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I have seen various uses of the word "thermal" recently (e.g. in black hole physics) that make me wonder what is the exact meaning of "thermal" in particle physics. There are also "thermal neutrons", for example, so my initial understanding of "thermal" as related "thermal radiation", i.e. propagated by photons, seems inadequate.

Can I apply "thermal" to any particle which meets the following criterion?

A thermal particle is a free particle with a kinetic energy corresponding to the most probable velocity at a temperature of 290 K (17 °C or 62 °F), i.e. the mode of the Maxwell–Boltzmann distribution for this temperature.

Or should the criterion be:

A thermal particle is a free particle with a kinetic energy corresponding to the most probable velocity at the temperature of its environment, i.e. the mode of the Maxwell–Boltzmann distribution for this temperature.

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    $\begingroup$ Your question is too broad to be answered. This question should be closed, in my opinion. $\endgroup$
    – Yashas
    Commented Feb 11, 2017 at 6:08
  • $\begingroup$ @YashasSamaga - made the question narrower. $\endgroup$
    – Frank
    Commented Feb 11, 2017 at 7:07

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Thermal particle is a particle which is in thermal equilibrium with its medium. For example, the neutrons produced in nuclear reactors have energy in the order of MeVs, it is the equivalent of some billions of K temperature. After they hit the water molecules of the cooling water many times, they are so slow as if they would be a neutron gas with the same temperature (around 0.01eV). Their temperature largely affects their reactions.

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  • $\begingroup$ So, the second definition would be correct? $\endgroup$
    – Frank
    Commented Feb 11, 2017 at 22:26
  • $\begingroup$ @Frank Yes, the second is exact, but also the first isn't so bad. For example, in a reactor, the cooling water is around 2-300C hot, thus we could say, the thermal neutrons have there higher energy as in a room temperature lab. But it only means that they have around 0.02eV energy instead 0.01eV, while the terminology is used mainly in a context we are talking about neutrons until the MeV regime. What is important: thermal particle means thermal equilibrium with its environment, which is not always room temperature (as the first definition says). $\endgroup$
    – peterh
    Commented Feb 11, 2017 at 22:37
  • $\begingroup$ @Frank Note, simetimes there are experiments with ultra-cool neutrons having kinetical energy of the superfluid He or more less. As I know, in this context, "thermal" means still room temperature and for them the terminology "cool" or "ultra-cool" is used. Thus also the first definition is, in my opinion, okay. $\endgroup$
    – peterh
    Commented Feb 11, 2017 at 22:43
  • $\begingroup$ Thanks again! What is striking for me here is that "thermal" is actually not related to "thermal radiation", or at least not directly. No photons are involved, except I guess, when the neutrons transfer some of their energy to the surrounding molecules/atoms ... which in turn emit photons when they go back to their ground state? $\endgroup$
    – Frank
    Commented Feb 11, 2017 at 23:12
  • $\begingroup$ @Frank Directly don't relate, indirectly yes: if any of the particles in the medium interacts electromagnetically, there will be also a "photon gas" around them, with the same temperature. And in practice, only in very rare cases has the whole experiment or simulation only electrically neutral particles (maybe neutrino or dark matter calculations could). $\endgroup$
    – peterh
    Commented Feb 11, 2017 at 23:31
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Thermal means a type radiation (i.e photons) which is given out by a body to reduce the temperature of itself. It is generally in microwave range.

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  • $\begingroup$ OK - then what is a "thermal neutron"? $\endgroup$
    – Frank
    Commented Feb 11, 2017 at 4:50
  • $\begingroup$ Thermal also can indicate the kinetic energy of a particle in thermal equilibrium with its surroundings. Thermal neutrons are neutrons whose K.E. is about the same as other particles in the same environment. $\endgroup$
    – K7PEH
    Commented Feb 11, 2017 at 5:21

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