We report on the generation of a narrow divergence (θ≈2.5 mrad), multi-MeV (EMAX=18 MeV) and ultra-high brilliance (≈2×1019
Nearby in the stack
photons s
−1
mm
−2
mrad
−2
0.1% BW)
γ
-ray beam from the scattering of an ultra-relativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude
a0≈2
). The spectrum of the generated
γ
-ray beam is measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear evidence of the onset of non-linear Thomson scattering. The photon source has the highest brilliance in the multi-MeV regime ever reported in the literature.