What is Neutron Activation Analysis?
NAA is an extremely sensitive technique used to determine the existence and quantities of major, minor and trace elements in a material sample. NAA differs from other methods in that it relies on the atom’s nucleus and ignores chemical formulation, unlike mass-spectrometry or chromatographic methods.
NAA requires a source of neutrons, gamma-ray detectors and a thorough understanding of how elements react to neutron bombardment.
Facilities at HFIR
Two Pneumatic Tubes:
PT-1: Thermal Neutron Flux: 4 × 1014 n cm-2 s-1
- Thermal-to-Resonance Ratio: 35
- Shielded sample loading station with remote manipulators
- Decay station in pool
- Rabbit travel time: 2.5 seconds
PT-2: Thermal Neutron Flux: 4 × 1013 n cm-2 s-1
- Thermal-to-Resonance Ratio: 250
- Loading station in hood
- Automated delayed-neutron counting station that will measure 20 - 30 picograms of 235U or other fissile material in 5 minutes
- Other characteristics of PT-1 apply
These are made of both high-density polyethylene or graphite. Internal volume for these is 1.5 cc.
- PC-based gamma spectrometer, six Germanium detectors (four coaxial, one well, one x-ray). One of the coaxials is an n-type detector that will detect gamma-rays with energies down to <50 keV.
- A laminar flow hood and glovebox are used to minimize contamination from ambient dust while samples are being prepared. The hood recirculates air through a HEPA filter in the sample preparation region.
- Liquid scintillation is available for analysis of Beta emitters.
- Drying oven with entry air filtered through a HEPA filter. Refrigerator for storing critical samples.
Research and collaboration with Universities and Industry are welcomed. Prospective users are encouraged to contact one of the staff to discuss potential projects, approaches, and arrangements.
Some materials are inappropriate for irradiation at HFIR. Among these are liquids (generally), materials highly corrosive to stainless steel or aluminum, materials that react violently with water, explosives and highly flammable substances, and materials that pose exceptional danger to humans. Irradiation container inner dimensions are 10mm × 20mm.
What Elements Can We Measure?
Approximately 65 elements can be determined at levels ranging from parts-per-million to parts-per-trillion or below.
If the matrix to be analyzed does not become too radioactive when activated with neutrons, or if the unwanted radioactivity decays quickly, then groups of trace elements can often be measured simultaneously. Examples of multi-element NAA are given below.
Personnel of the NAA laboratory have considerable experience in the forensic analysis of evidentiary materials. Bullet fragments, gunshot residue, plastic, hair and fingernails, and geological materials are included among recent examples. Comparing materials nondestructively is a chief advantage of NAA for forensics.
Materials such as high-purity silica, silicon, aluminum, other materials and their compounds that do not form long-lived radionuclides, cellulose air filters, as well as graphite are excellent matrices for high-sensitivity NAA. Such materials can be irradiated in graphite rabbits for many hours in PT-1 for determinations of many elements at the sub-ppb level. Silicon wafers and SiO2 used in fiber optics are examples that have been analyzed.
Extremely low quantities of certain elements (such as Ir) can be measured utilizing microwave digestion facilities available at ORNL and straightforward chemical separation techniques.