Experimental search for low energy nuclear excitation by femtosecond plasma

New experimental data on the secondary electron yield from a thin layer of ⁵⁷Fe irradiated by emission of the hot dense plasma source created by femtosecond laser pulses with intensity of 10¹⁷ W cm⁻² is presented. Plasma source hard x-ray and electron fluxes are measured for the source characterization. Thorough statistical analysis of the delayed secondary electron spectrum from the ⁵⁷Fe layer in the 20–100 ns temporal window allows exclusion of the ‘null’ hypothesis of the random character of the differences of this spectrum with that obtained using a ⁵⁶Fe target. Thus, statistically valid maxima at ∼5.8 and 7.4 keV can be attributed to the decay of the isomeric nuclear state of ⁵⁷Fe (3/2⁻, 14.41 keV, 98 ns) by the internal conversion process through the atomic K-shell (maximum at 7.4 keV), followed by the Auger process (maximum at 5.8 keV). (paper)