2. Non-Dispersive X-Ray Spectrometers

In this chapter I give a brief overview of the types of detectors that have been used to acquire the data used in this dissertation. These are all non-dispersive detectors. Their main advantage over dispersive detectors such as diffraction gratings and Bragg crystal spectrometers is their much greater quantum efficiency over a large energy band. I also describe novel cryogenic detectors which will be used in future missions. The driving force behind the development of these detectors is the need for better spectral resolution. To illustrate the importance of spectral resolution, Figure 2 shows the iron K region of a coronal spectrum as it might appear when observed with spectrometers with resolutions of 1, 10, and 100 eV. A resolution of 100 eV is slightly better than that of present-day X-ray observatories. With a resolution of 1 eV it is possible to resolve the K$\alpha$ lines of intermediate ionization stages of iron.

Figure 1: The iron line spectrum of a coronal plasma as it might appear when observed with detectors with energy resolutions of (top to bottom) 1, 10, and 100 eV.

X-rays are efficient ionizers and this is how they may be most easily detected. Early detectors were proportional counters. Positional information came from mechanical collimators, coded apertures, and shadow masks. The development of grazing-incidence X-ray mirrors such as those flown on the Einstein satellite allowed high resolution X-ray images of celestial objects to be made. X-ray telescopes use Wolter-type geometry (VanSpeybroeck and Chase (1972)), or an approximation, in which incident paraxial X-rays are reflected first by a paraboloid and then by a hyperboloid to form an image (see Figure 1).