Absolute calibration of various crystal types and geometries for National Ignition Facility x-ray spectrometers*

Absolute calibration of crystals within x-ray spectrometers fielded at the National Ignition Facility (NIF) is critical to understand the plasma source properties that they aim to diagnose. The spectrometer calibration station (SCS) located at the Lawrence Livermore National Laboratory was established in recent years to perform integrated spectrometer throughput measurements in order to fully characterize a given crystal's response. Full NIF spectrometer or duplicate surrogate hardware is placed in a vacuum chamber adjacent to the x-ray source chamber, such that the crystal-to-source standoff can be adjusted to its actual position from a target in the NIF. We use a steerable electron beam to bombard an anode foil as our x-ray source. The anode foil is housed in a rotatable turret, allowing for an easy selection of various elements without breaking vacuum. This combined with a voltage of up to 30 kV, for the electron beam power supply, allows for x-ray line emission of up to 30 keV photon energies. The crystal response is determined by taking a ratio of the crystal diffracted signal to the source emission spectrum for a given set of x-ray lines within the spectrometer's bandwidth. We use an absolutely calibrated Amptek silicon drift detector to infer the source emission signal. The SCS further characterizes crystal attributes by illuminating any spectral warping or defects that can only be seen with x-rays. Spectral dispersion is also mapped so that theoretical design can be vetted. Here we present the calibration data from various NIF x-ray spectrometers of different geometries and crystal types. We show that often each crystal is unique and requires individual calibration, regardless of identical design and fabrication specifications.