The detection of previously unknown nuclear aluminum-20 was achieved by observing attenuation during its flight.

Currently, more than 3,300 nuclides have been identified, yet fewer than 300 are stable and naturally occurring. The remaining nuclides are unstable and undergo radioactive decay.

By the mid-20th century, researchers had discovered several common decay modes, including α-decay, β+ decay, electron capture, γ-radiation, and nuclear fission.

In the last few decades, advancements in nuclear physics experimental facilities and detection techniques have enabled the discovery of various exotic decay modes, particularly in nuclei that are far from stability, especially neutron-deficient nuclei.

In the 1970s, the phenomenon of single proton radioactivity was identified, where the nucleus was attenuated by releasing a proton.

In the 21st century, the discovery of bipolar radioactivity emerged, attributed to the decay of highly neutron-deficient nuclei.

Recently, even rarer disintegration events have been recorded, including those resulting in three, four, and five products.

“Aluminum-20 is the lightest aluminum isotope ever discovered,” states Dr. Xiaodong Xu, a physicist at the Institute of Modern Physics, Chinese Academy of Sciences.

“It resides across the proton drip line and has seven fewer neutrons compared to stable aluminum isotopes.”

Employing in-flight damping techniques with fragment separators at the GSI Helmholtz Center for Heavy Ion Research, physicists assessed the angular correlation of the damping products of aluminum-20.

Their detailed analysis of these angular correlations revealed that the ground state of aluminum-20 initially decays by releasing one proton into an intermediate ground state of magnesium-19, which subsequently collapses through the simultaneous release of two protons.

Aluminum-20 marks the first observed tripolar emitter, classified as a bipolar radionuclide.

The research also indicated that the damping energy of the aluminum-20 ground state is significantly lower than anticipated based on isospin symmetry, suggesting a potential breaking of isospin symmetry between aluminum-20 and its mirror partner, neon-20.

This conclusion is endorsed by advanced theoretical calculations predicting that the spin parity of the aluminum-20 ground state differs from the spin parity of the neon-20 ground state.

“This research will enhance our understanding of the proton evaporation phenomenon and provide insights into the structural dynamics and collapse of nuclei beyond the proton drip line,” Dr. Xu remarked.

The team’s paper was published this month in the journal Physical Review Letters.

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X.-D. Xu et al. 2025. Isospin symmetry revealed through the attenuation of the three-proton emitter aluminum-20. Phys. Rev. Lett. 135, 022502; doi:10.1103/hkmy-yfdk