52 巻 (2017) 2 号 p. 79-84
The purity of niobium for a superconducting cavity refined using an electron-beam melting method was explained from RRR measurement, gas analysis and GDMS analysis. Metallic purity notation generally refers to the value obtained by subtracting the total of metal impurities other than gas components from100%. For example, it is 3N at 99.9% and 4N at 99.99%. In the case of niobium, since tantalum containing tens to hundreds of ppm is the majority of the impurities, it is 3N or 2N8 in a normal purity notation. However, with the agreement of customers, when tantalum, tungsten and molybdenum are excluded from the target elements, purity corresponding to 6N - 7N is obtained. Tantalum is not removed by electron-beam melting, so we select and purchase less tantalum at the raw materials stage.