Rapid determination of trace amounts of fluorine in silicon nitride by instrumental neutron-activation analysis

The rapid determination of ppm levels of F in silicon nitride powder and some reference materials by instrumental neutron-activation analysis (INAA), measuring the short-lived nuclide 20F (half life: 11.00 s), has been studied. The determination was carried out by single and cyclic activation methods using an activation analysis facility (PN-3) of the 20 MW reactor JRR-3M. Using this method, the sample was irradiated with a thermal neutron flux of 1.9 x 1013 n cm-2s-1 for 5 s, and the γ-ray spectrum was measured for 10 s after letting the sample stand for 7 s. In the cyclic-activation method, the single-activation method was repeated 5 times, including more than a 10 min cooling interval between each measurement. The effects of evaporating F during neutron irradiation and a side reaction due to 23Na (n, α) 20F were examined; it was confirmed that these effects can be neglected under the present experimental conditions. Due to the high Cd ratio of the irradiation field at PN-3, the effect of the side reaction from Na is 25- to 155-times lower than the values obtained by other reactors. The cyclic-activation method presents not only a lower detection limit, but also better analytical precision than the single-activation method. The analytical results of F in reference materials of Opal Glass (NIST SRM91) and Oyster Tissue (NIST SRM1566a) almost agreed within 5.1 to 5.9 % with the certified and reference values. The relationship between the detection limit of the F and Al contents was also clarified. (author)