The ion composition of dual-species beams in synchrotrons is typically inferred from invasive measurements performed after beam extraction. This paper introduces a complementary noninvasive method to determine the ion composition of such beams directly inside the synchrotron. The approach is applicable to low- and medium-energy ion synchrotrons and to small relative mass-to-charge ratio offsets, typically at the ${10}^{-4}$ level. The method exploits dispersive orbit offsets between the two species and corresponding frequency corrections applied by the synchrotron rf radial regulation loop. This capability is of particular interest for ongoing research on online monitoring in carbon ion beam therapy using mixed ${{}^4\text{He}}^{2+}$ and ${{}^{12}\mathrm{C}}^{6+}$ beams, which feature a relative mass-to-charge ratio offset of 0.065%. The proposed method is analytically derived and tested with particle tracking simulations using xsuite. Its applicability under realistic experimental conditions is demonstrated at the MedAustron facility using mixed ${{}^4\text{He}}^{2+}$ and ${{}^{12}\mathrm{C}}^{6+}$ beams. The results show good agreement with independent postextraction measurements.

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• Accelerator/storage ring control systems
• Beam diagnostics
• Medical physics & public health
• Proton therapy
• Radiation therapy
• Radio frequency calculations
• Radio frequency techniques
• Single-particle dynamics
• Accelerators & storage rings
• Advanced accelerator test facilities
• Machine protection
• Medical applications of accelerators
• Particle accelerator facilities
• Reliability & operability
• Particle accelerators