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Instrument: OSS : Open-Source Neutral Mass Spectrometer
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The objective of the Open-Source Netrual Mass Spectrometer was
to contribute to a study of the chemical, dynamic, and energetic
processes that control the structure of the thermosphere by
providing direct in situ measurements of both major and minor
neutral atmospheric constituents having masses in the range from
1 to 48 atomic mass units (u). A double-focusing,
Mattauch-Herzog magnetic deflection mass spectrometer with an
impact ion source was flown. Two ion collectors were included
to measure ions differing in mass by a factor of 8; i.e., the
two mass ranges covered were 1 to 6 and 6 to 48 u. In the ion
source the neutral species were ionized by means of electron
impact. The electron energies were selectable; 75 eV for the
high-eV mode and 25 eV for the low-eV mode. At altitudes greater
than 380 km, ion currents were measured with an electron
multiplier. Counts were accumulated for 1/20 s before
automatically switching to a different mass number. While
complete mass spectra could be swept, in the common mode of
operation peak stepping was employed; readings on principal
peaks in the mass spectrum were repeated approximately every 0.5
s and on other species less frequently. Data below 380 km were
measured using an electrometer. In addition to the peak
stepping mode, there were several other operating modes which
were selected by ground command. In the fly-through mode,
ambient particles striking the ion source retained energies less
than 0.1 eV, which was not high enough to overcome the negative
space charge potential holding the ions in the beam. Those
ambient particles that did not strike the ion source retained
their incoming energy of several eV after ionization and escaped
into the acceleration region of the analyzer.

[Summary provided by NASA]