NAA, discovered by G. Hevesy and H. Levi in 1936, is an activation analytical method (see Section 10.2.1).

In NAA, the sample is irradiated with neutrons, initiating nuclear reactions. Having no charge, neutrons can be captured easily by the different atomic nuclei. All elements, except for helium, have isotope(s) reacting with neutrons (as detailed in Section 6.2.1) in an (n, Y) nuclear reaction. As a result of the nuclear reaction, radioactive isotopes are produced. This process is called “activation.” These iso­topes have one more neutron than the inactive target, so they will typically decom­pose by emitting negative beta particles. Because of the continuous spectra, beta particles are measured with difficulty, especially when the sample contains many components. The beta decay, however, is frequently followed by the emission of gamma photons with discrete energies. The energy of these gamma photons is char­acteristic of the target elements and suitable for qualitative analysis. The activity/ intensity of gamma photons provides quantitative analytical information. The gamma photons are the result of radioactive decay, and they are present after the irradiation.

Besides neutrons, the samples can be activated by charged particles too. Table 10.2 describes this method as CPAA. The basic concepts (the detection of the emitted photons) are similar to NAA.

As discussed in Section 5.5.2, neutrons are produced in neutron sources, genera­tors, nuclear reactors, or spallation neutron sources. Since there are only a few spallation neutron sources all over the world, the irradiation is generally achieved in nuclear reactors (or sometimes neutron generators).

The way of neutron production determines the flux (neutron sources < neutron generators < nuclear reactors) and energy of the neutrons. In neutron generators, fast neutrons with 14 MeV energy are formed, which induce (n, Y), (n, p), (n, a), (n,2n) nuclear reactions. As seen in Figure 6.4, the general tendency is that the cross section of nuclear reactions with neutrons is inversely proportional to the neutron energy. For this reason, thermal neutrons produced in nuclear reactors are the most important in the (n, Y) nuclear reactions of NAA. In addition, the flux of the neutrons is highest in the nuclear reactors. As a result, neutron activation stud­ies are usually performed in the research nuclear reactors.

The high range of gamma radiation provides the possibility of nondestructive NAA. In this method, the samples are analyzed directly by an instrumental technique called instrumental neutron activation analysis (INAA). Of course, the instrumental measurements can be supplemented by radiochemical separation, if required. This method is called radiochemical neutron activation analysis (RNAA).

During activation, the inactive nuclei transform to radioactive ones via nuclear reactions. This means that the specific activity of the sample increases, usually from zero to a certain value. The sensitivity of the method is determined by the number of the radioactive nuclei formed. The number of the radioactive nuclei is deter­mined by the kinetic law of activation, as discussed in Section 6.1. Equation (6.11) gives the number of radioactive nuclei formed from any stable nuclide at a certain nuclear reaction characterized by cross section, flux of the irradiating particles, irra­diation, and cooling time. If all these factors are known, and the activity can be measured with high accuracy, absolute measurements can also be done.

These absolute measurements, however, are usually limited by the lack of accu­rate knowledge of the cross sections, the flux at the position of the sample, and activity after irradiation and cooling. In practice, mostly relative measurements are made; the samples are compared to a standard with known concentration of the ele­ments expected in the sample. The standard is simultaneously irradiated with the sample under the same conditions (flux, time, and irradiation position). The gamma radiation of the sample and the standard are measured under the same conditions as well. Standard samples may be homemade or purchased: commercial multielement Standard Reference Materials (SRMs) are available.

The advantages of NAA are that small sample sizes (1—200 mg) are suitable for the simultaneous analysis of many elements with low detection limits. The detec­tion limits of the elements are shown in Table 10.4. The accuracy of NAA is about 5%, and the relative precision may be better than 0.1%. INAA does not destroy the samples; thus, it can be useful when the sample has to remain intact (archeological, artistic, criminal, and other samples).

Because of the very small size of the sample, the sampling and preparation of the sample is critical. The sample must characterize the average composition of the object to be analyzed. Any contamination must be avoided.

The method’s major limitation is that, although almost all elements have iso­topes that can participate in nuclear reactions with neutrons, the produced radioac­tive isotopes of some light elements have very short half-lives, so they cannot be analyzed in neutron activation. Similarly, some elements have small neutron cap­ture cross sections, which cause difficulties in the analysis, and there are other ana­lytical methods, which often provide better sensitivities.

The nuclear reactions of the different nuclides can interfere with each other, for example, because the nuclear reactions produce the same radioactive nuclide. The

activation of bronze or brass results in the following nuclear reactions: 64Zn (n, Y)65Zn, 64Zn(n, p)64Cu, and 63Cu(n, Y)64Cu. As seen, MCu is produced from both zinc and copper. These interferences, namely, all possible nuclear reactions in the sample, always must be taken into consideration.

Energy (keV)

The neutron activation spectrum of sodium is shown in Figure 10.7. As a result of the irradiation, a 23Na(n, Y)24Na nuclear reaction takes place, and the spectrum shows gamma lines characteristic of 24Na.