Ultrahigh Brilliance Multi-MeVγ-Ray Beams from Nonlinear Relativistic Thomson Scattering

We report on the generation of a narrow divergence (${\theta }_{\gamma }<2.5\mathrm{mrad}$), multi-MeV ($E_{\max }\approx 18\mathrm{MeV}$) and ultrahigh peak brilliance ($>1.8\times {10}^{20}\mathrm{photons}{\mathrm{s}}^{-1}{\mathrm{mm}}^{-2}{\mathrm{mrad}}^{-2}$ 0.1% BW) $\gamma$-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude $a_0\approx 2$). The spectrum of the generated $\gamma$-ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature.