Why does a coincidence measurement in, for example, a scintillator paddle detector identify a particle as a muon? Couldn't it be some other particle that happens to travel through both detectors? Or say you have two Cherenkov detectors with one about a metre above the other. Then if they both detect a signal close together would you be able to say yep, that's a muon, and if so why?

Here's the link that seems to me to suggest that the coincidence measurement is what allowed them to say they've detected muons. They don't give details of any other identification method unless I've completely missed it.

Statistically I think most detections would be muons because of their sheer number, so is that what they're basing their statement on?


1 Answer 1


A simple coincidence counter provides no particle ID, so it is largely insensitive to what species has triggered it. So, yes, you assume the species from statistics, but muons make up an overwhelming fraction of the particle at ground level (about 45:1 over protons and neutrons according to the Particle Data Group).

Now we know the statistics of the population by building detectors that have particle ID. The simplest design would be a hodoscope telescope with a magnet but using a drift chamber or time-projection chamber (again with a analyzing magnet) with probably give better results.

  • $\begingroup$ Thank you for the clarification. In which case, what is the purpose of coincidence measurements? To reduce the angles from which a particle could be detected, since cosmic rays are largely incident vertically? $\endgroup$
    – user13948
    Nov 5, 2016 at 15:58
  • $\begingroup$ There is a lot of stray radioactivity in the environment. The counts from a single counter include contamination from local sources. A coincidence counter is reliably triggered only by some penetrating event going the right direction, so the contamination is cut way down (your detector will still record background events that happen to trigger the paddles at almost the same time, but there are fewer of those). Some cosmic ray lab setups I've seen have three detectors and let you demand either a 2 or 3 paddle events so you can measure the amount of contamination you are getting. $\endgroup$ Nov 5, 2016 at 16:06
  • $\begingroup$ Is the statistic of 45:1 from the particle data group published online anywhere? I'd very much appreciate a link if it is. And thank you for the detail in your answers, it's very useful. $\endgroup$
    – user13948
    Nov 5, 2016 at 17:48
  • $\begingroup$ @Karacoreable Of course. The PDG publishes the Particle Data Book every year. This is the go-to reference for established factual data in the particle physics world. There is a figure (24.3 in the 2011 edition, though the numbering might change from year to year) which shows the flux by particle species as a function of height. I just read off the values for muons and nucleons. $\endgroup$ Nov 5, 2016 at 19:04

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