Isotope-selective radiography and material assay using high-brilliance, quasi-monochromatic, high-energy photons.

We demonstrate the possibility to directly detect microgram amounts of the isotope 7Li using a quasi-monochromatic high-energy photon beam. The isotope selective detection is based on a witness scatterer absorbing and re-emitting photons via nuclear resonance fluorescence. This enables the detection of isotopes with microgram accuracy at long distances from the actual sample. Further, we demonstrate that the technique can deliver quantitative information without specific knowledge of the photon flux and no spectral capabilities or knowledge of the resonance fluorescence cross section. Detection of low-atomic-weight isotopes screened by heavy shielding is also shown. The techniques described are applicable to all next-generation, ultrahigh brilliance, laser-Compton light sources currently under construction.

Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser.

Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength. The introduction of X-ray free-electron lasers makes it possible to pump new atomic X-ray lasers with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.

Low-intensity nonlinear spectral effects in compton scattering.

Nonlinear effects are known to occur in Compton scattering light sources, when the laser normalized potential A approaches unity. In this Letter, it is shown that nonlinear spectral features can appear at arbitrarily low values of A, if the fractional bandwidth of the laser pulse Δϕ⁻¹ is sufficiently small to satisfy A²Δϕ≃1. A three-dimensional analysis, based on a local plane wave, slow-varying envelope approximation, enables the study of these effects for realistic interactions between an electron beam and a laser pulse, and their influence on high-precision Compton scattering light sources.

Betatron oscillations of electrons accelerated in laser wakefields characterized by spectral x-ray analysis.

Relativistic electrons accelerated by laser wakefields can produce x-ray beams from their motion in plasma termed betatron oscillations. Detailed spectral characterization is presented in which the amplitude of the betatron oscillations r is studied by numerical analysis of electron and x-ray spectra measured simultaneously. We find that r reaches as low as 1 mum in agreement with previous studies of radiation based on coherence and far-field spatial profile.

Imaging electron trajectories in a laser-wakefield cavity using betatron X-ray radiation.

We demonstrate that betatron x-ray radiation accurately provides direct imaging of electrons trajectories accelerated in laser wakefields. Experimental far field x-ray beam profiles reveal that electrons can follow similar transverse trajectories with typical excursions of 1.5 microm+/-0.5 microm in the plane of laser polarization and 0.7 microm+/-0.2 microm in the plane perpendicular.