on the HPS Ask the Experts Web site. If so, go to http://hps.org/publicinformation/ate/ and type "specific activity" in the search box. You might also check out question 4113 at the same site. Good luck.

George Chabot, PhD, CHP