ABSTRACT
Since the transition energy as well as the energy level of an atomic system can be scaled as Z2 and n−2 (where Z denotes the atomic number and n the principal quantum number) [1], most transitions fall in the x-ray range for highly charged heavy ions. Figure 4.1 shows examples of the Z dependence of transition wavelengths. As seen in Figure 4.1, not only Δn = 0 transitions, but also Δn = 0 transitions can fall in the x-ray range for heavy ions. Thus, x-ray spectroscopy is one of the most important methods in studying highly charged ions. There are two ways to analyze the energy (wavelength) of an x-ray photon. One is called the energy-dispersive method by which quantities
Ion
proportional to the photon energy are measured. For example, for semiconductor detectors, the number of electron-hole pairs produced in the semiconductor through the interaction with an x-ray photon is measured. The resolution E/ΔE of a semiconductor detector is usually limited by the statistical variability of the number of electron-hole pairs to the order of 102. Another way to analyze the x-ray energy (wavelength) is called the wavelength-dispersive method in which diffraction by a grating or a crystal is used. Generally, a grating is used for soft x-rays (>50 Å), and a crystal for hard x-rays (<50 Å). The resolution of the wavelength-dispersive method with a crystal is typically on the order of 103-104. Recently new types of energy-dispersive instruments show remarkable development. For example, the resolution of an x-ray micro-calorimeter [4] reaches the value comparable to that of the wavelength-dispersive spectrometer. However, the technique for such a detector is still state of the art so that it is rather difficult to acquire such detectors without collaborating with groups involved in research and development of these devices. In addition, the effective size of such a detector is generally very small (typically less than 1 mm2). Thus, crystal spectrometers are still the most important tools for the high-resolution spectroscopy of hard x-rays. In this chapter, various types of crystal spectrometers are described.
