| Popis: |
Bragg crystal spectroscopy has been used to study solar X-ray emission since the early 1960's. Studies of non-solar celestial X-ray emitters, however, require special techniques in order to achieve the requisite sensitivity. We review some of the Bragg spectrometers (all of which use concentration to reduce the geometric area of the detector) which have been proposed in the last 15 years, with special emphasis on two classes of spectrometers which are used in combination with grazing incidence telescopes: objective crystal spectrometers and focal plane crystal spectrometers. A special case of the latter class are focal plane spectrometers which operate in the Johann mode (the crystal is curved so as to intercept the diverging X-ray beam at a nearly constant Bragg angle, and refocuses the X-ray beam onto an imaging detector). The Bragg Crystal Spectrometer (BCS) for the Advanced X-ray Astrophysics Facility is such a device. We review the techniques which have been developed at MIT and by our colleagues at the Goddard Space Flight Center (Hakim et al. this volume) for achieving high spectral resolving powers (E/AE) for the BCS diffractors. Two techniques which we have studied, and which we find to be effective for decreasing AE, are a) the use of an exponential spiral crystal shape and b) placement of the imaging detector in an out-of-focus mode. Both of these minimize the line-broadening caused by the Johann geometry. The resolving power of the BCS exceeds 2000 at some energies; DES 1 eV over a large range of energies. The BCS achieves sensitivity adequate for the study of X-ray spectra for z 1000 cosmic X-ray sources. |
