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 In dosimetry, both the quartz fiber and film badge types are being superseded by TLDs and EPDs (Electronic Personal Dosimeter).
Neutron Thermoluminescent Dosimeter – Neutron TLD
The personnel neutron dosimetry continues to be one of the problems in the field of radiation protection, as no single method provides the combination of energy response, sensitivity, orientation dependence characteristics and accuracy necessary to meet the needs of a personnel dosimeter.
The most commonly used personnel neutron dosimeters for radiation protection purposes are thermoluminescent dosimeters and albedo dosimeters. Both are based on this phenomenon – thermoluminescence. For this purpose, lithium fluoride (LiF) as sensitive material (chip) is widely used. Lithium fluoride TLD is used for gamma and neutron exposure (indirectly, using the Li-6 (n,alpha)) nuclear reaction. Small crystals of LiF (lithium fluoride) are the most common TLD dosimeters since they have the same absorption properties as soft tissue. Lithium has two stable isotopes, lithium-6 (7.4 %) and lithium-7 (92.6 %). Li-6 is the isotope sensitive to neutrons. In order to record neutrons, LiF crystal dosimeters may be enriched in lithium-6 to enhance the lithium-6 (n,alpha) nuclear reaction. The efficiency of the detector depends on the energy of the neutrons. Because the interaction of neutrons with any element is highly dependent on energy, making a dosimeter independent of the energy of neutrons is very difficult. In order to separate thermal neutrons and photons, LiF dosimeters are mostly utilized, containing different percentage of lithium-6. LiF chip enriched in lithium-6, which is very sensitive to thermal neutrons and LiF chip containing very little of lithium-6, which has a negligible neutron response.
The principle of neutron TLDs is then similar as for gamma radiation TLDs. In the LiF chip, there are impurities (e.g. manganese or magnesium), which produce trap states for energetic electrons. The impurity causes traps in the crystalline lattice where, following irradiation (to alpha radiation), electrons are held. When the crystal is warmed, the trapped electrons are released and light is emitted. The amount of light is related to the dose of radiation received by the crystal.
Thermoluminiscent Albedo Neutron Dosimeter
Albedo neutron dosimetry is based on the effect of moderation and backscattering of neutrons by the human body. Albedo, the latin word for “whiteness”, was defined by Lambert as the fraction of the incident light reflected diffusely by a surface. Moderation and backscattering of neutrons by the human body creates a neutron flux at the body surface in the thermal and intermediate energy range. These backscattered neutrons called albedo neutrons, can be detected by a dosimeter (usually a LiF TLD chip), placed on the body which is designed to detect thermal neutrons. Albedo dosimeters have been found to be the only dosimeters which can measure doses due to neutrons over the whole range of energies. Usually, two types of lithium fluoride are used to separate doses contributed by gamma-rays and neutrons. LiF chip enriched in lithium-6, which is very sensitive to thermal neutrons and LiF chip containing very little of lithium-6, which has a negligible neutron response.
TLD – Principle of Operation
The following basic overview explains how a TLD works:
- When ionizing radiation passes through the detector (chip), the chip absorbs the radiation and its structure changes slightly.
- 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.
- 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.
- 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.
- 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.
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.
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.