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⁹He has not been observed: see (74AJ01). It is predicted to be particle unstable. Particle instability with respect to ⁸He + n, ⁷He + 2n and ⁶He + 3n implies atomic mass excesses greater than 39.667, 42.253 and 41.808 MeV, respectively. The calculated mass excess of ⁹He is 43.49 MeV based on the modified form of the mass equation (JE75). See also (TH74) and (IR74A, BE75Q, IR76C; theor.).