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Ultraprecise atomic mass measurement of the alpha particle and 4He.

The atomic masses of the alpha particle and 4He 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 ppt/h) of the magnet over hundreds of hours of observation. The leading systematic shift [at -202(9) ppt] is due to the image charge located in the trap electrodes. The new value for the atomic mass of the alpha particle is 4 001 506 179.147(64) nu and the corresponding value for the mass of 4He is 4 002 603 254.153(64) nu (nu=10(-9) u). The 16 ppt uncertainty is at least 20 times smaller than any previous determination.