Can relativistic heavy-ion collisions only probe the global shape of colliding nuclei, or their detailed internal structure as well? Taking ${}^{20}\mathrm{Ne}$ as an example, we attempt to directly probe its internal $\alpha$-cluster structure, by comparing experimentally measured observables in collisions at relativistic energies from density distributions of ${}^{20}\mathrm{Ne}$ with and without $\alpha$-cluster structure. Since the two density distributions give the same nucleus size and deformation, they lead to similar midrapidity observables. However, the $\alpha$-cluster structure may considerably reduce the free spectator nucleon yield and enhance the spectator light nuclei yield, as a result of more compact initial phase-space distribution of nucleons inside $\alpha$ clusters. We propose to measure the scaled yield ratio of free spectator neutrons to charged particles with mass-to-charge ratio $A/Z=3$, 3/2, and 2 in ultracentral ${}^{20}\mathrm{Ne}+{}^{20}\mathrm{Ne}$ collisions, which is found to be reduced by about $25\%$ at $\sqrt{s_{NN}}=7$ TeV and about 20% at $\sqrt{s_{NN}}=200$ GeV with $\alpha$-cluster structure in ${}^{20}\mathrm{Ne}$. This scaled yield ratio thus serves as a robust and direct probe of the existence of $\alpha$-cluster structure in ${}^{20}\mathrm{Ne}$ free from the uncertainty of midrapidity dynamics.

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• Cluster models
• Nuclear shapes and moments
• Relativistic heavy-ion collisions
• 20 ≤ A ≤ 38