Environmental dosimeters are used to measure
ionizing radiation exposures at offsite locations. The dosimeters
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 Site operations and other industrial
penetrating gamma radiation cannot be collected by filters or
chemically trapped in any media, but is directly measured using dosimeters.
ESER uses two types of dosimeters--thermoluminescent dosimeters (TLDs) and optically stimulated luminescent dosimeters (OSLDs).
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 and is reported in the unit of Roentgen.
TLDs have been used historically to measure exposures in air to ambient ionizing radiation. In November 2012, ESER began to collocate OSLDs with TLDs. The OSLDs are a relatively new technology. The method makes use of electrons trapped between the valence and conduction bands in the crystalline structure of certain minerals. The trapping sites are imperfections of the lattice. Radiation energy deposited in the material promotes electrons from the valence band to the conduction band. These electrons move to traps in the band gap. The greater the radiation energy absorbed (dose), the greater the number of trapped electrons. When it is time to assess the dose, the trapped electrons are freed by exposing the dosimeter to light. When the electrons are freed, they fall to a lower energy level and emit light photons. The intensity of the emitted light is measured and used to calculate the dose. The reported unit is mrem or ambient dose. Not all the electrons are freed from their traps. If the light output from the OSL dosimeter is analyzed over a short period of time, many electrons will remain trapped. This means that the dosimeter can be reread many times without a significant loss of signal. TLDs can only be read once.
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 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.