Noninvasive ion fraction quantification of dual-species beams in synchrotrons

Abstract

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 synchrotrons and to small relative mass-to-charge ratio offsets, typically at the 1e-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 4He2+ and 12C6+ 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 4He2+ and 12C6+ beams. The results show good agreement with independent post-extraction measurements.

Figures (6)

• Figure 1: Schematic mixed beam irradiation. The carbon beam is used for tumor treatment while the residual helium energy is measured downstream of the patient for diagnostic purposes.
• Figure 2: Mixed ^4He^2+ and ^12C^6+ beam charge distributions for different helium fractions $r$. The radial loop regulates the horizontal center of gravity of the mixed charge distribution, $\left\langle x\right\rangle_\mathrm{q,mix}$, to $x_\mathrm{RL}=\qty{27.5}{\milli\metre}$. Top: 1$\sigma_\mathrm{rms}$ beam envelopes along a PIMMS synchrotron. Bottom: Horizontal charge distributions at the radial loop pick-up with $D_x=\qty{-8.3}{\metre}$.
• Figure 3: Frequency and radial position offsets for mixed helium-carbon beams with different ion fractions $r$, estimated analytically (lines) and in simulations (grey diamond markers). The clinically relevant energy range is indicated by enhanced opacity. Left: relative frequency offsets with respect to a mono-isotopic ^12C^6+ beam. Center: Absolute frequency offsets in a PIMMS synchrotron. Right: Horizontal centroid of the mixed, carbon, and helium charge distributions at the radial loop pick-up with ${D_x = -8.3m}$. In this example the radial loop regulates to ${x_\mathrm{RL}=0mm}$.
• Figure 4: Schematic of the two complementary methods to determine the ion composition of the dual-species beam: the noninvasive RF-based measurement in the synchrotron and the post-extraction measurement with an ionization-chamber (IC) telescope.
• Figure 5: LLRF acquisitions at flat top for 262.3MeV^12C^6+, ^4He^2+ and mixed beams, with the radial loop regulating to ${x_\mathrm{RL} = 27.5mm}$. The gray-shaded time window is used to determine the average frequencies, shown by the solid lines. Left: Relative RF frequency offset with respect to a pure ^12C^6+ beam. The blue-shaded band marks the reference frequency $f_\mathrm{ref} \pm 3\sigma_{f_\mathrm{ref}}$. Right: Position measured at the radial loop pick-up.