Instrumentation IR Spectroscopy: Detectors
- 1. Detectors used in IR Spectroscopy Dr. Tambe V.S. (PES Modern College of Pharmacy, for Ladies)
- 2. Transducers convert radiant energy into electrical signal Ideal properties of detector • High sensitivity, • a constant response over range of wavelengths, • fast response time, • zero output signal in absence of illumination, • signal should be directly proportional to the power of incident radiation
- 3. Quantum detectors Thermal detectors Responds to photons Responds to heat Photoconductors 1. Thermocouple/ Therompiles 2. Bolometer / resistance thermometer 3. Thermistor 4. Golay cell/ gas thermometer 5. Pyroelectric Only photoconductors are used in IR, Other phototransducres are not used as IR lacks energy to cause photoemission Useful for IR radiation Quantum detectors responds to individual photons Thermal detectors responds to average power of incident radiation Sensitivity is dependent on wavelength but is higher than thermal detectors Sensitivity is independent on wavelength but lower than sensitivity of photoelectric detectors, slow speed, low dynamic range Most of the quantum detectors are used for measurement of UV, Visible and NIR radiations as they have sufficient energy. When they are used for long wavelengths, they must be cooled to dry ice or liquid nitrogen temperature to avoid interference from thermal background noise.
- 4. THERMAL TRANSDUCER Response depend upon the heating effect of radiation. Radiation is absorbed by a blackbody, and resultant increase in temperature is measured. 1. Thermocouples A pair of junctions formed when two dissimilar semiconductor (antimony and bismuth) fused to either end. Potential difference develops between the junctions as change in temperature. 2. Bolometers/ Resistance thermometers constructed of strips of metals such as platinum or nickel. Produce large change in resistance as a function of temperature. 3. Thermistors Similar to bolometer, but a semiconductor is used, produces decrease in resistance 4. Golay cell/ gas thermometer Heating of gas reflected by proportionate increase in current 5. Pyroelectric Temperature dependent polarisation reflected by proportionate increase in current.
- 5. Thermal noise from surrounding should be minimized by • Housing the detector in vacuum • Beams are chopped to minimize the effects of extraneous heat source.
- 6. A THERMOCOUPLE Several thermocouples in series is called thermopile (increased sensitivity) Has two junctions Difference in the temperature of two junctions, creates a voltage due to thermoelectric effect which provides a signal
- 7. • Voltage can be considered as the pressure that forces the charged electrons to flow in an electrical circuit. The flow of charge carriers between the hot and cold regions in turn creates a voltage difference.
- 8. A Reference junction 1. Temperature of reference junction should remain constant 2. Protected from IR radiation 3. Covered junction in insulating chamber 4. Its temperature should not change 5. High heat capacity: when heated, it is associated with small temperature rise 6. It is cooled in ice to get a measureable difference between reference and detector junction
- 9. A detector junction • Detects the intensity of radiation falling on it • Should absorb the heat completely from the IR radiation incident on it • Formed from a very fine metal wire such as platinum, silver, antimony or bismuth • A thin layer of metal can be deposited on a nonconducting material. • The junction is blackened as black bodies absorb maximum heat. • Junction has small heat capacity which causes large temperature change with small changes in heat. • The detector junction is sealed in an evacuated chamber having transparent window, entire IR beam is concentrated on its surface, so that the temperature change can be measured.
- 10. A BOLOMETER Made from strips of metals like platinum or nickel or semiconductor (Germanium). The metals have the high-temperature coefficient, i.e. their temperature increases with the increases in temperature Exhibit large change in resistance with temperature. The resistance of the metal is directly proportional to the temperature. Thermistor : The negative temperature coefficient means their resistance decreases with the increases in temperature.
- 11. The following are the requirements of the conductor The resistivity of the material is high so that the minimum volume of conductor is used for construction. The change in resistance of the material concerning temperature should be as high as possible. The resistance of the material depends on the temperature.
- 12. A GOLAY CELL
- 13. • Consists of a gas-filled enclosure with an infrared absorbing material and a flexible diaphragm or membrane. • When infrared radiation is absorbed, it heats the gas, causing it to expand. • The resulting increase in pressure deforms the membrane. Light reflected off the membrane is detected by a photodiode, and motion of the membrane produces a change in the signal on the photodiode. • The concept was originally described by Marcel J. E. Golay after whom it was named.
- 14. Pyroelectric detector Temperature fluctuations produce a charge change on the surface of pyroelectric crystals, which produces a corresponding electrical signal. This temperature gradient can be created by the absorption of light. Materials with special electric and thermal properties eg. DTGS (Deuterated Triglycine Sulphate), LiTaO3 Lithium tantalate, LiNbO3, Lithium Nibonate, Barium titanate Insulators (dielectric materials) and develop temperature dependent polarisation/ internal electric field below their curie point Pyroelectric material is sandwiched in the form of single crystalline wafers between two electrodes, one of which is IR transparent, a temperature dependent capacitor is formed. On absorption of IR, temperature changes, and alters the charge distribution across the crystal. This change in charge distribution can be detected as current in an external electric circuit connecting the two sides of the capacitor. Pyroelectric crystals lose their residual polarization when heated to a temperature called the Curie point. For TGS, Curie pt is 47 deg Celcius
- 15. Desired Properties of Pyroelectric materials: • The pyroelectric coefficient determines the ability to produce current from IR radiation. If it is bigger, the better. • The dielectricity constant determines the capacitance, thereby affecting the noise. The larger the capacitance, the lower the noise. • The specific heat capacity determines the temperature increase in the crystal depending on the absorbed radiation. A low value indicates larger temperature increase and thus a better signal.
- 16. A Pyroelectric detector No Current flows Flow of current depending on extent of polarisation
- 17. Photo conducting transducers These consists of a thin film of a semiconductor material , such as lead sulfide, mercury/cadmium telluride (MCT), or indium antimonide, deposited on a nonconducting glass surface and sealed into an evacuated envelope. Absorption of radiation by these materials promotes nonconducting valence electrons to a higher energy conducting state, thus decreasing the electrical resistance of the semiconductor. Must be cooled with liquid nitrogen to minimize thermal noise.
- 20. Intrinsic and Extrinsic Photoconductors • Intrinsic Materials When the photon energy is above the band gap energy, i.e., for a sufficiently short optical wavelengths, interband transitions, (optical transitions across the band gap from the valence band to the conduction band) takes place For example, lead salt, cadmium sulfide and mercury cadmium selenide detectors are based on this operation principle. • Extrinsic Materials In an extrinsic material additional energy levels are introduced by impurities. These are introduced by doping with additional chemical species and provide additional excitation pathways.
- 21. Thank You…