Neutron-proton bremsstrahlung from intermediate energy heavy-ion reactions as a probe of the nuclear symmetry energy?

TL;DR

This study investigates the potential of hard photon emissions from neutron-proton bremsstrahlung in heavy-ion collisions as a probe for the nuclear symmetry energy, highlighting the ratio of photon spectra from isotope reactions as a sensitive indicator.

Contribution

It demonstrates that the ratio of hard photon spectra from isotope reactions is a robust and sensitive observable for probing the nuclear symmetry energy in intermediate energy heavy-ion collisions.

Findings

Photon yield and spectra are affected by symmetry energy but less than other uncertainties.

The ratio of photon spectra from isotope reactions is insensitive to these uncertainties.

This ratio shows a significant rise (~15%) with a 20% reduction in symmetry energy at high density.

Abstract

Hard photons from neutron-proton bremsstrahlung in intermediate energy heavy-ion reactions are examined as a potential probe of the nuclear symmetry energy within a transport model. Effects of the symmetry energy on the yields and spectra of hard photons are found to be generally smaller than those due to the currently existing uncertainties of both the in-medium nucleon-nucleon cross sections and the photon production probability in the elementary process $pn\to pn\gamma$. Very interestingly, nevertheless, the ratio of hard photon spectra $R_{1/2}(\gamma)$ from two reactions using isotopes of the same element is not only approximately independent of these uncertainties but also quite sensitive to the symmetry energy. For the head-on reactions of $^{132}Sn+^{124}Sn$ and $^{112}Sn+^{112}Sn$ at $E_{beam}/A=50$ MeV, for example, the $R_{1/2}(\gamma)$ displays a rise up to 15% when the symmetry energy is reduced by about 20% at $\rho=1.3\rho_0$ which is the maximum density reached in these reactions.