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¹⁸C is particle stable. Therefore its atomic mass excess, M - A, must be < 29.84 MeV [¹⁶C + 2n] (WA70E). ¹⁸C has been observed in the bombardment of ²³²Th by 122 MeV ¹⁸O ions (AR69E, AR70D) and in the 3 GeV proton bombardment of Au (RA70). See also (ZE60A, PO68B, BU71E).