Charge-state manipulations and systematics of low-energy RI beams extracted from a low-pressure gas cell at JAEA-ISOL

Precision experiments with short-lived radioactive isotopes (RI) using ion trap techniques are being prepared at the the Isotope Separator On-Line (ISOL) facility at the Japan Atomic Energy Agency (JAEA). This offers the potential to expand new research areas in nuclear physics and chemistry, including mass measurements of neutron-rich transactinide nuclei and in-trap gas-phase ion chemistry of superheavy elements. To conduct such experiments, efficient and rapid production of low-energy RI beams with energies in the electron-volt range is required.

In this study, we have developed a windowless gas cell cooler and buncher (GCCB) designed to transport 30-keV ion beams from ISOL into a 100-Pa helium gas cell through a entrance hole. It subsequently decelerates and thermalizes ions solely via gas collisions with helium atoms, entirely obviating the need for electrostatic deceleration [1]. Thermalized ions are subsequently transported using radiofrequency (RF) ion guide techniques, which include RF carpet, sextupole ion guide, and quadrupole mass separator. To investigate the gas cell performance and the extraction charge-state systematics, we used both stable and RI beams with a wide range of elements from Rubidium (Z = 37) to Actinium (Z = 89). We found that their extraction charge states were distributed up to 3+ and showed a strong dependence on their atomic structure. Our results allowed us to calculate the charge-changing cross-section at low energies [2] and could help predict the charge state distribution for extracted ion beams of desired elements, such as superheavy elements with production rates of less than one particle per hour. We have also conducted preliminaly experiments to manipulate the charge state with reaction gases to achieve a single charge state. This is aimed to maximize efficiencies for experiments following the GCCB.

In this talk, we will present the details of the GCCB apparatus and experimental results, along with the prospects of future experiments.

[1] Y. Ito et al., JPS Conf. Proc. 6 (2015) 030112, Ion preparation systems for low-energy experiments at SLOWRI.

[2] I.Yu. Tolstikhina, Y. Ito, V.P. Shevelko, Nucl. Instrum. Meth. B 532 (2022) 27, Prediction of charge-changing cross sections of low-charged ⁸⁸Sr, ¹³⁸Ba and ¹⁴²Nd ions in a He-gas target at collision energies 50 eV/u-10 GeV/u.