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Environmental Dosimeters (TLDs)

Environmental Dosimeters, commonly called thermoluminescent dosimeters (TLDs), are used to measure ionizing radiation exposures at offsite locations. The TLDs measure ionizing radiation exposures from all sources, including natural radioactivity, cosmic radiation, fallout from nuclear weapons testing, radioactivity from fossil fuel burning, and radioactive effluents from INL operations and other industrial processes.

Direct penetrating gamma radiation cannot be collected by filters or chemically trapped in any media, but is directly measured using TLDs. The principle of TLD technology is that when certain crystals are exposed to penetrating gamma radiation, impurities are excited to high energy states and remain in these states at normal ambient temperature. When the TLDs are heated, electrons are released and the crystal returns to the lower state of energy. The released electrons are in the form of photon energy, which is measured with a photomultiplier tube; the light intensity is proportional to the absorbed dose of radiation.

Dosimeters are placed at 14 locations on the site perimeter and at more distant locations. If site operations were contributing significantly to the external radiation dose, the dosimeters at the site perimeter would show a higher dose that those at more distant locations.

At each location, a dosimeter card containing five individual chips is placed one meter above the ground. Dosimeters are changed twice per year.

ESER Offsite Dosimeter Locations

Other INL Surveillance Agencies