In-gas jet laser spectroscopy with S3-LEB

by Anjali AJAYAKUMAR, 3rd year PhD student at GANIL

The Superconducting Linear Accelerator (SPIRAL2-LINAC) facility of GANIL will extend the capabilities of studying exotic nuclei by producing beams of rare radioactive isotopes with the highest intensities achieved so far. The SPIRAL2-LINAC coupled with the Super Separator Spectrometer (S3) recoil separator will facilitate the production of neutron-deficient nuclei close to the proton dripline and super heavy nuclei via fusion-evaporation reactions and separate them from the intense background contamination. The Super Separator Spectrometer-Low Energy Branch (S3-LEB), currently under commissioning at LPC Caen, will sit at the focal plane of S3 and stop, thermalize, and then neutralize the radioactive isotopes using a buffer gas cell set-up. The LEB is dedicated to the study of the ground state and isomeric state properties using laser ionization with high efficiency and high selectivity.

Development work has been ongoing at the S3-LEB setup. It uses in-gas-jet laser ionization, to resonantly ionize the neutralized atoms, and ion guides to send them to the Piège à Ions Linèaire du GANIL pour la Rèsolution des Isobares et la mesure de Masse (PILGRIM), a Multi-Reflection Time Of Flight Mass Spectrometer (MR-TOF-MS) or to a decay spectroscopy study station, SEASON. A buffer gas cell with 400 ms extraction time has been coupled to the ion transport ensemble and a de-Laval nozzle of Mach number M ~ 8 (developed in KU Leuven) has been installed at the gas cell aperture which can create a hypersonic jet of narrow velocity distribution in a reduced collision environment. The hypersonic jet environment reduces the Doppler and pressure broadening by at least an order of magnitude compared to the gas cell. Laser spectroscopy with a suitable atomic transition scheme with S3-LEB thus offers improved spectral resolution (≤300 MHz) while maintaining high efficiency, enabling access to isotope shifts and hyperfine constants measurements and thus to nuclear structure such as nuclear spin, moments, and differences in nuclear charge radii for the exotic nuclei.

Here, I will  present the results from the offline commissioning of S3-LEB with first in-gas-jet ionization and high-resolution laser spectroscopy in-gas-jet results for erbium, performed to validate the potential of the setup. Characterization of the pressure-broadening effects in the gas cell, isotope shift, and hyperfine structure measurements of the atoms in the hypersonic gas jet will be reported. Additionally, the first optimization of the overall transport efficiency for the setup with laser-produced ions will be reported.