Secondary nuclei from spallation

Nuclear fragments are produced by spallation in the interactions between primary particles and atomic nuclei in the detector material. This process can be described as a three-step reaction:

1. intra-nuclear cascade

2. pre-equilibrium emission

3. evaporation

In the intra-nuclear cascade, the primary particle interacts with individual nucleons inside the target, which trigger a microscopic cascade inside the nucleus with following (forward) emission of nucleons. This process usually occurs with typical timescales of the order of 10^{-22} sec. The remnant nucleus is left in a highly excited state and the energy deposited by the primary particle is dissipated by evaporation, where nuclear fragments are emitted isotropically. Most of the simulations codes used to describe the intra-nuclear cascade treat individual nucleons as free particles. Such a simplified approach does not describe the fast forward emission of nuclear fragments which might be produced by surface coalescence or nuclear reactions inside the nucleus.

This additional component is called pre-equilibrium emission and is the object of many studies at the existing spallation facilities.

In ALICE, the light nuclei measurements are affected by a significant contribution of secondary nuclei which represents the largest background component at low transverse momentum. This contribution is usually estimated using Monte Carlo simulations which are based on GEANT transport code.