Chemical composition of T-21 silica tubing and implications for neutron activation analysis

Abstract

T-21 silica, a commonly used encapsulation material in neutron activation analysis of small samples, was analyzed by INAA to determine trace levels of the following impurities: Na, Sc, Cr, Fe, Co, Ni, Zn, Br, Sb, La, Ce, Sm, Tb, Hf and Au. In the unprocessed form supplied by the manufacturer, pieces of T-21 weighing 50 mg contain ∼6·10⁻² μg Na; ∼9·10⁻³ μg Fe; ∼3·10⁻⁷–7·10⁻⁶ μg light REE, Sc and Hf and ∼4·10⁻⁸ μg Tb and Au. In a series of glass-blowing steps, in which unfiltered gases were used for fuel, and cleaning steps, in which reagent grade aqua regia was used, irradiation vials were produced which contain higher average levels of Sc, La, Sm, Tb and Hf, by factors ranging from 1.3 for Sm to 11.5 for La, and lower average levels of Co, Na and Fe, by factors of 1.4, 2.0 and 4.0, respectively, than the unprocessed material. These contamination levels lead to blank corrections of 21% for La, 2.6% for Ce and 3.0% for Hf in 40 μg samples of refractory inclusions from carbonaceous chondrites, if counted in their irradiation vials.