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M R Howells

Publications and source records attributed to M R Howells.

7 recordsLinked to original sources
PubMed2002

Synchrotron soft X-ray and field-emission electron sources: a comparison.

The soft X-ray spectral region and the useful range of electron energy-loss spectroscopy are very similar, both including the energy range 100-1000 eV. Moreover, well-developed monochromators and parallel detection devices with comparable resolution exist for both. Despite the differing interactions of electrons and photons, many complementary experiments in imaging, spectroscopy and diffraction have been performed using both techniques. We therefore compare the brightness, degeneracy, monochromaticity, beam size, source size, spatial and temporal coherence of field-emission electron beams and soft X-ray synchrotron radiation from typical undulators. Recent brightness values for nanotip field emitters and undulators, both measured and calculated, are provided with examples from the Advanced Light Source synchrotron-radiation facility at Berkeley USA. The quantum mechanical upper limit on source brightness, as well as relationships among beam brightness, coherence parameters, and degeneracy, are discussed. Factors which limit these parameters and methods of measurement are reviewed, and the implications for diffraction, imaging and spectroscopic experiments as well as radiation damage are briefly commented on.

Journal Article↗
PubMed2001

Image reconstruction from electron and X-ray diffraction patterns using iterative algorithms: experiment and simulation.

The hybrid input-output iterative algorithm, which solves the phase problem for scattering from non-periodic objects, is reviewed for application to X-ray and electron diffraction data. Desirable convex constraints, including the sign of the scattering potential for electrons, and compact support, are discussed. The cases of complex and real exit-face wavefunctions, strong and weak phase objects, various supports, and the use of coherent focussed radiation are reviewed. Reconstruction of general complex objects requires accurate knowledge of the support, which should consist of two holes or a triangle in an opaque mask. The support boundaries should be as sharp as possible. Strong phase objects without absorption can be recovered if the support consists of one hole, is accurately known and has sufficiently sharp boundaries. Real and weak phase objects with absorption can be recovered without accurate knowledge of the support area if the support boundaries are sufficiently sharp and the support consists of one or more holes. A sign constraint on the scattering potential is used to recover weak phase objects. The experimental realization of theoretically desirable support conditions is discussed. A two-stage method of finding the support for complex objects is proposed. Experimental results from applying the Gerchberg-Saxton-Fienup HiO-algorithm to coherent electron diffraction patterns are presented, using specially made e-beam lithographed support structures. Images with a resolution of about 5 nm are thus recovered from the intensities alone in coherent electron diffraction patterns from non-periodic objects. Limitations of the present experiments are identified and suggestions made for development of both X-ray and electron work.

Journal Article↗
PubMed1998

An assessment of approximating aspheres with more easily manufactured surfaces.

In designing optical systems for synchrotron radiation, one is often led to conclude that optimal performance can be obtained from optical surfaces described by conic sections of revolution, usually paraboloids and ellipsoids. The resulting design can lead to prescriptions for three-dimensional optical surfaces that are difficult to fabricate accurately. Under some circumstances satisfactory system performance can be achieved through the use of more easily manufactured surfaces such as cylinders, cones, bent cones, toroids and elliptical cylinders. These surfaces often have the additional benefits of scalability to large aperture, lower surface roughness and improved surface figure accuracy. In this paper we explore some of the conditions under which these more easily manufactured surfaces can be utilized without sacrificing performance.

Journal Article↗
PubMed1986

Erythropoiesis in pregnancy.

The reticulocyte count, serum ferritin and serum erythropoietin concentrations were measured during the course of pregnancy in 41 women. A decrease in iron stores early in pregnancy was accompanied by a significant rise in the reticulocyte count until the 28th week. Erythropoietin levels did not rise significantly until the 28th week of pregnancy and had fallen to the post-natal level by 40 weeks. The results are consistent with an increase in erythropoietic activity early in pregnancy which did not appear to depend upon increased plasma erythropoietin levels.

Adult↗