Quantum dominance of coherent bremsstrahlung in $^{124}$Sn + $^{124}$Sn scattering at 25 MeV/u

Abstract

We present quantum-mechanical calculations of bremsstrahlung in the $^{124}$Sn + $^{124}$Sn at 25 MeV/u reproducing the measured photon spectrum over the full energy range. For the first time, we quantitatively determine the incoherent-to-coherent ratio in the photon spectrum. This ratio is extremely small, ranging from $10^{-11}$ to $10^{-4}$, which demonstrates that coherent emission dominates throughout the measured energy range. This behavior is in sharp contrast to proton-nucleus scattering, where incoherent emission dominates because of the leading role of nucleon magnetic moments. This contrast is illustrated by the TAPS Collaboration data for $p + ^{197}{\rm Au}$ collisions at a proton beam energy of 190 MeV, where the corresponding ratio reaches $10^{3}$-$10^{5}$. We find that incoherent-to-coherent ratios explain the difference between the two spectra in unified picture: (1) In proton--nucleus scattering, the spectrum contains a pronounced hump, (2) In $^{124}$Sn + $^{124}$Sn scattering, the spectrum decreases monotonically and has a nearly logarithmic shape. Our results identify a previously unexplored quantum regime of bremsstrahlung emission in nuclear reactions and open a new route for studying coherent effects in heavy-ion collisions.