Neutron Activation Analysis, Radionuclide Production, Radioassays,
and Other Spectroscopy
The MIT Nuclear Reactor Laboratory (NRL) has extensive capabilities for performing trace element analysis of a wide range of materials using Neutron Activation Analysis (NAA). These NAA facilities also enable the reactor’s research staff to produce small amounts of radio-isotopic tracers, and to detect and identify trace amounts of radioactive materials in many types of materials.
Basic Principles of Neutron Activation Analysis
Neutron Activation Analysis is based on two physical (as opposed to chemical) procedures. The first step entails the neutron irradiation of small amounts of the materials to be analyzed. The neutron irradiation causes a fraction of some stable isotopes in the samples to be transformed or ‘activated’ into radioactive isotopes. Most of these isotopes emit gamma rays of very specific energies as part of their radioactive decay. By detecting and quantifying the emitted gamma rays, the elemental concentration of the samples can be determined.
NAA has the following advantages over other methods for trace elemental analysis:
- Because the method detects, or ‘counts’, individual gamma rays emitted by the decay of single atoms, it is extremely sensitive.
- It can be applied to solids, liquids, suspensions, slurries, or even gases with little or no physical or chemical processing or the samples.
- It is largely non-destructive.
- It is intrinsically a multi-element analysis which is highly sensitive for many elements that are difficult to analyze for by other methods (e.g., lanthanides).
Application of Neutron Activation Analysis at the MIT Reactor Lab
Neutron irradiations are performed in the reactor’s reflector region which is accessed by two different pneumatic systems or at four manual insertion locations. For irradiations in the pneumatic systems, materials are placed in polyethylene sample holders or ‘rabbits’. The pneumatic rabbits have internal dimensions of either 1” diameter by 3-1/4” length (1-inch rabbits) or 1-3/8” diameter by 6-1/4” length (2-inch rabbits). For irradiations in the manually inserted locations, the total available sample space is 3” diameter by 24” length.
The pneumatic irradiation facilities have thermal neutron fluxes of up to 6x1013 n/cm2s and 8.4x1012 n/cm2s for the 2-inch and 1-inch rabbit irradiation locations respectively. The mechanically inserted irradiation facilities have a thermal neutron flux of up to 4.8x1012 n/cm2s. Lower fluxes can be achieved when the reactor is operated at less-than-full power. The 1” pneumatic and 3” manual irradiation locations have highly thermalized neutron spectra (cadmium ratios of 200), while the 2” pneumatic irradiation location has a fairly high fast neutron flux of up to 4.8 x1012 n/cm2s.
The pneumatic facility for 1-inch rabbits can be used to transfer samples between the irradiation location and a laboratory in the building adjacent to the reactor near where the gamma spectroscopy is performed. This allows for the analysis of activation products with fairly short half-lives (from tens of seconds to tens of minutes).
The laboratory in which irradiated samples are analyzed is currently equipped with three High Purity Germanium (HPGe) detectors. One of these is a well-type detector which provides for very high efficiencies in the collection of the emitted gamma rays. Output from the detectors is processed using multichannel analyzers and gamma spectrum analysis software on a dedicated desktop computer.
In addition to the NRL’s standard NAA facilities, a Prompt Gamma Neutron Activation Analysis (PGNAA) facility is also available. PGNAA is primarily used for the quantification of trace amounts of boron, but can also useful for the analysis of cadmium, samarium, and gadolinium. The PGNAA facility is installed at a horizontal beam ports within the reactor containment building where the thermal neutron flux is approximately 2x107 n/cm2s.
Studies which have either originated in, or been supported by the NAA lab include work on:
- Fine and coarse atmospheric particulate matter (PM2.5 and PM10);
- Atmospheric mercury in vapor and size-segregated particulate phases;
- Biological uptake and partitioning of minerals;
- Deep, dated ice-core samples;
- Historic human hair samples for forensic toxic exposure assessment;
- Raw and processed coal samples, and size-segregated coal-combustion particulate emissions;
- Tree-ring samples for historical toxic contamination estimations.
Radionuclide Production, Radioassays
The NRL’s NAA irradiation and gamma spectroscopy facilities can also be used separately for either the production of radionuclides or for radioassay services. The production of radiotracers can be useful for flexible lab-scale studies of elemental uptake and transport in physical, chemical, and biological systems. Radiotracers can be produced at the NRL in a variety of chemical and physical forms. The use of NRL’s gamma spectroscopic systems for detection and identification of trace amounts of radioactive materials can be useful for the study of either naturally occurring radioisotopes or radioisotopes present due to anthropogenic contamination.
The NRL is also equipped with an Inductively Coupled Plasma - Atomic Emission Spectrometer (ICP-AES) that is used to complement the NAA facilities. ICP-AES is a widely used multi-element analysis technique that offers good precision and speed and can be useful for samples that are not suitable for NAA, or where interfering activities are present. The NRL ICP-AES can be used for the analysis of radioactive samples. Note that ICP-AES analysis requires that the sample be in aqueous solution or uniform suspension, and that a standard is available within a matrix matched as closely as possible to the unknown.
For more information or to inquire about analyses, please contact:
Michael Ames, ScD