DIRECT MEASUREMENTS OF COMPRESSIVE AND TENSILE STRAIN DURING SHOCK BREAKOUT BY USE OF SUBNANOSECOND X-RAY-DIFFRACTION

Shock waves of order 100 kbar were launched into 50-μm-thick single crystals of silicon (111) by irradiation with nanosecond pulses of 1.05-μm laser light at irradiances in the region of 2×1010 W cm -2. A separate laser beam, synchronous but delayed with respect to the shock-driving beam, and containing approximately 25 J of 0.53-μm laser light in a pulse of 1 ns (FWHM), was focused to a tight (<100 μm) spot on a separate titanium target to produce a plasma which was a prolific source of He-like Ti x rays. The x rays were Bragg diffracted from the rear surface of the shocked crystal and the spectrum recorded on an x-ray streak camera. Changes in interatomic spacings in a region within several microns of the surface were thus deduced from the resultant shift in Bragg angle with a temporal resolution of 50 ps. Shock waves with compressions of order 6% were observed. We observed the crystal in a state of dynamic tension as the two rarefaction waves met. The results are in good agreement with hydrocode simulations in conjunction with x-ray diffraction calculations.