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

2. 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. .

3. Spectral Response Hamamatsu Catalogue Short limit – determined by window material Long limit – determined by photocathode material

4. Transmittance of Window Materials Hamamatsu Catalogue

5. 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 0.707 of the ideal is used to determine the time constant, .

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

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

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

9. The Work Function Limits the Spectral Response Hamamatsu Catalogue 2-5 eV = 250-620 nm  Use materials with lower work functions, e.g., alkali metals.

10. 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

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

12. 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

13. 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.

14. Photomultiplier Tube Douglas A. Skoog and James J. Leary, Principles of Instrumental Analysis, Saunders College Publishing, Fort Worth, 1992. 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 = 5 10  1 x 10 7 Typical Gain = 10 4 - 10 7

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

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

17. 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

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

19. Thermionic Emission is Dependent on Bias Voltage Hamamatsu Catalogue

20. 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.

21. 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

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

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