Neutron activation analysis
If NAA is conducted directly on irradiated samples it is termed instrumental neutron activation analysis (INAA).
In some cases, irradiated samples are subjected to chemical separation to remove interfering species or to concentrate the radioisotope of interest; this technique is known as radiochemical neutron activation analysis (RNAA).
NAA can perform non-destructive analyses on solids, liquids, suspensions, slurries, and gases with no or minimal preparation.
Due to the penetrating nature of incident neutrons and resultant gamma rays, the technique provides a true bulk analysis.
Until the introduction of ICP-AES and PIXE, NAA was the standard analytical method for performing multi-element analyses with minimum detection limits in the sub-ppm range.
[1] There are two noteworthy drawbacks to the use of NAA; even though the technique is essentially non-destructive, the irradiated sample will remain radioactive for many years after the initial analysis, requiring handling and disposal protocols for low-level to medium-level radioactive material; also, the number of suitable activation nuclear reactors is declining; with a lack of irradiation facilities, the technique has declined in popularity and become more expensive.
NAA was discovered in 1936 by Hevesy and Levi, who found that samples containing certain rare-earth elements became highly radioactive after exposure to a source of neutrons.
Following irradiation, the artificial radioisotopes decay with emission of particles or, more importantly gamma rays, which are characteristic of the element from which they were emitted.
A typical reactor used for activation uses uranium fission, providing a high neutron flux and the highest available sensitivities for most elements.
The newly formed radioactive nucleus now decays by the emission of both particles and one or more characteristic delayed gamma photons.
This decay process is at a much slower rate than the initial de-excitation and is dependent on the unique half-life of the radioactive nucleus.
For many workers in the field, a reactor is an item which is too expensive; instead, it is common to use a neutron source which uses a combination of an alpha emitter and beryllium.
Scintillation-type detectors use a radiation-sensitive crystal, most commonly thallium-doped sodium iodide (NaI(Tl)), which emits light when struck by gamma photons.
The germanium is processed to form a p-i-n (positive-intrinsic-negative) diode, and when cooled to ~77 K by liquid nitrogen to reduce dark current and detector noise, produces a signal which is proportional to the photon energy of the incoming radiation.
The semiconducting element silicon may also be used but germanium is preferred, as its higher atomic number makes it more efficient at stopping and detecting high energy gamma rays.
Gamma rays, however, are not absorbed or attenuated by atmospheric gases, and can also escape from deep within the sample with minimal absorption.
Some nuclei can capture a number of neutrons and remain relatively stable, not undergoing transmutation or decay for many months or even years.
Neutron Activation Analysis has a wide variety of applications including within the fields of archaeology, soil science, geology, forensics, and the semiconductor industry.
This method has proven to be very successful at determining trade routes, particularly for obsidian, with the ability of NAA to distinguish between chemical compositions.
In order to track the distribution of the fertilizers and pesticides, bromide ions in various forms are used as tracers that move freely with the flow of water while having minimal interaction with the soil.
NAA is used to detect trace impurities and establish contamination standards, because it involves limited sample handling and high sensitivity.
