Nuclear diagnostics for petawatt experiments (invited)

With the operation of successively more intense and powerful lasers, such as the NOVA petawatt laser with I∼3×10 20 W/cm 2 , several novel (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 constant, and intensity was about 40% of the incident laser energy was observed. New nuclear phenomenon included photonuclear reactions [e.g., (γ,xn)], photofission of 238 U and intense beams of ions. Photonuclear reactions were observed and quantified in Cu, Ni, and Au samples, and produced activation products as neutron deficient as 191 Au [a (γ,6n) reaction!], requiring gamma rays exceeding 50 MeV in energy. The spectral features of the gamma-ray source have been investigated by comparing activation ratios in Ni and Au samples, and angular distributions of higher energy photons have been measured with activation of spatially distributed Au samples. Extraordinarily intense beams of charged particles (primarily protons) were observed normal to the rear surface of the target and quantified using the charged particle reaction 48 Ti (p,n) 48 V , radiochromic film and CR39 plastic track detectors. Approximately 3×10 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.