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Space Applications: Radiation-Induced Effects

Radiation-induced spacecraft anomalies have been known since the Explorer I launch on January 31, 1958, when a Geiger counter put aboard by J.A. Van Allen suddenly stopped counting. It turned out that the counter was in fact saturated by an extremely high count rate. This led to the discovery of the Van Allen belts.

The inner belt, beginning at about 1,000 km above the surface of the Earth, contains primarily protons with energies between 10-100 MeV. The offset between Earth’s geographical and magnetic axes causes an asymmetry in the radiation belt above the Atlantic Ocean off the Brazilian coast, allowing the inner belt to reach a minimum altitude of 250 km. This "South Atlantic Anomaly" is important because it occupies a region through which low-orbiting satellites spend as much as 30% of their time. During a solar flare, which can happen anytime, the number of protons suddenly increases by more than a million.

The first spacecraft loss due to total radiation dose effects occurred unexpectedly in 1962. A satellite, Telstar, was launched just one day after a high altitude nuclear weapons test. This weapons test produced a large number of beta particles, which caused a new and very intense radiation belt that lasted until the early 1970s. Telstar and six other satellites were lost within a seven-month period after this weapons test. Telstar was well-studied and the loss was traced directly to breakdown of diodes in the command decoder due to the total radiation dose.

Radiation effects studies done at accelerators measure either total-dose effects, displacement damage, or single event effects (SEE). A SEE occurs when one ion passing through a semiconductor causes enough damage to upset the circuit in some way. In 1978, the first SEEs were observed at ground level when Intel Corporation discovered that anomalous upsets occurring in dynamic random access computer memories were being caused by alpha particles emitted from trace amounts of thorium and uranium in the materials from which the device’s packages were made. It was quickly found that massive ions, protons, and neutrons could all induce SEEs.

SEEs can result in the flip of a memory bit as in the case of upsets with little other effect on the circuit, or they can be more catastrophic, causing the chip to either latchup and have to be powered down to reset the circuit, or it might burn out altogether or rupture a gate.

  last updated: August 9, 2000 webmaster