An inverse photoemission spectroscopy apparatus is configured to detect a light generated by the relaxation of electrons to an unoccupied state of a sample. The apparatus includes an electron source for generating electrons with which a sample is irradiated, a wavelength selector for extracting a light having a certain wavelength from the light generated in the sample, a photodetector for detecting the light extracted by the wavelength selector; and a focusing optics disposed between the sample and the photodetector. The electron source contains yttrium oxide as a thermionic emission material.

According to one embodiment, an X-ray tube includes an anode target, a cathode including a filament and a convergence electrode which includes a groove portion, and an envelope. The groove portion includes a pair of first bottom surfaces which are located in the same plane as the filament and between which the filament is interposed in a width direction of the groove portion, and a pair of second bottom surfaces between which the filament and the pair of first bottom surfaces are interposed in a length direction of the groove portion and which are located closer to an opening of the groove portion than the pair of first bottom surfaces.

A field terminator includes a plurality of electrode plates positioned around a guide axis at a radial distance therefrom. The plates generate a quadrupole DC field such that a polarity on each plate is opposite to a polarity on the plates adjacent thereto. The plates may be positioned at an axial end of a quadrupole ion guide such as a mass filter. In addition to an RF field, the ion guide may generate a quadrupole DC field. The DC field of the plates may be opposite in polarity to that of the ion guide.