An algorithm of pre-equilibrium clustering of spectator matter based on the construction of the minimum spanning tree (MST) is presented. The algorithm was implemented in the Abrasion-Ablation Monte Carlo for Colliders (AAMCC) model designed to study the characteristics of spectator matter in collisions of relativistic nuclei. Due to accounting for the pre-equilibrium clusters in modelling 208Pb–208Pb collisions at the LHC, the agreement of simulation results with experimental data on the average multiplicities of spectator nucleons was improved. The results of the AAMCC-MST were compared with experimental data on the interactions of 197Au nuclei in nuclear photoemulsion. Comparison of the yields of spectator nuclei calculated for 16O–16O collisions with the yields measured in interactions of 16O with light nuclei of photoemulsion made it possible to estimate the effect of MST-clustering in small nuclear systems.

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We study the composition of spectator matter in ultracentral collisions of heavy relativistic nuclei with neutron-rich periphery known as a neutron skin (NS). The yields of spectator neutrons and protons in ultracentral 208 Pb-208 Pb collisions at the LHC were calculated within a new version of Abrasion-Ablation Monte Carlo for Colliders (AAMCC) model. Calculations demonstrate variations of the cross sections ( , ) to produce given numbers of spectator neutrons and protons depending on the parameters of NS in 208 Pb, in particular, for events with low . The relative difference between ( , ) obtained with different NS thickness reaches 150%. Furthermore, the values of ( , ) calculated with same difference between neutron and proton RMS-radii, but with different neutron half-density radius and diffuseness parameter, differ by 45%. The considered cross sections can be measured in the ALICE experiment at the LHC for 208 Pb-208 Pb to restrict the parameters of NS in colliding nuclei.