Mass spectrometer calibration of high-velocity impact ionization based cosmic dust analyzer

We are calibrating the time of flight mass spectrometer of the Cosmic Dust Analyzer (CDA) instrument aboard the Cassini spacecraft. The CDA measures t he flux of particles in the 10(-15) to 10(-9) g range at intersection veloc ities of up to 100 km/s. Of special interest are the chemical composition o f the particles in orbit about Saturn and/or its satellites that are expect ed to be captured by CDA during ring plane crossings and upon close encount er with the satellites. Upon impacting a rhodium plate, particles are expec ted to partially ionize and their chemical composition is expected to be de termined from mass analysis of the positive ions. In order to optimize impa ct ionization calibration experiments using a light gas-gun launched micros pheric particles, we have done initial testing with a short duration pulsed laser (4 ns duration nitrogen laser (337 nm)). The beam is focused to deli ver the 300 mu J energy per laser pulse onto a 33 mu m(2). The laser power density (similar to 10(10) W/cm(2)) simulates the impact of particles with various combinations of density and velocities, e.g., 8 g/cm(3) (Fe) projec tile at 23 km/s or 1 g/cm(3) projectile at 65 km/s. The CDA spectrometer wi ll operate in the near vacuum of Saturnian zone environment is housed in a laboratory chamber at 10(-6) mbar. The ions and electrons are separated by 680 V between target and grid. The laser ionization produces charge of 4.6p C (mostly Al+1) in aluminum and 2.8pC (Fe+1) in stainless steel. Estimating that each Al+1 and Fe+1 ion requires an energy of 5.98 and 7.90 eV/ion imp lies that similar to 10(-5) % of the laser pulse energy produces ions and t he present system has a 10% detection efficiency. Using multi-channel plate detector to detect ions from aluminum alloy and kamacite yields well defin ed peaks at 24(Mg+1), 27(Al+1) and 64 (Cu+1), and, 56(Fe+1), 58(Ni+1) and 6 0(Ni+1) amu, respectively. Also contaminant ions at 23 (Na+1) and 39(K+1)am u are detected. (C) 1999 Elsevier Science Ltd. All rights reserved.