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

Abstract

We are calibrating the time of flight mass spectrometer of the Cosmic Dust Analyzer (CDA) instrument aboard the Cassini spacecraft. The CDA measures the flux of particles in the 10^(−15) to 10^(−9) g range at intersection velocities of up to 100 km/s. Of special interest are the chemical composition of the particles in orbit about Saturn and/or its satellites that are expected to be captured by CDA during ring plane crossings and upon close encounter with the satellites. Upon impacting a rhodium plate, particles are expected to partially ionize and their chemical composition is expected to be determined from mass analysis of the positive ions. In order to optimize impact ionization calibration experiments using a light gas-gun launched microspheric particles, we have done initial testing with a short duration pulsed laser (4 ns duration nitrogen laser (337 nm)). The beam is focused to deliver the 300μJ energy per laser pulse onto a 33 μm^2. The laser power density (≈10^10 W/cm^2) simulates the impact of particles with various combinations of density and velocities, e.g., 8 g/cm^3 (Fe) projectile at 23 km/s or 1 g/cm^3 projectile at 65 km/s. The CDA spectrometer will 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 4.6pC (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 implies that ∼10−5 % of the laser pulse energy produces ions and the present system has a 10% detection efficiency. Using multi-channel plate detector to detect ions from aluminum alloy and kamacite yields well defined peaks at 24(Mg^(+1)), 27(Al^(+1)) and 64 (Cu^(+1)), and, 56(Fe^(+1)), 58(Ni^(+1)) and 60(Ni^(+1)) amu, respectively. Also contaminant ions at 23 (Na^(+1)) and 39(K^(+1)) amu are detected.

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