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

Abstract

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.

Laser Spectroscopy of Neutron-Rich Tin Isotopes: A Discontinuity in Charge Radii across the N=82 Shell Closure

“…In brief, protons from the proton synchrotron booster with an energy of 1.4 GeV and beam currents of up to 2 μA hit a neutron converter [44] in close proximity to a uranium carbide (UC x ) target. Neutron-induced fission provides an efficient production of tin isotopes that were subsequently ionized using resonant laser ionization [45] while at the same time reducing the number of unwanted contaminants. The tin ions were accelerated to an energy of either 40 (SP) or 50 keV (PP), mass separated, and transported to the radio frequency quadrupole cooler and buncher ISCOOL [46], where they were accumulated, cooled, and ejected towards the COLLAPS beam line as ion bunches of typically 5 μs pulse length.…”

Performance of Dye and Ti:sapphire laser systems for laser ionization and spectroscopy studies at S3

“…This technique was explored by Lethokov and Hurst [30,31] and features an inherent elemental selectivity as well as a high efficiency due to the large cross sections for resonant excitation processes and the efficient ion manipulation and detection. Because of these advantages the technique is employed for a multitude of applications, including trace detection [32,33], efficient ionization of rare isotopes [34][35][36], and spectroscopic investigations [18,37,38]. The bottleneck of this method is the final ion-izing transition.…”

“…Resonant laser ionization is used for efficient and selective ionization of exotic radionuclides from hot-cavity targets and gas filled stopping cells [7,20,27,[34][35][36]. The process of resonant laser ionization provides high efficiencies for a wide range of elements [21,36,41], limited by the availability of the required laser radiation for the first excitation transition. The unique and well-defined level structure of each element provides a Z-selective ionization, and in combination with a mass-to-charge-ratio separation, isotopic ion beams of high purity can be produced.…”

Progress of resonant ionization laser ion source development at GANIL

“…4 RILIS combines Z-selectivity with an efficient ionization process. 5 It is based on step-wise resonant photo-ionization of the elements of interest. RILIS is one of the priority ion sources which should be operational for the commissioning and very first physics runs at SPIRAL2 Phase 2 to satisfy beam requests from a large part of the submitted Letters of Intent.…”