KJ3055-Chapter 9 (Radiochemical methods)

3.3. NAA: experimental aspects

Fig. 10. Flow diagram for two types of neutron activation methods: non-destructive (instrumental) and destructive.

Basic steps of NAA are, as follows (Fig 10):

1.The sample is first carefully weighed into a plastic or quartz container.

2.The sample is then sealed with a high-speed friction sealer.

3.The sample, along with appropriate standards and blanks, is placed near the core of the nuclear reactor and irradiated with neutrons for a predetermined length of time. If a neutron approaches the nucleus of an atom, it may be absorbed. When this happens, the element will become a different isotope of the same element. This "new" isotope is almost always unstable (radioactive) and usually decays by emitting a gamma ray(s).

4.The sample is then pulled out of the reactor and then it is allowed to decay for a predetermined length of time appropriate to the radioactive half-life of the indicators for the elements being determined (“cooling” period).

5.The sample may be counted immediately, for elements with short half-lives, or counted after a delay for samples with longer half-lives. The reason for the delay for isotopes with longer half-lives is to allow the isotopes with a short half life to decay away, thereby preventing interference and allowing the isotopes with longer half lives to be more easily measured.

6.After the sample decays, it is "counted" using High Purity Germanium (HPGE) gamma ray detectors.

The above approach, which does not involve a chemical processing of the sample, is known as Instrumental Neutron Activation Analysis (INAA).

If a strong spectral background and large noise are disturbing, the destructive approach can be suitable in order to remove perform analyte separation from the interfering isotopes. The limit of detection can thus be improved significantly.

F.G. Banica, 2009-04-03