Responsivity and Sensitivity

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# Responsivity and Sensitivity - PowerPoint PPT Presentation

Responsivity and Sensitivity. Responsivity, R(  ): Ratio of the signal output, x, to the incident radiant power,  (in Watts). (voltage, current, charge). Sensitivity, Q(  ): Slope of a plot of x vs.  . Spectral Response. Short l limit – determined by window material

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Presentation Transcript
Responsivity and Sensitivity

Responsivity, R(): Ratio of the signal output, x, to the incident radiant power,  (in Watts).

(voltage, current, charge)

Sensitivity, Q(): Slope of a plot of x vs. .

Spectral Response

Short l limit – determined by window material

Long l limit – determined by photocathode material

Hamamatsu Catalogue

Response Speed

Consider a sinusoidal input into a transducer with a finite response time.

If the frequency, fc, of the sinusoidal input is high, the transducer response cannot keep up.

The frequency where R() drops to 0.707 of the ideal is used to determine the time constant, .

Dark Signal

Output in the absence of

Often limits S/N at low

signal intensities.

Hamamatsu catalog

Vacuum Phototube (“Vacuum Photodiode”)

Photosensitive material:

e.g. Cs3Sb, AgOCs

Ingle and Crouch, Spectrochemical Analysis

Photoelectric Effect

Photon must have some minimum energy to release an e-. Referred to as the work function.

lt = hc/Ec = 1240/Ec

For most metals the work function is ~2 – 5 eV.

Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

The Work Function Limits the Spectral Response

2-5 eV = 250-620 nm

 Use materials with lower work functions, e.g., alkali metals.

Hamamatsu Catalogue

Quantum Efficiency K()

# of photoelectrons ejected for every incident photon.

Typically K() < 0.5

Rate of electrons emitted from the cathode (rcp):

rcp = pK()

where p is the photon flux (photons / sec).

Multiply by electron charge to get current.

icp = ercp = eK()p

Ingle and Crouch, Spectrochemical Analysis

Efficiency with which photon energy is converted to photo-electrons.

Units: A / W

Ingle and Crouch, Spectrochemical Analysis

Anodic Current

Collection Efficiency () depends on the bias voltage (Eb).

Arrival Rate at the Anode

(collection rate):

rap = rcp = pK()

iap = icp = phR()

p = photon flux

Ingle and Crouch, Spectrochemical Analysis

Are you getting the concept?

A vacuum phototube has radiant cathodic responsivity of 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 105 photons/sec, find the anodic pulse rate and the photoanodic current for a collection efficiency of 0.90.

Photomultiplier Tube

8–19 dynodes (9-10 is most common).

Gain (m) is # e- emitted per incident e- () to the power of the # of dynodes (k).

m = k

E.g., 5 e- emitted / incident e-,10 dynodes.

m = k = 510 1 x 107

Typical Gain = 104 - 107

Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

Choosing a PMT
• Average anodic current
• Single photon counting

Hamamatsu Catalog

Modes of Operations
• Average anodic current
• Single photon counting

Hamamatsu Catalog

Single Photon Counting

Single photons give bursts of e-

The rise time of PMTs depends on the spread in the transit time of e- during the multiplication process.

FWHM: Full Width at Half of Maximum

Hamamatsu Catalogue

Single Photon Counting

Improved S/N at low p

Hamamatsu Catalogue

Sources of Dark Current: Glass Scintillation

Brief flash of light when an e- strikes the glass envelope.

Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992.

Ingle and Crouch, Spectrochemical Analysis

Sources of Dark Current:Thermionic Emission

Thermal energy releases e- from the cathode.

Reduced by cooling

Hamamatsu Catalogue