PRESENTATION OF thermoluminescence dosimetric properties

This presentation provides an overview of thermoluminescence dosimeters (TLDs), which are used in personal dosimetry to measure radiation exposure. TLDs work by storing energy from ionizing radiation in crystals and releasing it as light when heated, which is then measured by a TLD reader. The reader heats the dosimeter, collects the emitted light, and converts it into an electrical signal. The presentation emphasizes the importance of proper setup and thermal treatment in TL dosimetry, highlighting its versatility in medical and industrial applications.

1. Thermoluminescence dosimeter (TLDs) Dalal P hysics Department J azan University Jazan University, Physics Department, Date:

2. Luminescence – terminology Some materials have the ability to absorb a certain type of energy, store a part of it for a period of time and dissipate it in the form of optical radiation. These materials are called luminescent materials and the process is called luminescence. Depending on the means by which the luminescent material receives the excitation energy, a prefix is added to the term luminescence

3. There is an exception to that rule causing confusion and misunderstanding about the luminescent phenomenon. This is the case when the excitation energy is heat. A luminescent effect may be characterized as fluorescence or phosphorescence depending on the time elapsing after excitation until the emission of the optical radiation regardless of the means of excitation. Excitation occurs when a bound electron absorbs energy and is displaced to a higher energy state (excitation state). If the electron returns promptly either directly or via an allowed transition from an intermediate (relaxation) state to the initial energy state (ground state) with the excess in energy emitted as optical radiation, the process is called fluorescence.

4. If there is an allowed energy state (metastable state) in the forbidden energy gap and transition from this metastable state directly to the ground state (valence band) is not allowed, energy should be provided to electrons in this metastable state to reach an excitationstate (conduction band) and return to a ground state emitting the difference in energy as an optical photon. In such cases, with the return of the excited electrons to the valence band, consequently the emission of the optical photons is delayed by the metastable state and the process is called phosphorescence.

5. Thermoluminescence is a totally different process from incandescence. Light emission in incandescence results from the vigorous vibrations and collisions occurring in the heated material that lead to the excitation of all the atoms present. The emission spectrum in incandescence depends strongly on temperature, since the higher the temperature the more violent the vibrations and collisions are, similar to the black body emission spectrum at the same temperature. THERMOLUMINESCENCE

6. How TLDs work

7. Thermoluminescence dosimeters are routinely used in personal dosimetry. They make use of the ability of thermoluminescence crystals to store the energy of ionising radiation and release this energy again as light when the crystal is heated. A dosimeter along with its reader is referred to as a dosimetry system.

8. TLD READER The purpose of a TLD reader is to measure the output of thermoluminescent dosimeters after their irradiation. The reader must have the capability to controllably and reproducibly heat the TL dosimeters, collect the emitted light and measure the light output. The main components of a commonly used TLD reader (see figure) are the TL dosimeter heating system, the light collection system, the signal measuring system, and (d) display system. A commercially available TLD reader device is shown in figure. Heating system The primary purpose of the heating system in a TLD reader is to heat the TL sample (dosimeter) until the electrons are liberated from the dosimetry traps. In addition to this, it is sometimes used to fade rapidly the low-temperature glow peaks, by means of a pre-read anneal at low temperature. In cases when TLDs are used to measure high doses, TLD readers may be used to offer a high-temperature anneal, although usually this is performed by dedicated annealing ovens.

9. READER SYSTEM

10. Light Collection system The light collection system is responsible for the efficient collection of TL emitted from the heated TL dosimeters, rejecting if possible any other optical radiation (such as infrared radiation emitted from the heated tray, etc) and the conversion of the collected light to electrical signal. In all TLD readers the light collection system is based on a photomultiplier tube (PMT). PMTs make use of the phenomena of photoemission and secondary electron emission to detect very low light intensities. Photoemission is the ejection of electrons from a material as a result of exposure to optical radiation. The electrons emitted from the photocathode by incident light are accelerated and focused onto another emission surface. The PM tube acts as an electron multiplier, with 1–100 million electrons collected at the final electrode for each electron released from the photocathode.

11. Conclusion TL dosimetry is a versatile tool for the assessment of point doses and dose distributions in medicine and industry. However, considerable carE has to be taken in setting up a TLD measurement system. Of particular importance are thermal treatments of the TL material. Usage of different filters make possible to increase measurement range.

12. THANKS FOR YOUR KIND ATTENTION!