Inverse Compton X-rays from laser-accelerated multiple electron bunches are observed. A Ti:sapphire laser (pulse energy: 500 mJ; pulse width: 150 fs) beam is divided into two beams. The main beam is focused onto an edge of a helium gas jet to accelerate electrons to energies of 14 and 23 MeV, which inversely scattered the head-on colliding secondary laser beam into 6 and 12 keV X-rays; this agrees well with that calculated from the electron spectra obtained. This demonstrates a first on-axis inverse Compton scattering X-ray energy detection beyond 10 keV induced by laser-accelerated electrons.

Backscattered X-ray intensity was measured coincidently with a CPA laser-produced X-ray pulse. Results show that the coincident measurement is useful in reducing the effect of natural radiation. Even for signals at the photon counting level, we can distinguish the scattering materials and their thickness from the difference in backscattered X-ray counts. These results suggest that the backscattered X-ray can be used for imaging any distant object.

For the first time we illuminated the ultra-short pulse laser-produced X-ray radiations on living organisms and demonstrated the following preliminary but distinguished features: low dose laser irradiation at 20 mGy increased the germination ratio of Aspergillus awamori spores 12 times, while a 20 Gy dose decreased it to 0.5 %. The latter is 50 times less energy than that required for current X-ray tubes to decrease the germination of spores to 0.5 %.Key Words: Femtosecond laser-produced radiations, X-ray, Aspergillus awamori Received February 23, 2010 Fig. 1 Setup for laser-produced X-ray irradiation on Aspergillus awamori and angular distribution of radiations. Laser is focused from left on a 0.5-mm thick Al target, which emits electron and X-ray radiations. Radiations are detected with an imaging plate IP surrounding the Al target. The fi lters, holding A. awamori spores, were distributed circularly at angles of 0 , 30 , and 60 relative to the laser axis in a vacuum chamber. Emissions are symmetric relative to the laser axis. Al target is tilted 30 relative to laser axis. Peak emission at 0 is due mainly to the hot electrons. Emissions at other angles are due to X-rays. Infl uence of Femtosecond Laser Generated X-Ray Irradiation on Germination of Aspergillus Awamori