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K. Imamura, A. Takamine, K. Kikuchi, R. Mitsuyasu, S. Akimoto, M. Ito, K. Tsubura, A. Gladkov, M. Tajima, S. Go, M. Mukai, M. Doi, M. Nishimura, T. Yamamoto, H. Endo, Y. Fukuzawa, S. Sasamori, S. Takahashi, M. Hase, K. Kawata, H. Nishibata, Y. Ichikawa, A. Kitagawa, T. Wakui, H. Ueno, Y. Matsuo

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This version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature’s AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections. The Version of Record is available online at: https://doi.org/10.1007/s10751-024-01889-y.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

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[Injection of high energetic ion beams to superfluid helium as a hostmatrix of laser spectroscopic study of radioisotope atoms](https://mdr.nims.go.jp/datasets/2975d332-eff0-460a-ad47-f57bfa917adb)

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Injection of High Energetic Ion Beams toSuperfluid Helium as a Host Matrix of LaserSpectroscopic Study of Radioisotope AtomsK. Imamura1,2, A. Takamine2, K. Kikuchi2,3, R. Mitsuyasu2,3,S. Akimoto2,3, M. Ito2,3, K. Tsubura2,3, A. Gladkov2,M. Tajima1, S. Go2, M. Mukai4, M. Doi2,3, M. Nishimura2,3,T. Yamamoto2,3, H. Endo2,3, Y. Fukuzawa2,3, S. Sasamori2,3,S. Takahashi2,3, M. Hase5, K. Kawata6, H. Nishibata7,Y. Ichikawa7, A. Kitagawa8, T. Wakui8, H. Ueno2, Y. Matsuo2,3*1Japan Synchrotron Radiation Research Institute (JASRI), Hyogo,679-5148, Japan.2RIKEN Nishina Center (RNC), Saitama, 351-0198, Japan.3Department of Advanced Sciences, Hosei University, Tokyo, 184-8584,Japan.4Department of Applied Energy, Nagoya University, Aichi, 464-8603,Japan.5National Institute for Material Science (NIMS), Ibaraki, 305-0047,Japan.6Center for Nuclear Science (CNS), The University of Tokyo, Saitama,351-0106, Japan.7Department of Physics, Kyushu University, Fukuoka, 819-0395, Japan.8National Institutes for Quantum Science and Technology (QST),Chiba, 263-8555, Japan.*Corresponding author(s). E-mail(s): yukari.matsuo@hosei.ac.jp;Contributing authors: kimamura@spring8.or.jp;AbstractWe are developing a laser spectroscopic method for the study of nuclear struc-ture of radioactive isotope utilizing superfluid helium (He II). In the method, HeII is used as an efficient stopper for highly energetic ion beams and as an in-situ1laser spectroscopic environment. Recently, we conducted an ion stopping exper-iment for the energy of approximately 350 AMeV 84Rb37+ at QST-HIMAC.Radioactive 84Rb ions were produced via projectile fragmentation reaction usingaccelerated 84Kr36+ and 12-mm thickness Be target. As the first step, we mea-sured the stopping range of the injected ion beam using liquid N2 to estimatethe stopping range in He II and the spot size of the injected ion beam using aplastic scintillator. Then, laser-induced fluorescence (LIF) detection experimentwas performed using He II. We successfully observed the LIF from 84Rb atoms.We estimated stopping range of 84Rb ions in superfluid helium from the obtainedresults. The details of experiment at QST-HIMAC and results are given in thisreport.Keywords: Ion beam stopping in liquid, Superfluid helium, Nuclear physicsexperiment, Matrix laser spectroscopy1 IntroductionNowadays, experiments with cryogenic materials have been developed in manyresearch field such as atomic, molecular, nuclear physics, materials science and so on.Especially, liquid and solid helium provides a unique experimental environment forlaser-based research of atoms. ions. and molecules [1]. We are now developing a laserspectroscopic method using atoms in superfluid helium (He II) in order to utilize sucha unique environment to the study of radioactive isotopes generated as highly ener-getic ion beams at accelerator facilities. In our technique, an accelerated ion beamwhose kinetic-energy ranges from several tens to several hundreds MeV per nucleonis injected into He II. Owing to the relatively high stopping power of He II, injectedions are stopped in narrow region in the liquid [2]. The injected ions capture elec-trons from surrounding helium atoms during stopping process. Namely, the ions areneutralized. Moreover, it is known that superfluid helium environment has less influ-ence on nuclear properties, although it has much effect on the electronic transition ofimplanted atoms [3, 4]. In order to investigate nuclear structure, the production of theatomic spin polarization of the stopped atoms is conducted using optical pumping viaa circularly-polarized laser light. We can measure the Zeeman splitting and the hyper-fine structure splitting using double resonance method to deduce a nuclear spin anda nuclear electromagnetic moment. We have shown the validity of our method usingstable and radioactive isotopes of Rb (A = 84 − 87) produced at RIKEN projectilefragment separator (RIPS) in RIKEN Nishina Center for Accelerator Based Scienceso far. In a series of the previous experiments, 84,86Rb ions were produced using ionbeams of 85,87Rb which were accelerated approximately up to 66 AMeV. We success-fully observed double resonance spectra with the beam intensity of the order of 103particles per second [5]. As for the detection of the laser induced florescence (LIF)from the stopped atoms, we concluded that the beam intensity of approximately 200particles per second was enough for the detection using the current setup [6]. As thenext step, we are planning to investigate nuclear structure of far from stability line.However, such rare isotopes are often produced using ion beam energy of around 3502AMeV. Recently, we have been launching research and development experiments forapplying our technique to such higher energetic ion beams using Heavy Ion Medi-cal Accelerator in Chiba (HIMAC) at QST [7]. We experimentally investigated thestopping region of atoms using 84Rb37+ produced from 350 AMeV Kr36+ ion beamand Be target. In the experiment, we first used liquid N2 as stopping material. Then,LIF detection experiment was performed using He II. We estimated stopping range of84Rb ions in superfluid helium from the obtained results. We here show the details ofexperiment and the results in this report.2 ExperimentFig. 1 The schematic diagram of the setup. (a) top view. (b) side view.The experimental setup at QST-HIMAC is given in figure 1. The bird view of theSB2 beam line in HIMAC is given in ref [8]. The primary beam of 350 AMeV Kr36+beam was impinged on a 12-mm thickness Be target. A radioactive 84Rb37+ beamproduced via projectile fragment reaction was selected and delivered to the exper-imental apparatus. Note that the primary beam was generated as pulsed beam of200 ms pulse duration. The delivered 84Rb37+ ion beam was injected to the chamber(pre-cryostat chamber) at the upstream of the cryostat. In the pre-cryostat cham-ber, aluminum energy degraders of different thicknesses were mounted to optimize thebeam energy. The ion beam passed through the aluminum degraders and a collimatorplaced at downstream of the degraders were counted one by one using a plastic scintil-lator assembled with two photo-multiplier tubes (PMTs). Then, the energy degradedRb ions were injected into liquid N2 from the 10 mmϕ beam port which has 75 µmthickness Kapton foil for vacuum sealing. The ions injected into the cryostat weredetected using the plastic scintillator of 250 µm thickness fixed at the center of thecryostat. The luminescence from the plastic scintillator whose intensity is proportionalto an energy deposition from the ion beam was detected from the bottom window ofthe cryostat using the florescence detection system. In order to determine stopping3range of the injected ions in the liquid, we measured the luminescence intensity ateach degrader thickness by single photon counting.3 Results and DiscussionFigure 2(a) shows the observed luminescence intensity normalized by the beam inten-sity as a function of the aluminum degrader thickness (t). As seen in Fig. 2(a), thecurve consists of two components. The main component is the contribution of Rb ions.The second component appearing in thicker aluminum degrader region is due to 82Krions which can not be completely separated ion beam selection process. The peak ataround t = 600 µm is considered as so-called Bragg peak which appears because theenergy loss of charged particles in a material is inversely proportional to the square oftheir velocity. For the stopping range estimation, we calculate the slope of the obtainedcurve at each aluminum degrader thickness which given in Fig. 2(b). We deduced thethickness of the aluminum degrader to stop Rb ions at the center of the cryostat byfitting to Gaussian function. As a result we obtained aluminum degrader thickness oft = 769(27) µm as a required thickness for stopping Rb ion. The width of approxi-mately 214 µm at FWHM was the ion stopping range when we considered aluminumas a stopping material.Fig. 2 (a) The obtained curve in a stopping range measurement shown as a function of aluminumdegrader thickness (t). (b) Differential of Fig. 2(a). The first peak of the Fig. 2(b) is due to the Braggpeak structure which appears around t = 600 µm. Two depths at t = 800 µm and t = 1500 µmcorresponds to the ion stopping of 84Rb37+ and 82Kr36+, respectivelyAccording to the Bragg-Kleeman rule which is a semi-empirical formula giving arange of ions in a material. the relation between range of same ion species in differentmaterial expressed as follows [9],R1R2≈ ρ2√A1ρ1√A2where Ri, ρi, and Ai are range in material i, material density of i, and mass numberof material i, respectively. We estimate the stopping range of 84Rb ions in superfluid4helium using the values below: RAl = 214 µm, ρAl = 2.701 g/cm3, AAl = 26.98,ρHe = 0.146 g/cm3, AHe = 4.00. We obtained 1.52 mm at FWHM as the estimatedstopping range of 84Rb in superfluid helium. Next, laser induced fluorescence (LIF)detection experiment was performed using He II material and a laser that can exciteRb atomic transition. We successfully observed the LIF from 84Rb atoms in He II witha beam intensity of approximately 10k particles per second [10].The stopping range of84Rb in He II was evaluated approximately 1.1 mm at FWHM by comparing with theLISE++ calculation. Both results using liquid N2 and He II verified that the stoppingrange of 84Rb in He II resides within the diameter of conventional excitation lasers.Fig. 3 Schematic diagram of the beam transportation. Al degrader, collimator, and plastic scin-tillator in the left-hand side is stored in the pre-cryostat chamber. The right-hand side blue circleexpresses the horizontal cross-sectional view of the cryostat.In addition to the stopping range measurement using liquid N2, we measured theion beam size dependence using different collimator diameter with aluminum degraderthickness of 750 µm. Figure 3 shows the schematic image of the beam transportation.Assuming that the beam profile in the surface perpendicular to the beam propagat-ing axis available to be delivered to the cryostat corresponds to an integrated valueof a gaussian function, we investigated the collimator diameter dependence of thebeam intensity. Using the relation between Gaussian integration function and an errorfunction as below, ∫ d/2−d/2exp[− x22σ2]=√2πσerf[ d2√2σ]where d is collimator diameter, x is distance from the beam center, σ is fitting param-eter, we could deduce the transverse width of the 84Rb in beam to be 33(5) mm inFWHM from the fitting result using error function.4 ConclusionIn conclusion, we are developing a new laser spectroscopic method for investigatingnuclear spin and nuclear electromagnetic moment of rare-isotopes. The method utilizes5Fig. 4 The normalized luminescence intensity displayed as a function of the collimator diameter.Red line shows the resultant fitting curve using error functionsuperfluid helium (He II) as a efficient stopper for highly energetic ion beam and as aspectroscopic environment for in-situ laser spectroscopy. We have shown the feasibilityof our method using an accelerated 84−87Rb ion beam whose energy was 66 AMeV sofar. In order to investigate the stopping range of injected ion with higher kinematicenergy, we conducted a ion beam stopping experiment using a 350 AMeV 84Rb ionbeam produced from 84Kr in QST-HIMAC. In this experiment, we first used liquidN2 as a stopping material. The measurement results show that injected 84Rb ionswere successfully stopped in liquid N2 using an aluminum degrader whose thicknesswas 760(27) µm. We estimated the stopping range of the ions in He II using Bragge-Kleeman rule from the result. We obtained the estimated stopping range in He II of1.52 mm at FWHM of gaussian distribution. A subsequent experiment using He II asa stopping medium and a single frequency laser as a excitation light source for thestopping Rb at QST-HIMAC is in progress.Acknowledgments. The experiment conducted as a Research Project with HeavyIons at QST-HIMAC. The authors highly appreciate the earnest support form Depart-ment of accelerator and medical physics group and AEC support group. The work wassupported in part by JSPS KAKENHI Grants No. 18H05457 and 18H05462Declarations� Funding ‘H. Ueno was supported by JSPS KAKENHI Grants No. 18H05457 and18H05462’� Conflict of interest ‘Not applicable’� Ethics approval ‘Not applicable’� Availability of data and materials ‘The data that support the findings of this studyare available from the first or the corresponding authors upon reasonable request.’6� Authors’ contributions ‘All the authors contributed to the preparation and conduc-tion of the experiments. KI and AT performed data analysis. YM wrote applicationfor beamtime. KI wrote the main manuscript. All authors approve the manuscript.’References[1] Moroshkin, P., Hofer, A., Weis, A.: Atomic and molecular defects in solid 4He.Phys. 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