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Trapped Radiation Detector

NSSDCA ID: 1964-077A-05

Mission Name: Mariner 4
Principal Investigator:Prof. James A. Van Allen


A detector composed of three Geiger-Mueller (GM) tubes and a solid state device was used to detect low energy cosmic rays, protons, and electrons while the spacecraft journeyed to and passed Mars. The spacecraft was launched on November 28, 1964 into a heliocentric orbit. It encountered micrometeoroids and transmission from the satellite ended December 20, 1967.

The Trapped Radiation Detector (TRD) experiment was designed to search for magnetically trapped charged particles in the vicinity of Mars and to monitor the occurrence of solar cosmic rays and energetic electrons in interplanetary space, in order to study several scientific questions, including: 1) The magnitude and orientation of the magnetic moment of Mars; 2) The radial extent of the atmosphere of Mars; 3) Dilineation of the possibilities for aurorae and magnetic storms on Mars; 4) The interaction of the solar plasma with the magnetosphere, if any; 5) The relationship between solar phenomena and the emission of energetic particles; 6) The propogation of charges particles in interplnetary space; and 7) The relationship of the occurrence of energetic paricles in interplanetary space to solar and geophysical effects.

The TRD comprised three end-window Geiger-Mueller (GM) detectors, designated A, B, and C, and one 35-micron surface-barrier solid-state detector with two discrimination levels, designated D1 and D2. The instrument itself, a rectangular box, roughly 14 cm x 13 cm x 7 cm with 4 tubes protruding from one side, had a mass of about 1.2 kg. The four tubes were the conical collimators for the four detectors, and were set at angles to the side of the box. The instrument was mounted so that the axis of the B, C, and D collimators, all parallel, were at an angle of 70 degrees to the spacecraft roll angle, and the A collimator was at an angle of 135 degrees. The roll axis was maintained within 1 degree of direct pointing at the Sun, so collimators B,C, and D pointed 70 degrees off the sunward direction, and collimator A pointed 45 degrees in the anti-sunward direction. The full vertex angle of the collimators was 60 degrees, so particles would be detected within +- 30 degrees of the angles listed above. The sidewall shielding had a minimum thickness which would prevent the penetration of ~50 MeV protons. A nickel foil cover, with an air-equivalent thickness for alpha particles of 0.22 mg/square cm, was placed in front of the detector to shield against sunlight.

The A, B, and C detectors were 6213 GM counters with a dynamic range of 0.6 to 10,000,000 counts/sec and an omnidirectional geometric factor of 0.15 square cm. The A detector was sensitive to electrons with energies greater than approximately 45 keV and protons greater than 670 +- 30 keV. It had a unidirectional geometric factor of 0.044 +- 0.005 square cm steradian. Counter B was sensitive to electrons (~> 40 keV) and protons (> 550 +- 20 keV) with a unidirectional geometric factor of 0.055 +- 0.005 square cm steradian. Counter C was sensitive to electrons (~> 150 keV) and protons (> 3.1 MeV) and had a unidirectional geometric factor of 0.050 +- 0.005 square cm steradian. The counting rate of each GM tube was the sum of the rayes due to galactic cosmic rays (~0.6 counts/sec), electrons, X-rays, protons, alpha particles, etc. which passed through the collimators and in some instances to sidewall penetrations.

The D detector was designed to measure only protons and had two discrimination levels. Level D1 (lower discriminator) was sensitive to protons of energy 0.50 to 11.0 MeV and D2 (upper discriminator) could detect protons from 0.88 to 4.0 MeV. The detector had an in-flight calibration source of 95 Americium 241 which produced 5.477 MeV alpha particles at 0.071 counts/sec on D1 and 0.059 counts/sec on D2. The dynamic range of detector D was from this in-flight source rate up to 1,000,000 counts/sec. It had a unidirectional geometric factor of 0.065 +- 0.003 square cm steradian.

The output pulses from the GM counters are sent to an amplifier, a complementary NPN-PNP saturating device. From there, the output is sent to the data automation subsystem (DAS). A charged particle entering the solid state detector releases a number of electrons proportional to the energy of the particle lost in the detector. The total charge of the released electrons and the detector capacitance gives the magnitude of the output voltage pulse. The pulses go to preamplifiers, stable negative feedback voltage amplifiers, and then to discriminators, which reject all pulses under 1.4 V. An output and rate limiter circuit based on a monostable multivibrator limits the maximum counting rate to 50,000 counts/sec. The output is then passed to the DAS.

Alternate Names

  • Low Energy Cosmic Rays
  • Mariner4/LowEnergyCosmicRays
  • Solar Cosmic Rays and Energetic Electrons

Facts in Brief

Mass: 1.2 kg
Power (avg): 0.4 W


  • Planetary Science: Fields and Particles
  • Space Physics: Heliospheric Studies

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Mr. H. KielGeneral ContactUniversity of Iowa
Prof. Louis A. FrankOther InvestigatorUniversity of
Dr. Stamatios M. KrimigisOther InvestigatorApplied Physics
Prof. James A. Van AllenPrincipal InvestigatorUniversity of Iowa

Selected References

  • Van Allen, J. A., and S. M. Krimigis, Impulsive emission of 40-keV electrons from the sun, J. Geophys. Res., 70, 5737-5751, doi:10.1029/JZ070i023p05737, Dec. 1965.
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