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Cosmic-Ray Detector

NSSDCA ID: 1958-001A-01

Mission Name: Explorer 1
Principal Investigator:Prof. James A. Van Allen


An Anton 314 omnidirectional Geiger tube detector was used to measure the flux of energetic charged particles (protons E>30 MeV and electrons E>3 MeV). The detector was saturated much of the time. The experiment performed normally until March 16, 1958, at which time the batteries powering the Geiger tube circuits became exhausted. No usable data were received after that time. The experiment resulted in the discovery of the first of the zones of energetic charged particles trapped around the Earth, later named the Van Allen radiation belts after the Principal Investigator.

The instrument consisted of a single Geiger-Mueller tube, a scaling circuit to reduce the number of pulses, and a telemetry system to transmit the data to ground receiving stations. The Geiger-Mueller tube was a type 314 Anton halogen quenched counter with stainless steel (approximately 75% iron, 25% chromium) wall of approximately 0.12 cm thickness. The instrument was mounted within the spacecraft hull, which had 0.023 inch thick stainless steel walls. The counter was 10.2 cm long by 2.0 cm diameter and the internal wire was 10 cm in length. The tube had a very small variation in counting efficiency over the range -55 to +175 Celsius. It had approximately 85% counting efficiency for cosmic rays, and about 0.3% counting efficiency for photons of energy 660 keV. The "dead time" (time to reset to record the next count) of the counters was about 100 microseconds.

The counter was connected to a current amplifier, which directly fed a scaler stage, a bistable transistor multivibrator that could operate over a wide range of voltages and a temperature range of -15 to +85 Celsius, limited primarily by the supply batteries. The scaler resolving time was 250 microseconds. For pulse counts higher than 4000 per second, the scaler indicated a count of 4000. Results were sent to the ground through the telemetry system in real time. The experiment had no onboard data storage device, and Explorer 1 could only send telemetry to the ground when it was passing over an Earth receiving station, so some regions had no coverage during the flight.

Alternate Names

  • Explorer1/Cosmic-RayDetector

Funding Agency

  • NASA-Office of Space Science (United States)


  • Space Physics: Magnetospheric Studies

Additional Information

Questions and comments about this experiment can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Dr. George H. LudwigOther InvestigatorNOAA Environmental Research Laboratories
Prof. James A. Van AllenPrincipal InvestigatorUniversity of Iowa
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