Absolute calibration of a flat field spectrometer in the wavelength range 10–70 Å

We characterize an x-ray spectrometer in the wavelength range 10–70 Å. The instrument has been designed for the study of the soft x-ray emission from plasmas generated with intense sub-ps laser pulses. It uses a flat field reflection grating with 2400 lines per mm as dispersing element and an x-ray charged coupled device (CCD) as detector. The sensitivity of the spectrometer and CCD detector has been absolutely calibrated by means of a ns-laser generated gold plasma of well known x-ray conversion efficiency. The calibration procedure is explained in detail. Finally, we present examples of spectra emitted by a sub-ps laser plasma. From the spectra follows a resolving power of 150–350 and a low conversion efficiency into the second diffraction order of the flat field grating of only a few %. The measured frequency averaged total conversion of laser energy into x-ray energy emitted into 2π is in the range of 2.4×10−4 up to 6.2×10−3 depending on the material of the solid targets.