Development of a low-energy radioactive ion beam facility for the MARA separator

A low-energy radioactive ion beam facility for the production and study of nuclei produced close to the proton drip line is under development at the Accelerator Laboratory of the University of Jyv\"askyl\"a, Finland. The facility will take advantage of the mass selectivity of the recently commissioned MARA vacuum-mode mass separator. The ions selected by MARA will be stopped and thermalised in a small-volume gas cell prior to extraction and further mass separation. The gas cell design allows for resonance laser ionisation/spectroscopy both in-gas-cell and in-gas-jet. The facility will include experimental setups allowing ion counting, mass measurement and decay spectroscopy.Comment: Paper accepted for publication at Hyperfine Interaction

The science case of the FRS Ion Catcher for FAIR Phase-0

The FRS Ion Catcher at GSI enables precision experiments with thermalized projectile and fission fragments. At the same time it serves as a test facility for the Low-Energy Branch of the Super-FRS at FAIR. The FRS Ion Catcher has been commissioned and its performance has been characterized in five experiments with 238U and 124Xe projectile and fission fragments produced at energies in the range from 300 to 1000 MeV/u. High and almost element-independent efficiencies for the thermalization of short-lived nuclides produced at relativistic energies have been obtained. High-accuracy mass measurements of more than 30 projectile and fission fragments have been performed with a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) at mass resolving powers of up to 410,000, with production cross sections down to the microbarn-level, and at rates down to a few ions per hour. The versatility of the MR-TOF-MS for isomer research has been demonstrated by the measurement of various isomers, determination of excitation energies and the production of a pure isomeric beam. Recently, several instrumental upgrades have been implemented at the FRS Ion Catcher. New experiments will be carried out during FAIR Phase-0 at GSI, including direct mass measurements of neutron-deficient nuclides below 100Sn and neutron-rich nuclides below 208Pb, measurement of β-delayed neutron emission probabilities and reaction studies with multi-nucleon transfer.Peer reviewe

Experimental gas jet studies for the IGISOL LIST method and simulation modeling

The IGISOL Laser Ion Source Trap (LIST) is a method in which a supersonic gas jet is used to transport thermalized nuclear reaction products from a gas cell into a sextupole radio frequency ion guide (SPIG) while removing any non-neutral part of the jet by a positively biased repeller electrode. Specific atom species are resonantly re-ionized with laser radiation in the SPIG which enables the study of exotic nuclei without isobaric contamination. In the first part of this work the repelling effect is studied through a series of ion optical Monte-Carlo simulations. The simulations were able to support the hypothesis that the constant collisions between the fast moving buffer gas atoms and the ions provides additional momentum to overcome potential barriers of several tens of volts, explaining the need for high repelling electrode potentials in practice. The simulations were also used to investigate the differences between helium and argon buffer gases and different repelling geometries. The experimental behaviour of the repelling efficiency as a function of repelling potential was qualitatively reproduced. The second part of this work concentrated on the supersonic gas jet formation and the parameters that affect the gas jet properties, in particular the width. For an efficient transportation of atoms from the gas cell to the SPIG the gas jet needs to be well collimated. The gas jet behaviour was investigated with two nozzle shapes and diameters in varying pressure environments by photographing plasma afterglow in the gas expanding from an arc discharge ion guide. It was found that the jet width is strongly dependent on background pressure and independent of the nozzle shape and gas cell pressure. Because the gas jet exhibited a collimated behaviour only with increasing background pressure, a new de Laval type of nozzle was introduced. This nozzle produced a narrow collimated jet structure at background pressures as low as 0.33 mbar

Gas-phase chemistry, recoil source characterization and in-gas-cell resonance laser ionization of actinides at IGISOL

The underlying theme of this thesis focuses on buffer gas purification and relevant gas-phase ion chemistry which critically affects ion beam purity at gas cell-based radioactive ion beam facilities. The achievement of attaining a sub- parts-per-billion level of impurity at the IGISOL facility has enabled subsequent gas cell developments for production of the actinide elements, plutonium and thorium, required for a program of high-resolution optical spectroscopy. Firstly, the construction and characterization of the new IGISOL buffer gas purification system is presented. Off-line ion beam production of plutonium and thorium using in-gas-cell laser resonance ionization combined with filament dispensers has resulted in successful collinear laser spectroscopy of several long-lived plutonium isotopes and has revealed unique collisional phenomena significantly affecting resonance laser ionization in gaseous environments. Thorium is of particular interest due to 229Th and its low-energy nuclear isomeric state. By stopping 229Th recoils from the alpha decay of 233U in a helium-filled gas cell, a 229Th ground and isomeric state ion beam can be produced. The recoil efficiency determination of two 233U sources using direct and implantation foil gamma- and alpha-ray spectroscopy as well as surface characterization by Rutherford back scattering (RBS) measurements have shown the importance of good source quality. The development of a new gas cell to house several such recoil sources is also presented, emphasizing the interplay between gas pressure, size of the gas cell, and diffusion losses during extraction. Finally, this thesis presents the first on-line experiments for the production of 229Th via proton-induced fusion-evaporation reactions using a 232Th target

Observation of Collisional De-Excitation Phenomena in Plutonium

A program of research towards the high-resolution optical spectroscopy of actinide elements for the study of fundamental nuclear structure is currently ongoing at the IGISOL facility of the University of Jyväskylä. One aspect of this work is the development of a gas-cell-based actinide laser ion source using filament-based dispensers of long-lived actinide isotopes. We have observed prominent phenomena in the resonant laser ionization process specific to the gaseous environment of the gas cell. The development and investigation of a laser ionization scheme for plutonium atoms is reported, focusing on the effects arising from the collision-induced phenomena of plutonium atoms in helium gas. The gas-cell environment was observed to greatly reduce the sensitivity of an efficient plutonium ionization scheme developed in vacuum. This indicates competition between resonant laser excitation and collisional de-excitation by the gas atoms, which is likely being enhanced by the very high atomic level density within actinide elements.peerReviewe

Development of a saturated absorption spectroscopy setup at IGISOL for characterisation of Fabry-Pérot interferometers

A saturated absorption spectroscopy setup was developed and optimised for the characterisation of a home-built and a commercial Fabry-Pérot interferometer (FPI). The free spectral range of these FPIs has been determined with reliable statistical and systematic errors. These FPIs will be used for accurate wavelength determination of broad- and narrowband pulsed Ti:sapphire lasers used in resonance ionisation spectroscopy experiments.peerReviewe

A facility for production and laser cooling of cesium isotopes and isomers

We report on the design, installation, and test of an experimental facility for the production of ultra-cold atomic isotopes and isomers of cesium. The setup covers a broad span of mass numbers and nuclear isomers, allowing one to directly compare chains of isotopes and isotope/isomer pairs. Cesium nuclei are produced by fission or fusion-evaporation reactions using primary proton beams from a 130 MeV cyclotron impinging upon a suitable target. The species of interest is ejected from the target in ionic form, electrostatically accelerated, mass separated, and routed to a science chamber. Here, ions are neutralized by implantation in a thin foil, and extracted by thermal diffusion. A neutral vapor at room temperature is thus formed and trapped in a magneto-optical trap. Real-time fluorescence imaging and destructive absorption imaging provide information on the number of trapped atoms, their density, and their temperature. Tests with a dedicated beam of 133Cs ions at 30 KeV energy confirm neutralization, evaporation, and laser cooling to 150 K, with an average atomic density of 1010 cm−3. Availability of cold and dense atomic samples of Cs isotopes and isomers opens new avenues for high-precision measurements of isotopic and isomeric shifts thereby gaining deeper insight into the nuclear structure, as well as for sensitive measurements of isotopes’ concentration ratios in trace quantities. The facility also constitutes the core for future experiments of many-body physics with nuclear isomers.peerReviewe

In-gas-cell laser ionization studies of plutonium isotopes at IGISOL

In-gas-cell resonance laser ionization has been performed on long-lived isotopes of Pu at the IGISOL facility, Jyvaskyl ¨ a. This ¨ initiates a new programme of research towards high-resolution optical spectroscopy of heavy actinide elements which can be produced in sufficient quantities at research reactors and transported to facilities elsewhere. In this work a new gas cell has been constructed for fast extraction of laser-ionized elements. Samples of 238−242Pu and 244Pu have been evaporated from Ta filaments, laser ionized, mass separated and delivered to the collinear laser spectroscopy station. Here we report on the performance of the gas cell through studies of the mass spectra obtained in helium and argon, before and after the radiofrequency quadrupole coolerbuncher. This provides valuable insight into the gas phase chemistry exhibited by Pu, which has been additionally supported by measurements of ion time profiles. The resulting monoatomic yields are sufficient for collinear laser spectroscopy. A gammaray spectroscopic analysis of the Pu samples shows a good agreement with the assay provided by the Mainz Nuclear Chemistry department.peerReviewe

High-resolution laser spectroscopy of long-lived plutonium isotopes

Long-lived isotopes of plutonium were studied using two complementary techniques, high-resolution resonance ionization spectroscopy (HR-RIS) and collinear laser spectroscopy (CLS). Isotope shifts have been measured on the 5f67s27F0→5f56d27s(J=1) and 5f67s27F1→5f67s7p(J=2) atomic transitions using the HR-RIS method and the hyperfine factors have been extracted for the odd mass nuclei 239,241Pu. CLS was performed on the 5f67s8F1/2→J=1/2(27523.61cm−1) ionic transition with the hyperfine A factors measured for 239Pu. Changes in mean-squared charge radii have been extracted and show a good agreement with previous nonoptical methods, with an uncertainty improvement by approximately one order of magnitude. Plutonium represents the heaviest element studied to date using collinear laser spectroscopy.peerReviewe

The MARA-LEB ion transport system

A low-energy branch is under development for the MARA vacuum-mode recoil separator at the Accelerator Laboratory of the University of Jyväskylä. This development will allow for the study of proton-rich nuclei through laser ionisation spectroscopy and mass measurements. After stopping and extraction from a buffer gas cell, the ions of interest will be accelerated and transported to dedicated experimental setups by an ion transport system consisting of several focusing, accelerating and mass-separating elements. This article presents the current design and simulations for the ion transport.peerReviewe