Mass spectrometer calibration of Cosmic Dust Analyzer

Abstract

The time-of-flight (TOF) mass spectrometer (MS) of the Cosmic Dust Analyzer (CDA) instrument aboard the Cassini spacecraft is expected to be placed in orbit about Saturn to sample submicrometer-diameter ring particles and impact ejecta from Saturn's satellites. The CDA measures a mass spectrum of each particle that impacts the chemical analyzer sector of the instrument. Particles impact a Rh target plate at velocities of 1-100 km/s and produce some 10^(−8) to 10^(−5) times the particle mass of positive valence, single-charged ions. These are analyzed via a TOF MS. Initial tests employed a pulsed N2 laser acting on samples of kamacite, pyrrhotite, serpentine, olivine, and Murchison meteorite induced bursts of ions which were detected with a microchannel plate and a charge sensitive amplifier (CSA). Pulses from the N_2 laser (10^(11) W/cm^2) are assumed to simulate particle impact. Using aluminum alloy as a test sample, each pulse produces a charge of ∼4.6 pC (mostly Al^(+1)), whereas irradiation of a stainless steel target produces a ∼2.8 pC (Fe^(+1)) charge. Thus the present system yields ∼10^(−5)% of the laser energy in resulting ions. A CSA signal indicates that at the position of the microchannel plate, the ion detector geometry is such that some 5% of the laser-induced ions are collected in the CDA geometry. Employing a multichannel plate detector in this MS yields for Al-Mg-Cu alloy and kamacite targets 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)) dalton, respectively.