The mass excess of ${}_{}{}^{13}{}_{}{}^{}\mathrm{O}$ has been determined to be 23.107 ± 0.015 MeV. This yields a value for $d$, the coefficient of ${T_Z}^3$ in the isobaric multiplet mass equation, of -0.8 ± 3.4 keV for the $A=13\mathrm{}$ quartet. Additional values for $d$, obtained from new measurements of the masses of ${}_{}{}^{17}{}_{}{}^{}\mathrm{Ne}$, ${}_{}{}^{21}{}_{}{}^{}\mathrm{Mg}$, and ${}_{}{}^{25}{}_{}{}^{}\mathrm{Si}$, are also consistent with zero.

• Received 9 July 1970

DOI:https://doi.org/10.1103/PhysRevLett.25.533