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What is TLD – Principle of Operation – Definition

TLD – Principle of Operation The following basic overview explains how a thermoluminescent dosimeter works: When ionizing radiation passes through the detector (chip), the chip absorbs the radiation and its structure changes slightly. Radiation Dosimetry

A thermoluminescent dosimeter, abbreviated as TLD,  is a passive radiation dosimeter, that measures ionizing radiation exposure by measuring the intensity of visible light emitted from a sensitive crystal in the detector when the crystal is heated. The intensity of light emitted is measure by TLD reader and it is dependent upon the radiation exposure. Thermoluminescent dosimeters was invented in 1954 by Professor Farrington Daniels of the University of Wisconsin-Madison. TLD dosimeters are applicable to situations where real-time information is not needed, but precise accumulated dose monitoring records are desired for comparison to field measurements or for assessing the potential for long term health effects. In dosimetry, both the quartz fiber and film badge types are being superseded by TLDs and EPDs (Electronic Personal Dosimeter).

TLD – Principle of Operation

The following basic overview explains how a TLD works:

  1. When ionizing radiation passes through the detector (chip), the chip absorbs the radiation and its structure changes slightly.
  2. In thermoluminescent materials, electrons may reach the conduction band, when they are excited, for example, by ionizing radiation (i.e. they must obtain energy higher than Egap).  But in this case, defects in the material exist or impurities are added to trap electrons in the band gap and hold them there.
  3. These trapped electrons represent stored energy for the time that the electrons are held and the amount of this energy is dependent upon the radiation exposure.
  4. In order to obtain the dose received, the TLD chip must be heated in this TLD reader. The trapped electrons return to the ground state and emit photons of visible light. The amount of light emitted relative to the temperature is called the glow curve.
  5. After the readout is complete, the TLD is annealed at a high temperature. This process essentially zeroes the TL material by releasing all trapped electrons. The TLD is then ready for reuse.

TLD Reader

As was written, previously absorbed energy from electromagnetic radiation or other ionizing radiation in these materials is re-emitted as light upon heating of the material. The intensity of light emitted is measure by TLD reader and it is dependent upon the radiation exposure.  A typical basic TLD reader contains the following components:

  • Heater. Heater raises the temperature of the TL material
  • Photomultiplier tube. PMT amplifies and measures the light output.
  • Meter/Recorder. Recorder is able to display and record data.
Glow Curve - TLD Reader
Glow Curve Source: Dosimetry. Study Guide for Radiological control Technician. DOE-HDBK-1122-99. Department of Energy

In order to obtain the dose received, the TLD chip must be heated in this TLD reader. The trapped electrons return to the ground state and emit photons of visible light. The amount of light emitted relative to the temperature is called the glow curve. This curve is analyzed to determine the dose. After the readout is complete, the TLD is annealed at a high temperature. This process essentially zeroes the TL material by releasing all trapped electrons. The TLD is then ready for reuse. There are two types of readers. Automatic, and manual readers. The automatic TLD reader is a lot more complicated than it might expected.

References:

Radiation Protection:

  1. Knoll, Glenn F., Radiation Detection and Measurement 4th Edition, Wiley, 8/2010. ISBN-13: 978-0470131480.
  2. Stabin, Michael G., Radiation Protection and Dosimetry: An Introduction to Health Physics, Springer, 10/2010. ISBN-13: 978-1441923912.
  3. Martin, James E., Physics for Radiation Protection 3rd Edition, Wiley-VCH, 4/2013. ISBN-13: 978-3527411764.
  4. U.S.NRC, NUCLEAR REACTOR CONCEPTS
  5. U.S. Department of Energy, Instrumantation and Control. DOE Fundamentals Handbook, Volume 2 of 2. June 1992.

Nuclear and Reactor Physics:

  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
  9. Paul Reuss, Neutron Physics. EDP Sciences, 2008. ISBN: 978-2759800414.

See also:

TLD

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