| SSRL Users Newsletter | October 1996 |
- T. Cremer
- R. Tatchyn
At present, the primary applications of the SPEAR storage ring at SSRL involve the transport and utilization of synchrotron radiation (SR) extending from the ultraviolet (UV) regime to x-ray energies up to 100 keV. Fundamental to the performance of these functions is the interaction of SR photons with matter. A typical beam line, for example, may utilize thin beryllium windows to transmit hard x-rays; grazing-incidence mirrors to deflect, collimate, or focus the SR; and various instruments, such as monochromators, to provide sharply selective filtering of the SR spectrum. In assessing the performance of a beam line, or in planning or designing new ones, it is usually desirable to be able to make rapid quantitative estimates of these effects. In many experiments, a knowledge or estimation of the amount of light absorbed or transmitted by the experimental sample, or by optical instrumentation and optics upstream of the sample, can be critical to the optimal design and performance of the experiment. To these ends, a knowledge of the laws of the reflection, absorption, and transmission of light, together with a readily accessible compendium of the optical constants of the elements, are essential.
To date, a quick and convenient computer utility called ABSORPTION, developed by Sean Brennan on the basis of the optical constants tabulation of Cromer, Liberman, and McMasters has been in use at SSRL. In this article we describe the capabilities and features of a more recently installed utility, XREFLECTION, developed by us on the basis of the alternative optical constants compilations of Henke (30 eV - 30 keV) and Hubbell (1 keV - 100 + GeV).
XREFLECTION is an interactive computer utility with graphics and data file generation capabilities. The source code, written in C, is currently installed on SSRL's Digital mainframe. The graphics output, based on the GHOST package developed by Tony Cox for SSRL, is viewable on any Digital terminal, or graphics-capable Digital terminal emulator, logged into SSRL. The utility is called by typing XREFLECTION at the standard DCL terminal prompt. In addition to the graphics, selected menu choices offer the option of generating data files containing the requested information. These files, which are put automatically into the user's directory, can be deleted or ported out for more sophisticated or convenient graphing. A fuller description of XREFLECTION, including references and a number of worked examples, can be accessed by typing HELP XREFLECTION.
XREFLECTION has five basic menu choices. The first choice calculates the fraction of normally-incident light absorbed, transmitted, and back- reflected by a single homogeneous layer of an element. The next three selections calculate and graph the performance of an arbitrary three-finite-layer structure bounded by two arbitrary semi-infinite media a configuration that can be used to represent many components of practical interest.
With the fifth choice, a wide variety of useful constants can be graphed or tabulated for up to 200 selected (equi-spaced) energies. These include the f1 and f2 scattering constants, the dielectric constant, the complex index of reflection, the Rayleigh and Compton scattering cross sections, the photoelectric absorption cross section, the electron and nucleus mediated pair production cross sections, and the total cross section. Additional selected data and physical parameters that may be of interest to users of SR are also available.
It is our hope that the XREFLECTION utility will prove to be of use to the staff and users of SSRL, and to this end we would like to encourage your input for improvements, extensions, or corrections. As time and resources allow, we plan to keep updating and improving both the data base and source code. We would like to acknowledge the support of DOE for this work, in part through SLAC and SSRL, and in part through CRADA SLAC-9302.
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December 2, 1996
L. Dunn