A detector for low-energy (few hundred eV–20 keV) X-rays in the presence of a high background of high-energy X-rays

The Vanderbilt University FEL Center project for the production of monochromatic X-rays uses Compton backscattering of the infrared light from the FEL by the 40 MeV electron LINAC to produce nearly monochromatic pulses of X-rays in the 10–20 keV range. Detecting these requires the ability to distinguish the small flux of 15 keV X-rays produced by the backscattering from the large flux of X-rays from low energy to many MeV resulting from scattered electrons from the beam. We have constructed a detector using two thin silicon surface–barrier detectors as calorimeters, separated by an aluminum absorber, so that the front detector sees both high- and low-energy X-rays, and the back one sees only high-energy X-rays. Using a carefully balanced differential amplifer chain, we can detect a soft X-ray flux which is only about 5% of the broadband flux observed by our detector.