Thermoluminescent detectors (TLDs) are inorganic crystalline materials that record the total absorbed dose from ionizing radiation. As charged particles pass through the TLD they lose energy by ionizing (displacing electrons to a higher energy state) atoms of the crystal. When the electrons displaced by the charged particles attempt to return to their original state they become trapped in an area called the "electron trap". As the radiation dose to which the TLD is exposed to accumulates, so does the quantity of trapped electrons. The total absorbed dose is determined using a TLD reader. The TLD reader heats the TLD, which pushes the electrons out of their trap and back to a lower state of energy. This process releases energy and gives off visible light, which is measured by the TLD reader. The amount of light given off as the TLD is heated is proportional to the total absorbed radiation dose accumulated by the TLD over the duration of its exposure.
Because TLDs do not record any linear energy transfer (LET) information from charged particles, it is not possible to determine dose equivalent using TLDs alone. TLDs also record the dose from high-LET (heavy ion) particles with decreased efficiency compared to the dose from lower LET particles such as x-rays, gamma rays, electrons, and high-energy protons. When a heavy ion traverses a TLD, electron traps immediately surrounding the particle's trajectory quickly become saturated. Thus some of the dose deposited by the heavy ion is not stored as trapped signal and the dose read out from the TLD will be systematically too low. The degree to which the TLD under measures the dose from the heavy ions is proportional to the LET of the heavy ions. To accurately measure high-LET charged particles and to measure the LET spectrum needed to determine dose equivalent, TLDs are augmented by CR-39 PNTDs.
CR-39 plastic nuclear track detectors (PNTDs) are made of polymers that are sensitive to charged particles of LET greater than or equal to 5 keV/ micrometer. CR-39 PNTDs are usually made up of thin (approximately 0.6 mm) sheets which are assembled in multi-layer stacks exposed in three mutually-orthogonal orientations. Charged particles that pass through the CR-39 PNTD with enough LET to break the molecular bonds of the polymer form a pathway known as the "latent damage trail". As the amount of radiation the CR-39 PNTD is exposed to increases, so does the number of latent damage trails.
The CR-39 PNTDs are processed using a technique called etching. The CR-39 PNTDs are soaked in a bath of strongly alkaline solution (NaOH), for a predetermined length of time. The solution dissolves the highly reactive latent damage trails faster then the rest of the polymer surface. This produces etched pits at the location of the latent damage trails and the detector surface. The pits, which are the nuclear tracks, appear as dark cones when viewed using an optical microscope. The size of a nuclear track, when normalized to the quantity of bulk detector material removed during the etching process, is proportional to the LET of the charged particle that produced the original latent damage trail.
Since CR-39 PNTDs exposed in space contain nuclear tracks from charged particles possessing a wide range of LETs, it is the LET fluence spectrum, the number of particles per unit area and solid angle as a function of LET, which is usually measured. The measurement of the LET spectrum in a layer of CR-39 PNTD consists of first measuring all the nuclear tracks within a given area on the detector surface and then calculating the LET of the particles that produced these tracks. A number of corrections to the measured data are then made to compensate for the directional sensitivity of the detector.