A significant advantage of NAA over other techniques is the simplicity of sample treatment before analysis: typically the only requirement is that the sample be reduced to a size suitable for encapsulation prior to conducting the irradiation procedure.  Generally, a suitable sample mass may vary from milligrams to several grams of sample, depending upon the sensitivities required and specific elements to be analyzed.  In order to avoid the potential for contamination, samples are weighed into vessels appropriate for the type of irradiation to be performed under Class 1000 clean room conditions.  Given NAA’s excellent sensitivities, analysis is possible for nearly 70 elements, which under the appropriate experimental parameters, can also allow for the analysis of more than 30 elements on a simultaneous basis.

The actual irradiation process is performed by one of two methods.  Samples requiring short irradiation times (< 20 min.) and low to moderate flux densities (< 1013 n cm-2s-1), are packaged in virgin polyethylene vials.  Samples requiring longer irradiation times (hours or days), with moderate to high flux densities, are packaged in high-purity quartz vials.  Once packaged, the samples are placed near the core of the nuclear research reactor where the neutrons emitted by the reactor interact with the nuclei of the elements’ atoms causing them to become radioactive as depicted in the diagram on the previous page.

Elemental radioisotopes are formed through this interaction that subsequently decay by emitting gamma rays (γ-rays) that are unique in half-life and energy.  The distinct energy-signatures created are unique to each element and provide positive identification of the targeted element(s) in the sample, while the quantification of the element(s) are determined from the proportion of the intensity to the concentration of the respective element in the sample.  Since neutrons have no charge and will pass through virtually any materials, they will interact with the nucleus of the atom, rather than the electron shell.  In this respect, NAA is capable of providing a comprehensive determination of the samples total elemental content regardless of oxidation state, chemical form, or physical state of the desired element(s).

Placement or introduction of the packaged sample(s) for irradiation in the core of the research reactor is accomplished by one of two methods.  For analysis of short-lived nuclides requiring shorter irradiation times (< 20 min.), polyethylene vials are placed into a larger transport device know as a rabbit.  The rabbit travels from the laboratory into the reactor by an air-driven pneumatic transport system into the core of the reactor where the sample will reside until it has been subjected to the specified amount of radiation.  The rabbit is then extracted from the core and transported back to the laboratory where the samples are removed from the rabbit, and transferred to non-irradiated vials for completion of the analytical process.  For longer irradiation times, as many as 30 samples can be bundled into a watertight metal container called a can, which is mechanically (or manually) lowered into the reactor for the specified irradiation time.  After irradiation, the can(s) are removed from the core to cool, which allows the short-lived radioactivity to dissipate to safe handling levels.  The advantage of both methods is that it allows for the final analytical process to be conducted outside of the containment area, or in certain instances, by a laboratory outside the research reactor facility itself.

Upon return from the irradiation process and once the samples have been allowed to decay the specified length of time, the analyst will perform the final procedure known as counting.  The counting procedure is accomplished by placement of the activated sample in a device know as a detector, which is a specialized device that looks for the resultant γ-rays emitted from the irradiation process.  The detectors most commonly used for the counting procedure are either Lithium drifted Germanium [Ge(Li)] detectors, or High Purity Germanium (HPGe) detectors, with the latter being the most commonly used for multi-elemental analysis.  Since γ-rays emitted from the sample are very penetrating, the resultant γ-rays will create spectra similar to that of a gas chromatograph with peaks at different retention times (as depicted in the diagram below).  The position of each energy peak in the spectrograph determines the identification of the element, while the area underlying the peak is proportional to the concentration of the respective element.

Verifying the accuracy and quantification results for the NAA procedure is as important as with any viable analytical technique.  While in theory, it is feasible to calculate the target atoms mathematically (or perform absolute activation analysis), it is generally not acceptable since there can be significant uncertainties in values available for various nuclear parameters.  Equally important is the fact that the neutron energy spectrum and flux densities of the reactor are not always constant and can vary during different irradiation periods.  For these reasons, the comparator method is frequently incorporated, whereby a standard containing a known amount of the specified element is irradiated and counted along with the subject sample(s).  Since factors such as neutron flux, irradiation times, and other variables can be accounted for in the counting process, the results of the comparator standard can be used to verify the quantification and accuracy of the subject sample(s).  Since the creation of individual standards can become tedious as well as cost prohibitive stoichiometrically well-defined compounds can be obtained from sources that specialize in the preparation of standard reference materials (SRMs) such as the National Institute of Standards and Technology (NIST) and the U.S. Geological Society (USGS).  In this regard, either multi-elemental standards or the use of a single comparator standard for all elements can be incorporated for verification and quality control purposes.