2.2.2 Specific Activity

References: Sorenson and Phelps; Hendee.

Created October 6, 1995


2.2.2.1 Specific Activity

A sample of a nuclide may also contain other isotopes of the same element. For example, a sample of Sr-89 will contain stable Sr-84 and Sr-88 and contamination levels of radioactive Sr-85. The stable isotopes that accompany a sample are referred to as "carrier". When we have only the nuclide of interest, the sample is said to be "carrier-free".

The differentiating factor in determining whether or not a sample is carrier-free is its method of production. In neutron activation (n,gamma), we have stable carrier as the target remaining and chemically inseparable from the product nuclide. In cyclotron-produced nuclides, we produce carrier-free since the only atoms of the nuclide of interest are those produced by induced transmutation.

The ratio of nuclide activity to the total mass of the element present is known as the specific activity of the sample.

2.2.2.2 Carrier-Free Specific Activity

The maximum specific activity is known as the Carrier-Free Specific Activity - CFSA. The CFSA can be computed from Avadadro's number and is given by:

2.2.2.3 Other Specific Activity

When the radionuclide is attached as a label to a large molecule such as a protein, the specific activity may be expressed as activity per unit mass of protein.

2.2.2.4 Examples

(from Sorenson and Phelps) 1. What is the CFSA of 99m-Tc?

(from Hendee) 2. The physical half-life of In-113m is 1.7 hours.
a. A sample of In-113m has a 2 microgram mass. How many In-113m atoms are present?

b. How many In-113m atoms remain after 4.0 hr have elapsed?

c. What is the activity of the sample after 4 hours?

d. What is the specific activity of the In-113m sample after 4 hours?

e. Enough In-113m must be obtained at 4:00 pm Thursday to provide 10 uCi at 1:00 pm Friday. How much In-113m should be obtained?


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Douglas J. Wagenaar, Ph.D., wagenaar@nucmed.bih.harvard.edu