Electromagnetic probes of nuclear collisions at intermediate energy

Abstract

We show that emission of high-energy electrons and photons in nuclear collisions at intermediate energies is sensitive to the space-time evolution of the reaction. The electron and photon spectra measure related but complementary quantities connected with the nuclear charge distribution. We show that at 60 MeV/u beam energy production of 50 MeV electrons is predicted to measurable probability. (2) whereas the influence of the mean field is thought to dominate at the lower end [2], two-body collisions probably provide the main reaction mechanism at the higher energies [3]; (3) collective scattering of many nucleons is required to produce new particles, such as pions. Different models have been invoked to describe various aspects of these collisions, e.g. localized energy equilibrium ("hot spots") [4], statistical coalescence models [5] and coherent field methods [6]. However, no generally accepted unified picture of these reactions has emerged, because most existing experimental data provide a test for the results but not for the underlying assumptions of these models. We suggest to utilize energetic photons and electrons as probes of the nuclear reaction process at intermediate energies, because their emission mechanisms are well understood and their final state interactions are negligible. While lepton-pairs and photons are recognized as experimental signals for ultra-relativistic nuclear reactions [7], electrons, positrons and photons are known to reflect nuclear sticking times in low-energy collisions [8,22]. More than 20 years ago a bremsstrahlung experiment was proposed [9] to measure the nuclear time delay in order to offer an unambiguous separation between compound nucleus reactions and direct reactions by utilizing the fact, that the compound nucleus reactions are delayed. The usefulness of electromagnetically interacting particles at intermediate energies, however has not been widely discussed yet. The recently observed [10 13] subthreshold production of neutral pions motivates an independent determination of the reaction time scale. Since neutral pion production has been measured at bombarding energies as low as 20 MeV/u it is believed to be of cooperative resp. collective origin because the independent nucleon model with Fermi motion fails [14] in predicting pions below 50MeV/u. Hence the pions should depend strongly on the evolution of the nuclear collision zone [15,16], respectively on the deceleration time. In our treatment of photon and electron emission we shall assume that the nuclear charge and current density during the reaction can be described classically. This is probably a reasonable approximation as long as the energy carried away by the particle is