Observation of Soft-X-ray Superradiance in Hypervelocity (v$>$100 km/s) Impact of Nanoparticles

Anomalous particle detector signals were discovered by Bae and his colleagues in hypervelocity (v $>$ 100 km/s) impact of nanoparticles, such as water clusters and biomolecules, at Brookhaven National Lab in 1994. The estimated 1-D shock pressure range of the Bae et al's experiments is 20 Mbar -- 2 Gbar. Because the atomic range theory predicts that the nanoparticles cannot penetrate the detector window, thus cannot generate signals, the origin of the signals has been a mystery for more than a decade. In 2007, Winterberg proposed metastable quantum states can be formed with innershell electrons, which would decay by emitting intense x-rays, when atoms are under ``sudden'' compression with pressures in excess of 100 Mbar. The compression in Bae et al.'s experiments can be considered to be ``sudden'', because its time scale (10-100 fs) is much shorter than the ion-electron thermalization time scale ($>$1 ps). The detailed analysis of the anomalous detector signals revealed that they resulted from intense soft-x-rays generated in the nanoparticle impact, and the radiation energies are 75 -- 100 eV in agreement with Winterberg's. The conversion efficiency from the initial nanoparticle kinetic energy to the x-ray radiation energy was as high as 38 {\%}, owing to the Dicke superradiance mechanism, because the size of impact volumes is smaller than the radiation wavelength.