spectrum or part of a spectrum instead of pmts to detect a...
TRANSCRIPT
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Image Detectors – powerful detectors used instead of PMTs to detect a complete spectrum or part of a spectrum
PMT
Source
Prism monochromator
ImageDetector - locatedat the focal plane(no exit slit)
Exit slit
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Common Image Detectors1) Electron Image Intensifiers2) Image Dissectors3) Solid-State Imaging Systems
a) Vidicon tubesb) Optical Multichannel Analyzers (OMAs)c) Photo Diode Arrays (PDAs)
4) Charge Coupled Devices (CCDs)
These are often used with intensifiers –device to increase sensitivity
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Photodiodes, Linear Diode Array & Two Dimensional Arrays
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Charge Coupled Device (CCD)
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Photovoltaic Cell
Light excites electrons in Se at Se-Ag interface into “conduction band” and to metal conductor � current
Good only for high light levelsSubject to fatigue effects
A
thin layer of Agseleniumsemiconductor
iron or copper
hν
ammeter
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Another example of a Photovoltaic cell
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Photoconductive detector – semiconductor used with voltage applied across it
Photons � electrons promoted to conduction band � high conductivity (lower resistance)
PbS, PbSe, InSb good for 0.7 to 4.5 µm (near IR)
Ge activated with Cu, Au or Zn good from 2 to 15 µm – operated at ~5 oK
Considerably less sensitive than PMTsBetter than thermal detectors in IR
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Photographic detection – place film at focal plane and expose (integrating detector)
Advantages:1) good resolution2) fairly sensitive3) covers entire spectral region
Disadvantages:1) very old technique2) quantitatively very bad (can use densitometer)
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Thermal Detectors for IR – in IR region photons have lower energies � necessary to resort to thermal detectors – radiation absorbed and temperature change is detected
Response time is limited by rate of heat transfer � slow
Sensitivity is also much poorer
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Three types of thermal detectors:1) Thermocouples (most common) –
junction between dissimilar metals often covered with black substance to increase absorption
Voltage difference across junction is a function of temperature
Amplify signal and detectResponse time ~60 msec (i.e. slow)Sensitivity is greater using a thermopile = a
bundle of many thermocouples
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2) Bolometer (thermistor) – resistance is a function of temperature
Different kinds � Ni or Pt metal or oxides like NiO, CoO or MnO
Many have black coating on side toward source and a heat shield around them
Typically connected to a bridge circuit Johnson noise is importantRequires stable power supply
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3) Golay Pneumatic Detector (best performance characteristics)
Heat from radiation � gas expands �mirror position changes � amount of light reflected to photodiode changes
Best sensitivityResponse time ~4 msec � heat transfer in gas
phase faster than in solid
hν
window
pnuematic chamber
flexible mount & mirror
photodiodedetector
detectorsource
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Schematic diagramof a Double BeamSpectrophotometer
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Schematic diagramof a Double BeamSpectrophotometer
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Schematic diagramof a Single Beam
Spectrophotometer