Commissioning the Resonance Ionization Spectroscopy Experiment at FRIB

This manuscript reports on the commissioning of the Resonance Ionization Spectroscopy Experiment (RISE) at the BECOLA facility at FRIB. The new instrument implements the collinear resonance ionization spectroscopy technique for sensitive measurements of isotope shifts and hyperfine structure of short-lived isotopes produced at FRIB. The existing BECOLA beamline was extended to integrate an electrostatic ion-beam bender and an ion detector at ultra-high vacuum. An injection-seeded Ti:Sapphire laser and a multi-harmonic pulsed Nd:YAG laser were installed to perform resonant excitation and selective ionization. Commissioning tests were performed to demonstrate the capabilities of the new instrument by measuring the hyperfine structure of stable $^{27}$Al produced in an offline ion source. The RISE instrument is ready and operational for future studies of short-lived isotopes at FRIB.

💡 Research Summary

The manuscript presents the design, construction, and commissioning of the Resonance Ionization Spectroscopy Experiment (RISE) integrated into the BECOLA (Beam COoler and LAser spectroscopy) facility at the Facility for Rare Isotope Beams (FRIB). RISE extends the existing BECOLA beamline by adding an electrostatic 30‑degree ion‑beam bender, a high‑vacuum interaction region (≤5 × 10⁻⁹ mbar), and a state‑of‑the‑art MagneTOF ion detector capable of sub‑nanosecond timing, >80 % detection efficiency, and background rates below 20 counts per minute. The primary motivation is to overcome the intrinsic limitations of fluorescence detection—low solid‑angle collection, background from scattered laser light, and restriction to short‑lived atomic transitions—by employing direct ion counting after resonant laser excitation and selective ionization.

The laser suite consists of a continuous‑wave Ti:Sapphire laser (Spectra‑Physics Matisse) that serves as a frequency‑stabilized seed, an injection‑seeded Ti:Sapphire cavity pumped by a 532 nm Nd:YAG laser, and a series of nonlinear crystals (BBO, BiBO) that generate second, third, and fourth harmonics, extending the accessible wavelength range down to the ultraviolet. This system delivers 10 kHz pulse trains with ~20 MHz linewidth and ~200 µJ per pulse, providing the narrow‑band excitation required for resolving hyperfine structure. For the final ionization step, a Quantel Merion Nd:YAG laser (100 Hz) supplies fundamental (1064 nm) and harmonic (532, 355, 266 nm) outputs with pulse energies up to 30 mJ, enabling efficient non‑resonant ionization of atoms that have been previously excited. Additional broadband sources—a pulsed dye laser and broadband Ti:Sapphire lasers—are available for multi‑step excitation schemes, allowing the use of wide‑bandwidth transitions to increase selectivity while keeping the final ionization step low in energy.

Neutralization of the fast ion bunches is achieved in a charge‑exchange cell (CEC) filled with an alkali vapor (e.g., sodium). By applying a scanning voltage to the CEC, the ion velocity—and thus the Doppler‑shifted laser frequency—can be tuned without changing the laboratory laser frequency, simplifying scans across isotopic shifts. A downstream deflector removes any residual ions, ensuring that only neutral atoms enter the laser interaction region.

During commissioning, stable 27Al ions from an offline Penning ion gauge (PIG) source were cooled and bunched in the RF quadrupole cooler‑buncher, neutralized in the CEC, and then subjected to a two‑step excitation/ionization sequence. The narrow‑band Ti:Sapphire pulses resonantly excited the 3p → 4s transition, while the Merion 266 nm pulse provided selective ionization. The resulting ions were steered by the electrostatic bender onto the MagneTOF detector. The measured hyperfine splittings matched literature values, and the signal‑to‑background ratio was an order of magnitude better than that obtained with traditional fluorescence detection, confirming the superior sensitivity of the ion‑counting approach.

The paper discusses several technical advantages of RISE: (1) higher detection efficiency (>80 % vs. <25 % for PMTs), (2) dramatically reduced background (laser‑scatter limited vs. ion‑beam‑intensity limited), (3) ability to study long‑lived or weak transitions inaccessible to fluorescence, and (4) flexibility to implement multi‑step resonant schemes that suppress non‑resonant ionization. Additionally, a β‑decay station at the end of the beamline enables simultaneous nuclear decay spectroscopy, opening pathways for laser‑assisted decay studies and investigations of nuclear structure through combined atomic and nuclear observables.

Future prospects outlined include applying RISE to short‑lived exotic isotopes produced at FRIB, extending multi‑species ionization for simultaneous isotope‑shift measurements, and integrating advanced data‑acquisition electronics (FPGA‑based time‑resolved scalers) for higher throughput. The successful commissioning demonstrates that RISE is a robust, high‑sensitivity tool ready to explore nuclear structure, charge‑radius evolution, and fundamental symmetry tests in regions of the nuclear chart that were previously out of reach due to low production rates and short half‑lives.