Resonance Ionization Laser Ion Source Research Articles

RILIS applications at CERN/ISOLDE

Since 2011 the ISOLDE Resonance Ionization Laser Ion Source (RILIS) has comprised a dual system of three dye and three Ti:sapphire lasers capable of gap-free wavelength tuning from 210 to 950 nm. Temporal synchronization of the lasers has enabled several operating modes to be established which make optimal use of the complementary characteristics of each laser type. This flexibility of the system has presented several opportunities for additional atomic spectroscopy studies and ionization scheme development, whilst also enabling an increase in the number of operating hours for standard ion beam production. The extended capabilities of the dual-RILIS system are exemplified by the recent operational highlights. These include on-line ionization scheme development (At, Ca), measurements of ionization energies (At, Po), in-source resonance ionization spectroscopy of exotic isotopes (At, Au, Po), and the provision of a fibre-coupled narrow-linewidth Ti:sapphire beam for the on-line commissioning of the CRIS experiment (Fr).

In-source spectroscopy on astatine and radium for resonant laser ionization

At on-line isotope separator facilities, rare isotopes of radioactive elements such as astatine, radium or polonium are demanded for fundamental research on nuclear structure. These elements are generally suitable for a resonance ionization laser ion source, but more data on the atomic structure is necessary to develop efficient laser ionization schemes. Due to the missing stable reference isotopes spectroscopic investigation of the atomic structure can only be performed during on-line operation. At the Isotope Separator and ACcelerator (ISAC) facility at TRIUMF, the elements astatine and radium were investigated by in-source laser spectroscopy to optimize the laser ionization efficiency. For astatine, laser spectroscopy was performed to search for high lying bound states as well as for auto-ionizing resonances. This led to the identification of four new high lying bound states of odd parity, while no auto-ionizing resonances were observed in the investigated region. Furthermore, the feasibility and the impact of laser ionization on the yield of radium isotopes was investigated using an activated target after proton irradiation.

Resonance ionization laser ion sources for on-line isotope separators (invited)

A Resonance Ionization Laser Ion Source (RILIS) is today considered an essential component of the majority of Isotope Separator On Line (ISOL) facilities; there are seven laser ion sources currently operational at ISOL facilities worldwide and several more are under development. The ionization mechanism is a highly element selective multi-step resonance photo-absorption process that requires a specifically tailored laser configuration for each chemical element. For some isotopes, isomer selective ionization may even be achieved by exploiting the differences in hyperfine structures of an atomic transition for different nuclear spin states. For many radioactive ion beam experiments, laser resonance ionization is the only means of achieving an acceptable level of beam purity without compromising isotope yield. Furthermore, by performing element selection at the location of the ion source, the propagation of unwanted radioactivity downstream of the target assembly is reduced. Whilst advances in laser technology have improved the performance and reliability of laser ion sources and broadened the range of suitable commercially available laser systems, many recent developments have focused rather on the laser/atom interaction region in the quest for increased selectivity and/or improved spectral resolution. Much of the progress in this area has been achieved by decoupling the laser ionization from competing ionization processes through the use of a laser/atom interaction region that is physically separated from the target chamber. A new application of gas catcher laser ion source technology promises to expand the capabilities of projectile fragmentation facilities through the conversion of otherwise discarded reaction fragments into high-purity low-energy ion beams. A summary of recent RILIS developments and the current status of laser ion sources worldwide is presented.

Progress of resonant ionization laser ion source development at GANIL

SPIRAL2 (Système de Production d'Ions Radioactifs Accélérés en Ligne) is a research facility under construction at GANIL (Grand Accélérateur National d'Ions Lourds) for the production of radioactive ion beams by isotope separation on-line methods and low-energy in-flight techniques. A resonant ionization laser ion source will be one of the main techniques to produce the radioactive ion beams. GISELE (GANIL Ion Source using Electron Laser Excitation) is a test bench developed to study a fully operational laser ion source available for Day 1 operations at SPIRAL2 Phase 2. The aim of this project is to find the best technical solution which combines high selectivity and ionization efficiency with small ion beam emittance and stable long term operation. Latest results about the new ion source geometry will be presented.

ORNL developments in laser ion sources for radioactive ion beam production

The development of a resonant ionization laser ion source (RILIS) for the production of isotopically pure radioactive ion beams is reported. The application of the laser ion source calls for high elemental selectivity, high efficiency, and fast release of short-lived isotopes. A hot-cavity ion source and three Ti:sapphire lasers pulsed at a 10 kHz rate are employed for the RILIS. The Ti:sapphire lasers have been upgraded with individual pump lasers to eliminate intracavity Pockels cells and output losses due to synchronization delays. The development of ionization schemes for a wide range of elements is important to the success of Ti:sapphire-laser-based RILIS. In off-line studies with stable isotopes, resonant ionization of 14 elements has been studied, leading to new ionization schemes for ten elements. The absolute ionization efficiency of the hot-cavity RILIS has been measured to range from 0.9 % to 40 % for different elements. The mechanisms for ion transportation and confinement in the hot-cavity ion source have been studied using the temporal profiles of the laser-ionized ions. The hot-cavity RILIS has provided beams of neutron-rich \(^{83,85,86}\)Ga isotopes for beta decay studies and enabled the first measurement of the beta decay of the exotic \(^{86}\)Ga.

Narrow linewidth operation of the RILIS titanium: Sapphire laser at ISOLDE/CERN

A narrow linewidth operating mode for the Ti:sapphire laser of the CERN ISOLDE Resonance Ionization Laser Ion Source (RILIS) has been developed. This satisfies the laser requirements for the programme of in-source resonance ionization spectroscopy measurements and improves the selectivity for isomer separation using RILIS. A linewidth reduction from typically 10GHz down to 1GHz was achieved by the intra-cavity insertion of a second (thick) Fabry-Pérot etalon. Reliable operation during a laser scan was achieved through motorized control of the tilt angle of each etalon. A scanning, stabilization and mode cleaning procedure was developed and implemented in LabVIEW. The narrow linewidth operation was confirmed in a high resolution spectroscopy study of francium isotopes by the Collinear Resonance Ionization Spectroscopy experiment. The resulting laser scans demonstrate the suitability of the laser, in terms of linewidth, spectral purity and stability for high resolution in-source spectroscopy and isomer selective ionization using the RILIS.

β-delayed fission andαdecay of178Tl

A detailed nuclear-decay spectroscopy study of the neutron-deficient isotope 178Tl has been performed using the highly selective Resonance Ionization Laser Ion Source and ISOLDE mass separator (CERN), which allowed a unique isobarically pure beam of 178Tl to be produced. The first identification of the $\beta$-delayed fission of this isotope was made and its probability P$\beta$DF(178Tl) = 0.15(6)\% was determined. An asymmetric fission fragment mass distribution of the daughter isotope 178Hg (populated by the $\beta$ decay of 178Tl) was deduced based on the measured fission fragment energies. The fine-structure $\alpha$-decay pattern of 178Tl allowed the low-energy states in the daughter nucleus 174 Au to be studied.

ISOL facility for rare isotope beams at RAON

The Rare Isotope Science Project (RISP) in Korea is developing the Rare Isotope (RI) Accelerator, RAON. The RAON is planned to be an advanced RI beam using isotope separation on-line (ISOL) with a high-power target. The main goal of the ISOL facility is to deliver high-quality, intense, neutron-rich (n-rich) beams to the experimental hall; for example 108 atoms per second of the 132Sn n-rich reference isotope. The RAON ISOL facility consists of mainly two systems. One is the RI production system of a high-power uranium fission target combined with the Forced Electron Beam Induced Arc Discharge (FEBIAD) ion source, Surface Ionization (SI) ion source and Resonance Ionization Laser Ion Source (RILIS), in which system the final goal for the RI production rate is 1014 fission per second. The other is the RI beam purification system, which is comprised of an RF beam cooler, a high resolution mass separator (HRMS), a charge breeder and a charge state separator, and the design goal is to deliver the RI beam with a mass resolving power up to 45,000 and with a limit of 6 × 105 background counts per second. The current status of the ISOL facility of the RISP is reported.

Largeβ-Delayed One and Two Neutron Emission Rates in the Decay ofGa86

Beta decay of $^{86}\mathrm{Ga}$ was studied by means of $\ensuremath{\beta}$-neutron-$\ensuremath{\gamma}$ spectroscopy. An isotopically pure $^{86}\mathrm{Ga}$ beam was produced at the Holifield Radioactive Ion Beam Facility using a resonance ionization laser ion source and high-resolution electromagnetic separation. The decay of $^{86}\mathrm{Ga}$ revealed a half-life of ${43}_{\ensuremath{-}15}^{+21}\text{ }\text{ }\mathrm{ms}$ and large $\ensuremath{\beta}$-delayed one-neutron and two-neutron branching ratios of ${P}_{1n}=60(10)%$ and ${P}_{2n}=20(10)%$. The $\ensuremath{\beta}\ensuremath{\gamma}$ decay of $^{86}\mathrm{Ga}$ populated a 527 keV transition that is interpreted as the deexcitation of the first ${2}^{+}$ state in the $N=54$ isotone $^{86}\mathrm{Ge}$ and suggests a quick onset of deformation in Ge isotopes beyond $N=50$.

New developments of the in-source spectroscopy method at RILIS/ISOLDE

At the CERN ISOLDE facility, long isotope chains of many elements are produced by proton-induced reactions in target materials such as uranium carbide. The Resonance Ionization Laser Ion Source (RILIS) is an efficient and selective means of ionizing the reaction products to produce an ion beam of a chosen isotope. Coupling the RILIS with modern ion detection techniques enables highly sensitive studies of nuclear properties (spins, electromagnetic moments and charge radii) along an isotope chain, provided that the isotope shifts and hyperfine structure splitting of the atomic transitions can be resolved. At ISOLDE the campaign to measure the systematics of isotopes in the lead region (Pb, Bi, Tl and Po) has been extended to include the gold and astatine isotope chains. Several developments were specifically required for the feasibility of the most recent measurements: new ionization schemes (Po, At); a remote controlled narrow line-width mode of operation for the RILIS Ti:sapphire laser (At, Au, Po); isobar free ionization using the Laser Ion Source Trap, LIST (Po); isobar selective particle identification using the multi-reflection time-of-flight mass separator (MR-ToF MS) of ISOLTRAP (Au, At). These are summarized as part of an overview of the current status of the in-source resonance ionization spectroscopy setup at ISOLDE.

First application of the Laser Ion Source and Trap (LIST) for on-line experiments at ISOLDE

The Laser Ion Source and Trap (LIST) provides a new mode of operation for the resonance ionization laser ion source (RILIS) at ISOLDE/CERN, reducing the amount of surface-ionized isobaric contaminants by up to four orders of magnitude. After the first successful on-line test at ISOLDE in 2011 the LIST was further improved in terms of efficiency, selectivity, and reliability through several off-line tests at Mainz University and at ISOLDE. In September 2012, the first on-line physics experiments to use the LIST took place at ISOLDE. The measurements of the improved LIST indicate more than a twofold increase in efficiency compared to the LIST of the 2011 run. The suppression of surface-ionized francium contaminants has enabled the first in-source laser spectroscopy of 217Po and 219Po.

Data acquisition, remote control and equipment monitoring for ISOLDE RILIS

With a steadily increasing on-line operation time up to a record 3000h in the year 2012, the Resonance Ionization Laser Ion Source (RILIS) is one of the key components of the ISOLDE on-line isotope user facility at CERN. Ion beam production using the RILIS is essential for many experiments due to the unmatched combination of ionization efficiency and selectivity. To meet the reliability requirements the RILIS is currently operated in shift duty for continuous maintenance of crucial laser parameters such as wavelength, power, beam position and timing, as well as ensuring swift intervention in case of an equipment malfunction. A recent overhaul of the RILIS included the installation of new pump lasers, commercial dye lasers and a complementary, fully solid-state titanium:sapphire laser system. The framework of the upgrade also required the setup of a network-extended, LabVIEW-based system for data acquisition, remote control and equipment monitoring, to support RILIS operators as well as ISOLDE users. The system contributes to four key aspects of RILIS operation: equipment monitoring, machine protection, automated self-reliance, and collaborative data acquisition. The overall concept, technologies used, implementation status and recent applications during the 2012 on-line operation period will be presented along with a summary of future developments.

Resonant Ionization Laser Ion Source (RILIS) off-line developments on Ga, Al and Ca

The Resonant Ionization Laser Ion Source (RILIS) is an element selective, highly efficient and versatile tool for generation of radioactive ion beams at on-line mass separator facilities. Parallel to TRIUMF’s on-line RILIS at the Isotope Separator and ACcelerator (ISAC) facility, an off-line Laser Ion Source test stand (LIS STAND) is operated for systematic laser resonance ionization spectroscopy, ionization scheme and ion source development. Three titanium sapphire (Ti:Sa) lasers optionally equipped with harmonic frequency generation units are used to resonantly step-wise excite and ionize elements of interest. A grating tuned Ti:Sa laser allows continuous laser wavelength scans of up to Δ≈200nm. With this laser inventory and the LIS STAND, atomic Rydberg series and auto-ionizing levels can systematically be studied. The LIS STAND has been in use since 2009 and so far the spectroscopy on Ga, Al and Ca has been performed. The development of efficient laser resonant ionization schemes, their investigation and comparison using the LIS STAND are discussed.

In-source laser spectroscopy developments at TRILIS—towards spectroscopy on actinium and scandium

Resonance Ionization Laser Ion Sources (RILIS) have become a versatile tool for production and study of exotic nuclides at Isotope SeparatorOn-Line (ISOL) facilities such as ISAC at TRIUMF. The recent development and addition of a grating tuned spectroscopy laser to the TRIUMF RILIS solid state laser system allows for wide range spectral scans to investigate atomic structures on short lived isotopes, e.g., those from the element actinium, produced in uranium targets at ISAC. In addition, development of new and improved laser ionization schemes for rare isotope production at ISAC is ongoing. Here spectroscopic studies on bound states, Rydberg states and autoionizing (AI) resonances on scandium using the existing offline capabilities are reported. These results allowed to identify a suitable ionization scheme for scandium via excitation into an autoionizing state at 58,104 cm−1 which has subsequently been used for ionization of on-line produced exotic scandium isotopes.

Study of surface ionization and LASER ionization processes using the SOMEIL ion source: application to the Spiral 2 laser ion source development

SPIRAL2 is the new project under construction at GANIL to provide radioactive ion beams to the Nuclear Physics Community and in particular neutron rich ion beams. For the production of condensable radioactive elements, a resonant ionization laser ion source is under development at GANIL. In order to generate the ions of interest with a good selectivity and purity, our group is studying the way to minimize surface ionization process by using refractory materials with low work function as ionizer tube. To do those investigations a dedicated ion source, called SOMEIL (Source Optimisee pour les Mesures d‘Efficacite d‘Ionisation Laser) is used. Numerous types of ionizer tubes made in various materials and geometry are tested. Surface ionization and laser ionization efficiencies can be measured for each of them.

On-line commissioning of the HRIBF resonant ionization laser ion source

A highly selective resonant ionization laser ion source has been successfully commissioned at the Holifield Radioactive Ion Beam Facility, Oak Ridge National Laboratory, for the production of pure beams of short-lived nuclei for spectroscopic studies. The laser ion source provided beams of neutron-rich Ga isotopes to the Low-energy Radioactive Ion Beam Spectroscopy Station for beta decay measurements. The radioactive Ga isotopes were produced by 50-MeV proton induced fission of 238U and ionized by laser radiation using a two-step resonant ionization scheme. Isobarically pure 83Ga, 85Ga, and 86Ga beams were delivered to the experiment at approximate rates of 12,000ions/s, 100ions/s, and 3ions/s, respectively.

A test stand for off-line laser ion source development at TRIUMF

A test stand for ion source development and laser resonance ionization spectroscopy was built and commissioned at TRIUMF. The test stand is needed to develop efficient ion sources that can function reliably in the hostile, high temperature, high radiation environment of TRIUMF's isotope separator on-line (ISOL) production target ion source. In addition, it enables laser resonance ionization spectroscopy to develop laser excitation schemes suitable for the solid-state laser systems used with TRIUMF's resonant ionization laser ion source . Also, it allows for possible improvement of current ion sources and validation of new designs. The test stand employs a copy of the ion optics used on-line, so that results can be transferred directly to radioactive ion beam production. Due to space restrictions and the need for rapid mass scans, a quadrupole mass spectrometer is used as a mass separator. One of the first experiments conducted on the laser ion source test stand (LIS STAND) was resonant ionization spectroscopy of gallium to improve on the ionization scheme previously used on-line, so that low yield isotopes (e.g., (62)Ga) become available for experiments. Different Rydberg series in gallium were observed and autoionizing states were searched for. The overall LIS STAND system performance, characteristics, and the first resonant ionization spectroscopy are described.

Upgrade of the resonance ionization laser ion source at ISOLDE on-line isotope separation facility: New lasers and new ion beams

The resonance ionization laser ion source (RILIS) produces beams for the majority of experiments at the ISOLDE on-line isotope separator. A substantial improvement in RILIS performance has been achieved through a series of upgrade steps: replacement of the copper vapor lasers by a Nd:YAG laser; replacement of the old homemade dye lasers by new commercial dye lasers; installation of a complementary Ti:Sapphire laser system. The combined dye and Ti:Sapphire laser system with harmonics is capable of generating beams at any wavelength in the range of 210-950 nm. In total, isotopes of 31 different elements have been selectively laser-ionized and separated at ISOLDE, including recently developed beams of samarium, praseodymium, polonium, and astatine.

Laser ion source development at Holifield Radioactive Ion Beam Facility

This report describes the efforts made to develop a resonant-ionization laser ion source based on tunable Ti:sapphire lasers for nuclear physics and astrophysics research at Holifield Radioactive Ion Beam Facility. Three Ti:sapphire lasers have been upgraded with individual pump lasers to eliminate laser power losses due to synchronization delays. Ionization schemes for 14 elements have been obtained. Off-line studies show that the overall efficiency of the laser ion source can be as high as 40%. TaC surface coatings have been investigated for minimizing surface and bulk trapping of the atoms of interest.

Decay of ther-process nuclides137,138,139Sb, and theA=130solarr-process abundance peak

Half-life (${T}_{1/2}$) and \ensuremath{\beta}-delayed neutron branching (${P}_{n}$) values of 492(25) ms and 49(8)$%$, 350(15) ms and 72(8)$%$, and 93(13) ms and 90(10)$%$ for the $r$-process nuclei ${}^{137,138,139}$Sb, respectively, have been measured at the CERN On-Line Isotope Mass Separator (ISOLDE) facility by counting \ensuremath{\beta}-delayed neutrons. More precise ${T}_{1/2}$ and ${P}_{n}$ values of 300(15) ms and 27(4)$%$, and 273(7) ms and 50(8)$%$ for ${}^{136,137}$Sn, respectively, have also been measured. The sources were prepared by using the selective ionization of Sb or Sn with the Resonance Ionization Laser Ion Source and the high-resolution mass separator. The new data for Sb isotopes are compared with calculated ${T}_{1/2}$ and ${P}_{n}$ values for both spherical and nonspherical shapes. The data have been incorporated into parametrized nucleosynthesis calculations of the $r$ process in high-entropy winds of core-collapse supernovae in order to study the properties of the $A$ $=$ 130 solar-system $r$-process abundance peak.