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I have been studying cosmic ray muons lately, and I'm having trouble with making sense of all the eV values. What is the approxamate range (in eV) for "high-energy" and "low energy" cosmic rays? Does this differ with the particle? (e.g. between primary and secondary cosmic-rays or between particles such as muons, electrons, protons, pions etc).

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This depends on context. There are no generally-accepted definitions of energy ranges.

"Low energy" in a neutrino experiment like BOREXINO or JUNO will be 100s of keV, whereas for dark matter experiments, that's already "high energy". Similarly those neutrino experiments will think of 10MeV as "high energy" but that's just where the fun starts for SuperK. Not to mention IceCube and similar detectors which go way higher in energy, and even less to to mention dedicated ultra-high cosmic ray experiments like ANITA etc.

This is all just bad jargon, but there is an underlying reason for this confusion. Tom Gaisser assembled this now-famous plot showing the cosmic ray spectrum:

cosmic ray spectrum

Note that the horizontal axis spans 12 orders of magnitude! That's a huge energy range. Not surprisingly then, a large number of experiments are needed to cover all that. And this doesn't even consider the fact that there are different kinds of particles to measure, and that the flux drops by some 30 orders of magnitude from left to right on this plot (note the unit on the vertical axis). Thus, "low-energy" for one experiment is "high energy" for another. Don't get hung up on this, that's what the incorruptible electronvolts are there for.

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The energies of the primary cosmic rays range from around 1 GeV – the energy of a relatively small particle accelerator – to as much as 108 TeV, far higher than the beam energy of the Large Hadron Collider. The rate at which these particles arrive at the top of the atmosphere falls off with increasing energy, from about 10 000 per square metre per second at 1 GeV to less than one per square kilometre per century for the highest energy particles. The very high-energy cosmic rays generate huge showers of up to 10 billion secondary particles or more, which can be picked up by particle detectors when they are spread over areas as large as 20 square kilometres on the surface of the Earth.

Qualitatively, the energy is compared with energies available in the laboratory for experiments. The very high energy ones need the huge detectors in cosmic ray experiments . A list of cosmic ray experiments can be found here.

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