Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 25

2.8.4 Film Badge Dosimeter

The film badge dosimeter, or film badge, is a dosimeter used for monitoring cumulative
exposure to ionizing radiation. The badge consists of two parts: photographic film, and
a holder.
The film is sensitive to radiation and, once developed, exposed areas increase in
optical density (i.e., blacken) in response to incident radiation. One badge may contain
several films of different sensitivities or, more usually, a single film with multiple
emulsion coatings. This allows for separate measurement of neutron, beta, and gamma
exposure, and estimation of energy spectra. The holder may contain a number of filters
that attenuate certain types of radiation, such that only the target radiation is monitored.
To monitor gamma rays or X-rays, the filters are metal, usually tin or lead. To monitor
beta particle emission, the filters use various densities of plastic.

2.8.5 Thermoluminescent Dosimeter

A thermoluminescent dosimeter, or TLD, is a type of radiation dosimeter. A TLD
measures ionizing radiation exposure by measuring the amount of visible light
emitted from a crystal in the detector when the crystal is heated. The amount of
light emitted is dependent upon the radiation exposure.
A TLD is a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF),
in a solid crystal structure. When a TLD is exposed to ionizing radiation at ambient
temperatures, the radiation interacts with the phosphor crystal and deposits all or
part of the incident energy in that material. Some of the atoms in the material that
absorb that energy become ionized, producing free electrons and areas lacking one
or more electrons, called holes. Imperfections in the crystal lattice structure act as
sites where free electrons can become trapped and locked into place.
Heating the crystal causes the crystal lattice to vibrate, releasing the trapped elec-
trons in the process. Released electrons return to the original ground state, releasing
the captured energy from ionization as light, hence the name thermoluminescent.
Released light is counted using photomultiplier tubes and the number of photons
counted is proportional to the quantity of radiation striking the phosphor.
Instead of reading the optical density (blackness) of a film, as is done with film
badges, the amount of light released versus the heating of the individual pieces of
thermoluminescent material is measured.

2.9 Medical Sensing Devices

This topic puts a special emphasis on medical applications of wireless sensor net-
works. Therefore, a section of this chapter is devoted to devices used for monitoring
human vital parameters.