Description of the PorTL System
The PorTL dosimeter cell consists of crystal of radiation-sensitive thermoluminescent material mounted inside a sturdy, light-weight cylindrical metal cartridge. A memory chip inside each PorTL cell uniquely identifies the cell to the PorTL reader and also contains calibration data specific to that specific cell. Each PorTL cell weighs about 25 grams (less than 1 oz.) and measures 14 mm (0.55”) in diameter by 69 mm (2.7”) in length. One end of the PorTL cell is printed with the unique number identifying the individual cell, while the other end of the cell contains the electrical contacts needed to interface with the PorTL reader.
The PorTL reader contains all the hardware required to read out, analyze, and anneal the PorTL dosimeter cells. The PorTL reader weighs 2.6-3.2 kg (5.7-7.1 lbs., depending on battery capacity) and measures 210 mm (8.3”) x 170 mm (6.7”) x 80 mm (3.2”). The reader is powered by a rechargable battery and is capable of approximately 100 readouts between charges. Charging of the battery is by a standard power supply unit (provided with the reader) or by optional DC to DC converters for use with vehicles or on board aircraft. A total of 1920 data records (data from individual PorTL cell readouts) can be stored in the flash memory of the reader. Dose information for each readout is displayed on a 192 x 64 pixel graphic LCD screen on the front of the reader. The PorTL reader is controlled by microprocessor and control of the various options available in the reader is carried out through use of 6 push buttons on the front of the reader and a built-in, easy to use menu system.
The PorTL cell is read out by connecting on end of the cell to a special key and then inserting the other end of the cell into the readout chamber on the front of the reader.
The reader provides the electric power needed to heat the TL material thought the special key, while the light emitted by the TL material as it is heated is measured directly inside the readout chamber of the reader by means of a photomultiplier tube (PMT). As with all TLD systems, the amount of light emitted by the TL material during heating is proportional to the radiation dose to which the cell was exposed. The measured radiation dose, the unique identifier of the specific PorTL cell being read out, and the time and date of the PorTL cell readout are displayed on the screen of the PorTL reader. This information, plus more specialized information such as the TL glow curve, are recorded in the flash memory of the reader. The heating process also anneals the PorTL TLD cell, effectively resetting it to zero so that it is ready for immediate reuse. The complete record of dose information, times and dates of readouts, and the glow curve information for the individual readouts from each PorTL cell processed by the reader can then be downloaded via a standard serial port connection to a desktop or laptop PC for further analysis or addition to an archive database.
Currently, Al2O3:C, 6LiF:Mg,Ti and 7LiF:Mg,Ti are used as the active TL material in the PorTL detector cells, but future versions will use other TL materials as well.