Measurement of bromide ion used as a solute-transport monitor via epithermal neutron activation analysis

In soil science (ca. 1970), bromide ion (Br-) in various forms (e.g., KBr, NaBr, SrBr2) was introduced as a non-reactive stable tracer in solute transport studies normally moving freely with the flux of water without substantial chemical or physical interactions with the soil; Typically, Br is extracted from soil and quantified using either a bromide selective electrode (sensitivity is similar to 10 mu g/ml) or by high-performance liquid chromatography (sensitivity is similar to 0.010 mu g/ml). Where the sensitivity is adequate, the selective conductivity method, which is simple, affordable and fast, is preferred. More recently (ca. 1990), workers have reported that 20% of Br tracers, at low groundwater pH, may be adsorbed by iron oxides and kaolinite when present in the alluvial aquifer. We investigated the use of Epithermal Neutron Activation Analysis (ENAA) as a means of measuring Br directly in soil samples without an extraction. ENAA was chosen because of its high theoretical advantage factor over aluminum (i.e. similar to 20), the principal interfering soil constituent calculated for the Br-79(n,gamma)Br-80 reaction compared to Al-27(n,gamma)Al-28. Br- was measured (sensitivity is similar to 0.050 mu g/g) in one gram soil samples from a 5 s irradiation (phi(epi) = 2.5.10(12) n.cm(-2).s(-1)) using a BN capsule.