NASA Langley Research Center
Hampton, Virginia

Induced Radioactivity

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The induced radioactivity is produced by several sources of particle fluxes: galactic protons, trapped Van Allen protons (encountered in the South Atlantic Anomaly and accounting for the bulk of the activity), atmospheric and secondary neutrons, and to a small extent heavier ions. All of these particles induce radioactivity by colliding with a stable nuclide in the spacecraft material, and occasionally forming a radioactive nuclide. If its half-life is long enough, it can be detected in the laboratory following retrieval. The sources of external radiation flux, the nuclear reactions with the spacecraft material, and the spacecraft geometry can be combined, in principle, into a model to predict the experimentally measured activities.

The LDEF spacecraft structure and experiment materials acquired a low ( ~1 to ~100 pico Curies per kg) level of radioactivity. The 69 months of exposure caused the radioactivity induced by trapped protons and cosmic rays in aluminum, stainless steel, titanium, lead and other metals of the spacecraft structure to reach significant levels. The distribution of the induced radionuclides in samples of the LDEF structure, measured in sensitive gamma ray spectrosocpy facilities, allows significant additional studies of the radiation environment and its interaction with the spacecraft. The initial activation measurements were made of the full spacecraft (between 2 weeks and 2 months of LDEF recovery) with a cooled germanium detector array at Kennedy Space Center. Subsequently, about 400 samples of the metal structure of LEF (and some experiment samples) have been measured in shielded low background spectrometers at nine laboratories.

The study of induced radioactivity in spacecraft materials, and methods to model it, are of strong interest in gamma ray astronomy where the local background is often a limiting facting in detector sensitivity. A few nuclear transmutations occur principally by neutron absorption, and these may be used to study the secondary neutron flux in the presence of the large primary proton flux. The induced radioactivity, when measured carefully, converted to a specific activity and compared to calculations from environment models serves as a "dosimeter" separate from techniques such as thermoluminescence dosimetry (TLD). The induced activity allows extensive radiation mapping in the structure, an independent comparison with experiment dosimetric techniques, and significant studies of secondary effects. This data set will be a definitive benchmark for methods that are used to calculate activation in space.

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