Transducers Converts one type of energy into another. Light  Electrical (current, voltage, etc.) What characteristics should we look for in a transducer?

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 Hamamatsu Catalogue Short limit – determined by window material Long limit – determined by photocathode material

Transmittance of Window Materials Hamamatsu Catalogue

Response Speed Consider a sinusoidal input into a transducer with a finite response time. If the frequency, f c, of the sinusoidal input is high, the transducer response cannot keep up. The frequency where R( ) drops to of the ideal is used to determine the time constant, .

Dark Signal Output in the absence of input radiation. Often limits S/N at low signal intensities. Hamamatsu catalog

Vacuum Phototube (“Vacuum Photodiode”) Ingle and Crouch, Spectrochemical Analysis Photosensitive material: e.g. Cs 3 Sb, AgOCs

Photoelectric Effect Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, Photon must have some minimum energy to release an e -. Referred to as the work function. t = hc/E c = 1240/E c t = hc/E c = 1240/E c For most metals the work function is ~2 – 5 eV.

The Work Function Limits the Spectral Response Hamamatsu Catalogue 2-5 eV = nm  Use materials with lower work functions, e.g., alkali metals.

Quantum Efficiency K( ) # of photoelectrons ejected for every incident photon. Typically K( ) < 0.5 Rate of electrons emitted from the cathode (r cp ): r cp =  p K( ) where  p is the photon flux (photons / sec). Multiply by electron charge to get current. i cp = er cp = eK( )  p Ingle and Crouch, Spectrochemical Analysis

Radiant Cathodic Responsivity (R( )) Ingle and Crouch, Spectrochemical Analysis Efficiency with which photon energy is converted to photo- electrons. Units: A / W

Anodic Current Collection Efficiency (  ) depends on the bias voltage (E b ). Arrival Rate at the Anode (collection rate): r ap =  r cp =  p K( ) i ap =  i cp =  p h 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 10 5 photons/sec, find the anodic pulse rate and the photoanodic current for a collection efficiency of 0.90.

Photomultiplier Tube Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, –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 = 5 10  1 x 10 7 Typical Gain =

Choosing a PMT Hamamatsu Catalog 1.Average anodic current 2.Single photon counting

Modes of Operations Hamamatsu Catalog 1.Average anodic current 2.Single photon counting

Single Photon Counting Hamamatsu Catalogue 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

Single Photon Counting Improved S/N at low  p Hamamatsu Catalogue

Thermionic Emission is Dependent on Bias Voltage Hamamatsu Catalogue

Sources of Dark Current: Ionization of Residual Gases Ions formed when e - strike residual gas molecules. Gives a large noise spike when ion strikes cathode or one of the earlier dynodes.

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, Ingle and Crouch, Spectrochemical Analysis

Sources of Dark Current: Thermionic Emission Thermal energy releases e - from the cathode. Reduced by cooling Hamamatsu Catalogue

Sources of Dark Current: Leakage Current (Ohmic Leakage) Current from the glass base or the socket. Usually only significant at low bias.