Dependence of the yield of hard incoherent X-rays from femtosecond laser plasma on the atomic number of a target material

The yield of hard incoherent X-rays emitted by a dense femtosecond laser plasma is studied as a function of the atomic number Z of the target material. It is shown that for Z values in the range from 14 (Si) to 73 (Ta), the efficiency of laser radiation conversion to X-rays with quantum energy exceeding 8 keV varies as Z^3/2. The temperature of hot electrons formed in the plasma weakly depends on the atomic number of the target material and is equal to ~4 keV for both Si (Z = 14) and Ta (Z = 73) targets for laser radiation intensities ~10¹⁶  W/cm².