Abstract: It is an object of the present invention to obtain a high quality YB66 crystal by lowering the temperature of the molten zone and growing a crystal by deposition growth under an incongruent condition. A method for preparing a yttrium 66 boride crystal by the floating zone method by use of a YB66 polycrystalline rod. A YB66 crystal having a composition with an atomic ratio B/Y within a range of from 50 to 75, is grown under such conditions that the melt has a composition (an atomic ratio B/Y) different from the raw YB66 polycrystalline rod and the growing YB66 crystal, and that an equilibrium is maintained at the growth interface. When the atomic ratio B/Y of the starting material is within the range of from 50 to 62 and the atomic ratio B/Y of the melt is within the range of from 40 to 62, it is possible to attain the atomic ratio B/Y of the growing crystal within the range of from 50 to 62.

Abstract: The present invention relates to a lanthanum boride type single crystal having the chemical formula (La.sub.1-x M.sub.x)B.sub.6 (0.01.ltoreq.x.ltoreq.0.50) wherein M is at least one rare earth element selected from the group consisting of Ce, Pr, Nd, Sm and Gd, and further relates to a method for growing a lanthanum boride type single crystal by fusion method, which comprises using a lanthanum boride starting material containing from 1 to 50 mol % of at least one rare earth boride selected from the group consisting of CeB.sub.6, PrB.sub.6, NdB.sub.6, SmB.sub.6 and GdB.sub.6.

Abstract: A field emission type electron gun comprises a cathode, an anode placed opposing the cathode, a filament placed obliquely above the anode in a space between the cathode and anode to heat the anode, a control electrode and a vacuum container housing the cathode, the anode the control electrode and the filament. The anode has an elongated body with a through hole at its center, the control electrode is provided near the anode, and means for rendering the electric potential of the control electrode to be negative to the electric potential of the anode is provided to concentrate an electron beam to the top end of the anode.

Abstract: A field emission type electron gun comprises a cathode, a plurality of anodes opposing in series to the cathode and a filament for heating the anodes, all being contained in a vacuum tube. The filament is placed immediately before a first anode directly opposing the cathode in a symmetrical manner so as to serve both as a gas expelling member and as an anode.