Hard x-ray spectral energy distributions from pulsed power generators measured by transmission crystal spectrometers

Abstract

Summary form only given. Hard x-ray tungsten spectra from the NRL Gamble II generator were recorded by a transmission crystal spectrometer in the photon energy range > 10 keV. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> The crystal diffraction efficiencies were accurately calibrated,2 and this enables the accurate measurement of the absolutely-calibrated photon energy distribution from pulsed power generators. When measuring spectra in the 20 to 70 keV range, which is of interest for x-ray damage studies, the Gamble II experiments demonstrated the need to shield against higher energy photons up to the peak kilovoltage of the pulsed-power generator. In addition, a spectrometer optimized for measuring the x-ray fluence should be able to record the photon energy distribution from the entire area of a wide x-ray source on a single discharge. A new spectrometer has been designed that can record the absolutely-calibrated 20-70 keV photon energy distribution from a 12” diameter source such as produced by the Saturn generator. The spectra from the left and right sides of the source are spatially resolved on the detector, and a slot aperture provides spatial resolution in the perpendicular direction. The spectrometer can be deployed at standoff distances in the 2 to 4 meter range and can record the spatially resolved spectra from a single discharge on an image plate detector. The spectrometer has a linear geometry with a central axis, and the spectrometer is easily aligned using a laser propagating along the central axis and pointing to the center of the x-ray source. Spectrometers of this type have been custom designed for recording hard x-ray spectra at short-pulse laser facilities with shielding against MeV photons, and the same shielding techniques can be applied to recording spectra from energetic pulsed-power generators. Using an absolutely calibrated x-ray source at NIST, the individual spectrometer components such as crystal and filters can be calibrated to few percent accuracy. The spectrometer's final end-to-end sensitivity calibration, which relates the detector signal to the source fluence, can be performed to better than 20% absolute accuracy. Such accurate measurement of the x-ray fluence from pulse-power generators will greatly improve the analysis and mitigation of x-ray irradiation damage.