Compact γ-ray sources are of key importance not only for fundamental research but also for paramount practical applications such as cancer radiotherapy, active interrogation of materials, and high-energy radiography. Particular characteristics are required for meaningful implementation: multi-MeV energies per photon, a high degree of collimation, and a high peak brilliance. Laser-driven sources are theoretically expected to deliver such capabilities but experiments to date have reported either sub-MeV photon energies, or relatively low brilliance. By entering the non-linear regime of Thomson scattering, we report here on the first experimental realisation of a compact laser-driven γ-ray source that simultaneously ensures ultra-high brilliance (≈1019 photons s-1 mm-2 mrad-2 0.1% BW), low divergence (≈ mrad), and high photon energy (up to 18 MeV). The reported brilliance exceeds by two orders of magnitudes those of alternative mechanisms and it is the highest ever achieved in the multi-MeV regime in a laboratory experiment.
An ultra-relativistic electron beam passing through a thick, high-Z solid target triggers an electromagnetic cascade, whereby a large number of high energy photons and electron-positron pairs are produced. By exploiting this physical process, we present here the first experimental evidence of the generation of ultra-short, highly collimated and ultra-relativistic positron beams following the interaction of a laser-wakefield accelerated electron beam with high-Z solid targets. Clear evidence has also been obtained of the generation of GeV electron-positron jets with variable composition depending on the solid target material and thickness. The percentage of positrons in the overall leptonic beam has been observed to vary from a few per cent up to almost fifty per cent, implying a quasi-neutral electron-positron beam. We anticipate that these beams will be of direct relevance to the laboratory study of astrophysical leptonic jets and their interaction with the interstellar medium.
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