Betatron x-ray spectra recorded by a transmission crystal spectrometer in the 10 keV to 70 keV photon energy range

Abstract

Betatron x-ray spectra in the 10 keV to 70 keV energy range were recorded by a transmission crystal spectrometer. This work demonstrates for the first time the ability to accurately measure the absolutely-calibrated betatron photon energy distribution using a high-resolution wavelength dispersive crystal spectrometer. The betatron emission was generated by the interaction of intense pulses from the Rutherford Appleton Laboratory Astra-Gemini laser with a gas capillary waveguide. The spectrometer utilized a quartz (101) crystal that was cylindrically bent to a radius of 112 mm. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> The crystal was positioned 2.5 m from the photon source and covered a 12.6 mrad by 9.3 mrad field of view. Single and multiple shot spectra were recorded on an image plate detector. After subtracting the background resulting from energetic (> 300 keV) scattered photons, the energy-dependent shape of the spectrum and the absolute number of photons were measured using the crystal efficiency and image plate calibration curves. The experimental spectra in units of photons/keV/mrad <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , averaged over the spectrometer's field of view, were in good agreement with the synchrotron energy distribution. For optimal laser and betatron generation conditions, the fluence of photons having energies > 10 keV was about 3×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> photons/mrad2 per shot, and the critical energy of the fitted synchrotron distribution was 8.8 keV ± 0.5 keV. To further understand and optimize the betatron generation process, it will be necessary to measure the betatron photon distribution with increased accuracy and also with angular resolution in the laser polarization direction and perpendicular to this direction. Improved instrument accuracy and sensitivity can be accomplished by utilizing thicker shielding and improved baffling to reduce the background resulting from > 300 keV scattered photons. Owing to the Bragg condition that must be satisfied along the length of the bent crystal, spectral images present angular and energy dependence in the dispersion plane. Angular resolution in the orthogonal, out-of-plane direction occurs because the fan of x-rays from the small (effectively point) source passes through the crystal with small deflection and is projected onto the image plate.