Page 267 - Radiochemistry and nuclear chemistry

Uses of Radioactive Tracers 251

The specific activity is defined by (4.49): S = A/w Bq g-l; since only the ratio S O/S m is
used in (9.8), the activity A can be replaced by the measured radioactivity R when the
detection efficiency ~k is the same.
This technique is of particular advantage where quantitative separation of the desired
compound is not feasible, as illustrated already by de Hevesy in 1932: In determination of
micro amounts of lead by anodic precipitation, quite varying results were obtained. By
addition of a known amount of "radiolead" and measuring the radioactivity of lead at the
anode, the yield of the precipitation could be determined, and - although the electrolytic
precipitation was inefficient - an exact analysis was obtained.
In some cases the measurement of the final sample utilizes a technique other than
weighing, but the principle remains the same. Isotope dilution is used, for example, in the
determination of naphthalene in tar, of fatty acids in mixtures of natural fat, of amino acids
in biological material, etc.

9.3.3. Activation analysis

Activation analysis is a highly sensitive nondestructive technique for qualitative and
quantitative determination of atomic composition of a sample. It has been particularly useful
for determination of elements in complex samples (minerals, environmental samples,
biological and archeological objects, etc.), ~use it provides a simple alternative to much
more difficult, tedious and destructive techniques. Its main limitation is the demand for a
strong irradiation source.
In activation analysis advantage is taken of the fact that the decay properties such as the
half-life and the mode and energy of radioactive decay of a particular nuclide serve to
identify uniquely that nuclide. The analysis is achieved by the formation of radioactivity
through irradiation of the sample either by neutrons or charged particles. Neutron
irradiation is by far the more common technique, and hence this method is often referred
to as neutron activation analysis, NAA. A major advantage in activation analysis is that it
can be used for the simultaneous determination of a number of elements and complex
samples. If the counting analysis of the sample is conducted with a Ge-detector and a
multichannel analyzer, as many as a dozen or more elements can be measured quantitatively
and simultaneously (instrumental NAA, or INAA).
A sample is irradiated to form an amount R of radioactive nuclide according to the
relationship (cf. w
R = ~ ~ tr N (1 - e -xn') e -htc~176 (9.9)

We assume that irradiation is carried out by a homogeneous particle flux ~, in a neutron
reactor. The minimum amount of an element which can be detected increases with the
efficiency of the measuring apparatus ~b, the bombarding flux ~, the reaction cross-section
a, the irradiation time tit r (up to saturation activity), the decay constant, X, of the
radioactive nuclide formed, and the time from end of bombardment to start of counting,
tcool. By proper selection of tir r and tcool the sensitivity for any element can be changed and
interferences minimized. Table 9.1 shows the limits of detection in INAA.