The $Q$ value of the ${}^{76}\mathrm{Ge}$ double beta decay has been determined by measuring the masses of ${}^{76}\mathrm{Ge}$ and ${}^{76}\mathrm{Se}$ in a Penning trap using neon- and fluorinelike ions. The obtained masses are 75.921 402 758(96) u and 75.919 213 795(81) u, respectively. The systematic errors of these two determinations are nearly equal, and therefore, the remaining systematic uncertainty of the $Q$ value is drastically reduced. A $Q$ value of 2 039.006(50) keV was obtained improving the accuracy of the accepted value by a factor of 6.

• Received 11 September 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.4259