Detection of Indium as an Atmospheric Tracer by Neutron Activation

Abstract

In a test of the use of indium as a particulate atmospheric tracer, both tagged and untagged rain showers were sampled at ground level in Oklahoma during May 1967. In 29 samples of untagged rain 6±3 nanograms of In per liter were found, indicating a natural background somewhat above the reagent blank of about 2 ng liter?1. After aircraft injection at cloud base of 200 gm of indium, as finely divided particles from pyrotechnic flares, a maximum of 40 ng liter?1 was found in the rain, and a pattern of localization of the tracer indium was revealed in an array of 14 samplers spaced over 11 km. The procedure for analysis began in the field with 1) addition of HC1 to the polyethylene samplers before rain was collected to ensure solubility of the tracer, 2) filtration to remove suspended solids, 3) addition of lanthanum internal standard, Fe+++, and NH3 to each 1-liter sample to precipitate In and La with Fe(OH)3, and 4) membrane filtration of the Fe(OH)3. At The University of Michigan the procedure continued with 5) reactor neutron activation of the filters, 6) separation of In from La by isopropyl ether extraction from HBr solution, and 7) assay of In and La radioactivities by ? and ?, counting, respectively. The sensitivity of the method is determined by the natural background of the indium found.