In observing living cells, an optical microscope is generally used. However, a soft X-ray microscope with resolution several tens of times greater than an optical microscope is attracting much attention in the life sciences fields. When one hears the term “X-ray,” there probably are many people who think of an X-ray examination. Soft X-rays have longer wavelengths than even X-rays used for X-ray examinations, and are electromagnetic waves that are similar to those of ultraviolet rays. It is known that when a soft X-ray’s wavelength changes, permeability changes according to the element. For example, for oxygen soft X-rays become permeable at the wavelength of more than 2.3 nanometers (one nanometer is equivalent to 1,000,000,000 parts/1), and for carbon becomes permeable at that above 4.4 nanometers. That is, for the wavelength between 2.3 nanometers and 4.4 nanometers soft X-rays will not be absorbed by oxygen composed of water molecules. Conversely, they will be strongly absorbed by carbon composed of cells. Using this quality, we can observe in high contrast the internal composition of living cell structures that have entered a culture fluid. From this, we call the wavelength region the “water window.” A microscope that takes the soft X-rays of the “water window” region as the light source is being energetically developed. For example, at Utsunomiya University scientists are proposing to aim a laser at elements with high nuclear numbers such as bismuth and zirconium, convert them into plasma, and generate the soft X-rays in the “water window” region.
In order to convert a light source into a high luminescence-high efficiency light source it is necessary to investigate with high precision the emission spectrum from plasma. In a joint research project involving Utsunomiya University and the National Institute for Fusion Science, we injected a pellet (a small spherical object) bearing a candidate material into a high-temperature plasma in the Large Helical Device (LHD), which is utilized for fusion research. We observed a soft X-ray emission spectrum from the “water window” region. As a result, we observed for the first time the strong emission spectrum at a wavelength of 2.8 nanometers from zirconium ions in a plasma. This clearly was a soft X-ray from the “water window” region. Data gained this time from the LHD will be utilized in the future in light source development and in the design of the microscope system utilizing laser.