Answer to Question #103825 in Atomic and Nuclear Physics for John O'Renick

Question

I read that a neutron at 14.1 MeV “recoils” at 52,000 km/sec, 17% of C. I also read that 14.1 MeV equates to ~164 billion degrees K. That’s not at the same time, is it? Is that the temperature of the neutron when something stops it, when its kinetic energy becomes heat energy, assuming that it transfers none of that heat to its surroundings? If not, how does this work?

https://www.translatorscafe.com/unit-converter/en-US/energy/10-70/megaelectron-volt-kelvin/

https://en.wikipedia.org/wiki/Neutron_temperature#Fast

Answer 1

Your perplexity is understandable. The Wikipedia article you specified does not explain the concept of temperature. This concept refers to the field of statistical physics dealing with large ensembles of particles and not to the properties of a single particle. I would say That you have encountered the slang of nuclear physicists.

The comparison of neutron energy with temperature arose when analyzing the situation most often encountered in nuclear reactors with thermal neutrons. In these reactors, fast neutrons that are born in the process of fission of atomic nuclei are slowed down by special substances-retarders, their movement becomes chaotic, similar to the movement of molecules in an ideal gas, and the concept of temperature can be applied to this state of the neutron cloud in full. The vast majority of modern nuclear power plants operate on such neutrons.

For ultrafast neutrons, which are described in the article, the concept of temperature is applied conditionally. Of course, no substance can come into thermal equilibrium with them, because it would instantly evaporate long before this. However, they fly out of the thermonuclear reactor also randomly, in different directions, and have some energy distribution vaguely resembling thermal. In this sense, we can talk about their temperature. Of course, this value strictly speaking can not relate to an individual neutron.