Injection of high energetic ion beams to superfluid helium as a host matrix of laser spectroscopic study of radioisotope atoms

We are developing a laser spectroscopic method to study the nuclear structure of radioactive isotopes utilizing superfluid helium (He II) as an efficient stopper for highly energetic ion beams and as an in-situ laser spectroscopic environment. Recently, we conducted an ion stopping experiment for the ^84 Rb ^37+ beams with the energy of approximately 350 A MeV at QST-HIMAC with a cryostat system used for 66 A MeV at RIKEN-RIPS experiment (Appl. Phys. Express 12, 016502 (2019)). The radioactive ^84 Rb ions were produced via a projectile fragmentation reaction using accelerated ^84 Kr ^36+ beams and a 12-mm thick Be target. As a first step, we measured the stopping range distribution of the injected ion beams in liquid N _2 to estimate the stopping range distribution in He II and the spot size of the injected ion beams using a plastic scintillator. Then, a laser-induced fluorescence (LIF) detection experiment was performed using He II. We successfully observed the LIF from ^84 Rb atoms. We estimated the longitudinal range straggling of ^84 Rb ions in superfluid helium from the obtained results. The details of experiment at QST-HIMAC and results are given in this report.