Radiochemical Neutron Activation Analysis (RNAA)

Radiochemical neutron activation analysis (RNAA) extends the capabilities of instrumental neutron activation analysis (INAA) by adding a chemical separation to isolate the element(s) of interest from the matrix, thus producing better detection limits.  The basic method is as follows:  Samples are encapsulated in polyethylene or some other suitable packaging, packed into an irradiation capsule (usually a polyethylene “rabbit”) and irradiated in the reactor core.  During irradiation elemental nuclei capture neutrons and produce radioactive nuclei.  After a suitable decay period the samples are dissolved (e.g.mineral acid or molten flux) or otherwise decomposed (e.g. by combustion), and the element(s) of interest are separated from the sample matrix.  Chemical separations and purifications are typically performed by precipitation, solvent extraction, by passage through ion exchange resin or other ion retention media, or by trapping of volatile elements after combustion of the sample.  Elements may be separated and counted individually or in groups.   Quantification of the elemental nuclei of interest is usually performed by gamma ray spectroscopy (high resolution germanium detector), or by beta counting (low background proportional or liquid scintillation counting) when pure beta emitters are measured.  Quantification of elements is accomplished by comparison with standards typically processed in the same manner.  RNAA procedures that have been used at NIST are summarized below.

HMD = Hydrated Manganese Dioxide; DDC = diethyldithiocarbamate; BPC = Beta Proportional Counting; LSC = Liquid Scintillation Counting.