An instrument for elemental and isotopic abundance characterization of extra-terrestrial materials

Samples returned from the Genesis and Stardust missions of NASA's Discovery Program require quantitative analysis at sensitivities unobtainable with present instruments. This has driven development of a new generation of instruments for laser-post-ionization secondary neutral mass spectrometry (laser-SNMS). Construction of a prototype time-of-flight (TOF) SNMS instrument has been completed recently at Argonne National Laboratory (ANL) and testing began in August 2002. This instrument is optimized for laser post-ionization of sputtered neutrals and is capable of locating and analyzing individual sub-micrometer interstellar particles on a sample stage for Stardust or determining elemental concentrations in shallow implants at ultra-trace levels for Genesis. Post-ionization can be accomplished with a variety of laser sources. These include high repetition rate tunable Ti-sapphire lasers for ultra-trace analysis of a single element and two vacuum ultraviolet (VUV) light sources for simultaneous ionization of most atomic and molecular species in the sample. The two VUV lasers are an F2 laser with a fixed wavelength of 157 nm and the self-amplified spontaneous emission free-electron laser (SASE FEL) capable of generating tunable VUV at wavelengths down to 60 nm. Fundamental physical theory of ion sputtering forms the scientific basis of the approach used to design the instrument. An ion optics design for the instrument was perfected through extensive three-dimensional computer simulations using SIMION software. Realistic sets of photo-ions were calculated using formalisms derived from sputtering theory. Their trajectories in various instrument designs were then traced by SIMION. Finally, results of the simulations were processed to estimate instrument capabilities including resolution and useful yield. This same approach proved accurate and quantitative during tests of an existing TOF SNMS instrument demonstrating the reliability of the simulation method. The completed prototype instrument and results of recent tests will be presented. This work is supported by the U. S. Department of Energy, BES-Materials Sciences, under Contract W-31-109-ENG-38 and in part by a contract from NASA's Office of Space Science's Cosmochemistry Program.