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 a nd some reference materials by instrumental neutron-activation analysi s (INAA), measuring the short-lived nuclide F-20 (half life: 11.00 s), has been studied. The determination was carried out by single and cyc lic activation methods using an activation analysis facility (PN-3) of the 20 MW reactor JRR-SM. Using this method, the sample was irradiate d with a thermal neutron flux of 1.9 x 10(13) n cm(-2) s(-1) for 5 s, and the gamma-ray spectrum was measured for 10 s after letting the sam ple stand for 7 s. In the cyclic-activation method, the single-activat ion 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 Na-23 (n, alpha) F-20 were examined; it was confirmed that these effects can be neglected un der the present experimental conditions. Due to the high Cd ratio of t he 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 meth od. 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 bet ween the detection limit of the F and Al contents was also clarified.