COSMIC RAY SUBSYSTEM
Cosmic Ray Detector
As its name implies, the Cosmic Ray Subsystem (CRS) was
designed for cosmic ray studies [STONEETAL1977B]. It consists
of two high Energy Telescopes (HET), four Low Energy Telescopes
(LET) and The Electron Telescope (TET). The detectors have
large geometric factors (~ 0.48 to 8 cm^2 ster) and long
electronic time constants (~ 24 [micro]sec) for low power
consumption and good stability. Normally, the data are
primarily derived from comprehensive ([Delta]E, [Delta]E
and E) pulse- height information about individual events.
Because of the high particle fluxes encountered at Jupiter and
Saturn, greater reliance had to be placed on counting rates in
single detectors and various coincidence rates. In
interplanetary space, guard counters are placed in
anticoincidence with the primary detectors to reduce the
background from high-energy particles penetrating through the
sides of the telescopes. These guard counters were turned off
in the Jovian magnetosphere when the accidental anticoincidence
rate became high enough to block a substantial fraction of the
desired counts. Fortunately, under these conditions the
spectra were sufficiently soft that the background, due to
penetrating particles, was small.
The data on proton and ion fluxes at Jupiter were obtained with
the LET. The thicknesses of individual solid-state detectors
in the LET and their trigger thresholds were chosen such that,
even in the Jovian magnetosphere, electrons made, at most, a
very minor contribution to the proton counting rates
[LUPTON&STONE1972;]. Dead time corrections and accidental
coincidences were small (< 20%) throughout most of the
magnetotail, but were substantial (> 50%) at flux maxima within
40 R[J] Of Jupiter. Data have been included in this package
for those periods when the corrections are less than ~ 50% and
can be corrected by the user with the dead time appropriate to
the detector (2 to 25 [micro]sec). The high counting rates,
however, caused some baseline shift which may have raised
proton thresholds significantly. In the inner magnetosphere,
the L counting rate was still useful because it never rolled
over. This rate is due to 1.8- to 13-MeV protons penetrating
L (0.43 cm^2 ster) and > 9-MeV protons penetrating the
shield (8.4 cm^2 ster). For an E^-2 spectrum, the two groups
would make comparable contributions; but in the magnetosphere,
for the E^-3 to E^-4 spectrum above 2.5 MeV [MCDONALDETAL1979],
the contribution from protons penetrating the shield would be
only 3 to 14%.
The LET LLL and LLL coincidence-
anticoincidence rates give the proton flux between 1.8 and 8
MeV and 3 to 8 MeV with a small alpha particle contribution (~
10^-3). Corrections are required for dead time losses in L,
accidental LL coincidences and anticoincidence losses
from L. Data are given only for periods when these
corrections are relatively small. The energy lost in detectors
L, L and L was measured for individual particles. For
protons, this covered the energy range from 0.42 to 8.3 MeV.
Protons can be identified positively by the [Delta]E vs. E
technique, their spectra obtained and accidental coincidences
greatly reduced. Because of telemetry limitations, however,
only a small fraction of the events could be transmitted, and
statistics become poor unless pulse-height data are averaged
over a period of one hour.
HET and LET detectors share the same data lines and pulse-
height analyzers; thus, the telescopes can interfere with one
another during periods of high counting rates. To prevent such
an interference and explore different coincidence conditions,
the experiment was cycled through four operating modes, each
192 seconds long. Either the HETs or the LETs were turned on
at a time. LET-D was cycled through L only and LL
coincidence requirements. The TET was cycled through various
coincidence conditions, including singles from the front
detectors. At the expense of some time resolution, this
procedure permitted us to obtain significant data in the outer
magnetosphere and excellent data during the long passage
through the magnetotail region. Some of the published results
from this experiment required extensive corrections for dead
time, accidental coincidences and anticoincidences
([VOGTETAL1979A], [VOGTETAL1979B]; [SCHARDTETAL1981];
[GEHRELSETAL1981]). These corrections can be applied only on a
case-by-case basis after a careful study of the environment and
many self-consistency checks. They cannot be applied on a
systematic basis and we have no computer programs to do so;
therefore, data from such periods are not included in the Data
Center submission. The scientists on the CRS team will,
however, be glad to consider special requests if the desired
information can be extracted from the data.
Principal Investigator: R.E. Vogt
The preceding section on instrumentation has been extracted
from the NSSDC documentation for the Voyager Cosmic Ray
Subsystem (Reference_ID = NSSDCCRS1979).
Gehrels, N., E.C. Stone, and J.H. Trainor, Energetic oxygen and sulfur in theJovian magnetosphere, J. Geophys. Res., 86, 8906, 1981.
Lupton, J.E., and E.C. Stone, Measurement of Electron DetectionEfficiencies in Solid-state Detectors, Nucl. Instr. and Meth. 98, 189,1972.
McDonald, F.B., A.W. Schardt, and J.H. Trainor, Energetic Protons in theJovian Magnetosphere, J. Geophys. Res. 84, 2579, 1979.
Data and instrument description document provided by the NSSDC for dataset 77-048A-08A, Voyager 1 and 2 Cosmic Ray Subsystem Description ofJupiter Encounter Data. 1979.
Schardt, A.W., F.B. McDonald, and J.H. Trainor, Energetic Particles in thePre-dawn Magnetotail of Jupiter, J. Geophys. Res., special Voyager issue,1981.
Stilwell, D.E., W.D. Davis, R.M. Joyce, F.B. McDonald, J.H. Trainor, W.E.Althouse, A.C. Cummings, T.L. Garrard, E.C. Stone, and R.E. Vogt, The VoyagerCosmic Ray Experiment, IEEE Trans. on Nuclear Science, Vol. 26, p. 513, 1979.
Stone, E.C., R.E. Vogt, F.B. McDonald, B.J. Teegarden, J.H. Trainor, J.R.Jokipii, and W.R. Webber, Cosmic ray investigation for the Voyager missions;energetic particle studies in the outer heliosphere--and beyond, Space Sci.Rev., 12, No. 3, 355-376, Dec. 1977.
Vogt, R.E., W.R. Cook, A.C. Cummings, T.L. Garrard, N. Gehrels, E.C. Stone,J.H. Trainor, A.W. Schardt, T. Conlon, N. Lal, and F.B. McDonald, Voyager 1:Energetic Ions and Electrons in the Jovian Magnetosphere, Science, 204, 1003,1979.