National Aeronautics and Space Administration

The Pioneer Venus Multiprobe was launched on 8 August 1978. It encountered Venus on 9 December 1978. It consisted of 5 separate probes: the probe transporter (referred to as the Bus), a large atmospheric entry probe (called Sounder), and 3 identical small probes (called North, Day, and Night). The Sounder released from the Bus on 15 November 1978; the 3 small probes released on 19 November 1978. All probes entered the Venusian atmosphere within 11 minutes of each other, and descended toward the surface over approximately an hour long period sending back data to the Earth.

The complete system, including connecting cables, weighed 2.9 kg and operated on 1 Watt of electrical power. The sensors were mounted at the periphery of the Orbiter spacecraft equipment platform, diametrically opposite each other. Thus, there was nearly uniform omnidirectional coverage. The sensors were actively guarded scintillation photon counters, each containing a 3.8 cm diameter x 3.2 cm long CsI scintillation crystal. The crystal was optically bound to a 0.5 cm shell of plastic scintillator. The composite scintillator (or phoswich) was passively shielded from low energy radiation by a jacket of 0.25mm lead foil and the sensor housing.

In the absence of a gamma ray event, the instrument operated in a real time background mode. These data included background count rates, spectra, state of health information, and source calibration. The background spectral information was taken in 4 energy channels. Events were determined by a statistically significant increase in counting rate. Since it is important to see the beginning of a burst, a pre-trigger memory was included. The data stream was continuously routed to the pretrigger memory, which kept it for about 3 seconds. In background mode, the oldest data were continuously replaced by newer data. However, once a trigger occurred, the contents of the pretrigger memory were transferred to the main memory. In this way, the onset of an event was recorded even though it occurred before the event was recognized by the system. The event data were accumulated into 5 energy channels. Channel 1 covered the energy range 0.1-2.0 MeV. Channels 2-5 were defined by thresholds at 0.1, 0.2, 0.5, 1.0,and 2.0 MeV. The normal accumulation time for Channel 1 was 11.7 ms, although 0.25 ms resolution was achievable.

The homogeneity of the burst source population in space was studied on the basis of a sample of 225 observed gamma-ray bursts between September 1978 and July 1988 from the Pioneer Venus Orbiter. The results were consistent with a uniform distribution in space of the parent population of burst sources.

A PVO gamma-ray burst catalog was published (Chuang 1990). This catalog gives information on the triggered events detected by the PVO instrument from 9/14/78 to 7/21/88.