14 MeV中性子放射化分析法による微粒鉄粉の全酸素含有量の定量

抄録

Finely divided metal powders have been attracting a special attention for their physical and chemical characteristics. These metal powders frequently contain large amounts of gases through oxidation and surface adsorption due to their reactivity and very large specific areas. Many difficulties will be supposed to be in the analysis of oxygen in such metal powders by the usual methods of gas analysis in metals; a hard sample handling due to their bulkiness and very low apparent specific gravity; difficulties of replacement of dead volume by an inert gas or degassing when a sample is stuffed in a metal capsule or wrapped in a metal foil for easier sample handling in the usual methods; and tendency of scattering of samples in the instrument when such a powder sample of low apparent specific density and high gas content was thrown into the high temperature metal bath and by the following hard chemical reaction.
The 14 MeV neutron activation analysis is considered to be effective to analyze such samples as fine powders, due to the unnecessity of the chemical reaction at the high temperature which is different from the usual methods. The analysis by the above method for iron powders prepared by a forced vaporization method with a plasma jet has been studied. The amount of a sample, 100 mg, is sufficient for the analysis, and there is no any obstructive radioactive nuclide simultaneously produced from the matrix iron in the measurement of ¹⁶N produced by the ¹⁶O (n, p) reaction with the 14 MeV neutron irradiation if the measured γ-ray energy range was limited to lie between 4.5 and 8.0 MeV.
A calibration curve prepared from the standard series of oxygen-graphite can be used, and it is favorable because of the unnecessity of consideration of residual radioactive nuclides when repeated irradiation of samples is required.
The analysis resulted in 2.47-12.6 mass% oxygen for three kinds of the samples, suggesting the importance of the control of residual gases in the instrument, that of the purification of the plasma gas, and that of prevention of oxidation of the products during storage.
When an amount of oxygen corresponding to its statistical deviation is supposed to be a lower limit of detection, it becomes ca. 38 μg of oxygen or 0.038 mass% in a fine iron powder of 100 mg.
A total duration of ca. 62 s is required for the analysis of one sample, and the external standard method is adopted to improve the accuracy and the reproducibility of this method.