Ultraprecise Atomic Mass Measurement of theαParticle andHe4

The atomic masses of the $\alpha$ particle and ${}^4\mathrm{He}$ have been measured by means of a Penning trap mass spectrometer which utilizes a frequency-shift detector to observe single-ion cyclotron resonances in an extremely stable 6.0 T magnetic field. The present resolution of this instrument approaches 0.01 ppb [10 ppt (parts per trillion)] and is limited primarily by the effective stability ($<5\mathrm{ppt}/\mathrm{h}$) of the magnet over hundreds of hours of observation. The leading systematic shift [at $-202(9)\mathrm{ppt}$] is due to the image charge located in the trap electrodes. The new value for the atomic mass of the $\alpha$ particle is $4001506179.147(64)\mathrm{nu}$ and the corresponding value for the mass of ${}^4\mathrm{He}$ is $4002603254.153(64)\mathrm{nu}$ ($\mathrm{nu}={10}^{-9}\mathrm{u}$). The 16 ppt uncertainty is at least 20 times smaller than any previous determination.