Absorption and phase imaging with synchrotron radiation

Abstract

All users agree on two essential virtues of synchrotron radia­ tion: 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 ofthe 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 ofits appli­ cations 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 ofthe trajectory in the bending magnets, or spatially oscillat­ ing in the insertion devices. The spectrum of the light thus produced extends from the infra red into the X-ray range, the lat­ ter part being to most users the more valuable one. Emission is strongly concentrated in the forward direction with respect to the velocity ofthe emitting electrons, the characteristic angular open­ ing being mC-lE, with E the energy of the electrons, m their rest mass. Three machines in the world belong to the category ofthird generation, high energy sources : the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, at 6 GeV; the