Photon Counting Task

Reader:  Hello. My company assigned me to a project which involves doing photon counting. We’re just getting started on this. During our first staff meeting, some people suggested using an expensive professional-grade CCD camera as our prime detector. But one engineer suggested a photomultiplier tube was the best detector to use, since we don’t need to acquire any spatial-resolved data for this task. Some staff at the meeting said this was an old-fashioned solution. What’s your take on this?

Tony’s Reply:  Since I don’t know any of the details about your project, except that you stated that you “Don’t need to acquire any spatial-resolved data . . .” my reply to your question is pretty easy.

A CCD camera, regardless of its quality or cost, is not mandated, because you have no need to record photon arrival along with spatially-resolved information, i.e. imagery data. If you basically need to do photon-counting of some weak light flux, then a photomultiplier tube, suitably chosen for this task based on its specs, and calibrated by the tube manufacturer is an ideal way to go. Here’s why:

The classical photomultiplier tube (a glass vacuum tube with a photocathode and electron-multiplier dynodes) is inherently the closest thing to a perfect photon detector. This detector may be considered “old fashioned” by some younger engineers, but the fact is, the photomultiplier tube (or PMT) is still the ideal photon detector. For this short blog I won’t go into all the reasons why it is unbeatable.

As for doing photon-counting, there’s a list of questions you will need to answer before you buy components and set up your photon-counting system. PMTs come with many different photocathodes (hence different spectral responsivities); you need to decide which is best for your application. PMTs come in different physical package sizes and shapes. You need to decide, etc. For most serious photon-counting jobs the PMT will need to be cooled to a low temperature (we’re not talking liquid nitrogen here) so it probably ought to be installed inside a special cooling chamber with a suitable window, which will transmit the spectral band you are interested in. This affects what kind of form-factor PMT is appropriate to use. Other specs for the PMT need to be considered, based on the photon-counting task at hand. Finally there is the topic of the electronics hardware you need to support the PMT, in order to get reliable results for the actual photon-arrival-rates which you want to record. But lots of researchers have done this with great success, most notably astronomers and nuclear instrumentation engineers!

So, my quick answer was:  PMT – Yep, that’s the way to go. But the real answer for your task will be longer, because there are a lot of engineering decisions you need to make, based on your application. Best thing to do is contact some PMT manufacturers and talk with one of their Application Engineers about this. They are going to be really knowledgeable on PMTs.

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