exposure from radiations of different types and energies. Filters of metals
of different densities stop different energy radiations, thus discriminating
exposures from them. After exposure the optical density of the developed
film is measured by a densitometer and compared with that of a calibrated
film exposed to known radiation. Film badges are usually changed monthly
for each radiation worker in most institutions. Film badges provide an inte-
gral dose and a permanent record. The main disadvantage of the film badge
is the long waiting period (a month) before the exposed personnel know
about their exposure. The film badge also tends to develop fog resulting
from heat and humidity, particularly when in storage for a long time, and
this may obscure the actual exposure reading. The film badges of all workers
are normally sent to a commercial firm that develops and reads the density
of the films and sends back the report of exposure to the institution. The
commercial firm must be approved by the National Voluntary Laboratory
Accredition Program (NVLAP) of the National Institute of Standards and
Technology.
Thermoluminescent Dosimeter
A thermoluminescent dosimeter (TLD) consists of inorganic crystals
(chips) such as lithium fluoride (LiF) and manganese-activated calcium flu-
oride (CaF 2 : Mn) held in holders like the film badges and plastic rings. When
these crystals are exposed to radiation, electrons from the valence band are
excited and trapped by the impurities in the forbidden band. If the radia-
tion-exposed crystal is heated to 300°C to 400°C, the trapped electrons are
raised to the conduction band; they then fall back into the valence band,
emitting light. The amount of light emitted is proportional to the amount
of radiation absorbed in the TLD. The amount of light is measured and read
278 16. Radiation Regulations and Protection
Fig. 16.2.(A)Film badge holder.(B)Film badge.