High-energy differential-filtering photon spectrometer for ultraintense laser-matter interactions.

Large quantities of ultrahigh-energy x-rays are produced by petawatt-class lasers; however, spectroscopy in this range of 0.1-1 MeV is difficult due to the long photon mean free path. A novel geometry step filter to measure the high-energy bremsstrahlung emission tail has been developed for use in high energy density, short-pulse laser-matter interaction experiments. The grid design of the filters allows for the independent determination of a local background, which reduces systematic errors in the reconstructed spectra. This spectrometer was used to measure x-ray spectra for various laser and target conditions at intensities near 1 × 10 ¹⁸ W/cm ² where single-exponential bremsstrahlung spectra were fit to the data and show an increasing photon temperature with pulse duration for a fixed laser intensity.