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
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 in
May and again in November after six months in the field.