B. B. Dhal

X-ray phase imaging: Demonstration of extended conditions with homogeneous objects

We discuss contrast formation in a propagating x-ray beam. We consider the validity conditions for linear relations based on the transport-of-intensity equation (TIE) and on contrast transfer functions (CTFs). From a single diffracted image, we recover the thickness of a homogeneous object which has substantial absorption and a phase-shift of --0.37 radian.

Measurement of the x-ray mass attenuation coefficient of silver using the x-ray-extended range technique

We used the x-ray-extended range technique to measure the x-ray mass attenuation coefficients of silver in the 15–50 keV energy range with a level of uncertainty between 0.27% and 0.4% away from the K-edge. The imaginary part of the atomic form factor of silver was derived by subtracting the scattering component from the measured total mass attenuation coefficients. Discrepancies between the measured mass attenuation coefficients and alternative theoretical predictions are discussed.

X-ray phase contrast tomography with a bending magnet source

X-ray radiography and x-ray tomography are important tools for noninvasive characterization of materials. Historically, the contrast mechanism used with these techniques has been absorption. However, for any given sample there are x-ray energies for which absorption contrast is poor. Alternatively, when good contrast can be obtained, radiation damage from an excessive dose may become an issue. Consequently, phase-contrast methods have in recent years been implemented at both synchrotron and laboratory facilities. A range of radiographic and tomographic demonstrations have now been made, typically utilizing the coherent flux from an insertion device at a synchrotron or a microfocus laboratory source. In this paper we demonstrate that useful results may be obtained using a bending magnet source at a synchrotron. In particular we show that the same beamline can be used to make and characterize a sample made by x-ray lithographic methods.

Phase-space reconstruction of focused x-ray fields.

We apply the method of phase-space tomography to reconstruct x-ray beams focused using a compound refractive lens. We show that it is possible to decouple the effect of aberrations in the optical system from the field and hence measure both them and the original field. We recover the complex coherence function and find that it is consistent with expectations.

Experimental evidence in favour of the Stark mixing of atomic L-subshell states in the boron impact of Au and Bi

L-subshell ionization of Au and Bi induced by boron impact has been investigated for impact energies ranging from 0.48 to 0.88 MeV/u. The energy dependence of the measured ionization cross section shows, for the first time, a plateau structure for all three subshells. The plateau structure revealed by previous data for proton and helium impact was for the L1 subshell only and this had been attributed to the bimodal nature of the 2s electron density. The observed plateau structure for all the three subshells and its occurrence at a somewhat lower energy signifies a considerable amount of Stark mixing of target 2s and 2p atomic wavefunctions. Fresh calculations incorporating the Stark mixing effect in target atomic wavefunctions are necessary to improve agreement with the present data. The existing theories, however, are found to be inadequate.

L-subshell ionization studies of Au for alpha -particle and lithium-ion bombardment

L-subshell ionization of Au has been investigated for alpha -particle and lithium-ion bombardments with energies 0.54-1.74 MeV u-1 and 0.65-1.44 MeV u-1, respectively. Comparison of experimental X-ray production cross sections with the predictions of the ECPSSR and SCA theories shows reasonably good agreement for Lalpha and Lbeta X-rays, whereas for Lgamma and Lgamma 1+5 the ECPSSR theory underestimates the cross sections by about 60% for both alpha -particle and lithium-ion impact, and the SCA theory agrees reasonably well. The ECPSSR theory underestimates the L1- and L2-subshell ionization cross sections and gives good agreement for the L3-subshell, whereas the SCA theory overestimates the L3-subshell ionization cross sections and gives good agreement for L1 and L2. The experimental data for the total-ionization cross sections are within 25% of the predictions of both the theories.

Complete recovery of synchrotron wave fields using phase-space tomography and its applications in x-ray imaging

We discuss the role of coherence in X-ray imaging and consider how phase-space tomography can be used to extract information about partial coherence. The application of phase-space tomography to X-ray imaging will also be discussed.

X-ray extended-range technique for precision measurement of the X-ray mass attenuation coefficient and IM(F) for molybdenum using synchrotron radiation

We reconsider the long-standing problem of accurate measurement of atomic form factors for fundamental and applied problems. We discuss the X-ray extended-range technique for accurate measurement of the mass attenuation coefficient and the imaginary component of the atomic form factor. Novelties of this approach include the use of a synchrotron with detector normalisation, the direct calibration of dominant systematics using multiple thicknesses, and measurement over wide energy ranges with a resulting improvement of accuracies by an order of magnitude. This new technique achieves accuracies of 0.27– 0.5% and reproducibility of 0.02% for attenuation of copper from 8.84 to 20 keV, compared to accuracies of 10% using atomic vapours. This precision challenges available theoretical calculations. Discrepancies of 10% between current theory and experiments can now be addressed.  2001 Elsevier Science B.V. All rights reserved. PACS: 78.20.Ci; 32.80.Cy; 78.70.Dm; 78.20.Bh