Commissioning the Resonance ionization Spectroscopy Experiment at FRIB

TL;DR

RISE enables MHz-level, Doppler-free collinear resonance ionization spectroscopy of short-lived isotopes at FRIB by integrating a dedicated ionization beamline with BECOLA, including an electrostatic bender, MagneTOF detector, and a versatile laser system for resonant excitation and selective ionization, complemented by a beta-decay station for decay spectroscopy. Commissioning with stable $^{27}$Al confirms high spectral resolution, robust long-term stability, and effective background suppression, with a Doppler-free centroid obtained from collinear/anticollinear measurements such as $nu0 = sqrt(nu_a nu_c)$. The results demonstrate strong potential for isotope-shift and hyperfine-structure studies across exotic nuclei and establish RISE as a sensitive, background-suppressed tool capable of operating at low online yields, paving the way for future FRIB experiments and decay-spectroscopy integration.

Abstract

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, as well as 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.

Figures (8)

• Figure 1: Layout of the BECOLA facility at FRIB. In the BECOLA facility, radioactive (via FRIB) or stable (via PIG source) ion beams are injected into the RFQCB, extracted at an energy of $\sim$30 keV, and neutralized in-flight. Collinear laser spectroscopy is then performed either via fluorescence detection, or via single ion counting with the new RISE beamline extension.
• Figure 2: Layout of the RISE extension to the BECOLA beamline. Bunches of cooled ions from the RFQCB are sent to the CEC for neutralization. For fluorescence experiments, a laser is overlapped with the neutral bunch directly after the CEC, and the resulting fluorescence is collected and detected by a set of PMTs. For resonance ionization experiments, the non-neutral bunch fraction is filtered out by the deflector, while the neutral fraction is overlapped with lasers in the interaction region, causing resonant excitation, followed by selective ionization. The reionized bunch is then steered onto a retractable single ion counter after a 45$\degree$ electrostatic bender. A $\beta$-decay station is located at the end of the beamline, for beam characterization during experiments with rare isotope beams and for laser-assisted nuclear decay spectroscopyLynch2016Combined198.
• Figure 3: Schematic diagram of RISE/BECOLA laser systems. White boxes indicate wavemeters, while red ($650-1000$ nm), green($532$ nm), blue ($350-500$ nm), and violet ($\lesssim350$ nm) boxes indicate lasers at various wavelengths. Outgoing arrows indicate coupling into the beamline. The helium neon (HeNe) laser is used to keep the HighFinesse WSU30 (Wavemeter 1) calibrated.
• Figure 4: $\beta$ decay stationSchematic of the $\beta$-decay station at the end of RISE, consisting of a $\Delta$E-E $\beta$ telescope mounted on a quartz viewport. The setup provides $\beta$ Q-value selectivity while suppressing muon and $\gamma$-ray background. The $\Delta$E detector is configurable for different decay energies. See text for more details.
• Figure 5: Example hyperfine spectrum for $^{27}$Al. The four distinct peaks are due to the hyperfine splitting of the $^{2}P_{1/2}$ and $^{2}S_{1/2}$ states. The colors and types of fitted line indicate the corresponding transitions shown in the right-hand side of the figure. The individual peaks have a shared asymmetric line shape due to the energy exchange during the charge exchange process (see text for more details). The error bars are of a similar size or smaller than the markers.