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We analyze the time evolution of the kinetic properties of nuclear matterproduced in heavy-ion collisions at Fermi energies. The collision system issimulated using Constrained Molecular Dynamics (CoMD) transport calculationswhose output is the isospin, position, and momentum of the nucleons. Focusingon central 35$\,$A$\cdot$MeV $^{40}$Ca+$^{40}$Ca collisions we utilize thisinformation to extract localized momentum distributions in volume elements of8$\,$fm$^3$ and time steps of 5$\,$fm/$c$. We then parameterize thesingle-particle momentum distributions with thermally motivated fit functionsin the local rest frame of each cell. While the transverse-momentumdistributions are well reproduced by thermal ones, the longitudinal ones carrya marked imprint of the initial nuclear motion which we capture by introducinga centroid motion into our fit functions. In particular, we find that Fermidistributions yield significantly better fits than Boltzmann ones, aconsequence of the Pauli blocking implemented in CoMD. From the fits we extractthe time dependence of the thermodynamic and collective properties of theexcited nuclear medium. We find that the transverse temperature gradually risesto about 6$\,$MeV, which is accompanied by a dissipation of the initialcentroid motion of the incoming nuclei which vanishes at about 100$\,$fm/$c$after initial impact. We are therefore able to track the transition of beamenergy into random kinetic energy for nucleons, suggesting a three-dimensionalequilibration of energy in the late stages of the collision.

Thermalization of Nuclear Matter in Heavy-Ion Collisions at Fermi-Energies