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Neutron Activation Analysis

Neutron Activation Analysis (NAA)

Neutron activation analysis uses the absorption of neutrons to create radioactivity in a sample. The resulting radioactivity can be measured using gamma spectrometry where the elements are identified and quantified. The SLOWPOKE reactor was designed with NAA in mind. The flux of neutrons produced (the number of neutrons that can contact a sample in a given time) is very stable. The temperature inside the reactor is near room temperature for most applications. Neutron activation is a nuclear reaction and the chemical state of an element (electronic structure) has no effect.

Advantages

• Matrix independent
• Non-destructive
• Simultaneous multi-elemental analysis
• Solids and liquids can be analyzed
• Analysis of a neat (unmodified) sample
• Minimal sample preparation
• Large sample sizes, up to 27 mL
• Organic elements pose no interference (C, H, O, N)

Disadvantages

• Independent of chemistry, strictly elemental analysis
• Some elements can't be determined (for example, Pb, C, N, O)
• Require a source of neutrons (we've got this one covered)
• Analysis of gases is difficult
• Analysis must take into account the time constraints of isotope half-lives

Applications

NAA is applicable to many different materials. Since both neutrons and gamma-rays easily penetrate matter the state of a material is usually of no consequence, thus both a liquid and a solid can be analyzed in the same manner. For example, low concentrations of elements can be determined in water and weight percents of elements can be determined in a metal with only small modifications to the procedure. The high resolution gamma-ray spectrometry equipment allows the detection of many different gamma-ray energies simultaneously, so many elements can be determined simultaneously.

The sample preparation is very easy. Samples are weighed or pipetted into plastic vials which are then sealed. In most cases the sample is irradiated and counted while in the vial. Samples do not need to be diluted or modified in any way so the risk of contamination from handling is greatly reduced. If sample irradiation produces only short-lived isotopes then the sample can be returned unchanged once it has been analyzed, which is convenient if one is analyzing precious gems or artifacts.

Elements

Not all elements can be determined in this manner. Some nuclei do not absorb a neutron readily or the resulting isotopes don't emit gamma-rays. The half-lives of isotopes can also be a problem if a sample must be returned after analysis. Common elements that can't be determined are:

• Carbon
• Hydrogen
• Oxygen (only ¹⁸O can be determined)
• Nitrogen
• Phosphorus
• Lead

Since the primary organic elements (C, H, O, and N) don't interfere it is easy to determine trace elements in organic samples, such as oils and plastics, and plastic containers can be used when irradiating the samples.

Sample Size

The maximum sample size is 7 mL but in some cases it is possible to have a sample size of 27 mL. There really isn't a minimum sample size and it is quite common to analyze only a milligram of a sample, however, the potential for interference from trace elements in the plastic containers becomes more of a concern when sample size gets too small.

Cost

The price of an analysis depends mostly on the elements being determined and the requested turnaround time. Please don't hesitate to contact us for an estimate.

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