Thomas Onyango | Searcharxiv

arXiv2025

Photon Emission from Nucleon-Nucleon Bremsstrahlung in Fermi-energy Heavy-Ion Collisions

The emission of direct (hard and thermal) photons from nucleon-nucleon Bremsstrahlung in heavy-ion collisions at Fermi energies is analyzed. We utilize a photon emission rate based on a quantum-field-theoretical model together with nucleon distribution functions extracted from a coarse-graining method of transport model simulations. The latter accounts for off-equilibrium effects during the early stages of nuclear collisions primarily occurring in the beam direction while the transverse-momentum distributions are amenable to a thermal description. With this setup, we quantify the contributions from first-chance nucleon-nucleon collisions and the subsequent transition to a thermal source to the photon energy spectrum measured from Ca-Ca collisions at 35 A$\cdot$MeV bombarding energy. We find that most of the hard photons are produced in the initial stages of heavy-ion collisions from primordial collisions where nucleons move with the initial collective nuclear motion while the emission from the later stages plays a sub-dominant role. We compare our calculations to experimental measurements of a differential photon-energy spectrum from a collision system of similar size and beam energy, thereby including acceptance cuts as applied in the detectors.

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arXiv2021

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

We analyze the time evolution of the kinetic properties of nuclear matter produced in heavy-ion collisions at Fermi energies. The collision system is simulated using Constrained Molecular Dynamics (CoMD) transport calculations whose output is the isospin, position, and momentum of the nucleons. Focusing on central 35$\,$A$\cdot$MeV $^{40}$Ca+$^{40}$Ca collisions we utilize this information to extract localized momentum distributions in volume elements of 8$\,$fm$^3$ and time steps of 5$\,$fm/$c$. We then parameterize the single-particle momentum distributions with thermally motivated fit functions in the local rest frame of each cell. While the transverse-momentum distributions are well reproduced by thermal ones, the longitudinal ones carry a marked imprint of the initial nuclear motion which we capture by introducing a centroid motion into our fit functions. In particular, we find that Fermi distributions yield significantly better fits than Boltzmann ones, a consequence of the Pauli blocking implemented in CoMD. From the fits we extract the time dependence of the thermodynamic and collective properties of the excited nuclear medium. We find that the transverse temperature gradually rises to about 6$\,$MeV, which is accompanied by a dissipation of the initial centroid motion of the incoming nuclei which vanishes at about 100$\,$fm/$c$ after initial impact. We are therefore able to track the transition of beam energy into random kinetic energy for nucleons, suggesting a three-dimensional equilibration of energy in the late stages of the collision.

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