Optical Calibration of TLD Readers
Trident Scholar project rept.
NAVAL ACADEMY ANNAPOLIS MD
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The Navy uses thermoluminescent dosimeters TLDs on all of its nuclear warships. TLDs measure the amount of radiation a person receives through the use of a small internal crystal. Electrons in the crystal are excited to a higher energy state when they absorb radiation. If the crystal is later taken out of the TLD and heated to a high temperature, the electrons drop back to the lower energy state, emitting light. A TLD reader determines the radiation exposure by measuring the emitted optical energy. Presently, TLD readers need to be calibrated frequently to ensure the measurements are accurate. A reference light source is used to calibrate the output of the reader. However, the power emitted by this light source is not stable over time, resulting in poor calibration. For this project, four light sources were compared as possible alternatives. The sources are a commercial light emitting diode, a commercial laser diode, a scintillating C-14 doped radioactive source, and a tritium source. The first part of this project was to determine the cause of the instability in the output of the TLD reader. This included understanding the effects of environmental temperature variations on both the reader components and light sources. Consequently a temperature data acquisition system DAQ was developed to record the time variation of these temperatures. A second DAQ was then implemented to measure the optical power stability of each light source. After the power stability of each source was measured, the stability of the photomultiplier tube output in the reader was verified. Finally, since the Navy prefers to eliminate the use of unnecessary radioactive sources, various methods of guiding the light from the diode sources to the TLD reader using optical fiber were considered. The research determined that the Ocean Optics LED and the C-14 source are relatively