  Relativistic electrons, accelerated by the laser ponderomotive force, can be
focused onto a high-Z convertor to generate high-brightness beams of
gamma-rays, which in turn can be used to induce photonuclear reactions. In this
work, the possibility of photo-transmutation of long-lived radionuclide Cs-135
by laser-plasma driven electron source has been demonstrated through Geant4
simulations. High energy electron generation, bremsstrahlung and photonuclear
reaction have been observed at four different laser intensities of 10^{20}
W/cm^2, 5 times 10^{20} W/cm^2, 10^{21} W/cm^2 and 5 times 10^{21} W/cm^2,
respectively. It was shown that the laser intensity and the target geometry
have strong effect on the transmutation reaction yield. At different laser
intensities the recommended target sizes were found to obtain the maximum
reaction yield. The remarkable feature of this work is to evaluate the optimal
laser intensity to produce maximum reaction yield of 10^8 per Joule in laser
pulse energy, which is 10^{21} W/cm^2. Our study suggests photo-transmutation
driven by laser-based electron source as a promising approach for experimental
research into transmutation reactions, with potential applications to nuclear
waste management.
