The digitized Green's function (DGF) algorithm and the underlying theory are described. This finite element algorithm models dielectric particles of arbitrary shape and arbitrary optical structure. DGF predictions of differential and total cross sections are compared with predictions of Mie and EBCM algorithms for several homogeneous spheres and spheroids. Results of tests of convergence of the DGF calculation as the number of elements are increased are presented. Computer time and storage requirements as functions of wavelength and particle size, shape, and optical structure are discussed.

We have measured the neutron capture cross sections of the stable magnesium isotopes 24,25,26 Mg in the energy range of interest to the s process using the neutron time-of-flight facility n_TOF at CERN. Capture events from a natural metal sample and from samples enriched in 25 Mg and 26 Mg were recorded using the total energy method based on C 6 2 H 6 detectors. Neutron resonance parameters were extracted by a simultaneous resonance shape analysis of the present capture data and existing transmission data on a natural isotopic sample. Maxwellian-averaged capture cross sections for the three isotopes were calculated up to thermal energies of 100 keV and their impact on s-process analyses was investigated. At 30 keV the new values of the stellar cross section for 24 Mg, 25 Mg, and 26 Mg are 3.8±0.2 mb, 4.1±0.6 mb, and 0.14±0.01 mb, respectively.

The neutron capture cross section of 139 La has been measured relative to that of 197 Au by means of the activation method. The sample was irradiated in a quasistellar neutron spectrum for kTϭ25 keV generated via the 7 Li(p,n) 7 Be reaction with the proton energy adjusted 30 keV above the threshold. Maxwellian averaged neutron capture cross sections were calculated for energies kTϭ5 -100 keV. The new value for kT ϭ30 keV is found to be 31.6Ϯ0.8 mb, 18% lower and considerably less uncertain than the previously recommended value of 38.4Ϯ2.7 mb. With these results the s-and r-process components could be more accurately determined, making lanthanum a reliable s-and r-process indicator in stellar spectroscopy.

The useful compromise between resolution and penetration power of the submillimeter or terahertz (THz) spectral region has long made it attractive for a variety of imaging applications. However, many of the demonstrations of imaging in this spectral region have used strategically oriented targets, especially favorable concealment materials, proximate imaging geometries, etc. This paper reports the results of studies aimed at better understanding the phenomenology of targets, the impact of this phenomenology on various active and passive imaging strategies, and most importantly, the development of imaging strategies that do not require the aforementioned special circumstances. Particular attention is paid to the relationship between active and passive images, especially with respect to how they interact with the illumination-and detector-mode structures of various imaging scenarios. It is concluded that the very large dynamic range that can be obtained with active single-mode systems (including focal-plane arrays) can be used in system designs to overcome the deleterious effects that result from the dominance of specular reflections in single-mode active systems as well as to strategically orient targets to obtain recognition. This will aid in the development of a much more robust and generally useful imaging technology in this spectral region.

Neutron resonance analyses have been performed for the capture cross sections of 186 Os, 187 Os, and 188 Os measured at the n TOF facility at CERN. Resonance parameters have been extracted up to 5, 3, and 8 keV, respectively, using the SAMMY code for a full R-matrix fit of the capture yields. From these results average resonance parameters were derived by a statistical analysis to provide a comprehensive experimental basis for modeling of the stellar neutron capture rates of these isotopes in terms of the Hauser-Feshbach statistical model. Consistent calculations for the capture and inelastic reaction channels are crucial for the evaluation of stellar enhancement factors to correct the Maxwellian averaged cross sections obtained from experimental data for the effect of thermally populated excited states. These factors have been calculated for the full temperature range of current scenarios of s-process nucleosynthesis using the combined information of the experimental data in the region of resolved resonances and in the continuum. The consequences of this analysis for the s-process component of the 187 Os abundance and the related impact on the evaluation of the time duration of galactic nucleosynthesis via the Re/Os cosmochronometer are discussed.