iglis-net newsletter no. 8 june 20202 iglis-net newsletter no. 8 june 2020 resonance fluorescence...

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June 2020 IGLIS-NET Newsletter No. 8

Introduction This is the eighth issue of the IGLIS-NET (In-Gas Laser Ionization and Spectroscopy NETwork) newsletter. The IGLIS-NET was launched on Dec. 2012, and is now constituted by 17 participating research groups and institutes from 11 countries, aiming to regularly exchange communications among these participants. The periodic issuance of this newsletter summarizing the status of the research activities of the participating groups is an important means to facilitate this communication. The present issue includes six status reports from the IGLIS laboratory at KU Leuven, IGISOL facility at JYFL, GALS at Dubna, SLOWRI at RIKEN, KISS at KEK, and a first contribution from the slow and stopped beam program at FRIB.

IGLIS-NET News

★ The IGLIS laboratory at KU Leuven reports on the development of a new nozzle contour designed at the Von Karman Institute (VKI) for Fluid Dynamics which has been machined with a high precision and minimal surface roughness. The local flow properties of gas jets produced by such a nozzle have been fully characterized by PLIFS as well as by a new method based on Resonance Ionization Spectroscopy (RIS), and the results are being analyzed and will be published soon. The new RIS method is complementary to PLIFS but about 300 times more efficient. The transverse beam emittance of the ions extracted from the RFQ ion guides in the IGLIS beam line has been determined and a new gas cell designed to extract 229Th isotopes has been installed and tested. The performance of the mass separator on the mass resolution of heavy isotopes has been demonstrated.

★ At the IGISOL facility, JYFL, the completion of the nuClock FET-OPEN project

has not deterred general progress towards investigations in actinide elements. A new decay spectroscopy setup will be used in a detailed study of the fusion-evaporation products produced in the interaction of high-energy protons with 232Th targets. The team is also involved in a new EU Marie Curie project, LISA (Laser Ionization and Spectroscopy of Actinides), which will focus on the training of a new generation of young scientists using state of the art tools in laser spectroscopy to focus on understanding of the atomic and nuclear properties of the actinides. Other long-term projects include the construction of the MARA Low Energy Branch with initial characterization tests reported using the new gas cell. Further development of the laser spectroscopy capabilities, including upgrades to the laser systems, are exploited in recent optical

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resonance fluorescence spectroscopy of neutron-rich Pd isotopes for the first time. A fruitful beginning to a new program of high-precision laser-RF double resonance spectroscopy is initiated with studies of the octupole moment of scandium. An unusually large magnetic octupole moment in stable 173Yb has also been investigated using conventional collinear laser spectroscopy.

★ The GALS project at JINR, Dubna, which is devoted to the production and study

of heavy neutron-rich nuclei in the region of N = 126 by multinucleon transfer reactions, reports on their recent progress. The laser laboratory was supplemented with new TiSa lasers. They were tested and their optimization for the first experiments is in progress. Works on other GALS subsystems and components continue. More precise simulations and more detailed comparison of SPIG and S-shaped RFQ ion guides are reported.

★ SLOWRI at RIBF of RIKEN reports that from April 1, 2019, they have entered

into official collaboration to jointly operate devices under auspices of both RNC and WNSC/KEK. In this year, the off-line testing of a 50-cm-long He gas cell with new RF carpets, as well as a new multi-reflection time-of-flight mass spectrograph (MRTOF) dedicated to on-line commissioning and systematic mass-measurements for radioactive isotopes at F11 of BigRIPS in FY2020, have nearly been completed. Since F11 is located at the end of ZeroDegree Spectrometer (ZDS), symbiotic utilization of RI beams, which can pass through detectors for particle identification in ZDS, together with other (main) experimental programs such as HiCARI project, can be realized. Parallelly, the on-line commissioning for an Ar gas cell with resonant laser ionization (PALIS) has been continuously performed at F2 of BigRIPS. For the optimization of optical components on SLOWRI beam line, a compact Cs ion source has been installed at the upstream of the beam line.

★ The KISS group at KEK reports on their recent online experiments and

continued R&D efforts toward nuclear spectroscopic studies of nuclei around N = 126. In terms of R&D they have implemented MRTOF-assisted laser resonant ionization spectroscopy and optimized the YAG laser path length to suppress the odd-order replica peaks in the excitation laser produced by dye amplifier successfully, making the response function much simpler. They provide detailed reports on three online experiments for (1) in-gas-cell laser

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spectroscopy of 194,196Os isotopes for the study of nuclear structure, (2) b-g spectroscopy of 192mOs, 192g,192mRe (Spokesperson : H. Watanabe) to perform the decay spectroscopy from the nuclear astrophysical interest and (3) b-g spectroscopy of 186g, 186m, 187g, 187mTa isotopes (Spokesperson : P. Walker) to study nuclear structure of the high-K isomers.

★ The low-energy and stopped beams group at Michigan State University’s Facility

for Rare Isotope Beams (FRIB) provides their first report to this newsletter. They provide an overview of their facility and describe their activities in Penning trap-based mass measurements at LEBIT and SIPT, laser spectroscopy at BECOLA, and their efforts with reaccelerated beams.

Recent publications

Please look at IGLIS-NET website (http:/research.kek.jp/group/wnsc/iglis-net/), where the publications are grouped according to the topics defined in the IGLIS-NET framework.

Any requests or comments about the IGLIS-NET newsletter are welcome. Please send them via [email protected].

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Status Report (1) On the characterization and optimization of the IGLIS technique at KU Leuven A. Claessens, K. Dockx, R. Ferrer*, S. Kraemer, Yu. Kudryavtsev, P. Van den Bergh, P. Van Duppen, M.

Verlinde and E. Verstraelen. KU Leuven

I. Development of well-collimated, high Mach number gas jets

New theoretical and experimental investigations are ongoing at KU Leuven to improve the production of collimated gas jets with uniform low temperatures and low densities. Following up the previous experimental campaign [1], a new set of calculations of a nozzle contour with Mach number M=8.5 was accomplished by using an advanced simulation code at the von Karman Institute for Fluid Dynamics (Brussels). Unlike in previous studies, these results revealed the need for a nozzle to have a conical geometry to overcome the thick boundary layer and obtain a collimated gas jet with a homogenous temperature at M=8.5. Figure 1. (Left) Nozzle machined out of brass reproducing with a high precision the contour obtained by simulations. (right) Plot of the deviation of the machined contour from the calculated one.

Machining of such a nozzle contour was realized on campus with a high

precision in order to rule out mechanical imperfections from a non-optimal performance in experimental conditions. Two nozzles made out of brass with a precision of < 5 µm

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in the inner contour and a surface (peak-to -valley) roughness of 100 nm were obtained by CNC machining.

Flow properties such as velocity, relative density and temperature of the gas jet

produced by this nozzle were obtained by Planar Laser Induced Fluorescence Spectroscopy (PLIFS) and also by Resonance Ionization Spectroscopy (RIS). In the latter, copper atoms seeded in the argon buffer gas is resonantly ionized in the gas jet using a two-step ionization scheme. The first step laser is sent (anti)collinear to the gas jet while the second step laser, with a small beam spot diameter, interacts in transverse geometry probing different areas of the jet with the help of a set of mirrors placed on motor-controlled movable platforms (see Fig. 2). In this way, laser spectroscopy is performed by scanning the first step laser around the 2S1/2 à 2P1/2 excitation at 327 nm and the local flow properties defining the Mach number can be obtained. By changing the laser beam paths, one can additionally obtain full radial cross sectional images of the jet properties that complement the axial view obtained by the PLIFS method. Furthermore, the characterization of the jet properties by RIS is about 300 times more efficient than with the PLIFS technique.

First results of the local flow properties of jets formed by the new nozzle show a good agreement between the PLIFS and RIS data and reveal a Mach number M~8

L1

L2

Vertical motion

Horizontal motion

Figure 2. Schematic layout of the

laser arrangement used to

characterize the flow properties by

Resonance Ionization Spectroscopy

(RIS) of copper atoms.

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with very homogeneous temperature and density distributions. The results of these measurements are currently being analyzed and will soon be published. [1] A. Zadvornaya et al. Phys. Rev. X 8, 041008 (2018)

II. Characterization of the transverse beam emittance In a former set of studies, we reported the design and commissioning tests, including the longitudinal ion beam emittance, of the radiofrequency quadrupole (RFQ) ion guides used in the IGLIS laboratory at KU Leuven [1]. The RFQ ion guides are important devices to efficiently transport the laser ionized species in the gas jet (P≈10-1 mbar) up to the acceleration region (P≈10-5 mbar). Owing to ion-atom collisions within the first RFQ structure, the original ion beam distribution is cooled down, thus improving the transverse ion beam emittance ε. In recent studies we were able to determine ε and fully characterize the optical properties of the extracted ion beams.

Figure 3 shows schematically the gas cell and the electrode configuration in the IGLIS beam line used in these studies. Laser ionized copper atoms were extracted from the gas cell and transported through the S-shaped RFQ (S-RFQ) and the differential pumping RFQ (DP-RFQ) before passing through the last ion guide structure (IG-RFQ). Leaving the IG-RFQ they were accelerated to an energy of 40 keV and focused by means of an Einzel lens on an MCP detector inserted in the path of the ion beam.

Figure 3. Schematic layout of the partial IGLIS beam line used to determine the transverse beam emittance of the ion beams extracted from the RFQ ion guides.

A phosphor screen, serving as the MCP anode, was used to produce an image of the ion beam, which was recorded by a CCD camera. From a series of recorded beam images (see Fig. 4, left), each of them for a different voltage at the Einzel lens, the

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spatial distribution of the beam was obtained. Fitting the spatial distributions as a function of the voltage in the Einzel lens (see Fig. 4, right), along with the results obtained in simulations of the focusing properties of the Einzel lens, the ion beam emittance could be determined as explained in [2].

From these studies the beam emittance was found to be ex= 2.5(2) p mm mrad for the horizontal component and ey= 2.8(2) p mm mrad for the vertical component.

Figure 4. (Left) Image of the beam spot on the phosphor screen recorded by a CCD camera and the corresponding fit of the Gaussian intensity distributions in the vertical and horizontal dimensions. (Right) Spatial width of the beam spots for the vertical and horizontal components as a function of the Einzel lens voltage.

[1] S. Sels, R. Ferrer et al. Nucl. Instr. Meth. B 463, 143 (2019)

[2] R. Ferrer et al. Nucl. Instr. Meth. A 735, 382 (2014)

III. A fast gas cell for the production of 229mTh ion beams A new gas cell of smaller dimensions than those of the S3-prototype used in former off-line studies has been installed and tested (see Fig. 5 (left)). This new gas cell is specifically designed for efficient thermalization and fast extraction of 229Th isotopes produced in the decay of 233U deposited on the inner walls of hollow cylindrical sources placed around the gas cell exit channel. Additionally, the cell accommodates both a filament and an atomic/ionic ablation target that can be used for stable material evaporation. In the first commissioning tests, resistive heating of a copper filament was used to produce an atomic copper vapor that

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Figure 5. (Left) CAD drawing of the fast gas cell designed to extract 229mTh. (Right) Simulated time profies of 229mTh in the gas cell. An evacuation time of the volume in which the 229Th will be stopped of ~1 ms can be inferred, in good agreement with the measured value for copper atoms.

was subsequently laser ionized in the gas cell to characterize the gas cell extraction time, essential in the 229mTh research. A number of time profiles were obtained and compared to gas-flow-dynamics simulations in COMSOL, in which we could confirm the fast extraction time of about 1 ms, predicted by the same simulations for a realistic 229mTh recoil distribution in the gas cell (see Fig. 5 (right)).

In the next measurements we plan on loading the gas cell with the 233U sources and investigating the production of the so-far-not-observed 229mTh+1 charge state. Based on the measured nuclear moments available in the literature and assuming a FWHM ~160 MHz in laser spectroscopy studies in the gas jet (T~ 20 K) produced by our newly tested nozzles, we foresee a successful separation of the ground nuclear state from the isomeric state in 229Th. This would lead us to the production of purified 229mTh ion beams that could later on be used to measure with a somewhat higher precision the energy of the isomer by VUV spectrometry studies.

In line with these measurements, we also tested out the performance of the IGLIS dipole magnet on the mass resolution of heavy masses. To this end, we performed laser ablation of a gold target and analyzed the products of ablation by mass (m/q) scans with the dipole magnet. A typical mass scan over 20 mass units around the expected mass of the stable gold isotope is shown in Fig. 6. From a finer scan of the gold peak (shown in the inset) we could extract a resolving power R=m/Dm= 385 (12),

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which might be sufficient to resolve the 229Th isotopes from neighboring molecular contamination in the planned laser spectroscopy studies on thorium.

Figure 6. Mass scan obtained with dipole magnet of the IGLIS beam line. Mass peaks at practically every mass, likely originating from molecular species from the ablation process, are observed around the 197Au peak. The inset shows a finer mass scan of the 197Au peak from which a resolving power R = 385(12) can be deduced.

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Status Report (2) Optical spectroscopy for nuclear and atomic physics at JYFL S. Geldhof1, W. Gins1, R. de Groote1, A. Koszorus1,2, S. Kujanpää1, I.D Moore1, I. Pohjalainen1, M. Reponen1, S. Zadvornaya1 1University of Jyväskylä, 2University of Liverpool Introduction The following report shortly summarizes the various activities of the JYFL team and collaborators during the period since the seventh IGLIS-NET newsletter. In the Fall of 2019 we welcomed a new addition to our team, Agi Koszorus, who joined following her PhD at the University of Leuven. Employed by the University of Liverpool, UK, Agi is based at JYFL and will focus on the collinear laser spectroscopy program. Sonja Kujanpää has also joined our team as a Master’s student working under the supervision of Ruben de Groote.

In addition to the individual reports presented in the following, we note that only minimal progress has been made on the Cs atom trap project, mainly due to a current lack of personnel. New efforts to apply for funds are underway within the UK. Despite this, local activity focused on the Cs ampoules received from the reactor at ILL, Grenoble. Some of the samples delivered were reference 133Cs sources and were used to develop safe handling procedures for the 134Cs samples as well as to determine the efficiency of producing ion beams at IGISOL. Samples were prepared on Ta foils, dried and inserted into the hot cavity catcher ion source. The mass-separated focal plane current was recorded as a function of time, and by integrating and comparing to the estimated number of atoms in the initial sample, an efficiency of close to 20% was obtained. In parallel, the activity of all ampoules was measured (separately) using a Ge detector, with several MBq obtained.

In other news, the HV area of the RF cooler-buncher and JYFLTRAP was modified in order to house the ion trap for MORA, which is due to arrive in 2021. More details on this project can be found in [1]. In the previous newsletters we have presented the ongoing technical studies in the continuous wave laser laboratory connected with frequency stabilization and frequency determination. These efforts are summarized in the PhD thesis of Sarina Geldhof who will defend her thesis, titled “Developments for high-resolution laser spectroscopy and application to palladium isotopes”, in May 2020 [2]. The performance of different wavelength meters at several EU institutes and facilities has recently been published in two parts [3, 4], of which part 1 is connected

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with the work in Sarina’s thesis. The general upgrades of the collinear laser spectroscopy beamline have also been published [5].

[1] P. Delahaye, E. Liénard, I. Moore et al., Hyp. Int. 240 (2019) 63.

[2] S. Geldhof, PhD thesis, University of Jyväskylä,

http://urn.fi/URN:ISBN:978-951-39-8156-3

[3] M. Verlinde, K. Dockx, S. Geldhof, K. König, D. Studer et al, Appl. Phys. B 126 (2020) 85.

[4] K. König et al., Appl. Phys. B 126 (2020) 86.

[5] R. de Groote et al, Nucl. Instr. and Meth. B 463 (2020) 437.

I. Actinide studies at IGISOL In the summer of 2019, the nuClock FET-OPEN project (www.nuclock.eu) officially finished. Our final contributions to that project include the recent publication of the spectroscopic characterization of 233U samples [1], and submission of our manuscript entitled “Gas cell studies of thorium using filament dispensers at IGISOL”, in 2020. Our actinide project continues however. The analysis of the preliminary online experiment (presented in the previous newsletter) with high-energy protons on a 232Th target, used to produce neutron-deficient actinide isotopes via fusion-evaporation reactions, is almost complete. Currently we are preparing for another online beam time with the focus on the testing of a new spectroscopy setup, allowing for gamma-gamma and alpha-alpha correlations, as well as the detection of conversion electrons. Up to four Ge detectors will be positioned in a frame around an implantation chamber (Fig. 1), three of which are broad energy-type to optimize efficient detection of low-energy gamma-rays. Two quadrant Si detectors and a standard 300 mm2 Si detector for the detection of alphas, and a liquid-nitrogen-cooled Si(Li) detector will be available. The mass-separated ion beam will be implanted at 30 keV into a thin C foil mounted in the center of the chamber. The goal is to obtain additional data on the radioactive decays of A=227, 226 and 225 chains, in particular the decay of Pa and Th isotopes and respective daughters. In a second phase of the beam time, we will use the PI-ICR method at JYFLTRAP for both yield exploration as well as mass measurements of key isotopes. These measurements will support the mass evaluation of superheavy elements which are linked to the lighter actinides via alpha-decay chains. In 2019, the EU funded a new 4-year Marie Curie Innovative Training Network (ITN) for the career development of 15 Early Stage Researchers (ESRs) across 12 host institutions throughout Europe. LISA (Laser Ionization and Spectroscopy of Actinides) will focus on the training of a new generation of young scientists using state of the art

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tools in laser spectroscopy focused on an understanding of the atomic and nuclear properties, as well as applications of, the actinides. We are delighted to be within this project and look forward to welcoming our new ESR PhD student, Andrea Raggio, later this year. More details of the LISA network can be found in [1].

Figure 1. Schematic of the new decay spectroscopy setup planned for the study of light actinide isotopes produced in proton-induced fusion-evaporation reactions. [1] https://lisa-itn.web.cern.ch/

II. Collinear laser spectroscopy developments In the previous newsletter, we presented the status of the mass spectrometry and laser spectroscopy of neutron-rich Ag fission fragments, with additional data obtained in 2019. The charge radii extracted from this work have been used to test the predictions of a new Fayan’s energy density functional, in a similar manner as discussed in the following section for Pd isotopes.

The spectral coverage of the continous wave laser system was extended to the deep UV range, from 210 nm to 250 nm. This can be achieved by inroducing a second frequency doubling unit, thus generating the fourth harmonic of the fundamental output of the laser. This development enables us to produce the deep UV light necessary for the study of e.g. Co and other refractory species which have not been studied online before. First tests indicated that e.g. 3 mW of 235 nm light can easily be generated, sufficient for collinear laser spectroscopy.

We furthermore wish to upgrade the light collection region in the beamline. The goal is to improve the geometrical effiency of the detection system, and the pressure in the vacuum chamber by moving to conflat flanges. The design includes several PMTs, an

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upgrade compared to the single PMT currently in use. Ray tracing simulations are being performed to inform the CAD design of the new chamber and lens mounting system. III. Collinear laser spectroscopy of fission fragments of palladium Between the region of nuclear structure associated with atomic number Z~40 and mass A~100, famous for sudden changes of nuclear shape, and the more smoothly evolving behaviour in the Sn, In, Cd and Ag nuclei, lie elements whose radioactive isotopes have yet to be probed via laser spectroscopy. The elements Tc, Ru, Rh and Pd are difficult to produce for conventional ISOL facilities and have complex atomic structure, contributing to challenges for optical measurements.

In a collaboration involving researchers from JYFL, Liverpool, Manchester, GANIL, ISOLDE-CERN and TU Darmstadt, collinear laser spectroscopy of neutron-rich fission fragments of Pd has been performed for the first time. As discussed in the previous newsletter, four different optical transitions, accessible to the Ti:sapphire laser system, were tested on stable isotopes of Pd for efficiency and sensitivity to atomic parameters prior to the main experiment [1]. In 2019, optical spectra were then obtained on a series of even-A radioactive isotopes (112,114,116,118Pd) as well as data on the odd-A isotopes of 113,115Pd. Figure 2 highlights the shift observed in the optical transition for stable and radioactive even-A isotopes.

Figure 2. Optical resonance fluorescence spectra of even-A isotopes of Pd.

Conventional nuclear spectroscopy indicates a change of deformation in the Pd

isotopes however the origin and character of such evolution remains an open question.

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With our new data, changes in mean-square charge radii have been determined and compared to a set of nuclear energy density functionals. A range of Skyrme functionals have been tested as well as calculations from recently developed Fayans functionals, proved to reproduce charge radii in spherical nuclei. In our work, we have been able to test these latter functionals in deformed nuclei for the first time. The analysis is presented in the recent PhD thesis of Sarina Geldhof and will shortly be prepared for publication [2].

In April 2020, a new proposal to the JYFL PAC to study neutron-deficient isotopes of Pd in light-ion fusion-evaporation reactions was awarded beam time. This work will form part of the PhD thesis of Alejandro Ortiz Cortes, who will be working for one year at IGISOL on a cotutelle agreement with GANIL, France. [1] S. Geldhof et al., Hyp. Int. 241 (2020) 41.

[2] S. Geldhof, PhD thesis, University of Jyväskylä,

http://urn.fi/URN:ISBN:978-951-39-8156-3

IV.Resonance laser ionization of neutron-deficient Ag isotopes The neutron-deficient silver isotopes around the N=Z line remain an attractive goal for many facilities worldwide, including the future S3 facility at SPIRAL-2, the Ion Catcher collaboration at the FRS, GSI, and the future MARA-LEB facility, JYFL. In the previous newsletter we presented a short summary of the preliminary beam time in 2018 using a rf inductively-coupled hot cavity laser ion source for the production and study of 104-97Ag. Following this, further developments have been made, in particular a re-design of a dual-etalon Ti:sapphire laser in order to produce laser light with a better frequency overlap with the Doppler-broadened ensemble of atoms. Synchronized etalon scanning for almost 25 GHz in the fundamental is realized with a so-called look-up table. The team achieved an important milestone in the summer of 2019. Selective and efficient resonance laser ionization of 104-96Ag produced, stopped and extracted from the graphite catcher, has been combined with the mass purification capabilities of the JYFTRAP Penning trap and the detection of ions using the Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique.

The frequency of the ground state 328.2-nm atomic transition was scanned as part of a three-step resonant laser ionization scheme while recording the mass-purified ions using a position-sensitive detector. In this way, background-free laser spectroscopy measurements were performed. An example of a frequency scan for 99Ag, shown in Fig. 3, reveals clear evidence of the production of the high-spin ground state from the

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heavy-ion fusion reaction 92Mo(14N, 2pxn)104−96Ag. The optical data have allowed for the extraction of changes in mean-square charge radii across the N=50 shell closure below 100Sn for the first time and will provide a critical test for nuclear theory. In addition, JYFLTRAP was used to measure the mass of the high-spin ground state of 97Ag with high precision.

Figure 3. In-source laser spectroscopy of 99Ag using JYFLTRAP as a mass purifier and the PI-ICR method for ion detection. Top: Imposing a mass gate based on the ion impact coordinates on the 2D MCP allows the simultaneous RIS of the ground state (blue) and the 506 keV isomer (red). Bottom: Even though the low-spin isomer (red) is produced weakly compared to the ground state (blue), the ultra-low background achieved through the PI-ICR allows the state to be probed optically.

V. First characterization tests of the MARA-LEB gas cell Work towards realization of the MARA Low-Energy Branch (MARA-LEB) is ongoing with simulations of the ion-optical elements, designs of the radiofrequency (RF) ion guides and a large number of technical drawings being prepared. Preliminary aspects of the ion-transport system were published in the recent EMIS conference series [1] and additional information on the separator MARA can be found in [2]. Three second-hand pump lasers (Lee Laser) have been purchased in the past month and are currently on their way to JYFL to be used both for the IGISOL pulsed laser system as well as to pump the Ti:sapphire lasers for MARA-LEB.

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In the Fall of 2019, the first offline tests of the gas cell for MARA-LEB were performed following adaptation to the IGISOL target chamber as shown in Fig. 4. A radioactive 223Ra α-recoil source (T1/2= 11.4 d) was installed at different locations within the cell. Survival efficiency and evacuation time of ions transported by subsonic helium gas flow were studied. Cleanliness of the gas cell was monitored via ionization of impurities caused by the recoiling ions. A maximum ion survival efficiency of 12% and shortest evacuation time of about 100ms were measured using an exit orifice of 1.3 mm in diameter. The results have been compared with numerical simulations using COMSOL Multiphysics software.

Figure 4. The gas cell designed for MARA-LEB undergoing offline testing at IGISOL (left). A mass scan showing stable isotopes of tin following in-gas-cell resonance laser ionization (right).

The tests continued in 2020 with a bronze filament (about 91% copper and 9%

tin by mass) installed and resistively heated to produce an atomic vapor. Two-step laser resonance ionization has been used to selectively ionize stable isotopes of tin inside the gas cell. The right panel in Fig. 4 shows a mass scan of the isotopes with their expected isotopic abundances. Pressure broadening and shift of the second step atomic transition have been probed in grade 6 argon gas. Evacuation time profiles will be compared with gas flow simulations. The ion collector electrodes, used to collect non-neutralized species entering the gas cell from the MARA separator have also been successfully tested with a 223Ra alpha recoil source in helium gas. Suppression of the ion signal was achieved with 5 V applied to one of the electrodes. This work is an important step towards an implementation of in-gas-jet laser ionization and spectroscopy for

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neutron-deficient isotopes of tin at MARA-LEB in the future and a publication is in preparation. In the Fall of 2020 we will start to construct parts of the facility in an offline area. This will allow the vacuum system to be fully tested, the efficiency of the RF guides as well as transport through the mass separator. A new dual-doctoral PhD student from the University of Liverpool, Jorge Romero, will start working in our team in September 2020. We were also grateful for the two visits of Dimitar Simonovski from PNPI, Russia, who was working with the gas cell to realize the above tests alongside Sasha Zadvornaya and Wouter Gins. [1] P. Papadakis et al., Nucl. Instrum. and Meth. B463 (2020) 286.

[2] J. Uusitalo et al., Acta Physica Polonica B50 (2019) 319.

VI.High-precision laser-RF double resonance spectroscopy A new program is now underway at the IGISOL facility, to develop and exploit higher precision atomic techniques for the study of exotic nuclei. In a first proof-of-principle of such a measurement, we extracted the magnetic octupole moment, Ω, of 45Sc [1]. Current methods of optical spectroscopy at radioactive ion beam facilities generally provide measurements of hyperfine frequency splitting with a precision of ~1 MHz, with the exception of some specialized methods [2, 3]. This limitation in precision restricts the exploration of higher-order nuclear moments. In a push to address this limitation, we have recently demonstrated the feasibility of combining the efficiency of resonance ionization spectroscopy (RIS) with the precision of radiofrequency (RF) spectroscopy, the latter promising a dramatic improvement in precision of measurements of exotic nuclei by three orders of magnitude.

The magnetic octupole moment can be extracted from the third-order term in the hyperfine multipole expansion, provided accurate atomic theory calculations of the required atomic matrix elements are performed. The required state-of-the-art atomic calculations were provided by our collaborators.

Scandium was chosen out of a nuclear structural interest, as it has one proton outside of the magic Ca (Z=20) closed proton shell. The sensitivity of the atomic structure of Sc to its octupole moment was however found to be rather small. Nevertheless, Ω could be extracted. Detailed comparisons to nuclear shell-model and density functional theory (DFT) calculations were also performed. The goal here was twofold; to explore possible explanations for the somewhat large value of Ω that was obtained experimentally, and secondly, for the first time to evaluate the use of DFT to

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investigate magnetic octupole moments. Since DFT provides a global description of isotopes throughout the nuclear chart, such developments will be valuable once measurements on radioactive systems become possible.

Figure 5: Overview of experimental results. The top-left panel shows a laser scan of the first step of the three-step RIS scheme. The top-right plot shows an RF scan of one of the transitions within the ground-state hyperfine manifold. Different lines can be seen due to the Zeeman splitting. The bottom plots each show one of these Zeeman components for all five transitions within the ground-state hyperfine manifold, with (F, mF) values indicated. More details can be found in [1]. [1] R. P. de Groote et al., arXiv preprint arXiv:2005.00414, (2020).

[2] A. Takamine et al., Phys. Rev. Lett. 112 (2014) 162502.

[3] X. Yang et al., Phys. Rev. A 90 (2014) 052516.

VII. Precise collinear laser spectroscopy of stable 173-Ytterbium In parallel to the developments of rf-spectroscopy in combination with laser resonance ionization spectroscopy, we performed a confirmation measurement of an unusually large magnetic octupole moment in 173Yb, recently reported in [1]. The value of this moment is sufficiently large to be measured even with conventional collinear laser spectroscopy. As a part of the Masters’ thesis of Sonja Kujanpää, we therefore performed collinear laser spectroscopy on fast, neutral beams of 173Yb. These neutral beams were produced using a charge-exchange cell, using a potassium neutralizer. This charge-exchange process populates the 3P2 atomic state of interest, albeit with a very

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low efficiency (<1:10000). Nevertheless, this state could be studied using three different transitions.

These spectra are currently under analysis, searching for a non-zero contribution of the magnetic octupole moment. At the current stage in the data analysis process, no statistically significant sign can be found thus raising questions on the value reported in the literature.

Figure 6: Hyperfine structure scans of three different atomic lines in 173Yb, starting from the 3P2 metastable state.

[1] A.K. Singh, D. Angom, and V. Natarajan, Phys. Rev. A 87 (2013) 012512.

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Status Report (3) GALS – setup for production and study of heavy neutron rich nuclei at JINR S.G. Zemlyanoy1*, V.I. Zagrebaev1, K.A. Avvakumov1, B. Zuzaan1, T. Tserensambuu1, G.V. Myshinsky1, V.I. Zhemenik1, Yu. Kudryavtsev2, V. Fedosseev3, R. Bark4, Z. Janas5 1JINR, 2KUL, 3CERN, 4NRF, South Africa, 5University of Warsaw, Poland

The implementation of the GALS project, devoted to the production and study of heavy neutron-rich nuclei by multinucleon transfer reactions, is in progress. The main results obtained during the period since the previous IGLIS-NET newsletter are the following.

1. Laser laboratory preparation is almost finished. Recently, three new Photonics Industries TU-H TiSa lasers were installed and tested (see Fig. 1). Optimization of their working parameters for performing our first off-line experiments is in progress.

2. Our work on GALS subsystems located in the experimental room within the cyclotron U-400M hall is continuing. Figure 2 presents the view of the GALS facility with its subsystems. A significant part of the subsystems is already manufactured and ready for installation (e.g. front-end vacuum chamber, gas cell, Einzel lens, mass-separator analyzing magnet, focal chamber, gas purifying system), although some of the parts are still being designed or manufactured (e.g. ion guide, detection and DAQ systems).

Figure 1. Three new Photonics Industries TU-H Titanium-sapphire lasers are put into operation and optimized for first experiments.

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Figure 2. GALS facility view in

experimental room within the FLNR

U-400M cyclotron hall.

3. Our first offline experiments are planned to be performed on Os laser ionization with preliminary experiments on the best ionization scheme determination. For these offline experiments, our existing reference cell is planned to be used, and also a new, more compact one will be built. Also, modernization of the vacuum pumping system of the existing reference cell is currently being performed.

4. As was mentioned in the previous report, both the sextupole ion guide (SPIG) and the quadrupole ion guide with an S-shaped piece can be used as an ion extraction and guiding system (see Fig. 3, 4). We performed more precise SIMION simulations [1, 2] and comparison of these two options (see Table 1).

Figure 3. Two options for the the ion guide: SPIG (on the left-hand side) and S-shaped RFQ (right-hand side).

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Table 1. The comparison of the output parameters of the simulated ions for the two ion

guide options.

SPIG SRFQ + mRFQ + LRFQ

Before the End Plate Mean longitudinal kinetic energy KE 0.2 eV 12 eV Mean time of flight 3138.5 μs 484.8 μs Efficiency 98.8 % 99.4 % At analyzing magnet entry Mean longitudinal kinetic energy KE 39783.7 eV 39766.4 eV Standard deviation of kinetic energy σKE

183.8 eV 2.7 eV

Mean time of flight t 3154.1 μs 487.2 µs Standard deviation of time of flight σt 3409.5 μs 58.6 µs

Horizontal emittance component 1 π ∙ mm ∙ mrad

6.3 π ∙ mm ∙ mrad

Vertical emittance component 1 π ∙ mm ∙ mrad

6.4 π ∙ mm ∙ mrad

Efficiency 97 % 97 %

Comparison of these two options of the ion guide system shows the following. The advantage of SPIG is quite simple construction (single 6-rod structure ion guide through the whole front-end vacuum chamber), at the same time it has a disadvantage of high risk that the ions can get stuck due to collisions with the residual gas molecules and absence of a longitudinal electric field, which drastically increases the transport time. The RFQ ion guide with S-shaped ion guide can provide much better time of flight and efficiency. On the other hand, it is a much more complicated system. Nevertheless, in order to provide the best experimental transport efficiency of studied ions, it was decided to use the second option with segmented SRFQ. The system design is already finished and it is currently being manufactured.

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Figure 4. The quadrupole ion guide SIMION model within the front-end vacuum chamber.

[1] https://simion.com/

[2] S. Zemlyanoy et al., Hyperfine Interact 241 (2020) 38.

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(a) (b)

Status Report(4) SLOWRI development H. Ishiyma1, A. Takamine1, M. Rosenbusch2, T. Sonoda1, K. Kojima1 for the SLOWRI collaboration 1RIKEN Nishina Center (RNC), 2Wako Nuclear Science Center (WNSC), KEK

I. Development of an RF ion guide gas cell Section Title

We have continued the development of an RF ion guide gas cell of 500-mm-length for the SLOWRI facility at RIBF in RIKEN. This gas cell has a two-stage RF ion guide scheme; ions are firstly collected onto the 1st stage RF carpet (RFCP) by DC fields, carried by the 1st RFCP (RF+DC type) onto the 2nd RFCP, and then guided by the 2nd RFCP to a small exit hole at the center of the 2nd RFCP (traveling wave type). In FY2019, we modified the 1st RF carpet to have a finer pitch which was reduced by 20% compared to the previous version because the transport efficiency for the previous one was limited up to about 60% in a He gas of 133 mbar and even lower in a He gas of a higher pressure, e.g., less than 40% in 188 mbar, when the RF frequency was set to 4 MHz. We tested the transport performance of the new 1st RF carpet using Cs+ ions produced from a surface ionization ion source attached at the inner wall of the gas cell. As a result of the offline measurements, we successfully achieved 80 % transport efficiency in a He gas of 133 mbar and 70 % in a He gas of 267 mbar, at the RF frequency of 4.3 MHz (see Fig. 1). This transport performance was even more improved

Figure 1. 1st stage RFCP transport efficiency as a function of the RF voltages for (a) the previous version and (b) the new version (a finer pitch one)

if we use a higher RF frequency. As combined with a transport efficiency on the 2nd RFCP, we confirmed > 70% extraction efficiency from the gas cell for the Cs+ ions which were collected onto the RFCPs. The transport test for K+ ions is also in progress.

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We installed the RFCPs to the gas cell which can be cooled down by a cryocooler, and an offline transport test is proceeding. We will later combine the RF ion guide gas cell with a multi-reflection time-of-flight mass spectrograph (MRTOF) to conduct an online commissioning and systematic mass measurements at the F11 focal point of the BigRIPS beam line where symbiotic measurements with other BigRIPS experiments can be performed in FY2020.

II. High resolution mass separation with the new MRTOF-MS

In a collaborative work between the SLOWRI project of RIKEN and the WNSC of KEK, a new multi-reflection time-of-flight mass spectrograph (MRTOF-MS) has been assembled [1] and set up to be coupled to a He gas cell placed behind the Zero Degree spectrometer of BigRIPS at RIKEN (see Fig. 2). The purpose of the new setup is to allow for low-energy high-precision mass separation and mass measurements behind RIBF's SRC accelerator for the first time. The new MRTOF-MS has recently been set into operation and has now been optimized using 39K+ ions from a surface-ionization

source. Figure 2. Overview of the MRTOF setup with the three ion traps connected and the new pulsed drift tube allowing for dedicated mirror optimization.

A trap chamber containing ion source and a three-fold Paul trap system [2] has been attached to the main MRTOF chamber to create well cooled ions for injection into the MRTOF-MS. As described in [2], the central Paul trap, referred to as flat ion trap in Fig. 2, has a planar geometry which allows for perpendicular ion ejection with a well-shaped dipole field leading to excellent emittance properties of the ion bunch after extraction. After extraction from the flat ion trap, the ions pass a double-deflector unit and enter a pulsed drift tube utilized to adapt the kinetic energy of ions towards the MRTOF-MS. Through the new pulsed drift tube, the front-end section containing the Paul traps and the section of the mass spectrometer are electrically decoupled, which allows to vary the ion energy while maintaining the central drift tube in the

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MRTOF-MS at ground potential and thus enables high stability. Injection of ions into and ejection of ions from the MRTOF-MS is enabled by pulsing down the outermost ion mirrors at each side to a lower voltage for a short moment, whereas the ions are trapped between both mirrors and perform motional oscillations for a chosen number of laps as long as both mirrors are at closing (higher) voltage. After ejection, the ions' mass-dependent time-of-flight (TOF) is detected by a TOF detector, event-by-event.

Figure 3. Ion TOF spectrum of 39K+ ions after 500 full axialoscillations in the MRTOF-MS and a flight duration of more than 9ms. After thorough optimization and application of an algorithm for TOF drift correction, it was achieved to map the events to a FWHM of less than 8 ns.

The mass resolving power Rm = m / ∆m = TOF / (2 ∆TOF) for mass separation is related to the focal width (∆TOF) of the event distribution and depends crucially on the optimization of the mirror voltages, which are the generators of the TOF-energy dispersion function TOF(Ekin) of the system, while at the same time, the potentials also determine ion-optical aberration effects due to spatial focusing.

A voltage setting with low aberrations has been optimized earlier [2] and has been used as first voltage distribution to initialize the optimization procedure. Exploiting the opportunities of the pulsed drift tube in the transfer section, a large distribution of ion energies could be investigated and the effect of each individual electrode in the MRTOF-MS could be studied for the first time by measuring the entire TOF(Ekin) distributions with the aim to satisfy ∂TOF/∂ Ekin = 0 being equivalent with a TOF focus for the whole energy interval. The optimization took place applying 500 laps in the MRTOF-MS with the 39K+ test ions.

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After addition of high-voltage low-pass filters with time constants in the order of seconds to all relevant electrodes, and application of concomitant referencing and subsequent software drift correction [3, 4] an impressive mass resolving power of Rm = 570 000 has been achieved with a TOF of only 9 ms (see Fig. 3). To the knowledge of the authors this achievement is a first, and denotes the fastest high-resolution mass spectrometry result available at present. This result is of very high interest for nuclear mass measurements of "cocktail" beams (mixtures of isotopes and their excited states) produced by SRC. In the present configuration, nuclear isomers of <80keV excitation energy can be resolved for this mass region. Hence, our achieved performance allows for future measurements not only of ground-state masses, but also of low-lying isomers, which further enables measurements of branching ratios for population upon nuclear decay using a fast ion gate for post separation. [1] M. Rosenbusch et al., Nucl. Instr. Meth. B 463, 184 (2019).

[2] P. Schury et al., Nucl. Instr. Meth. B 335, 39 (2014).

[3] Y. Ito et al., Phys. Rev. Lett. 120, 152501 (2018).

[4] P. Schury et al., Nucl. Instr. Meth. B 433, 40 (2018).

III. Development of PALIS More than 99.9% of RI produced in projectile fission or fragmentation are simply dumped in the first dipole magnet and the slits at BigRIPS. A new scheme, named PALIS, meant to rescue such precious RI using a compact gas catcher cell and resonance laser ionization, was proposed as a part of SLOWRI. The thermalized RI implanted in a cell filled with Ar gas can be quickly neutralized and transported to the exit of the cell by a use of gas flow. Irradiation of resonance lasers at the exit ionizes neutral RI atoms efficiently and selectively. The PALIS gas cell is under off- and on-line commissioning. In FY2019, two on-line experiments have been done at BigRIPS. Please see the following references: [1] “Conceptual study on parasitic low-energy RI beam production with in-flight separator

BigRIPS and the first stopping examination for high-energy RI beams in the parasitic gas cell”, T.

Sonoda, I. Katayama, M. Wada, H. Iimura, V. Sonnenschein, S. Iimura, A. Takamine, M.

Rosenbusch, T. M. Kojima, D. S. Ahn, N. Fukuda, T. Kubo, S. Nishimura, Y. Shimizu, H. Suzuki,

H. Takeda, M. Tanigaki, H. Tomita, K. Yoshida, H. Ishiyama, Prog. Theor. Exp. Phys. 113D02

(2019).

[2] T. Sonoda, et al, RIKEN ACC. Prog. Rep. Vol 53, (2019) to be published.

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IV.Construction of SLWORI beamline: a compact Ion source for ion transportation test

A beamline which transports slow RIs from PALIS and from the gas cell to the experimental area in the next building of BigRIPS facility is being constructed [1]. The hardware construction has been almost finished and the duct is already evacuated and kept at a low pressure of 10-5 Pa or better. The construction is now turning into the phase for testing ion transport, the beam diagnosis equipment, and the total control system. At present neither PALIS nor the gas cell is completed, thus no ion can be extracted. An ion source is truly required to examine ion transport at the beamline. However, there is no room for installing a usual ion source at the upstream end of the beamline, since PALIS has been already installed at that place. Therefore, we have designed and assembled a simple compact ion source which is used as a side-inserted type device. It can be installed at any of the particular CF114 flanges for the beam profile monitor on the duct.

Figure 4. Pictures of the ion source: (a) Assembly of the system. (b) Battery box set on the top of the assembly. It is isolated from GND potential. (c) Configuration of the electrodes.

The ion source consists of ion emitter part and extraction part. The former is biased at a few ten kilovolts acceleration voltage so that it must be isolated with a CF70 isolating nipple. A commercial thermal ionization type alkali ion emitter (HeatWave Labs, Model 101139 Cs+) is wired and its position is designed to be on the axis of beamline duct. Heater power of the emitter is supplied by a lithium ion battery (GlobalTech, GANGAN GT5) with an output regulating circuit. The battery and the circuit are isolated from GND potential. The battery, when fully charged, can operate

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the ion source for more than ten hours in usual condition. The extraction part is assembled on the CF114 flange at GND potential.

The ion source was tested along a very short (40 cm) region of the beamline, which includes only one set of 20 cm-long electrostatic quadrupoles between the ion source and a Faraday cup. With 10 kV acceleration and 6.8 W heater power, the ion current extracted through 4 mmf "collimator" was about 1 nA on the Faraday cup when the electrostatic quadrupole was optimized. That intensity is enough for our purpose. Besides, higher intensity is expected with higher power, and the rated power of the heater is 11.3 W. With this ion source, the ion transport properties of the beamline are going to be examined and adjusted. [1] M. Wada et al, RIKEN Acc. Prog. Rep. 47 (2014) 203.

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Status Report (5) Status of KISS project Y. Hirayama*1, Y.X. Watanabe1, P. Schury1, H. Miyatake1, M. Wada1, S.C. Jeong1, M. Rosenbusch1, Y. Kakiguchi1, M. Oyaizu1, J.Y. Moon2, T. Hashimoto2, M. Mukai3, H.S. Choi4, A. Taniguchi5, S. Iimura6, H. Ishiyama6, H. Watanabe6, P. Walker7 1 KEK, 2 IBS RISP, 3 Univ. of Tsukuba, 4 Seoul National U., 5 KUR, 6 RIKEN, 7 Univ. of Surrey

I. In-gas-jet laser ionization spectroscopy at KISS

In order to determine electromagnetic moments and isotope shifts with the higher precision permitted by in-gas-jet laser ionization spectroscopy, we have developed a De Laval nozzle and S-shaped RFQ, and installed new a laser system. The laser system now consists of a Nd:YAG pumping laser (EdgeWave, 355nm, 60W), narrow-band seed laser (TOPTICA, DLC DL PRO HP), and dye-amplifier (Sirah). As we reported in the last newsletter, we successfully achieved a resonance width of 0.6(1) GHz in FWHM by applying the in-gas-jet laser ionization technique. However, there were replica peaks generated by the multi-mode of YAG pumping laser, as was also reported by the KU Leuven group.

Figure 1. Measured resonance spectra of 194Pt (Ip = 0+) before (blue line) and after (red line) the YAG laser path modification. Top and bottom figures show the same spectra with linear and logarithmic vertical scales, respectively. We used the combination of

l1 = 225.000 nm and l2 = 355 nm for in-gas-jet laser ionization spectroscopy. Horizontal axis shows the

deviation from each l1 of the applied ionization schemes.

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In order to suppress the odd-order replica peaks, we optimized the YAG laser path to

the dye amplifier as in the KU Leuven group’s report. Figure 1 shows the measured laser resonance spectrum of 194Pt (Ip = 0+) isotope. The wavelength of the excitation laser was 225.00092 nm at the peak position. Blue and red lines indicate the measured spectra before and after the YAG laser path modification, respectively. We observed the replica peaks with a period of 1.2 GHz. Therefore, we made the half-period delay of the YAG laser path for the suppression. After the modification, we could successfully suppress the odd-order replica peaks by one order of magnitude, and make the response function much simpler.

Further R&D works are in progress for an online experiment. II. MRTOF-MS system at KISS

We installed the MRTOF-MS system at KISS in FY2018, and started the offline study from FY2019. The MRTOF requires very low-energy ions. To make the 20 keV/q KISS beam useable by the MRTOF, a gas cell cooler-buncher (GCCB) [1] has been constructed. This device is similar in construction to a small rf carpet gas cell [2], but is windowless and pressurized to only 1 mbar He. The beam from KISS stops in the GCCB, and is then extracted to vacuum and transferred to the MRTOF. The MRTOF makes rapidly interleaved time-of-flight measurements of analyte (the beam from GCCB) and reference ions. We are optimizing the parameters to transport the beam to the MRTOF-MS. We also

installed an offline ion source (85,87Rb+ and 133Cs+) in both the GCCB and the trap system to test the system without KISS beam. So, we can effectively optimize each components of the system, such as the extraction from the GCCB, the mass resolution of the MRTOF-MS and so on. The present overall efficiency of the MRTOF-MS is about 10%, and we will improve it by optimizing the KISS beam transport to the He gas cell cooler.

Figure 2 shows the measured TOF spectrum by using 133Cs+ from the local ion-source. We have achieved a mass resolving power of Rm = 200,000 successfully. Such Rm value allows us to determine the atomic masses of nuclei with a precision of 100 keV (required from nuclear astrophysics) by accumulating only 100 ions.

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Figure 2. Measured TOF spectrum of 133Cs+.

In the initial test of the GCCB, a 20 keV/q beam of 198Pt+ was delivered from KISS. Figure 3 shows a time-of-flight spectrum of measured during that study. It was presumed that the peak at A/q=99 was 198Pt2+ and a high-resolution follow-up measurement confirmed as much.

Figure 3. Time-of-flight spectra from online and test of the KISS gas cell cooler-buncher (GCCB) using a 20 keV/q beam of 198Pt+. The black spectrum shows the reference ions,85;87Rb+ and 133Cs+. The red spectrum shows the ions delivered from the GCCB, dominantly A/q=99.

There are two ion traps between the GCCB and MRTOF. While the first of these has a very broad-band response, the second trap (referred to as "flat trap") is somewhat mass selective. The amplitude of the flat trap was systematically varied to determine the probability of conversion to a doubly-charged state. A time-of-flight spectrum was measured at each amplitude, and the number of singly- and doubly-charged A=198 ions was recorded. The result, shown in Fig. 4, indicates that 80% of incoming Pt ions were converted to the doubly-charged state. From the occurrence of such charge-stripping

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reactions we presume the GCCB will be highly effective at breaking molecular contaminants which are a common problem for gas cell-based studies. We hope to report on such an effect in the near future.

Figure 4. Effect of varying the RF amplitude of the final preparation trap before the MRTOF, indicating that 80% of the incoming beam is converted to a doubly-charged state.

[1] DOI:10.7566/JPSCP.6.030112

[2] Y. Ito et al., RIKEN Accel. Rep. 49(2016)183

III. In-gas-cell laser spectroscopy of 194,196Os isotopes by using MRTOF-MS

To study nuclear structure at KISS, we measured the hyperfine structure (HFS) of the nuclei 194Os (Ip = 0+, T1/2 = 6.0 y) and 196Os (Ip = 0+, T1/2 = 34.9 m) to determine the change in charge radius by using the in-gas-cell laser ionization spectroscopy technique assisted by the multi-reflection time-of-flight mass spectrograph (MRTOF-MS). The MRTOF-MS can identify the isotopes from the mass-dependent time-of-flight (TOF) spectrum, and was recently installed at KISS successfully as discussed above.

We have measured the HFS of short-lived (T1/2 ≲ 30 min) isotopes by detecting the b- and g-rays at KISS, and it is difficult to measure the HFS of isotopes with T1/2 > 1 h by detecting the decay radiations in a limited beam time. However, we can efficiently measure the HFS of these isotopes by applying ion counting with the MRTOF-MS without waiting for the radiation decays. Here, we report the HFS measurement of 194Os by using the MRTOF-MS.

The 194Os isotopes were produced in multi-nucleon transfer reactions by impinging a stable 136Xe beam (50 pnA) with an energy of approximately 10 MeV/nucleon on a 198Pt target (12.5 mg/cm2). The singly-charged isotopes, produced with the in-gas-cell laser

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ionization technique, were extracted from the KISS gas cell at 20 keV/q for the HFS measurements. The extracted isotopes were injected into the GCCB and thermalized. Then, doubly-charged ions were mainly produced by the charge exchange reaction with helium atoms in the stopping process, and were extracted from the GCCB. After that, the bunched isotopes were injected into the MRTOF-MS for particle identification.

Figure 5. Measured TOF spectrum of 194Os2+.

Figure 5 shows the measured TOF spectrum of 194Os2+ by using the MRTOF-MS at

KISS. We can clearly identify the 194Os2+ isotope with the contaminant peak of 194Pt2+ ions, which were elastic events emitted from the production target and transported to the MRTOF-MS. By fitting the TOF spectrum, we can deduce the number of ions extracted from the KISS gas cell. The resultant HFS spectrum shown in Fig. 6 was obtained by measuring the number of laser-ionized 194Os as a function of the laser wavelength. There appears one resonance peak from one atomic transition of 194Os due to Ip = 0+. The

Figure 6. Measured HFS spectrum of 194Os (Ip = 0+).

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constant background (dashed line) in Fig. 6 comes from the contamination of the 194Os2+ peak by 194Pt2+. From the peak position, we can determine the isotope shift value of 194Os to deduce the change in the charge radius and discuss the nuclear deformation. Further analysis is in progress.

IV. b-g spectroscopy at KISS

As KISS collaborative research, we performed two b-g spectroscopy studies. First, decay spectroscopy from the nuclear astrophysical interest was performed for 192mOs, 192g,192mRe (Spokesperson : H. Watanabe, NP1612-RRC44). To study nuclear structure of the high-K isomers, b-g spectroscopy of 186g, 186m, 187g, 187mTa isotopes was performed (Spokesperson : P. Walker, NP1712-RRC37R1).

The 192mOs, 192g,192mRe were produced by the MNT reactions between a 136Xe beam (10.75 MeV/nucleon) and natural Pt (5 µm in thickness) target. The reactions produced both ground and isomeric states. Isomers with a longer half-life (typically more than 100 ms) than the extraction time from the gas cell can be laser-ionized and transported to the KISS decay station. In this measurement, we shared the KISS beam between the decay station and the MRTOF-MS. During the decay curve measurement at the decay station, the KISS beam was transported to the MRTOF-MS to measure the extracted beam intensity. 192mOs and 192gOs were clearly identified in the TOF spectrum, and the intensity of 192mOs was measured to deduce the decay branching ratio by coupling with the g-ray measurements at the decay station. Thus, the MRTOF-MS proved very useful for not only mass measurements but also particle identification. The b-decay scheme of 192gRe was measured precisely for the first time. Further analysis is in progress.

The Ta isotopes were produced by the MNT reactions between 136Xe beam (7.2 MeV/nucleon) and natural W (5 µm in thickness) target, and were laser-ionized [1] for the extraction from the KISS gas cell successfully. We could measure the decays from the isomeric states of 186mTa (T1/2 = 1.5 m) [2] and 187mTa (T1/2 < 22 s) [2] successfully, and determined the decay schemes and the half-lives more precisely. Further analysis is in progress. [1] Y. Hirayama et al., Rev. Sci. Instrum. 90 (2019) 115104.

[2] M.W. Read et al., Phys. Rev. C86 (2012) 054321.

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V. Deexcitation γ-ray transitions from the isomeric state of 195Os

We have analyzed the data of the β-γ spectroscopy of 195Os reported in the IGLIS-NET Newsletter No. 6 (2018) and recently published the results of de-excitation γ-ray transitions from an isomeric state of 195Os [1]. The 195g+mOs beam extracted from KISS was implanted in the aluminized Mylar tape for β-γ spectroscopy. The MSPGC was used to detect β-rays, conversion electrons and X-rays from the implanted radioactive isotopes [2]. The MSPGC is sensitive to X- rays and low-energy conversion electrons only with hit pattern multiplicity “M = 1” (see details in Ref. [1] and [2]). De-excitation γ-rays were detected by four high-purity germanium clover detectors, which were on loan to KISS as part of the collaboration between KEK WNSC and IBS RISP.

Figure 7(a) shows the measured γ-ray energy spectrum in coincidence with the MSPGC hit pattern multiplicity “M = 1”. Two peaks corresponding to the characteristic X-rays of osmium, Kα1 (63.1(1) keV) and Kβ1 (71.3(1) keV), are clearly observed. It indicates that the MSPGC hit pattern “M = 1” selects for the isomeric decay of 195mOs.

(a)

(b)

(c)

Figure 7. (a) γ-ray energy spectrum in coincidence with the MSPGC hit pattern “M = 1”. (b) Time distributions of detected counts for the sum of x rays (63.1 keV and 71.3 keV) and γ rays (111.0 keV, 148.8 keV, 168.8 keV and 279.0 keV). (c) Possible decay scheme of isomeric state of 195Os. Labels with arrows indicate the γ-ray energies in keV.

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Four γ-ray peaks at 111.0(1), 148.8(2), 168.8(2) and 279.0(2) keV were newly found to be associated with the isomeric decay of 195Os. It was confirmed by analyzing the time spectra gating two x-ray peaks and four γ-ray peaks. The KISS beams were pulsed in the time cycles of 120-s beam-on and 240-s beam-off periods to analyze the growth and decay curves of implanted radioactivity. Half-lives obtained from the fitting of six time spectra gating x rays and γ rays observed in Fig. 7(a) agree with each other within their fitting errors. Figure 7(b) shows the sum of all gated time spectra and the fitting result with the half-life of 47(3) s, which is shorter than the half-life of the ground state of 195Os, 6.5(11) min, and is considered assigned to its isomeric decay.

The γ-γ coincidence analysis for those four γ-ray transitions provides a hint for the decay scheme from the isomeric state. It suggests that the 111.0- and 168.8-keV transitions occur in series, which are in parallel to the 279.0-keV transition. The 148.8-keV transition occurs in series of those three transitions. This gives an energy difference of 428 keV between the initial and the final state of those cascade transitions, which is 26 keV smaller than the excitation energy of the isomeric state, 454(10) keV, as measured by the ESR at GSI [3]. It suggests a possible decay scheme from the isomeric state as shown in Fig. 7(c), where the γ-rays measured at KISS follow the transition with 26-keV energy difference from the previously reported isomeric state 195mOs. The discrepancy in half-lives of the isomeric state, 47(3) s for the neutral atom 195mOs at KISS and >9 min for the fully-stripped ion 195mOs76+ at GSI, would be caused by the dominance of the internal conversion in such a low-energy transition.

[1] Y.X. Watanabe et al., Phys. Rev. C 101 (2020) 041305(R).

[2] M. Mukai et al., Nucl. Instrum. Methods Phys. Res. A 884 (2018) 1.

[3] M. Reed et al, Phys. Rev. C 86 (2012) 054321.

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Status Report (6) Status of Low-Energy projects at FRIB Ryan Ringle*, Georg Bollen, Stefan Schwarz, Antonio Villari FRIB I. Introduction The Facility for Rare Isotope Beams, FRIB, a DOE Nuclear Physics facility, nears completion at Michigan State University. User operation and the start of the science program are planned in early 2022. The 2015 NSAC LRP [NPR15] recommended this timely completion of FRIB as one of DOE’s accelerator facility for Nuclear Science, also noting that “FRIB will provide intense beams of rare isotopes through in-flight fragmentation and fission of fast heavy ion beams on thin targets. The rare isotopes will be collected and separated by a high-efficiency fragment separator for fast beam experiments. They will also be delivered to a gas cell for collection, combined with gas stopping and subsequent reacceleration using an EBIS for charge breeding before injection into the ReA heavy ion linac. Beams not readily available at facilities using the complementary ISOL production method can be produced, and nearly any isotope can be made available with limited development time. The full complement of fast, stopped, and reaccelerated beams will be available for experiments with a broad suite of equipment.”

Based on a 400 kW, 200 MeV/u heavy-ion driver linac FRIB will deliver beams of rare isotopes with unprecedented intensities. FRIB will enable new science opportunities at the frontiers of nuclear structure, nuclear astrophysics, fundamental symmetries, and societal applications. These will be able to be addressed with FRIB’s broad range of isotopes, unprecedented beam rates, and the availability of stopped and reaccelerated beam in addition to fast beams.

Maximizing the science opportunities with rare isotopes mandates their delivery over a wide range of beam energies. In-flight rare isotope production is a very powerful approach in which a heavy-ion beam with high energy (>100 MeV/u) impinges on a thin target and the reaction products are separated in-flight and delivered directly to experiments. The production is chemically independent and fast, and thus provides access to the shortest-lived isotopes far from the valley of beta stability. Experiments with these fast beams have provided great insight into nuclear structure very far from the valley of stability, helped delineate the borders of stability, study the equation of state of nuclear matter and are expected to continue to do so. Major and

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world-unique advances in nuclear science will be possible in a variety of experiments that rely on stopping and reaccelerating rare isotopes produced in flight with high efficiency and on a fast time scale. Beams of rare isotopes with energies of 0.01- 100 keV, “stopped beams”, are used in high-precision experiments with traps and lasers, to measure nuclear binding energies (masses), to determine nuclear radii and moments, and to test fundamental symmetries at levels complementing much more expensive high-energy experiments. Precision beams of rare isotopes with energies of 0.1-20 MeV/u are used to measure cross sections of key reactions that are critical for the understanding nuclear synthesis in the cosmos and for nuclear reaction studies that help determine detailed nuclear properties needed for further advances in nuclear theory. The availability of intense stopped and reaccelerated beams will be a key feature of FRIB at MSU.

The ability to effectively stop beams produced in the 100-200 MeV/u range was demonstrated at the NSCL [Wei04, Mor07, Bol06] more than a decade ago and led to a unique program of Penning trap mass measurements with stopped beams [Rin13] as well as a facility for laser spectroscopy [Min13]. Since 2015 the NSCL has operated a unique facility, ReA [Vil18], to reaccelerate the rare isotopes for nuclear reaction studies. A new ReA upgrade [Gad16] is scheduled to be completed by end of 2020 will enhance the energy of the reaccelerated beams from 6 MeV/u to a maximum of 12 MeV/u for A/Q = 2.

In order to ensure that this emerging world-unique research direction with stopped and reaccelerated beams from projectile fragmentation at FRIB remains successful, the highest performance and most reliable conversion of fast, rare-isotope beams into stopped beams is of utmost importance. Key requirements are efficient stopping over a range from the lightest to the heaviest ions, rapid (<<100ms), efficient extraction for incoming fast beam rates >108/s, and a high purity of the stopped and extracted ions. FRIB will employ a multi-facetted approach that will maximize performance and minimize risks that builds on major upgrades of its beam stopping facilities and important developments of novel, powerful techniques for beam stopping and manipulation. Fig. 1 shows the present layout of the beam stopping facilities at NSCL that will also be used at FRIB. Following the successful use of a compact linear high-pressure gas cell [Wei05], a linear Gas Catcher [Sum20] was commissioned and brought into operation at NSCL in 2012 and has served beam to many experiments since then. Based on operational experience with the gas catcher a next-generation system, the Advanced Cryogenic Gas Stopper (ACGS) [Lun20], was developed to

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overcome identified shortcomings by combining novel RF-carpet ion-transport techniques [Bol11, Bro12, Bro13], an innovative, simple geometry, and cryogenic operation. The ACGS is operational since 2018. In parallel, a cyclotron gas stopper specialized for providing light ions beam was developed. Construction and off-line tests have been completed. In addition, a “solid stopper” device is currently being considered for delivery of the most intense beams FRIB will be able to provide but will be limited to certain elements.

Addressing the two main challenges of gas stopping, efficiency and purity, will have significant benefit for FRIB science. The areas most directly affected are (1) precision mass measurements and (2) laser spectroscopy of rare isotopes, and (3) astrophysical and nuclear reaction studies using reaccelerated beams.

II. High precision Penning trap mass measurements High precision Penning trap mass measurements of rare isotopes provided by FRIB will benefit directly from higher beam purity provided. The Low Energy Beam and Ion Trap (LEBIT) facility [Rin13]at the NSCL was the first, and remains the only, facility in the world to perform mass measurements or rare isotopes produced by projectile fragmentation [Blo08, Izz18, Val18]. A recent addition is the Single-Ion Penning Trap (SIPT) spectrometer [Ham19] for mass measurements of the most exotic isotopes only produced at extremely low rates. An important aspect of successful Penning trap mass measurements is the availability of relatively clean beams. At present LEBIT uses time-consuming in-trap purification techniques to remove unwanted ions. An improvement of the purity of stopped beams will reduce measurement time and enable

Figure 1 - Overview of the stopped beam facility and associated experimental end stations at FRIB/NSCL.

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measurements that were jeopardized by excessive isobaric beam contamination, or loss of beam rate due to fractionation.

III. Laser Spectroscopy Laser spectroscopy of rare isotopes provided by FRIB will benefit directly from the elimination of isotopes delivered as radiomolecules. The Beam Cooling and Laser spectroscopy (BECOLA) facility [Min13] uses collinear laser spectroscopy (CLS) to enable the study of ground state properties of rare isotopes such as charge radii and electromagnetic moments. BECOLA can also be used to produce highly polarized beams with optical pumping techniques that can be used for beta-decay nuclear-magnetic/quadrupole resonance (b-NMR/NQR) measurements [Min08]. Polarized beams from BECOLA are also available for tests of fundamental symmetries via the measurement of correlations in nuclear beta decay. IV. Reaction studies with reaccelerated beams Reaction studies with reaccelerated beams of rare isotopes provided by FRIB will benefit directly from an increase in the beam intensities available from gas stopping. With ReA, FRIB is in the position to provide rare-isotope beams at an energy range that enables a wealth of reaction studies aiming at the understanding of reaction mechanisms, determining reaction rates critical to astronuclear processes, and for nuclear structure studies. As examples, one can cite studies of the evolution of single-particle excitations across the nuclear chart, which are a key focus of exotic-beam physics programs aiming at establishing a unified theoretical picture for stable and exotic nuclei. High intensity heavier beams with A>100 will be available with FRIB, allowing long chains of closed-shell isotopes and isotones to be studied at energies in the proximity of the Coulomb barrier. Studies on single proton excitations on 103-135Sn would be possible with FRIB and ReA, taking advantage of high-intensity beams provided by a suitable gas stopper. Another example would be studies of shape evolution in neutron-rich nuclei near the r-process path, as for example probing the N=82 shell closure by spectroscopy of neutron rich nuclei, taking advantage of high intensities of beams of heavy nuclei. Finally, astronuclear reaction studies with SECAR, a DOE and NSF-funded large recoil separator, and SOLARIS, a new DOE-funded spectrometer,

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will also benefit from improved purity and beam rates reaching or exceeding 107 pps after reacceleration.

The conversion of high-energy projectile fragmentation beams to low-energy, or stopped, beams via thermalization in a buffer gas is an essential feature of the NSCL and will only become more important for nuclear science in the era of FRIB. These stopped beams can be used directly for experiments or reaccelerated by the ReA facility. However, in order to make optimal use of all beams that will be available when FRIB comes online next-generation gas stopping devices are required that can accommodate high-intensity incident beams in order to deliver low-energy, rare-isotope beams with high efficiency and good beam purity. Developments are currently being pursued to this end.

[Bol06] G. Bollen, D. Davies, M. Facina, J. Huikari, E. Kwan, P. A. Lofy, D. J. Morrissey, A. Prinke, R. Ringle, J. Savory, P. Schury, S. Schwarz, C. Sumithrarachchi, T. Sun, and L. Weissman, Experiments with thermalized rare isotope beams from projectile fragmentation: A precision mass measurement of the superallowed beta emitter Ca-38, Phys. Rev. Lett. 96 (2006) 152501. DOI:10.1103/PhysRevLett.96.152501

[Bol11] G. Bollen, "Ion surfing" with radiofrequency carpets, Int. J. Mass Spectrom. 299 (2011) 131. DOI:10.1016/j.ijms.2010.09.032

[Blo08] M. Block, C. Bachelet, G. Bollen, M. Facina, C. M. Folden, C. Guenaut, A. A. Kwiatkowski, D. J. Morrissey, G. K. Pang, A. Prinke, R. Ringle, J. Savory, P. Schury, and S. Schwarz, Discovery of a nuclear isomer in Fe-65 with penning trap mass spectrometry, Phys. Rev. Lett. 100 (2008) 132501. DOI:10.1103/PhysRevLett.100.132501

[Bro12] M. Brodeur, A.E. Gehring, G. Bollen, S. Schwarz, D.J. Morrissey, Experimental investigation of the ion surfing transport method, Int. J. Mass Spectrom. 336 (2013) 53. DOI:10.1016/j.ijms.2012.12.011

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[Bro13] M. Brodeur, A.E. Gehring, G. Bollen, S. Schwarz, D.J. Morrissey, Traveling wave ion transport for the cyclotron gas stopper, Nucl. Instr. Meth. In Phys. Res. B 317 (2013) 468. DOI:10.1016/j.nimb.2013.06.033

[Gad16] A. Gade, H. Iwasaki, ReA Energy Upgrade, https://fribusers.org/documents/2016/ReAEnergyUpgradeWP.pdf

[Ham19] A. Hamaker, G. Bollen, M. Eibach, C. Izzo, D. Puentes, M. Redshaw, R. Ringle, R. Sandler, S. Schwarz, I. Yandow, SIPT - An ultrasensitive mass spectrometer for rare isotopes, Hyperfine Interactions 240 (2019) 34. DOI:10.1007/s10751-019-1576-9

[Izz18] C. Izzo, G. Bollen, M. Brodeur, M. Eibach, K. Gulyuz, J. Holt, J. Kelly, M. Redshaw, R. Ringle, R. Sandler, S. Schwarz, S. Stroberg, C. Sumithrarachchi, A. Valverde, A. Villari, Precision mass measurements of neutron-rich Co isotopes beyond N=40, Phys. Rev. C 97 (2018) 014309. DOI:10.1103/physrevc.97.014309

[Lun20] K. Lund, G. Bollen, D. Lawton, D. Morrissey, J. Ottarson, R. Ringle, S. Schwarz, C. Sumithrarachchi, A. Villari, and J. Yurkon, Online tests of the Advanced Cryogenic Gas Stopper at NSCL, Nucl. Instr. Meth. In Phys. Res. B 463 (2020) 378. DOI:10.1016/j.nimb.2019.04.053

[Min08] K. Minamisono, R. R. Weerasiri, H. L. Crawford, P. F. Mantica, K. Matsuta, T. Minamisono, J. S. Pinter, and J. B. Stoker, Fast-switching NMR system for measurements of ground-state quadrupole moments of short-lived nuclei, Nucl. Instr. Meth. In Phys. Res. A 589 (2008) 185. DOI:10.1016/j.nima.2008.01.105

[Min13] K. Minamisono, P.F. Mantica, A. Klose, S. Vinnikova, A. Schneider, B. Johnson, B.R. Barquest, Commissioning of the collinear laser spectroscopy system in the BECOLA facility at NSCL, Nucl. Instrum. Methods Phys. Res. A 709 (2013) 85. DOI:10.1016/j.nima.2013.01.038

[Mor07] D. J. Morrissey, Extraction of thermalized projectile fragments from gas,

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Eur. Phys. J. Special Topics 150 (2007) 365. DOI:10.1140/epjst/e2007-00348-7

[NPR15] Reaching for the Horizon, The DOE/NSF Nuclear Science Advisory Committee, 2015. https://science.osti.gov/-/media/np/nsac/pdf/2015LRP/2015_LRPNS_091815.pdf?la=en&hash=F731E22D31731E61C64E4B684377314FD4A0D6C7

[Rin13] R. Ringle, S. Schwarz, and G. Bollen, Penning trap mass spectrometry of rare isotopes produced via projectile fragmentation at the LEBIT facility, Int. J. Mass Spectrom. 349 (2013) 87. DOI:10.1016/j.ijms.2013.04.001

[Sum20] C. Sumithrarachchi, D. Morrissey, S. Schwarz, K. Lund, G. Bollen, R. Ringle, G. Savard, and A. Villari, Beam thermalization in a large gas catcher, Nucl. Instrum. Methods Phys. Res. B 463 (2020) 305. DOI:10.1016/j.nimb.2019.04.077

[Val18] V. Valverde, M. Brodeur, G. Bollen, M. Eibach, K. Gulyuz, A. Hamaker, C. Izzo, W. Ong, D. Puentes, M. Redshaw, R. Ringle, R. Sandler, S. Schwarz, C. Sumithrarachchi, J. Surbrook, A. Villari, I. Yandow, High-Precision Mass Measurement of Cu56 and the Redirection of the rp-Process Flow, Phys. Rev. Lett. 120 (2018) 032701. DOI:10.1103/physrevlett.120.032701

[Vil18] A. Villari, G. Bollen, D. Crisp, M. Ikegami, A. Lapierre, S. Lidia, D. Morrissey, S. Nash, R. Rencsok, R. Ringle, S. Schwarz, R. Shane, C. Sumithrarachchi,T. Summers, Q. Zhao, On the Acceleration of Rare Isotope Beams in the Reaccelerator (ReA3) at the National Superconducting Cyclotron Laboratory at MSU, Proceedings of LINAC2016

[Wei04] L. Weissman, P. A. Lofy, D. A. Davies, D. J. Morrissey, P. Schury, S. Schwarz, T. Sun and G. Bollen, First extraction tests of the NSCL gas cell, Nucl. Phys. A 746 (2004) 655C. DOI:10.1016/j.nuclphysa.2004.09.045

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