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¹³Be has not been observed. ¹³Be is predicted to have an atomic mass excess of 35.35 MeV (TH74), 34.60 MeV (JE75). It is then unstable with respect to decay into ¹²Be + n by 2.20 MeV or by 1.45 MeV, respectively, based on the atomic mass excess of ¹²Be, 25.078 MeV (AL78E). See also (76AJ04) and (SE77B; theor.).