Charge resolution in the isochronous mass spectrometry and the mass of $$^{51}$$Co

Isochronous mass spectrometry (IMS) of heavy-ion storage rings is a powerful tool for the mass measurements of short-lived nuclei. In IMS experiments, masses are determined through precision measurements of the revolution times of the ions stored in the ring. However, the revolution times cannot be resolved for particles with nearly the same mass-to-charge (m/q) ratios. To overcome this limitation and to extract the accurate revolution times for such pairs of ion species with very close m/q ratios, in our early work on particle identification, we analyzed the amplitudes of the timing signals from the detector based on the emission of secondary electrons. Here, the previous data analysis method is further improved by considering the signal amplitudes, detection efficiencies, and number of stored ions in the ring. A sensitive Z-dependent parameter is introduced in the data analysis, leading to a better resolution of $$^{34}$$ Ar $$^{18+}$$ and $$^{51}$$ Co $$^{27+}$$ with $$A/Z=17/9$$ . The mean revolution times of $$^{34}$$ Ar $$^{18+}$$ and $$^{51}$$ Co $$^{27+}$$ are deduced, although their time difference is merely 1.8 ps. The uncorrected, overlapped peak of these ions has a full width at half maximum of 7.7 ps. The mass excess of $$^{51}$$ Co was determined to be $$-27{,}332(41)$$ keV, which is in agreement with the previous value of $$-27{,}342(48)$$ keV.