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Photomultipliers

Photomultipliers. Radiant Measurement Flux (W) Energy (J) Irradiance (W/m 2 ) Emittance (W/m 2 ) Intensity (W/sr) Radiance (W/sr m 2 ) These are pure physical quantities. Luminous Measurement Luminous flux (lumen; lm) Quantity of light (lm s) Illuminance (lux; lx)

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Photomultipliers

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  1. Photomultipliers

  2. Radiant Measurement Flux (W) Energy (J) Irradiance (W/m2) Emittance (W/m2) Intensity (W/sr) Radiance (W/sr m2) These are pure physical quantities. Luminous Measurement Luminous flux (lumen; lm) Quantity of light (lm s) Illuminance (lux; lx) Luminous emittance (lm/m2) Lum. intensity (candela; cd) Luminance (cd/m2) These relate to visual sensation. Measuring Light

  3. Radiant flux (W) measures the energy flowing at a point per unit time. Luminous flux (lm) weights the flux for its impact on a visual system. Peak efficiency constant Km = 683 lm/W Spectral luminous efficiency V(l) Flux

  4. The flux per unit area is the irradiance (illuminance) E. lm/m2 = lux Flux from a luminous area is the emittance M. Lux not used Luminous Area dF dF dA dA Irradiance Emittance

  5. Intensity relates to the flux from a point source. Flux per unit solid angle Definition of the candela Intensity is calibrated by a current from a known source with a known response s. Intensity

  6. Counting photons requires conversion to electrons. The photoelectric effect can eject electrons from a material into a vacuum. Exceed gap energy EG and electron affinity energy EA Compare to work function y Photoelectron Emission vacuum energy e- EA conduction band y Fermi energy hn EG valence band

  7. There is a probability that a photon will produce a free electron. Depends on bulk material properties Depends on atomic properties This is expressed as the quantum efficiency h(n). Reflection coefficient R Photon absorption k Mean e escape length L Probability to eject from surface Ps Probability to reach vacuum energy Pv Quantum Efficiency

  8. Photocathodes are designed to maximize the quantum efficiency. Layer of semiconductor or alkali compound on glass. Quantum efficiency dominated by L and Ps. Thin material that passes electrons easily for L Material with low EA to improve Ps Photocathode Factors

  9. Different photocathodes vary in response to frequency and in quantum efficiency. Alkali for UV detection (Cs-I, Cs-Te) Bialkali for visible light (Sb-Rb-Cs, Sb-K-Cs) Semiconductors for visible to IR (GaAsP, InGaAs) Commercial Photocathodes Hamamatsu.com

  10. Single photoelectrons would produce little current. Electrons can be multiplied by interaction with a surface. Emitter: BeO, GaP Metal substrate: Ni, Fe, Cu This electrode is called a dynode. Electron Multiplier e emissive surface substrate electrode

  11. Dynodes need good electron multiplication. Emission material accelerating potential for the incident electron Dynodes typically operate around 100 V. Factor d of 2 to 6 Multiplication Factor Hamamatsu.com

  12. Photomultiplier Tube • A photomultiplier tube (phototube, PMT) combines a photocathode and series of dynodes. • The high voltage is divided between the dynodes. • Output current is measured at the anode. • Sometimes at the last dynode

  13. Dynode gain d depends on the material and potential E. k typically 0.7 to 0.8 Multiple dynodes are staged to increase gain. Photocathode current Id0 Input stage current Idn Total gain is a product of stage gain. Collection efficiency a Gain

  14. Photomultiplier tubes often have 10 to 14 stages. Gain in excess of 107 A single photon can produce a measurable charge. Single photoelectron Qpe ~ 10-12 C Fast response in about 1 ns. Ipe ~ 1 mA Amplifier

  15. Photocathodes are not uniform. Variations in response on the surface. Angular response falls off beyond 40°. Position Response

  16. Dark Current • Phototubes have “dark” current even with no incident light. • Thermionic emission • Anode leakage • Case scintillation • Gas ionization • This increases with applied voltage.

  17. Dark current contributes to the noise in a measurement. Equivalent noise input For Df = 1 Hz, P ~ 10-15 W Signal to noise depends on the statistical fluctuations, dark current and readout circuit. Dominated by statistics Noise

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