The temperature is achieved by slamming two nuclei together. There is enough time for the various particles inside to scatter off each other many times and establish a thermal distribution of energies between themselves, but not enough time for any of them to interact with anything outside the two nuclei. After the collision takes place, theses particles will stream out of the interaction region just like in normal proton-on-proton collisions, going through decays and physically traveling through various detector elements.

Presumably, the temperature is inferred by measuring the various energies of the outcoming particles (just like in a normal collision) and noting that they are distributed thermally (i.e. according to Bose-Einstein or Fermi-Dirac statistics). This distribution is tell-tale evidence that the particles exchanged energy through scattering many times.

I guess you could call this thermal imagining, although "thermal imaging camera" denotes something that looks at infrared radiation and measures its distribution (in order to infer the temperature of the matter that radiated it). In this case, the particles themselves (and their decay products) are exploding outwards and having their energies measured.