Shock compression of molybdenum to 2.0 TPa by means of a nuclear explosion

The principal Hugoniot of molybdenum has been determined at a pressure of 2.0 TPa by measuring directly both the shock velocity and the particle velocity behind the shock. Neutrons from an underground nuclear explosion were used to generate a high-pressure shock in a slab of molybdenum by rapidly fission heating an adjacent slab of enriched uranium. A shock velocity of 18.2 mm/μs (±5%) was obtained by determining the transit time of the planar shock between two points in the molybdenum separated by 9.87 mm. A particle velocity of 10.7 mm/μs (±5%) was obtained by observing the Doppler shifts of six neutron resonances in the energy region from 200 to 800 eV in the moving shocked molybdenum. The pressure and density derived from this pair of measurements are 2.0 TPa (20 Mbar) and 24.8 g cm−3, respectively. This experiment represents the first direct determination of a point on the Hugoniot of any material in this pressure region, and the resulting data point is in good agreement with theoretical estimates. This measurement was a successful demonstration that the Doppler-shift technique can be used to obtain particle velocities in this pressure region. It appears that errors in both the shock velocity and the particle velocity can be reduced to approximately ±2% in an improved measurement, resulting in a well-defined Hugoniot for molybdenum, which can be used as a standard in future impedance-matching experiments.