Last year, the X27A beamline at the National Synchrotron Light Source (NSLS) became dedicated solely to X-Ray Computed Microtomography (XCMT). This is a third-generation instrument capable of producing tomographic volumes of 1-2 micron resolution over a 2-3mm field of view. Recent enhancements will be discussed. These have focussed on two issues: the desire for real-time data acquisition and processing and the need for highly monochromatic beam (.1 % energy bandpass). The latter will permit k-edge subtraction studies and will provide improved image contrast from below the Cr (6 keV) up to the Cs (36 keV) k-edge. A range of applications that benefit from these improvements will be discussed as well. These two goals are somewhat counterproductive, however; higher monochromaticity yields a lower flux forcing longer data acquisition times. To balance the two, a more efficient scintillator for X-ray conversion is being developed. Some testing of a prototype scintillator has been performed; preliminary results will be presented here. In the meantime, data reconstruction times have been reduced, and the entire tomographic acquisition, reconstruction and volume rendering process streamlined to make efficient use of synchrotron beam time. A Fast Filtered Back Transform (FFBT) reconstruction program recently developed helped to reduce the time to reconstruct a volume of 150 x 150 x 250 pixels 3 (over 5 million voxels) from the raw camera data to 1.5 minutes on a dual R10,000 CPU. With these improvements, one can now obtain a "quick look" of a small tomographic volume (~l0 6 voxels) in just over 15 minutes from the start of data acquisition.

No abstract

The general kinematic properties of vertex functions which follow from the transformation properties of the initial and final single-particle states, and of the vertex (current) operator under proper and improper Lorentz transformations, are studied for the pseudoscalar (pion) and vector (electromagnetic) vertices. The treatment, which relies strongly on the helicity representation for the states of a relativistic particle introduced by Jacob and Wick, is fully relativistic, and applies to particles of arbitrary spin. The number of independent vertex functions or form factors is determined in each case, and the analogs of the nonrelativistic multipole expansions are obtained. One obtains thereby a complete specification of the dependence of the matrix elements on the spins, helicities, and relative parities of the particles, and some information on the limiting behavior of the form factors for small momentum transfers. A theorem first proved by Ernst, Sachs, and Wali in the special case of spin J, that the matrix elements of the 4-divergence of the electromagnetic current between states of the same (single) particle vanish independently of the assumption of gauge invariance or current conservation, is extended as a byproduct of the general results to the case of arbitrary spin and to any vector current having the same transformation properties as y M . Finally, the parametrizations obtained for the pseudoscalar and vector vertex functions are used to calculate the cross sections for the general two-particle scattering process a-\-b -> c-\-d in the single-quantum-exchange approximation for relativistic particles of arbitrary spin.

No abstract

No abstract