Hard X-ray emission from laser-produced plasmas of U and Pb recorded by a transmission crystal spectrometer

Hard X-ray spectra from laser-produced plasmas were recorded by a transmission crystal survey spectrometer covering the 12– 60 keV energy range with a resolving power of E/ΔE≅100. This emission is of interest for the development of hard X-ray backlighters and hot electron diagnostics. Foils of U and Pb were irradiated at the OMEGA laser facility by 24 beams (12 on each side), each with an energy of ≅500 J , a pulse duration of 1 ns , and no beam smoothing. The beams were focused to a 50 μm diameter spot on the target plane. The spectra typically exhibit a few intense and relatively narrow features in the 12– 22 keV energy range. Initial analysis suggests that these hard X-ray features are inner-shell transitions resulting from L-shell vacancies created by energetic electrons. The observed transition energies are slightly higher than the neutral-atom characteristic X-ray energies. Calculations suggest that the transitions are in the Ni-like or lower ionization stages. The analysis further indicates that opacity effects play an important role in producing the spectra.