Speaker
Description
Gaseous active targets are a key technology for nuclear physics experiments requiring full kinematic reconstruction of beam and low-energy reaction products. However, their application to high-intensity heavy-ion beams is limited by severe background from delta-electrons, which significantly degrade the signal-to-noise ratio and trigger performance in conventional readout systems. In experiments with heavy-ion beams exceeding $10^5$ counts per second at $\sim 100$~MeV/nucleon, such as tin isotopes, a large number of energetic delta-electrons reach the sensitive region of the gaseous active target CAT-M. The accumulation of ionization electrons from these backgrounds leads to severe signal degradation, making stable operation with standard electronics difficult.
Suppression of delta-electrons is therefore essential for extending active target techniques to high-intensity environments. Electrons produced in these reactions are light particles with a maximum energy of about $300$~keV, corresponding to a magnetic rigidity of about $0.002$~Tm. To confine these electrons within $10$~mm of the beam path, a magnetic field of approximately $0.2$~T is required. While traditional coils are impractical for an active volume of $28 \times 32 \times 20$~cm$^3$, recent high-remanence permanent magnets enable a compact and efficient solution.
To address this issue, we developed an ``embedded permanent magnet delta-electron sweeper'' integrated directly inside CAT-M to sweep delta-electrons away from the active region. The magnet is coupled with grading strip electrodes to maintain a uniform electric field. Since the dipole field prevents direct beam tracking in the main TPC, auxiliary small-scale TPCs are installed at the entrance and exit to recover precise beam trajectory information.
The system was commissioned at HIMAC using a $^{132}$Xe beam at $\sim 100$~MeV/u (21H445). The implementation of this magnetic suppression system improved the signal-to-noise ratio by more than a factor of 450. The dead region, which was caused by setting the higher threshold and was 40% of the active area for the high energy particles, is recovered. The system was successfully applied to deuteron scattering on $^{86}$Kr in inverse kinematics (21H445). These results demonstrate that a compact permanent-magnet-based approach enables gaseous active targets to operate under high-intensity heavy-ion beams, achieving stable operation that was previously not feasible. This development opens a path toward experiments that were previously limited by severe background conditions.