As mentioned above, the design of the XSS was driven by the performance capability of the AXAF. The source size is constrained by the HRMA point spread function. The spectral purity is set by the requirement of analyzing calibration data from the two microchannel plate detectors (HRC and HSI), which have poor energy resolution. The calibration of the HETG (High Energy Transmission Grating) and LETG (Low Energy Transmission Grating) requires line sources with E/dE greater than 1000. A requirement for the dynamic range of the XSS flux level to be over 10,000 is driven by the ACIS point response function measurements and the HRMA point response function inner core mapping with the SSD and a 5 micrometer diameter pinhole. The effective area measurements across the absorption edges of the mirror, gratings, and the SIs require a tunable monochromatic source. In this section, the XSS capabilities and its operating principles will be discussed.
The basic operating principle of an Electron Impact Point Source (EIPS) is simple. First, electrons are generated by a heated tungsten cathode; they are then accelerated and focused in an electric field. Picking up enough energy, the electrons will kick the inner shell electrons out when they hit an anode. As a result, X-ray photons at energies characteristic of the anode material will be emitted. Bremsstrahlung continuum radiation is also produced. For the XRCF calibration, using an EIPS with continuum-removing filters, one can get clean lines from different anodes. The available anode materials can be found via the World Wide Web at http://wwwastro.msfc.nasa.gov/xray/ axafps_xss/tnf.html, and in the paper presented by Jeff Kolodziejczak at the SPIE meeting in 1995. In general, the X-ray line emission from K-shells is narrow for anode materials of high atomic number (Z), and K-lines for low Z anodes and L-lines are broad and blended. For example, the Mg-K line at 1.25 keV has E/dE of about 3000; this line is a good X-ray source for the calibration of High Energy Gratings (HEG) of the HETG, since the low-energy limit of the HEG is 1 keV. Carbon K-alpha at 0.277 keV has E/dE of only about 35.5.
For the LETG and the Medium Energy Gratings (the other subsystem of the HETG), a Penning source will provide narrow lines. A Penning source is a continuous gas discharge source. The Penning source available at the XRCF has two cathodes, one at each end, and two annuli anodes in the middle. An external magnetic field is applied along the direction perpendicular to the two cathode surfaces. Electrons emitted from the cathodes are constrained to follow the magnetic field lines in very tight helices until they are scattered to the anode. An anomalously high energy electron distribution is thereby developed in the discharge region between the two anodes. These electrons excite a variety of lines, including both lines intrinsic to the discharge gas and lines produced by the excitation of highly ionized material sputtered from the cathode faces. For LETG and MEG calibration, it is planned to use a Penning source with argon gas and aluminum cathodes to provide the Al-IV and Ar-I lines at 0.095 keV and 0.22 keV. Both lines have resolving power E/dE greater than 1000. The corrosion of the cathode of a Penning source limits its continuous useful time to about 4-5 hours. Its cathode needs to be refurbished for further use, and the refurbishing takes about half an hour. There are three Penning sources available at the XRCF.
Two monochromators provide tunable monochromatic sources. The grating monochromator is called the High Resolution Erect Field Spectrometer (HIREFS). It covers an energy range between 0.09 to 1.5 keV. It can be run at a high or a low resolution mode, which will offer resolving powers, E/dE, of 1000 and 100, respectively. The resolving power is controlled by the widths of the slits. The HIREFS has contributions from the higher orders. This effect will be characterized during the XSS calibration. For a Double Crystal Monochromator (DCM), the dispersion elements are crystal pairs. The diffraction efficiency of each crystal obeys Bragg's law, therefore only the photons with selected energies are reflected at the Bragg angle off two crystals into a parallel but offset beam. The DCM at the XRCF has five sets of crystals to cover an energy range of 1 - 10 keV. The energy resolution of a DCM mainly depends on the crystal properties, and incident and outgoing beam divergence. The DCM at the XRCF will provide a resolving power from 100 to 1000.
Both monochromators will be fed by Rotating Anode Sources (RAS). The operation principle of a RAS is similar to an EIPS. Because a RAS has a rotating anode (6000 rpm), it is able to take higher anode current and generate higher fluxes than an EIPS. At the XRCF, one of the two RASs has both a carbon anode and a tungsten anode. This RAS will be used with the HIREFS. The other RAS, with only a tungsten anode, will feed the DCM and it can be used alone as a continuum source. The continuum source is necessary for measurements such as the molecular contamination study, the fluorescence study, and the ghost image search.