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Two-proton relative momentum (q pp ) and opening angle (θ pp ) distributions from the three-body decay of two excited proton-rich nuclei, namely 23 Al → p + p + 21 Na and 22 Mg → p + p + 20 Ne, have been measured with the projectile fragment separator (RIPS) at the RIKEN RI Beam Factory. An evident peak at q pp ∼ 20 MeV/c as well as a peak in θ pp around 30 • are seen in the two-proton break-up channel from a highly-excited 22 Mg. In contrast, such peaks are absent for the 23 Al case. It is concluded that the two-proton emission mechanism of excited 22 Mg is quite different from the 23 Al case, with the former having a favorable diproton emission component at a highly excited state and the latter dominated by the sequential decay process.

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