3/2003 Rev 1 II.3.7 – slide 1 of 40 Session II.3.7 IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources Part IIQuantities and Measurements Module 3Principles of Radiation Detection and Measurement Session 7Thermoluminescent Detectors

3/2003 Rev 1 II.3.7 – slide 2 of 40  Upon completion of this section, the student will be able to describe:  The band theory of materials including the conduction band, valence band, band gap, and the role of impurities; and,  The differences between thermoluminescent dosimetry (TLD) and optical stimulated luminesence (OSL) Overview

3/2003 Rev 1 II.3.7 – slide 3 of 40 TLD  The energy levels of electrons are called “shells”  The higher the shell, the higher the energy level of the electrons that are occupying the shell  For example, an electron in the L-shell has more energy than an electron in the K-shell, and less energy than an electron in the M- shell

3/2003 Rev 1 II.3.7 – slide 4 of 40 K- shell L- shell M- shell Energy Electron Shells

3/2003 Rev 1 II.3.7 – slide 5 of 40  When atoms are brought closer together, the highest shell occupied by electrons, the valence shell, splits  The greater the number of atoms, the greater the number of splitting Electron Shells

3/2003 Rev 1 II.3.7 – slide 6 of 40 Valence Shell Splitting e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e-

3/2003 Rev 1 II.3.7 – slide 7 of 40  When many atoms combine to form a solid, the valence shell has split so many times that it forms a continuum of energies, that is, a “band”  The highest energy band occupied by electrons is called the “valence band” Valence Shell Splitting

3/2003 Rev 1 II.3.7 – slide 8 of 40 Valence Shell Splitting e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- Valence Band

3/2003 Rev 1 II.3.7 – slide 9 of 40 Band Theory of Solids  The empty energy band above the valence band is called the “conduction band”

3/2003 Rev 1 II.3.7 – slide 10 of 40 Valence Shell Splitting e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- e-e-e-e- Valence Band Conduction Band

3/2003 Rev 1 II.3.7 – slide 11 of 40 Band Theory of Solids  The range of energies between the valence band and the conduction band is called the “band gap,” or “forbidden band”  In a pure material, electrons cannot possess energies in the band gap. If there are impurities or defects, they can.

3/2003 Rev 1 II.3.7 – slide 12 of 40 Band Theory e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band BandImpurity/Defect e-e-e-e-

3/2003 Rev 1 II.3.7 – slide 13 of 40 Band Theory  Insulator – the valence band in an insulator is full. For this reason the electrons are immobile and the material does not conduct electricity  Conductor – the valence band of a conductor is not full so electrons are mobile so the material conducts electricity  Semiconductor – the valence band in the semiconductor is full. The electrons are immobile so the material does not conduct electricity

3/2003 Rev 1 II.3.7 – slide 14 of 40 Band Theory - Insulators and Semiconductors  The difference between a semiconductor and an insulator is the band gap  In an insulator, the band gap is greater than 5 electron volts. It is less than this value in a semiconductor  A semiconductor is like a switch – a small amount of energy can promote electrons to the conduction band where they become mobile

3/2003 Rev 1 II.3.7 – slide 15 of 40 Semiconductor Impurities  If an impurity atom in the solid has an extra unpaired electron, it is referred to as a hole trap  If an impurity atom in the solid results in an incomplete bond (a vacancy), it is referred to as an electron trap  Defects in the crystalline lattice can have the same effect as impurities

3/2003 Rev 1 II.3.7 – slide 16 of 40 Luminescent Dosimetry  The process of using luminescence for dosimetry (either optical or thermal) is:  Radiation energy deposited in the material promotes electrons from the valence band to the conduction band  These electrons move to high energy electron traps  More radiation energy absorbed results in more electrons in the traps

3/2003 Rev 1 II.3.7 – slide 17 of 40  Dose is determined by trapped electrons being freed by exposing the dosimeter to light (for optical stimulated luminescence, OSL) or heat (for thermoluminescent dosimetry, TLD)  When an electron is freed, it falls to a lower energy level and emits a photon of light  The number of photons emitted is proportional to the dose, (the number of trapped electrons) Luminescent Dosimetry

3/2003 Rev 1 II.3.7 – slide 18 of 40 Electron in Valence Band Absorbs Radiation Energy e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Hole Trap e Electron Trap +

3/2003 Rev 1 II.3.7 – slide 19 of 40 Electron Promoted to Conduction Band Moves to Trap + e - e - e - e - e - e - e - e - Valence Band BANDGAP Conduction Conduction Band Band Hole Trap e Electron Trap e-e-e-e-

3/2003 Rev 1 II.3.7 – slide 20 of 40 + e - e - e - e - e - e - e - e - Valence Band BANDGAP Conduction Conduction Band Band Hole Trap e-e-e-e- Filled Electron Trap Filled Electron Trap e-e-e-e- Electron Promoted to Conduction Band Moves to Trap

3/2003 Rev 1 II.3.7 – slide 21 of 40 e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Filled Hole Trap Filled Electron Trap Filled Electron Trap e-e-e-e- + Electron Promoted to Conduction Band Moves to Trap

3/2003 Rev 1 II.3.7 – slide 22 of 40 Electron Promoted in Valence Band Moves to Hole Trap e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap e-e-e-e- +

3/2003 Rev 1 II.3.7 – slide 23 of 40 Electron Promoted in Valence Band Moves to Hole Trap e - e - + e - e - e - e - e - e - Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap e-e-e-e- e-e-e-e-

3/2003 Rev 1 II.3.7 – slide 24 of 40 Electron Falls to Valence Band Causing Luminescence e - e - + e - e - e - e - e - e - Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap e-e-e-e- e-e-e-e- LightPhoton

3/2003 Rev 1 II.3.7 – slide 25 of 40 Recombination Centers e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap+ e-e-e-e- RecombinationCenter

3/2003 Rev 1 II.3.7 – slide 26 of 40 Recombination Centers e - e - e - e - e - e - e - + e - Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap e-e-e-e- e-e-e-e- RecombinationCenter

3/2003 Rev 1 II.3.7 – slide 27 of 40 Recombination Centers e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap e-e-e-e- + RecombinationCenter

3/2003 Rev 1 II.3.7 – slide 28 of 40 Recombination Centers e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap e-e-e-e- + RecombinationCenter

3/2003 Rev 1 II.3.7 – slide 29 of 40 Recombination Centers e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap e-e-e-e- + RecombinationCenter e-e-e-e-

3/2003 Rev 1 II.3.7 – slide 30 of 40 Recombination Centers e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap e-e-e-e- + RecombinationCenter e-e-e-e- +

3/2003 Rev 1 II.3.7 – slide 31 of 40 Recombination Centers e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e-e- e- e- e- e- e- e- e- e- Valence Band BANDGAP Conduction Conduction Band Band Hole Trap Electron Trap Electron Trap Light Photon Emitted e-e-e-e- +

3/2003 Rev 1 II.3.7 – slide 32 of 40 TLD Glow Curve

3/2003 Rev 1 II.3.7 – slide 33 of 40 TLD’s  There are 4 TLD’s on each badge. They measure H p(0.07) ; H p(3) ; and H p(10). Two of the dosimeters are used to measure H p(10) – one has a 6 Li element which is sensitive to neutrons and 7 Li with is not sensitive to neutrons. The difference between these two provides the neutron dose.

3/2003 Rev 1 II.3.7 – slide 34 of 40 TLD Characteristics  Depending on the material composition, TLD’s characteristics include:  Measuring Radiation Types:  Photons > 5 keV;  Beta energies > 70 keV; and  Neutrons from thermal to 100 MeV;  Linear response from 10  Gy to 10 Gy;  Detection threshold of 1  Gy;

3/2003 Rev 1 II.3.7 – slide 35 of 40 TLD Characteristics  Detection threshold of 1  Gy;  Reusable from 100 to 1000 times;  Fading of the signal of <20% over 3 months;  Residual signal of <1%;  Composition is similar to tissue equivalent with LiF:Mg,Ti; and  Environmental sensitivity with CaF 2 : Dy.

3/2003 Rev 1 II.3.7 – slide 36 of 40 Optically Stimulated Luminescence (OSL)  Aluminum oxide (Al 2 O 3 ) is the only material being used in OSL dosimeters  Little is known about the identity of the electron traps is aluminum oxide  More is known about the recombination centers and the hole traps

3/2003 Rev 1 II.3.7 – slide 37 of 40 Characteristics of Aluminum Oxide  Extremely durable  Available in many forms – powders, crystals, thin layers bonded to a substrate  Extreme sensitivity to radiation  Low fading at room temperature  Light output is linearly related to dose  Simple emission spectrum  Low effective atomic number reduces energy dependence

3/2003 Rev 1 II.3.7 – slide 38 of 40 Optically Stimulated Luminescence  Aluminum oxide may be used in either a OSL or as a TL dosimeter  The light output, (the signal provided), is greater when operated in the OSL mode rather than in the TL mode  The intensity of the OSL emissions are greatest when stimulated by light with a wavelength of 500 nm

3/2003 Rev 1 II.3.7 – slide 39 of 40 Optically Stimulated Luminescence  Argon lasers are often used as the stimulating light because they emit light with a wavelength of 514 nm  Green light emitting diodes (LEDs) at 525 nm are used when there is concern with using lasers  Luxel dosimeters use copper and tin filters to correct for over response at low energies

3/2003 Rev 1 II.3.7 – slide 40 of 40 Advantages of OSL Dosimeters  OSL is performed at room temperature, which simplifies the design of the equipment  The detector can be reread multiple times – unlike TLDs which may be read only once

3/2003 Rev 1 II.3.7 – slide 41 of 40 Disadvantages of OSL Dosimeters  The OSL system is more expensive than TLDs  Workers might be concerned why doses are being reported using the OSL system when no dose was reported when the TLD system was used

3/2003 Rev 1 II.3.7 – slide 42 of 40 OSL Performance  Energy Response  Photons: 5 keV to in excess of 40 MeV  Beta:150 keV to in excess of 10 MeV  Sensitivity  Photons:0.01 mSv to 10 Sv  Beta:0.1 mSv to 10 Sv

3/2003 Rev 1 II.3.7 – slide 43 of 40 OSL Dosimeter Different filters are used to provide dose for the skin, lens of the eye, and deep dose. A TLD is needed to provide information on neutron dose.

3/2003 Rev 1 II.3.7 – slide 44 of 40 Where to Get More Information  Cember, H., Introduction to Health Physics, 3 rd Edition, McGraw-Hill, New York (2000)  Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8 th Edition, 1999 update), Wiley, New York (1999)  International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)