Extremity TLDs are small discs of LiF which are sealed in plastic holders. These are worn on the fingers or taped to the ankles to measure the external equivalent dose to the extremities i.e. hands, forearms, feet and ankles. They are made of LiF powder spread very thinly into a plastic wafer so that they may be worn comfortably.
Extremity TLDs are designed to measure beta radiation
at a depth equivalent to
that of the sensitive basal layer of the skin (about 0.3 mm). The holder,
either a ring or a finger stall, shields the TLD from the lower energy betas
(70 keV and less) which can not penetrate to the basal layer. Gamma
radiation
is also recorded by extremity TLDs, and delivers a dose to the bones of the
extremities in addition to the skin dose .
The danger from a high skin dose,
and above, is that it may lead to burns which are slow
to heal.