4.5 A New Technique for Ionization of Elements with a High Melting Point
-An Ion Source Using SF6 Plasma-

Fig. 4-9 Ion source and mechanism for ionization of elements with a high melting point by SF6 plasma
In the plasma chamber, electrons emitted from tungsten filaments move around within the magnetic field, and then ionize SF6 gas to generate plasma. The sample metals with a high melting point, such as B, Mo, and Si, react with active fluorine ions in the plasma to convert into the metal fluoride with sufficiently high vapor pressure. Those metal fluorides are vaporized and further decomposed into metal ions and fluorine ions by electrons and ions activated in the SF6 plasma.

In order to use ion beams for experimental studies in various fields, such as frontier materials science, bio-engineering, and the production of radioisotopes, it is essential to provide scientists with various kinds of ions accelerated in a wide range of energy which meet their requirements. First and foremost, various ion species are required to be generated by ionizing various elements and compounds. In the case of solid compounds, they have to be vaporized by heating and then introduced into a plasma chamber of the ion source to be ionized. However, the elements with a higher melting point (mp), such as boron (B:mp 2,300 degrees), molybdenum (Mo:mp 2,610 degrees), and silicon (Si:mp 1,414 degrees), are scarcely vaporized with sufficiently high vapor pressure only by heating them. On the other hand, their fluorides can be easily vaporized and ionized, since melting points of the fluorides are much lower than those of the elements. From a practical point of view, however, the fluorides are not useful on account of their great drawbacks, such as toxic and corrosive properties.
We developed a new ionizing technique for various elements with a high melting point. In this technique, sulfur fluoride (SF6) gas, which is chemically stable and nontoxic, is used. The elements with a high melting point are transformed into gaseous fluorides by fluorine ions (F+) isolated in the SF6 plasma. The active fluorine ions are very reactive so that the various kinds of elements can be safely and effectively ionized without using any toxic gases. The fluorides thus obtained are also easily ionized in plasma into the element ions and the fluorine ions. Thus, we succeeded in accelerating the charged ions of B, Mo, Nb, W, and Ta.

Reference
Y. Saitoh, Y. Ohkoshi, W. Yokota, Production of Multiply Charged Metallic Ions by MINI-ECR Ion Source with SF6 plasma, Rev. Sci. Instrum., 69 (2), 703 (1998).

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Persistent Quest-Research Activities 1998
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