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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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responsivity and sensitivity
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
Spectral Response

Short l limit – determined by window material

Long l limit – determined by photocathode material

Hamamatsu Catalogue

response speed
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
Dark Signal

Output in the absence of

input radiation.

Often limits S/N at low

signal intensities.

Hamamatsu catalog

vacuum phototube vacuum photodiode
Vacuum Phototube (“Vacuum Photodiode”)

Photosensitive material:

e.g. Cs3Sb, AgOCs

Ingle and Crouch, Spectrochemical Analysis

photoelectric effect
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
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
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

radiant cathodic responsivity r
Radiant Cathodic Responsivity (R())

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

Units: A / W

Ingle and Crouch, Spectrochemical Analysis

anodic current
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
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
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
Choosing a PMT
  • Average anodic current
  • Single photon counting

Hamamatsu Catalog

modes of operations
Modes of Operations
  • Average anodic current
  • Single photon counting

Hamamatsu Catalog

single photon counting
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 counting1
Single Photon Counting

Improved S/N at low p

Hamamatsu Catalogue

sources of dark current glass scintillation
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
Sources of Dark Current:Thermionic Emission

Thermal energy releases e- from the cathode.

Reduced by cooling

Hamamatsu Catalogue