Information on mass chains and nuclides available on this website:

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⁹He is predicted to be particle unstable: its calculated mass excess > 40.17 MeV (WA70E, WA72A), = 43.54 MeV (TH72K). Particle instability with respect to ⁸He + n, ⁷He + 2n and ⁶He + 3n implies atomic mass excesses greater than 39.7, 42.25 and 41.812 MeV, respectively. See also (CE68A). ⁹He has not been observed in a pion experiment [⁹Be(π^-, π⁺)⁹He] (GI65E) nor in the spontaneous fission of ²⁵²Cf (CO67A).