Absorption and phase imaging with synchrotron radiation

Abstract

All users agree on two essential virtues of synchrotron radiation: its high intensity in the X-ray range, and its continuous spectrum. The new, third generation, sources have another, at first sight less spectacular feature: the small divergence of the beam as seen from the sample. This characteristic is due to the very small cross-sectional area of the electron beam that acts as the source of radiation, and to the large source -sample distance. All three of these qualities lead to novel possibilities, among others in X-ray imaging. We will discuss some of the approaches developed in hard X-ray synchrotron radiation imaging, and some of its applications in quite diverse fields. Synchrotron radiation refers to the electromagnetic radiation emitted by ultrarelativistic electrons (energies of several GeV), circulating in storage rings, at those parts of the rings where they are accelerated by a magnetic field. This can be uniform over a part of the trajectory in the bending magnets, or spatially oscillating in the "insertion devices". The spectrum of the light thus produced extends from the infra red into the X-ray range, the latter part being to most users the more valuable one. Emission is strongly concentrated in the forward direction with respect to the velocity of the emitting electrons, the characteristic angular opening being mc 2 /E, with E the energy of the electrons, m their rest mass. Three machines in the world belong to the category of third generation, high energy sources : the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, at 6 GeV; the Advanced Photon Source in Argonne, IL, USA at 7 Gev; and SPRING-8 in Japan, at 8 GeV. They are characterised by the thinness of the electron beam that produces the radiation (source dimensions < 0.1 mm), and by the provision, in between the bending magnets, of many straight sections. These allow the positioning of insertion devices, viz. wigglers or undulators, which can provide each experiment with the best suited beam. Imaging is normally associated, in our minds, with lenses. Unlike visible light or electrons, efficient lenses are not (yet?) available for hard X-rays, essentially because they interact weakly with matter, resulting in a refractive index very close (to within 10 -5 or 10 -6) to unity. Nevertheless X-ray imaging plays an immense role. Radiographs of hands of attendants made at the lectures on X-rays in 1896 are the historical banner of X-rays, while each of us benefited from medical radiography and enjoys the leak-tightness which industrial radiography controls in pipelines. X-ray tomography is used on a routine basis, under the name of computed axial tomography (CAT) or medical scanner, to visualise virtual cuts through human anatomy, obtained from attenuation measurements performed under different viewing angles. In radiography (two-dimensional images) as well as in tomography (three-dimensional exploration), only contrast associated with local variations in X-ray absorption was, until recently, considered. On the other hand, a different approach, based on Bragg diffraction ("X-ray topography"), reveals isolated defects, such as dislocations,