Comparison between performances of optical grating and Si-PIN detectors in soft X-ray (2-7 keV) spectroscopy

In soft x-ray spectroscopy the region from 2 to 7 keV is the most critical, because optical gratings are considered to become inefficient and solid-state detectors exhibit a relatively low energy resolution. We investigated the performances of ion-etched blazed gratings and of Silicon detectors as components for soft x-ray spectroscopy. The gratings were mounted in a Rowland geometry at angles of incidence varying from 89 degrees to 89.45 degrees. We used a microfocus soft x-ray as source and as the detector an uncoated channel electron multiplier sliding along the Rowland circle to scan the spectrum. We recorded several spectra between 2 and 7 keV. The spectral resolution obtained shows essentially almost slit limited line widths; the diffraction efficiency ranges from 1 to 3% at 89.35 degrees and up to 4.2% at 89 degrees. As a comparison, we recorded the same spectra with a Si-PIN diode, cooled at -30 degrees C by a Peltier circuit. The diode exhibits good sensitivity and low noise; the resolution obtained is around 280 eV, almost constant at the various energies. This experiment shows that it is possible to extend the classical spectroscopic techniques to the soft x-ray region in order to obtain complementary performances with respect to solid-state detectors.