Neutron Activation Analysis

Neutron Activation Analysis (NAA) is a non-destructive method for determining the elemental composition of a sample. The sample is irradiated with thermal neutrons in a nuclear reactor, which creates radioactive forms of some of the elements within the sample. The radioactive isotopes undergo decay to reach stable ground state configurations, emitting gamma rays as they do so. As a given radioactive isotope always emits gamma rays at certain specific energies, the radioisotopes present, and hence the parent element(s) present in the sample can be determined by gamma ray spectroscopy. Quantitative information is obtained by gamma counting – recording the intensities of the emitted gamma rays using a high purity germanium detector.

With the high neutron fluxes available at the McMaster Nuclear Reactor, NAA becomes a highly sensitive analytical technique. It can be applied to any element that possesses a suitable activation product (radioisotope); with appropriate experimental parameters, excellent sensitivity is possible for some 70 elements (see Figure). In some cases, multi-element analysis at the ppm level for up to 30 elements can be accomplished with a single sample weighing less than a gram. NAA is an established analytical technique for determining trace elements in a wide variety of materials, including rocks and other geological samples, as well as ceramics, oils, plastics, metals, water, biological and botanical materials.

A significant advantage of NAA over other analytical techniques is the simplicity of sample treatment before analysis: in most cases, the only requirement is that the sample be reduced to a size suitable for encapsulation. Activation analysis is also non-destructive, and can therefore be used for expensive or irreplaceable samples such as archaeological artifacts. Several variations on NAA are routinely performed at MNR’s Centre for Neutron Activation Analysis, and are described in detail below.

Short Lived Activation Analysis

The greatest use of NAA at MNR has been in the analysis of short-lived isotopes, particularly for Al, Ca, Cl, Dy, I, In, K, Mg, Mn, Na, Rh, Se, Ti, and V. Each irradiation terminal is accessed with a computer-controlled pneumatic transport system which was designed and built in-house. This irradiation system is integrated with the gamma counting facilities by delivering samples directly to an automatic sample positioner, interfaced to a PC-based multi-channel analyzer for data collection.

Delayed Neutron Counting (DNC)

Delayed neutron counting is a sensitive, selective and rapid technique for determining uranium at ppm levels or lower. The MNR DNC system is computer-controlled and fully automated, integrating sample irradiation, data analysis and data logging into a single system that is designed for efficient throughput of large numbers of samples.

Prompt Gamma Neutron Activation Analysis

Prompt gamma activation analysis is a method complementary to conventional NAA. Prompt gamma activation analysis looks at gamma rays emitted during neutron capture, rather than the gamma rays that are emitted as the resulting radioisotope undergoes decay. It can determine major, minor and trace elements, particularly boron, cadmium, europium, gadolinium and samarium. Boron is readily analyzed in concentrations from a few percent to the ppm level. Since the technique is completely instrumental, losses of volatile boron compounds are avoided.

The neutron flux during a typical PGNAA determination is several orders of magnitude lower than for conventional NAA. Consequently, both residual activity and the potential for radiation damage to samples are dramatically reduced, rendering this technique ideally suited for the analysis of valuable or sensitive materials.