Elemental Analysis Laboratory

Department of Chemistry  -  Texas A&M University

Neutron Activation Analysis -  A general description of the method 

William D. James

Introduction:

Neutron activation analysis is a sensitive multielement analytical method for the accurate and precise determination of elemental concentrations in unknown materials. Sensitivities are sufficient to measure certain elements at the nanogram level and below, although the method is well suited for the determination of major and minor elemental components as well.

The method is based on the detection and measurement of characteristic gamma rays emitted from radioactive isotopes produced in the sample upon irradiation with neutrons. Depending on the source of the neutrons, their energies and the treatment of the samples, the technique takes on several differing forms. It is generally referred to as INAA (instrumental neutron activation analysis) for the purely instrumental version of the technique. RNAA (radiochemical neutron activation analysis) is the acronym used if radiochemistry is used to separate the isotope of interest before counting. FNAA (fast neutron activation analysis) is the form of the technique if higher energy neutrons, usually from an accelerator based neutron generator, are used. Epithermal activation analysis (EPNAA) uses neutrons of intermediate energy for activation.

Typically, unknown samples together with standard materials of known elemental concentrations are irradiated with thermal neutrons in a nuclear reactor. After some appropriate decay period, high resolution gamma ray spectroscopy is performed to measure the intensity and energies of the gamma lines emitted. A comparison between specific activities induced in the standards and unknowns provides the basis for computation of elemental abundances.

Instrumentation:

The process of measurement of the gamma ray spectra following neutron irradiation is generally performed using a high resolution germanium semiconductor detector. These devices provide sufficient resolution to differentiate between most all commonly occurring gamma lines. Our laboratory operates four EG&G Ortec germanium detectors, each with nominal full width at half maximum (FWHM, a routine measurement for peak resolution) of 1.65 to 1.80 keV for the 1332 keV line of cobalt-60. The signals produced from these detectors are refined with various electronic modules to amplify and shape the pulses prior to input to a high speed analog to digital converter (ADC). The resulting digital information is stored in a multichannel pulse height analyzer which accumulates the spectrum. Our laboratory operates primarily EG&G Ortec modular electronics. Our data acquisition is based on a Canberra AlphaVAX Genie system. The software which then performs the spectral analysis, as well as neutron activation analysis calculations, was written by Canberra as an integral part of the Genie.

One of our gamma spectroscopy systems is physically housed at the Nuclear Science Center where irradiated samples are returned to the counting lab in a matter of a few seconds. This system is used for determination of those elements which undergo neutron activation reactions which result in relatively short half-lives (from seconds to a few hours). Spectral data for this particular system is accumulated on a personal computer version of Canberra's Genie, the Genie2000. Files containing the data as well as various sample parameter information are transferred via ethernet back to our Alpha system for analysis.

Two gamma spectroscopy systems, located in the Olin E. Teague Building on the main campus of Texas A&M, are automated. This allows for the unattended, 24 hour sequential counting of irradiated samples. The automation is done using a Toshiba M-40 programmable logic controller (PLC) and is based on pneumatic transfer of the encapsulated samples from a load wheel, to the detector for counting and finally to a storage location. These systems do the bulk of the counting for intermediate and long lived radioactive species in our laboratory.

Additional information:

Favorite NAA references: "Activation Analysis", Volumes I and II, Zeev B. Alfassi, editor, CRC Press, 1990, Boca Raton, Florida; "Radiochemistry and Nuclear Methods of Analysis", William D. Ehmann and Diane E. Vance, John Wiley & Sons, Inc., 1991, New York; "Activation Analysis with Fast Neutrons", Sam. S. Nargolwalla and Edwin P. Przybylowicz, John Wiley & Sons, 1973, New York.