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Sensitivity and ResponsivitySensitivity and Responsivity
Responsivity, R(Responsivity, R():): Ratio of the signal output, x, to the Ratio of the signal output, x, to the incident radiant power, incident radiant power, (in Watts). (in Watts).
x
R
x
R(voltage, current, charge)(voltage, current, charge)
Sensitivity, Q(Sensitivity, Q():): Slope of a plot of x vs. Slope of a plot of x vs. ..
d
dx Q
d
dx Q
Spectral ResponseSpectral Response
Hamamatsu CatalogueHamamatsu Catalogue
Short Short limit – determined by window material limit – determined by window materialLong Long limit – determined by photocathode material limit – determined by photocathode material
Transmittance of Window MaterialsTransmittance of Window Materials
Hamamatsu CatalogueHamamatsu Catalogue
Response SpeedResponse Speed
Consider a sinusoidal input into a transducer with a Consider a sinusoidal input into a transducer with a finite response time.finite response time.
If the frequency, fIf the frequency, fcc, of the sinusoidal input is high, , of the sinusoidal input is high,
the transducer response cannot keep up.the transducer response cannot keep up.
The frequency where R(The frequency where R() drops to 0.707 of the ) drops to 0.707 of the ideal is used to determine the time constant, ideal is used to determine the time constant, ..
cf2
1
cf2
1
Dark SignalDark Signal
Output in the absence ofOutput in the absence ofinput radiation.input radiation.
Often limits S/N at lowOften limits S/N at lowsignal intensities.signal intensities.
Hamamatsu catalogHamamatsu catalog
Vacuum Phototube (“Vacuum Photodiode”)Vacuum Phototube (“Vacuum Photodiode”)
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
Photosensitive material:Photosensitive material:e.g. Cse.g. Cs33Sb, AgOCsSb, AgOCs
Photoelectric EffectPhotoelectric Effect
Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.
Photon must have some Photon must have some minimum energy to release minimum energy to release an ean e--. Referred to as the . Referred to as the work function.work function.
tt = hc/E = hc/Ecc = 1240/E = 1240/Ecc
For most metals the work For most metals the work function is ~2 – 5 eV.function is ~2 – 5 eV.
The Work Function Limits the Spectral ResponseThe Work Function Limits the Spectral Response
Hamamatsu CatalogueHamamatsu Catalogue
2-5 eV = 250-620 nm2-5 eV = 250-620 nm
Use materials Use materials with lower work with lower work functions, e.g., alkali functions, e.g., alkali metals.metals.
Quantum Efficiency K(Quantum Efficiency K())# of photoelectrons ejected for # of photoelectrons ejected for every incident photon.every incident photon.
Typically K(Typically K() < 0.5) < 0.5
Rate of electrons emitted from Rate of electrons emitted from the cathode (rthe cathode (rcpcp):):
rrcpcp = = ppK(K())
where where pp is the photon flux is the photon flux
(photons / sec).(photons / sec).
Multiply by electron charge to Multiply by electron charge to get current.get current.
iicpcp = er = ercpcp = eK( = eK())ppIngle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
Radiant Cathodic Responsivity (R(Radiant Cathodic Responsivity (R())))
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
Efficiency with which photon Efficiency with which photon energy is converted to photo-energy is converted to photo-electrons.electrons.
h
e K R
h
e K R
Units: A / WUnits: A / W
Anodic CurrentAnodic Current
Collection Efficiency (Collection Efficiency () depends ) depends on the bias voltage (Eon the bias voltage (Ebb).).
Arrival Rate at the AnodeArrival Rate at the Anode
(collection rate):(collection rate):
rrapap = = rrcpcp = = ppK(K())
iiapap = = iicpcp = = pphhR(R())
pp = photon flux = photon flux
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
Are you getting the concept?Are you getting the concept?A vacuum phototube has radiant cathodic responsivity of A vacuum phototube has radiant cathodic responsivity of 0.08 A/W at 400 nm. (a) Find the quantum efficiency at 0.08 A/W at 400 nm. (a) Find the quantum efficiency at 400 nm. (b) If the incident photon flux at 400 nm is 2.75 x 400 nm. (b) If the incident photon flux at 400 nm is 2.75 x 101055 photons/sec, find the anodic pulse rate and the photons/sec, find the anodic pulse rate and the photoanodic current for a collection efficiency of 0.90.photoanodic current for a collection efficiency of 0.90.
First, convert First, convert to to →→ = 7.5 x 10 = 7.5 x 101414 s s-1-1
K(K() = R() = R()h)h/e = (0.08 As/J)(6.63 x 10/e = (0.08 As/J)(6.63 x 10-34-34 Js)(7.5 x 10 Js)(7.5 x 101414 s s-1-1)) 1.602 x 101.602 x 10-19-19 As As
K(K() = 0.248) = 0.248
rrapap = = ppK(K() = (0.90)(2.75 x 10) = (0.90)(2.75 x 1055 photons/s)(0.248) photons/s)(0.248)
rrapap = 6.15 x 10 = 6.15 x 1044 photons/s photons/s
iiapap = = pphhR(R() )
=(0.90)(2.75 x 10=(0.90)(2.75 x 1055 h h/s)(6.63 x 10/s)(6.63 x 10-34-34 Js)(7.5 x 10 Js)(7.5 x 101414 s s-1-1)(0.08 As/J) )(0.08 As/J)
iiapap = 9.7 x 10 = 9.7 x 10-15-15 A A
Photomultiplier TubePhotomultiplier Tube
Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.
8–19 dynodes (9-10 is 8–19 dynodes (9-10 is most common).most common).
Gain (m) is # eGain (m) is # e-- emitted emitted per incident eper incident e-- ( () to the ) to the power of the # of power of the # of dynodes (k).dynodes (k).
m = m = kk
E.g., 5 eE.g., 5 e-- emitted / incident e emitted / incident e--,,
10 dynodes.10 dynodes.
m = m = kk = 5 = 51010 1 x 10 1 x 1077
Typical Gain = 10Typical Gain = 1044 - 10 - 1077
Choosing a PMTChoosing a PMT
Hamamatsu CatalogHamamatsu Catalog
1.1. Average anodic currentAverage anodic current2.2. Single photon countingSingle photon counting
Modes of OperationsModes of Operations
Hamamatsu CatalogHamamatsu Catalog
1.1. Average anodic currentAverage anodic current2.2. Single photon countingSingle photon counting
Single Photon CountingSingle Photon Counting
Hamamatsu CatalogueHamamatsu Catalogue
Single photons give Single photons give bursts of ebursts of e--
The rise time of PMTs The rise time of PMTs depends on the depends on the spread in the transit spread in the transit time of etime of e-- during the during the multiplication process.multiplication process.
FWHM: Full Width at Half of MaximumFWHM: Full Width at Half of Maximum
Single Photon CountingSingle Photon Counting
Improved S/N Improved S/N at low at low pp
Hamamatsu CatalogueHamamatsu Catalogue
Sources of Dark Current:Sources of Dark Current:Thermionic EmissionThermionic Emission
Thermal energy releases Thermal energy releases ee-- from the cathode. from the cathode.
Reduced by coolingReduced by cooling
Hamamatsu CatalogueHamamatsu Catalogue
Thermionic Emission is Thermionic Emission is Dependent on Bias VoltageDependent on Bias Voltage
Hamamatsu CatalogueHamamatsu Catalogue
Sources of Dark Current: Sources of Dark Current: Glass ScintillationGlass Scintillation
Brief flash of light when an eBrief flash of light when an e-- strikes the glass envelope.strikes the glass envelope.
Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
PhotodiodesPhotodiodes
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysis
Photons incident on the Photons incident on the depletion layer induce a depletion layer induce a current.current.
In most cases, best In most cases, best response in the NIR.response in the NIR.
Response is linear over 6 Response is linear over 6 – 7 orders of incident – 7 orders of incident radiant powerradiant power
Spectral Response of PhotodiodesSpectral Response of Photodiodes
Shinya Inoue and Kenneth Spring, Shinya Inoue and Kenneth Spring, Video MicroscopyVideo Microscopy, Plenum Press, New York, 1997., Plenum Press, New York, 1997.
Avalanche PhotodiodeAvalanche Photodiode
http://micro.magnet.fsu.edu/primer/java/digitalimaging/avalanche/index.htmlhttp://micro.magnet.fsu.edu/primer/java/digitalimaging/avalanche/index.html
Hamamatsu CatalogHamamatsu Catalog
Photodiode Arrays (PDA or DAD)Photodiode Arrays (PDA or DAD)
Douglas A. Skoog and James J. Leary, Principles of Instrumental Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.Analysis, Saunders College Publishing, Fort Worth, 1992.
Simultaneous Simultaneous detection detection in a spectrophotometer.in a spectrophotometer.
Charge Coupled Device (CCD)Charge Coupled Device (CCD)
Ingle and Crouch, Ingle and Crouch, Spectrochemical AnalysisSpectrochemical Analysiswww.piacton.comwww.piacton.com
CCD ArchitectureCCD Architecture
http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/ccdanatomy.html http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/ccdanatomy.html and Bryce Marquis (Haynes Lab)and Bryce Marquis (Haynes Lab)
Image Area
serial registeramplifier
SiO2 backing
Pixel Array
CCD ArchitectureCCD Architecture
Bryce Marquis (Haynes Lab)Bryce Marquis (Haynes Lab)
One pixel
“Channel Stops” form horizontal pixel boundaries
3 electrodesform verticalpixel boundaries
Top View
Cross section
Insulating oxiden-type silicon
p-type silicon
Electrode
Charge Generation/CollectionCharge Generation/Collection
N-type
P-type
-V-V +V
Incident photons excite electron-hole pairs,electrons gather in potential wells in each pixel
-V-V +V
CCD Rain Bucket AnalogyCCD Rain Bucket Analogy
Shinya Inoue and Kenneth Spring, Shinya Inoue and Kenneth Spring, Video MicroscopyVideo Microscopy, Plenum Press, New York, 1997., Plenum Press, New York, 1997.
1. Generation 2. Collection3. Transfer4. Measurement
Charge Generation/CollectionCharge Generation/Collection
N-type
P-type
-V-V +V
N-type silica is doped with pentavalent species = excess electrons
P-type is doped with trivalent species = excess holes
-V-V +V
Potential Well
One Pixel
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Time-slice shown in diagram
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Charge TransferCharge Transfer
Every third electrode is coupled, charge packetsare walked towards Serial registry
to serial registry
http://spiff.rit.edu/classes/phys445/lectures/ccd1/ccd1.htmlhttp://spiff.rit.edu/classes/phys445/lectures/ccd1/ccd1.html
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Charge TransportationCharge Transportation
• Pixels at end of the array dump charge into serial register
• Serial register walks charge packets to amplifier, where it is measured.
amplifier
Quantum EfficiencyQuantum Efficiency
www.piacton.comwww.piacton.com
Noise Sources in CCDNoise Sources in CCD
• Shot Noise– Statistical variation of signal over time
• Increases with the square of the intensity
• Dark Signal Noise– Caused by thermal liberation of electrons
• Strongly coupled to temperature
• Readout Noise– Summation of noise associated with amplification
of signal, and conversion from analogue to digital• Increases with the processing speed
222 )()()( readdarkshottotal NNNN
Other Issues: Bad PixelsOther Issues: Bad Pixels
• Hot Pixels– Increased charge accumulation due to variations in chip
surface.• Dead Pixels
– Defective pixels that do not respond.
Increasing hot pixels
Other Issues: BloomingOther Issues: Blooming
Other Issues: Cosmic RaysOther Issues: Cosmic Rays
* indicates cosmic rays
Other Issues: EtaloningOther Issues: Etaloning
Etaloning
www.piacton.comwww.piacton.com
Other Features: BinningOther Features: Binning
• On Chip Pixel Binning
– Increases S/N
• Shot noise decrease
– Increased speed
– Less storage space needed
– Decreases resolution
http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/binning.htmlhttp://micro.magnet.fsu.edu/primer/digitalimaging/concepts/binning.html