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Neutron Activation Analysis

Instrumental Neutron Activation Analysis (INAA or NAA)

INAA is a method to determine the concentration of trace (1 to 100 ppm), minor (0.1 w/o to 1.0 w/o), and major (1.0 w/o and above) elements in a variety of matrices. Samples are exposed to neutrons, producing radioactive nuclides in the sample (neutron activation). When radioactive atoms in the sample decay, gamma rays with characteristic energies are emitted by each nuclide. Using a gamma-ray spectroscopy system (GRSS), the quantity and energy of emitted gamma rays can be measured. The energy of the measured gamma rays can be used to “fingerprint” the radioactive nuclides, and therefore the elements that were activated. In addition, the concentration of these elements can be determined quantitatively by comparison of gamma ray intensities with those emitted by standards for the various nuclides. This sensitive analytical technique is useful for a variety of purposes, including geological, environmental, industrial, and forensic.

Neutron activation and decay diagram

Gamma-ray spectroscopy system setup

Advantages of INAA

  • Many elements can be analyzed simultaneously
  • Very low detection limits for many elements
  • Small sample sizes (1 – 200 mg)
  • No chemical preparation
  • Non-destructive
  • Insensitive to matrix elements such as C, N, O, H, and Si, making detection of trace elements possible
  • Often complementary to other analytical techniques
  • Inexpensive analytical technique if a strong source of neutrons, such as a research reactor, is available

NAA at the NRL

At the NRL, the neutrons are supplied by the OSURR, for which irradiations can be scheduled and tailored to meet individual experimenter requirements. Samples should be supplied ready for analysis in small sealed polyethylene vials, and while the OSU-NRL has a limited selection of standards materials, the user may need to provide standards (samples with known concentrations of the elements of interest).

Irradiations are typically performed using the rabbit pneumatic facility, which uses air flow to deliver samples to a position adjacent to the core. After a pre-determined irradiation time (typically on the order of minutes), the system automatically returns the sample to a shielded holding box.

Following an appropriate decay period, the gamma-ray spectrum is measured and recorded using the OSU-NRL 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 the requested level of analysis. (Some customers only need raw data, while others will want a more formal report.)

Detection Limits

This table provides approximate detection limits for several elements. Actual detection limits depend on major components of the material.