A low energy proton spectrometer for directional distribution measurements in space.

Measurements of the directional distribution of charged particles are at present considered as a powerful tool to study particle propagation in interplanetary space and the physical processes involved. This paper describes the design and operational characteristics of an instrument capable of measuring accurately three-dimensional proton flux anisotropies and their energy dependence in the energy range 35-1600 keV. The instrument provides 180 independent values of the proton distribution function in momentum space every 16 s. For the detection of protons, silicon surface barrier detectors are employed. The detectors operate in the temperature range -35 C to -15 C to reduce their noise width. The elimination of 'instrumental count rate modulations' in a multiple telescope instrument is discussed in some detail. Such effects are readily induced at proton energies below 100 keV, in particular if the differences in dead layer of the detectors in the particle telescopes are not sufficiently known. The instrument is flown aboard the NASA ISEE-C spacecraft which was launched on August 12, 1978.