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: