| Summary: | We use an electric-dipole laser-driven transition to precisely measure the cyclotron-frequency ratios of the pairs ^{42}Ca^{+}−^{40}Ca^{+}, ^{44}Ca^{+}−^{40}Ca^{+}, and ^{48}Ca^{+}−^{40}Ca^{+} in a 7-tesla Penning trap. A single laser-cooled (T≈1 mK) ion serves, together with photon-counting and photon-imaging units, as a radiofrequency detector covering a broadband frequency spectrum, in the present case from kHz to a few MHz. Such detectors (^{40,42,44,48}Ca^{+}) allow measuring extremely small forces increasing the sensitivity in Penning-trap mass spectrometry. The direct determination of the ions' amplitudes makes a cyclotron-frequency measurement process more robust against inhomogeneities of the magnetic field and/or deviations of the electric quadrupole field due to mechanical imperfections of the trap.
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