Selected Publications
David D. Allred, Matthew B. Squires, and R. Steven Turley (et al.)
The reported optical constants of uranium differ from that of vacuum significantly more than other elements do over the range of about 150 to 350 eV. This suggests that uranium could be used to produce high reflectance imaging mirrors for many soft x-ray applications. Elemental uranium is too chemically active to be used as a front surface mirror without protection. We computed the expected reflectance of carbon-coated uranium films and of uranium-nickel alloys for low-angle reflectors. Carbon is mostly transparent below its K absorption edge at about 283 eV. The reflectance at 10 degrees from grazing is computed to be greater than 50% at 277 eV (C Kα). For comparison, about 5 degrees is the maximum grazing incidence angle for which conventional materials are computed to have comparable reflectance. We sputter deposited and measured the reflectance of carbon-coated uranium layers at 44.7 Å (C Kα). Sample reflectance was a factor of two greater than that of nickel, the material used for low-angle mirrors. The initial oxidation behavior of sputtered uranium-nickel alloys is similar to pure U so their reflectance was not determined. Coatings based on uranium should be considered for all applications where high-reflectance, broadband, low-angle soft x-ray mirrors are required
We have prepared our own very small, Martian test tubes and flasks which possess many of the conditions of Mars’ surface (up to the first three of five conditions listed below) and at a cost well below $100**. We did several experiments of interest to us, namely: lightening on Mars in a jar (12-13 year olds) and what might happen to ice cream on Mars (an 8 year old). Student, amateur, and professional researchers alike need Mars-like conditions in which to test their ideas and to help answer their questions. It has been proposed that vacuum stations like those used in industry and university labs could be employed to produce Mars-like conditions to test student ideas for whole classrooms of children.1 It is our goal to find simpler and lower cost ways to help researchers set up their own small test stations where they can test out their ideas. The test stands need only meet those characteristics of Mars which are pertinent for testing the ideas.
M. B. Squires, David D. Allred, and R. Steven Turley
Shannon Lunt, R. Steven Turley, and David D. Allred
We designed multilayer mirrors for the IMAGE/Explorer mission that were intended to reflect well at 304 * and poorly at 584 *. The best designs utilized the novel materials U and Y_2O_3with Al or Si as spacer layers. The highest design reflectivities were obtained with aperiodic multilayers, although these were too hard to grow in practice. We found these novel designs using a genetic algorithm calling on a database of 43 promising materials with the freedom of aperiodic multilayer stacks.
D. D. Allred and R. S. Turley (et al.)
The Extreme Ultraviolet Imager (EUV) of the IMAGE Mission will study the distribution of He+ in Earth's plasmasphere by detecting its resonantly-scattered emission at 30.4 nm. It will record the structure and dynamics of the cold plasma in Earth's plasmasphere on a global scale. The 30.4-nm feature is relatively easy to measure because it is the brightest ion emission from the plasmasphere, it is spectrally isolated, and the background at that wavelength is negligible. Measurements are easy to interpret because the plasmaspheric He+ emission is optically thin, so its brightness is directly proportional to the He+ column abundance. Effective imaging of the plasmaspheric He+ requires global 'snapshots' in which the high apogee and the wide field of view of EUV provide in a single exposure a map of the entire plasmasphere. EUV consists of three identical sensor heads, each having a field of view 30 degrees in diameter. These sensors are tilted relative to one another to cover a fan-shaped field of 84 degrees x30 degrees, which is swept across the plasmasphere by the spin of the satellite. EUV's spatial resolution is 0.6 degrees or similar to 0.1 R-E in the equatorial plane seen from apogee. The sensitivity is 1.9 count s(-1) Rayleigh(-1), sufficient to map the position of the plasmapause with a time resolution of 10 min.