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Instrument: BIMS : Bennett Ion-Mass Spectrometer
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The Bennett Ion-Mass Spectrometer (BIMS) experiment was flown to
measure, throughout the orbit, the individual concentrations of
all thermal ion species in the mass range 1 to 72 atomic mass
units (u) and in the ambient density range from five to 5.E6
ions/cc. The mass range was normally scanned in 1.7 s, but the
scan time per range could be increased by command. Laboratory
and inflight determination of spectrometer efficiency and mass
discrimination permitted direct conversion of measured ion
currents to ambient concentrations. Correlation of these
measured data with the results from companion experiments, CEP
(75-107A-01) and RPA (75-107A-04) permitted individual ion
concentrations to be determined with high accuracy. The
experiment's four primary mechanical components were guard ring
and ion-analyzer tube, collector and preamplifier assembly,
vent, and main electronics housing. A three-stage Bennett tube
with 7-to 5-cycle drift spaces were flown; it was modified to
permi t ion concentration measurements to be obtained at low
altitudes. The balance between ion-current sensitivity and mass
resolution in a Bennett spectrometer may be altered by changing
appropriate voltages. These voltage changes were controlled
independently by ground command for each one of the three mass
ranges: 1 to 4, 2 to 18, and 8 to 72.

[Summary provided by NASA]