Absolute energy distribution of hard x rays produced in the interaction of a kilohertz femtosecond laser with tantalum targets

Abstract

Previous reports have indicated the anomalous excitation rate for the 6.2keV nuclear level of Ta181 in a plasma produced with a femtosecond laser. A detailed characterization of the electrons and x-ray sources produced in such a plasma is required to interpret these results. In a preliminary work, the continuous energy distribution of hard x rays (10–500keV) produced in the interaction of a kilohertz femtosecond laser beam with a tantalum solid target is investigated in the 3×1015–6×1016W∕cm2 range of intensity. A sodium iodide detector with appropriate shielding is used. Strong collimation and absorption filters are used to avoid the pileup of photons in the detector. The response function of this setup is calculated with the GEANT3 simulation code. We demonstrate the necessity to quantify the Compton scattered events in the raw spectra in order to restore the absolute x-ray energy distribution.