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I still see photomultiplier tubes (PMT) used for even benchtop spectroscopy experiments. How much more sensitive are they to CCDs?

I found a post which claims about 1.5 x more sensitivity for PMTs, but any references or more complete understanding of the problem would be really appreciated.

I'm also wondering if they need to be cooled or specialized chips or if commodity imaging chips like from a camera has reasonable S/N characteristics for detection?

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Photo-multiplier tubes, specially those with III-V semiconductor photo-cathodes, are able to detect single photons, in the main part of the visible spectrum. (blue to red)

CCDs are not capable of doing that.

Gallium Arsenide, and Gallium Phosphide are the principal III-V semi-conductor photo-cathode types. The highest quantum efficiency is obtained with "Dormer Window" types, where the light is introduced at grazing angles to the photo-cathode, so the photons see a thicker layer of absorbing material than the resulting emitted photo-electrons do, as they are extracted from the face of the cathode film.

Cooling of the photo-multiplier tubes is often used to reduce thermionic emission from both the photo-cathode and the multiplier dynodes. Dry ice, and liquid nitrogen being two available methods. Some photo-multiplier tubes also include accessory magnets, often ceramic, to steer the emitted photo-electrons, to the multiplier structure, while generally deflecting cathode thermionic emission electrons, which come from a wider area of the cathode. This results in lower noise.

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  • $\begingroup$ nice description of high performance PMTs - any idea of the specific characteristics of CCDs in comparison? I'm looking at qPCR machines and was surprised to find a four channel PMT, so not cooled, but perhaps still cheaper (~$800 used) than getting a CCD to meet spec... $\endgroup$ – shigeta Sep 28 '13 at 15:27
  • $\begingroup$ Shigeta, CCDs are almost always used as pixel arrays for imaging, so it is uncommon to see them for single point detectors. PMTs of course are non imaging, so are generally preferred for applications such as spectroscopy. I don't have any good data on high performance CCDs even though my namesake received the 2009 Nobel Physics prize for their invention (he was at Bell Labs). Today of course CMOS sensors are being used for digital cameras, displacing CCDs except for very high performance applications such as astronomy. SS sensor noise is capacitance limited; PMTs are not. $\endgroup$ – user26165 Sep 28 '13 at 20:36
  • $\begingroup$ I see the point, and don't really question it. Still would like to try to understand and quantitative difference. numbers are good to have if they can be found. After all I did some homework - I already found a quantitative comparison else where, but without any reference or substantiation (see the link in my question). Was looking to see if I could find a better answer. $\endgroup$ – shigeta Sep 29 '13 at 5:17
  • $\begingroup$ A lot depends on what you mean by "sensitivity". A PMT can detect a single arriving photon, and emit a single photo-electron, with about 85% max quantum efficiency depending on the frequency of the photon, and the PMT cathode material. That single photo-electron can be amplified as much as a million times or more, depending on the electron multiplier design and materials, and generate an extremely fast anode current pulse , that has comparatively low noise. A single arriving photon on a CCD pixel, will create a signal that will get swamped by subsequent amplifier noise. $\endgroup$ – user26165 Sep 30 '13 at 18:58

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