This experiment measured in situ the spatial distribution and temporal changes of the
concentrations of the neutral atmospheric species. In addition, new insight into in situ measurement
techniques was obtained from comparisons of these measurements with those obtained from other onboard
experiments; namely, open source spectrometer (75-107A-07), solar EUV spectrometer (75-107A-06), and
atmospheric density accelerometer (75-107A-02). The mass-spectrometer sensor had a gold-plated stainless
steel thermalizing chamber and ion source, a hyperbolic rod quadrupole analyzer, and an off-axis
electron multiplier. When operating in the "normal" format, the analyzer measured all masses in the
range 1 to 44 atomic mass units with emphasis on hydrogen, helium, oxygen, nitrogen, and argon.
Another format was optimized for minor constituent studies of gas species in the measured range.
Spatial resolution was determined primarily by the mode of spacecraft operation. In orbit, the presealed
spectrometer was opened, and the atmospheric constituents passed through a knife-edged orifice into the
thermalization chamber and ion source. Selected ions left the quadrupole analyzer through a weak
focusing lens and were accelerated into an electron multiplier, where they were turned 90 deg to
strike the first dynode. The spectrometer had a resolution of better than 1 u for all masses between
1 and 44, and the measurement system had a planned dynamic range of approximately 1.E8. There was
provision for the instrument orifice to be covered during spacecraft thruster operations. More
experiment details can be found in D. T. Pelz et al., Radio Sci., v. 8, n. 4, p. 277, 1973.