Giant collimated gamma-ray flashes

Bright sources of high-energy electromagnetic radiation are widely employed in fundamental research, industry and medicine(1,2). This motivated the construction of Compton-based facilities planned to yield bright gamma-ray pulses with energies up to(3) 20 MeV. Here, we demonstrate a novel mechanism based on the strongly amplified synchrotron emission that occurs when a sufficiently dense ultra-relativistic electron beam interacts with a millimetre-thickness conductor. For electron beam densities exceeding approximately 3 x 101(9) cm(-3), electromagnetic instabilities occur, and the ultra-relativistic electrons travel through self-generated electromagnetic fields as large as 10(7)-10(8) gauss. This results in the production of a collimated gamma-ray pulse with peak brilliance above 10(25) photons s(-1) mrad(-2) mm(-2) per 0.1% bandwidth, photon energies ranging from 200 keV to gigaelectron-volts and up to 60% electron-to-photon energy conversion efficiency. These findings pave the way to compact, high-repetition-rate (kilohertz) sources of short (less than or similar to 30 fs), collimated (milliradian) and high-flux (> 10(12) photons s(-1)) gamma-ray pulses.