Neutron Activation Analysis
Neutron Activation Analysis (NAA) is a technique in which neutrons from a reactor are used to determine the chemical composition of materials. The basic idea is quite simple. Take a sample of the material to be analyzed and place it in (or near) a reactor core. Some of the neutrons from the core are absorbed in the sample and make it radioactive. By studying the spectrum of gamma rays emitted by the sample, one can determine which elements are present in the sample. If a material of known composition is irradiated with the unknown sample, one can also determine the amount of each element in the sample.
Of course, the details are much more complicated than the basic idea. Like all tools, NAA is well suited to some tasks and very poorly suited to others. Some elements don't absorb neutrons very well. Some do absorb neutrons, but don't become radioactive. Some become radioactive, but don't emit gamma rays when they decay. Even if the element you're looking for is a prime candidate for NAA, it may be difficult to detect if its concentration is small.
Fortunately, there are many applications for which NAA is quite well suited. It has been used to look for trace elements in water, soil, biological samples, rocks and minerals, plastics, styrofoam, ...
NAA at the NRL
At the NRL, the neutrons are supplied by the OSU Research Reactor (OSURR). It has a maximum steady-state thermal power of 500 kW, which provides a peak in-core thermal neutron flux of 1.4x1013 nv. The OSURR operates on a flexible duty cycle, determined almost entirely by user demand. Irradiations can be scheduled and tailored to meet individual experimenter requirements. Operations are normally performed during standard business hours, but other schedules can be arranged as necessary. Samples may be supplied ready for analysis, or the preparation can be done by the NRL staff. Irradiation is typically done in the rabbit facility. This 2" tube pneumatically delivers samples to a position adjacent to the core. In this position, the thermal neutron flux is considerably smaller (2.1x1012 nv), but it is sufficient for short-lived isotope production. After a pre-determined amount of irradiation time (typically on the order of minutes), the system automatically removes the sample.
Following an appropriate decay period, the gammay ray spectrum is recorded on the NRL gamma ray spectroscopy system (GRSS). The decay period depends on the element of interest and the other elements present in the material. In some cases, the spectrum is recorded as soon after irradiation as possible. In others, the sample may decay for a day, week, or month before analysis.
Finally, a staff member will take the information from the sample and the standard(s) and perform as much or as little analysis as requested. Some customers like to have the raw data, while others only want to see the final numbers. Results are presented in the form of a final report detailing the procedures, standards, and methods used. If time is critical, a preliminary report of results can be delivered via fax or e-mail.
This table provides approximate detection limits for several elements. Actual detection limits depend on major components of the material.