A TLD is used often used instead of film badges. These dosimeters can measure low dose rate the only limitation for using TLD’S is a time required for readout and store the information. In TLD’s there is a imperfect lattice structure is used which can easily absorb and store ionization radiation and when heated up and reemission is done inform of light and that reemitted light is detected and and output form of light is correlated to absorbed dose that is received earlier. when a T LD material is exposed to ionizing radiation at some certain value of temperatures then radiation interacts with crystal and deposit all incident energy in material. some of atoms get ionized by energy that get absorbed and free electrons will produce and holes will also be created in TLD material.so now electrons will move free in conduction band and may be get trapped at sites that are some imperfections in crystal lattice and release trapped electrons which fall down to ground state and will release energy in form of light .Released light is counted by PM tubes and counted number of photons are proportional to quantity of ionizing radiation .The number of resulting events are measured by repeated process again and again so total number of events in total time often repeating can be get integrated.——–1(a),1(b)
The scintillator materials posses the property of scintillation when excited by ionizing radiation .when an ionizing radiation struck the luminescent materials absorb energy and absorbed energy get reemitted in form of light .In in organic scintillators the scintillation processes are possible to electron band structure of crystals .so an incoming particle excite an electron from valence band to conduction band and excitation band that is located between the conduction band and valence band and holes are formed in valence band. Impurities in crystal lattice structure causes electronic levels in forbidden gap. These excitations are loosely band pairs of electron holes pairs and moved in crystal lattice until they are captured in impurity center of crystal lattice structure, rapidly de excited in form of an emission of light scintillating .The activator impurities are used for reemission to be done in visible light and PMT tubes response is effective. The holes that are associated with conduction band electrons can move freely and these holes and electrons are captured by impurity centers under some metastable states .The delayed de excitation of theses impure metastable states results in slow scintillation light emission mechanisms.——2(a)
The scintillators are used because of the properties like high density and low cost fast operation speeds and durability parameters .High operating speeds will posses the good resolution of spectra. Particles deposit their energy that is directly proportional to response to scintillators and short decay times are important for measurement of time intervals and for output results that coincide or occurring at same time interavls.—–2b
REFRENCES:
1.(a) https://www.nde-ed.org/EducationResources/CommunityCollege/RadiationSafety/radiation_safety_equipment/thermoluminescent.htm