Two-dimensional beam profile monitor for the detection of alpha-emitting radioactive isotope beam

Ions with similar charge-to-mass ratios cannot be separated from existing beam profile monitors (BPMs) in nuclear facilities in which low-energy radioactive ions are produced due to nuclear fusion reactions. In this study, we developed a BPM using a microchannel plate and a charge-coupled device to differentiate the beam profiles of alpha-decaying radioactive isotopes from other ions (reaction products) produced in a nuclear reaction. This BPM was employed to optimize the low-energy radioactive francium ion (${\mathrm{Fr}}^{+}$) beam developed at the Cyclotron and Radioisotope Center (CYRIC), Tohoku University, for electron permanent electric dipole moment (e-EDM) search experiments using Fr atoms. We demonstrated the performance of the BPM by separating the ${\mathrm{Fr}}^{+}$ beam from other reaction products produced during the nuclear fusion reaction of an oxygen (${}^{18}O$) beam and gold (${}^{197}\mathrm{Au}$) target. However, as the mass of Au is close to that of Fr, separating the ions of these elements using a mass filter is a challenge, and a dominant number of ${\mathrm{Au}}^{+}$ renders the ${\mathrm{Fr}}^{+}$ beam profile invisible when using a typical BPM. Therefore, by employing the new BPM, we could successfully observe the ${\mathrm{Fr}}^{+}$ beam and other ion beams distinctly by measuring the alpha decay of Fr isotopes. This novel technique to monitor the alpha-emitting radioactive beam covers a broad range of lifetimes, for example, from approximately 1 s to 10 min, and can be implemented for other alpha-emitter beams utilized for medical applications.