Soft X-ray spectroscopy with sub-electron readnoise charge-coupled devices

We demonstrate use of a charge-coupled device (CCD) with sub-electron readnoise performance as a non-dispersive X-ray spectrometer. The exceptionally low readnoise (0.9 electrons rms) was achieved by applying a floating gate output amplifier with 16 readouts per pixel. The soft X-ray quantum efficiency was enhanced over other front-side illuminated devices by using a novel thin-poly gate structure. The combination of sub-electron noise and good soft X-ray quantum efficiency have enabled us to detect photons in the EUV energy range (E < 100 eV) in photon counting mode with this detector. These low energy events are well separated from the readnoise floor. The measured energy resolution is 16.3 eV (FWHM) at Al L (E = 72 eV), 33.8 eV at C Kα (E = 277 eV) and 120 eV at Mn Kα (E = 5.9 keV).