To understand neutron activation analysis, you should be acquainted with a few basic concepts. The nuclei of atoms are stable only when they contain certain numbers of neutrons and protons. The number of protons in an atom’s nucleus determines an element’s identity; the number of neutrons usually determines whether or not that atom is radioactive or nonradioactive (stable).[1]

Thus, while all sodium atoms contain 11 protons, only those sodium atoms that contain 12 neutrons are stable. A radioactive sodium atom contains a different number of neutrons. For other elements, there may be more than one number of neutrons that results in stability; for instance, there are 10 stable atoms (isotopes) of tin, each containing a different number of neutrons in their nuclei.

The fact that nuclei can absorb additional neutrons, which, in many cases, results in the conversion of a stable nucleus to a radioactive one, makes neutron activation analysis possible. Because radioactive nuclei decay in unique ways and yield radiations that are often distinct and can be measured even in very small amounts, measurements of these radiations can determine the kind and the number of radioactive atoms that are present.

In the most common type of activation analysis, the neutron bombardment of a sample is performed in a nuclear reactor where the neutrons that strike the target atoms have been slowed down so that they have very little energy of motion. In this case, the usual reaction between the target atoms and a neutron results in the capture of the neutron and this creates a nucleus with an atomic weight of one more unit than it started with. Thus for sodium as found in nature (symbol ²³Na)

sodium-23 + a neutron ? radioactive sodium-24 + gamma rays[2]

The numbers denote the atomic weight of the atom, which is the total number of protons and neutrons in its nucleus.

In a nuclear reactor, there are many, many neutrons that can be used in this reaction; approximately 10¹² to 10¹4 (10¹² is a million million; 10¹4 is a hundred times 10¹²) pass through each square centimeter of target area every second. Not all these will strike the nuclei of sodium atoms. Of those that do, not all will be captured. A mathematical relationship that tells how many atoms of sodium-24 will be created in a cubic centimeter of the target in one second is:

N24 = N23fst

where N24 is the number of sodium-24 atoms created during each second in a cubic centimeter of the target; N23 is the number of atoms of sodium-23 in a cubic centimeter of the target; f is the number of neutrons crossing a square centimeter per second (called the neutron flux); t is the time in seconds that the target is in the reactor; and s is a number that represents the probability that the conversion of sodium-23 to sodium-24 will occur. This last number is called a “cross section” and it is expressed in “barns”. One barn is equal to 10^-24 square centimeter, which is approximately the cross-sectional area of a typical atomic nucleus.

In an activation analysis experiment, the analyst wants to determine the number of target atoms (N23 in the above example). He can measure how long the target was in the nuclear reactor; there are ways of measuring the neutron flux, f; and the cross section is fixed and generally known for each target nucleus. So, by measuring the number of radioactive atoms created (N24), he can calculate the number of target atoms. See the figure on the next two pages.

Actually, to get the most accurate results, there are certain practical tricks he can use that increase the accuracy. Some of these will become apparent in later sections of this booklet.

The most important of these “tricks” is the use of a “standard” or “comparator”. This comparator is similar in form and composition to the sample to be measured but contains a known quantity of the element to be determined. The steps used for the analysis are simple.

1. Put the sample and comparator together into a reactor and bombard them with neutrons.

2. Remove them and measure the radioactivity produced from the sample.

3. Compare the radioactivity of the sample and the comparator and calculate the amount of the element in the sample as a proportion: