Nuclear diagnostics for petawatt experiments (invited)

With the operation of successively more intense and powerful lasers, such a s the NOVA petawatt laser with I similar to 3 x 10(20) W/cm(2), several nov el (to laser physics) nuclear diagnostics were used to determine the nature of the laser/matter interaction at the target surface. A broad beam of hot electrons, whose centroid varied from shot to shot, width was remarkably c onstant, and intensity was about 40% of the incident laser energy was obser ved. New nuclear phenomenon included photonuclear reactions [e.g., (gamma , xn)], photofission of U-238 and intense beams of ions. Photonuclear reactio ns were observed and quantified in Cu, Ni, and Au samples, and produced act ivation products as neutron deficient as Au-191 [a (gamma ,6n) reaction!], requiring gamma rays exceeding 50 MeV in energy. The spectral features of t he gamma-ray source have been investigated by comparing activation ratios i n Ni and Au samples, and angular distributions of higher energy photons hav e been measured with activation of spatially distributed Au samples. Extrao rdinarily intense beams of charged particles (primarily protons) were obser ved normal to the rear surface of the target and quantified using the charg ed particle reaction Ti-48(p,n)V-48, radiochromic film and CR39 plastic tra ck detectors. Approximately 3 x 10(13) protons, with energies up to 55 MeV were observed in some experiments. Collimation of this beam increases with increasing proton energy. Correlations of activation with laser performance will be discussed. (C) 2001 American Institute of Physics.