How does a TLD works

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How does a TLD work?

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What type of detector it is? Why no circuitry is involved? Could it be used at any exposure rate! Why should they be given frequently for analysis?

CONTEXT : 

CONTEXT Luminescence Mechanism of Luminescence Concept of Activators & killers Characteristics of phosphor Evaluation of TLDs.

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The property of emission of light when a energetic particle impinges on material (semiconductors) leading to creation of electron hole pairs and excitation of carriers. When these carriers come to their equilibrium states they emit light. What is luminescence?

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Three processes are involved in the Luminescence Phenomenon Excitation Absorption Emission

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Luminescence PHOSPHORESCENCE FLOURESCENCE

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Phosphorescence phenomenon :- conductor band Valence band a (a) (b) (c) (d) (e)

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An incoming photon with hν = Eg is absorbed creating an EHP An excited electron gives up energy to the lattice by scattering until it nears the bottom of conduction band. The electron is trapped by impurity level Et and remains trapped until it can be thermally re excited to conduction band. The re-excited electron falls in a lower level of conduction band. (e) Finally direct recombination occurs as the electron falls to an empty state in valence band giving off hv of approximately same band gap energy. BACK

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MECHANISM OF RECOMBINATION RADIATIVE RECOMBINATION NON RADIATIVE RECOMBINATION

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Non-Radiative recombination :- When the excited excess carriers reach equilibrium positions by emission of phonons due to surface / bulk defects / other defects it is said to be non-radiative recombination

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The total life time of the excess carriers ( t) is given by the relation as follows: 1/t = 1/tr + 1/tnr tr --------------------- radiative life tnr ------------------------non- radiative life time. Total recombination rate Rtotal = Rr + Rnr Internal quantum efficiency or radiative recombination efficiency is defined as ηr = Rr / (Rr + Rnr) Surely ηr < 1 ; For high internal efficiency Rnr should be very less.

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What Is Activator? Impurity atom occurring in relatively small concentrations in host material or a small stoichometric excess of one of constituents of material which exhibits the property of Luminescence.

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What Is Killer? Presence of certain type of impurity may also inhibit Luminescence of other centers, in which case they are referred as killers.

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Compounds which exhibit Luminescence in pure state. - According to Randall, such compounds should contain one ion or ion group per unit cell with an incompletely filled shell of electrons which is well screened from its sorroundings. eg:- Manganous halides, Samarium, Gadolinium sulfate, and platino cyanides. The Alkali halides activated with Thallium or other heavy metals. ZnS and CdS activated with Cu, Ag, Au, Mn or with an excess of one of their constituents. The silicate phosphors, such as Zinc Ortho silicate activated with Divalent manganese which is used for Oscilloscope screens. Oxide phosphors, such as self activated ZnO and Al2O3 activated with Transition metals. Organic crystals such as anthracene activated with napthacene ; these materials are often used in scintillation counters. Luminescent Crystalline Solids

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Dependence of luminescence efficiency on Activator concentration?

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After a particular level of concentration, the efficiency decreases as excess carrier returns to the ground state only if there is no other activator with in a sphere of radius ‘R’ around central activator atom. η = C(1-C)z / [C+(β/α)(1-C)]; β/α - ratio of capture crossection of photon of given wavelength by a lattice atom. This factor also depends on temperature also. z – Metallic positions with in sphere of radius ‘R’. For smaller concentrations of activator, ‘η’ increases proportionally with ‘C’. At higher concentrations mutual quenching takes over, leading to decrease in η.

THERMALLY STIMULATED LUMINESCENCE : 

THERMALLY STIMULATED LUMINESCENCE When substances are irradiated by energetic particles/rays are heated they show an increased glow. This glow is called Thermo Luminescence or Thermally stimulated Luminescence. This is due to recombination of electrons which are thermally reactivated from deep traps. This emission is called thermo luminescence emission and the temperature dependence of emission on intensity of light is called glow.

Measurement of Glow Curve : 

Measurement of Glow Curve The measurement of glow curve of a phosphorescence process is as follows: i) The sample is kept at room temperature. ii) The sample is excited by UV rays /X-rays/gamma rays until the traps are filled with electrons or holes. iii) The excitation is terminated and the temperature of sample is raised at a constant rate of heating while the intensity is recorded.

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How a TLD Works? When a substance is irradiated, the phosphor absorbs some of the radiant energy there by free electrons and free holes are generated with in the phosphor host. The electrons are captured by traps (mostly color centers). If the phosphor in that state is heated the captured electron is released and recombines with a trapped hole. During recombination process photon is ejected. For analysis a photomultiplier tube is used in which a photoelectric material is used to eject electrons. The electrons ejected would be accelerated and collected at end of the tube, thus leading to electric signal which would be fed to amplifier stage. The glow curves obtained would be studied to know the field exposure.

EXAMPLES OF PHOSPHORS FOR TLD : 

EXAMPLES OF PHOSPHORS FOR TLD LiF : Mg2 CaF2 : Mn+2 The most commonly used TLD phosphors are Lithium Fluoride (LiF), Calcium Fluoride (CaF2), Lithium Borate(Li2B4O7), Calcium Sulfate(CaSO4) and Aluminum Oxide(Al2O3) activated with trace quantities of Transition metal or Rare Earth metal ions. TLD phosphors are available in a variety of forms, including powders, compressed chips, Teflon impregnated disks, single crystals, extruded rods and thin films.

Characteristics of a Phosphor for Thermo-Luminescence devices : 

Characteristics of a Phosphor for Thermo-Luminescence devices High concentration of carrier traps. High emission efficiency. Large trap depth. Traps, Luminescence & lattice are not to be damaged by repeated irradiation & heating process.

GLOW CURVE : 

(I) GLOW CURVE Temperature Intensity

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GLOW CURVE Thermo Luminescence glow curve of LiF phosphor activated with trace amounts of Mg2+ and Ti4+.

THERMOLUMINESCENCE EMISSION SPECTRA : 

(I) (λ) THERMOLUMINESCENCE EMISSION SPECTRA Wavelength Intensity

SUPRA LINEARITY : 

SUPRA LINEARITY Dose Sensitivity When the irradiation dose is increased at a constant radiation energy if the sensitivity do not vary then it is said to be supra linear.

SUPER LINEARITY : 

SUPER LINEARITY Energy Sensitivity When the irradiation dose is increased at a constant radiation energy, if the sensitivity do not vary, it is said to be super linearity.

FADING : 

FADING Time Optical density Thermo- Luminescence fading should be as less as possible.

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Routine monitoring of occupational radiation exposure. (Dose range of interest: 0.1 to 1 mGy) To determine patient exposure as a result of X-ray diagnostics. (Dose range of interest : 1.0 to100mGy) To determine patient exposure cancer radiotherapy treatment. (Dose range of interest :1 to 10 Gy) Application of TLDs

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INSTRUMENTATION TO EVALUATE TLDs Heating setup for TLD Photo multiplier tube Amplifier COMPUTER for analysis Power Heater

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What type of detector it is? Why no circuitry is involved? Could it be used at any exposure rate! Why should they be given frequently for analysis?

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P. Girish Kumar MSc Solid state physics Senior Technical Officer E-mail: palvaigirish@conceptualphsyicstoday.com THANK YOU