**1. Introduction**

On ion beam experiments including materials analysis and modification with ion beams, a beam monitoring system is installed in the sample chamber to monitor the beam position and uniformity in the beam spot. Most of the beam monitor consists of a fluorescent plate, enabling real time visualization of a beam spot on the plate. A SiO<sup>2</sup> plate, for example, has been used for beam monitoring because of strong emission [1–5] in the visible range when irradiated with MeV-ion beams. A Cr-doped Al2O<sup>3</sup> (e.g., AF995R, Desmarquest) is also suitable for beam profiling [6,7] for ion beams with energies larger than several hundred keV. The aforementioned materials are insulators and therefore electric charging takes places on the fluorescent plate irradiated with ion beams, resulting in deflection of ion beams in the vicinity of the fluorescent plate if their acceleration voltage is comparable to the charged potential of a few tens kilovolts [8]. This means that the fluorescent point would be different from the real position, and further the fluorescent point would not appear at all in the case of low energy ion beams with <10 keV. In addition to the fluorescent materials, phosphors such as ZnS based materials [9–11], which have been widely used for screens of a cathode-ray tube, are applicable to beam monitoring. Other candidates of inorganic luminescent materials can be found in the literature [12]. Powders of such materials, mixed with a conducting paste and deposited onto a conducting plate, are a candidate for ion beam monitoring materials. However, such powders cost very high, but their lifetimes are very short because radiation damage causes degradation of light-emitting efficiency. It is, therefore, not easy to view a beam spot of a low energy ion beam with energy of several keV on real-time.

On the other hand, non-real time beam monitoring can be conducted by the color change of materials irradiated with ion beams. A polyimide film is, for example, widely used to check both the position and uniformity of an ion beam, because blackening due to graphitization [13–15] occurs when the film is irradiated with ion beams. A polyimide

film is, however, non-conducting and is inapplicable to the beam viewer for low energy ion beams with energies of several keV. The favorable beam viewer for low energy ion beams should be composed of electrically conducting materials. A metallic copper plate, even if a thin upper layer of copper oxide is present, is a good conducting material. The color of the copper plate covered with thin oxide is reddish-brown, largely different from polished metallic copper. The present authors made an attempt to fabricate a beam viewer in which the appearance of a beam spot turns shiny due to removal of the oxide layer by physical sputtering. In the irradiation apparatus with base pressure of 2 <sup>×</sup> <sup>10</sup>−<sup>4</sup> Pa, the shiny beam spot could be clearly recognized after irradiation with 5 keV Ar<sup>+</sup> ions to a fluence of 1 <sup>×</sup> <sup>10</sup><sup>15</sup> Ar<sup>+</sup> cm−<sup>2</sup> . Surprisingly, in the other irradiation equipment with base pressure of 2 <sup>×</sup> <sup>10</sup>−<sup>6</sup> Pa, the color of the beam spot turned dark blue-purple after irradiation under the same conditions above. In the present work, the chromatic change observed in the irradiation equipment with such a high vacuum is examined to fabricate a new type of beam viewer for low energy ion beams.
