BECOLA Facility at FRIB

• BECOLA is a facility that employs Resonance Ionization Spectroscopy Experiment (RISE) to achieve high-sensitivity isotope and hyperfine structure measurements.
• It uses a sophisticated beamline with ultrahigh vacuum, state-of-the-art laser systems, and precise electrostatic controls to enable MHz-scale resolution.
• The integration of decay tagging and optimized ion-detection dramatically improves signal-to-background ratios, facilitating studies of rare, short-lived isotopes.

The BECOLA (BEam COoler and LAser spectroscopy) facility at the Facility for Rare Isotope Beams (FRIB) has advanced from a fluorescence-based collinear laser-spectroscopy station to a combined high-efficiency, low-background ion-detection system through the commissioning of the Resonance Ionization Spectroscopy Experiment (RISE). The integration of collinear resonance ionization spectroscopy (CRIS) enables sensitive measurements of isotope shifts and hyperfine structure for short-lived isotopes produced at FRIB, considerably enhancing the spectroscopic reach into rare and exotic nuclear systems (Brinson et al., 12 Nov 2025).

1. Beamline Configuration and Vacuum Infrastructure

Prior to the implementation of RISE, the BECOLA beamline supported two experimental inputs: a Penning Ion Gauge (PIG) offline ion source and the FRIB gas-catcher beam inlet. Ion beams traversed a Radio-Frequency Quadrupole Cooler Buncher (RFQCB), operating with high-pressure ($10^{-2}\ \mathrm{torr}$) He-buffer gas in the cooling section and $10^{-5}\ \mathrm{torr}$ in the bunching section under differential pumping. Subsequent to cooling and bunching, 30 keV ion bunches underwent in-flight neutralization in a sodium-vapor charge-exchange cell (CEC), before entering the fluorescence-detection region, which utilized an ellipsoidal mirror and photomultiplier tube for optical detection.

The RISE upgrade extended the beamline downstream of the fluorescence chamber by incorporating:

• Electrostatic deflector plates post-CEC for residual ion removal,
• A 2-meter interaction tube held at $<$5×10$^{-9}$ mbar (UHV) with CEC heated,
• Orthogonal electrostatic steering and a high-precision 45° electrostatic bender, routing reionized atoms to a MagneTOF single-ion detector,
• A retractable $\beta$-decay telescope at the terminal position for decay-tagged spectroscopy.

Beamline voltages were stabilized and controlled to better than $10^{-4}$ precision, ensuring stable drift-tube and CEC operations. The total beamline from ion production to MagneTOF detection spanned approximately 15 meters. All UHV integrity was preserved via upgrades to ceramic breaks, conflat flanges, and ion-pump stages (Brinson et al., 12 Nov 2025).

2. Laser Systems for Collinear Resonance Ionization

Two laser systems underpin the RISE experiment:

A. Injection-seeded Ti:Sapphire pulsed laser (Spectroscopy Step)

• Continuous-wave Spectra-Physics Matisse Ti:Sapphire laser (700–1000 nm), locked to ±1 MHz via HighFinesse WSU30 wavemeter, regularly calibrated to a He–Ne standard,
• Seed light guided by fiber into an injection-locked Ti:Sa oscillator, pumped by a Photonics TU-H Nd:YAG at 532 nm, delivering 30–50 ns pulses at up to 10 kHz, ~20 MHz linewidth, and ~200 μJ pulse energy,
• Nonlinear optics chains (BBO/BiBO crystals) producing second, third, or fourth harmonics (down to ≤265 nm) at conversion efficiencies of 10–30%.

B. Multi-harmonic pulsed Nd:YAG laser (Selective Ionization)

• Quantel Merion MW 7-100 system, 100 Hz, <10 ns pulse width,
• Output: 1064 nm (≤300 mJ), 532 nm (≈160 mJ), 355 nm (≈90 mJ), 266 nm (≈30 mJ); harmonic choice depends on the ionization threshold and selectivity requirements.

This laser infrastructure supports both single-step and multi-step resonant excitation and nonresonant ionization for precise isotope selection.

3. Collinear Resonance Ionization Spectroscopy Methodology

The RISE methodology exploits the overlap of a fast, neutralized atomic beam with laser pulses propagating in either collinear or anti-collinear geometry. The Doppler-shifted laser frequency in the ion rest frame is given by:

$$\nu_\mathrm{lab} = \nu_0 \frac{1 \pm \beta}{\sqrt{1-\beta^2}}, \quad \beta = \frac{v}{c}$$

where $v$ is the atomic velocity and $c$ the speed of light. By tuning the CEC voltage ($\Delta V$), the beam velocity, and thus the Doppler shift, are modulated, allowing for frequency scans without retuning the laser.

A typical two-step ionization scheme for $^{27}\mathrm{Al}$ involves:

1. Excitation $$(3\mathrm{s}^2 3\mathrm{p})^2P_{1/2} \rightarrow (3\mathrm{s}^2 5\mathrm{s})^2S_{1/2}$$ at $\approx$265 nm (frequency-tripled Ti:Sa laser).
2. Single-photon nonresonant ionization from $5s^2S_{1/2}$ via 532 nm (Merion laser).

The hyperfine Hamiltonian for the excited state is:

$$\hat{H}_\mathrm{hfs} = A\, \mathbf{I}\cdot \mathbf{J} + B \frac{3(\mathbf{I}\cdot \mathbf{J})^2 + \frac{3}{2}(\mathbf{I}\cdot \mathbf{J}) - I(I+1)J(J+1)}{2 I(2I-1)J(2J-1)}$$

Stepwise ionization rates in the low-saturation regime follow:

$$R_2 = N_1 \sigma_2 \Phi_2 = N_1 \sigma_2 \left(\frac{I_2}{h\nu_2}\right)$$

with overall multi-step efficiency scaling as

$$Y \propto (1-e^{-\sigma_1 \int I_1 dt}) (1-e^{-\sigma_2 \int I_2 dt}) \ldots$$

Reionized ions are deflected by the 45° bender to the MagneTOF detector, with timing electronics tagging each ion event to its bunch release time (Brinson et al., 12 Nov 2025).

4. Commissioning Results with $^{27}$Al

Testing with stable $^{27}\mathrm{Al}$ utilized ions from the PIG source, cooled and bunched at 30 Hz and 30 kV, neutralized in the sodium-vapor CEC. The Ti:Sa laser was alternated between collinear and anti-collinear alignments, locked to rest-frame centroid frequencies near 377.2148 THz and 376.0529 THz, respectively. Spectroscopy was performed with 265 nm (third harmonic), and a 532 nm Merion pulse arrived approximately 40(20) ns after excitation to optimize signal yield and suppress AC-Stark effects.

A combined collinear/anti-collinear analysis (6+6 spectra, total integration 1500 s) determined the rest-frame centroid as $$\nu_0 = 1\,129\,899.838(2)_\mathrm{stat}(30)_\mathrm{sys}$$ GHz. Fitting all 40 spectra from a cumulative 5580 s yielded the $^2P_{1/2}$ hyperfine magnetic constant $A = 502.85(14)_\mathrm{stat}(36)_\mathrm{sys}$ MHz, consistent with reference values (502–503 MHz).

Measured line shapes, modelled by an asymmetric pseudo-Voigt profile, demonstrated control of side-band and width parameters across all transitions. Stability studies over 90 h evidenced beam-energy drifts of 0.0036(3) eV/h, correctable via linear calibration, yielding centroid instabilities of ±1.5 MHz and A-constant fluctuations of ±0.6 MHz.

Comparison of detection efficiency—bounded from below by Faraday-cup current versus on-resonance counts—established $\eta \geq 10^{-5}$. The RISE method achieved signal amplitudes of $\approx$200–300 counts/s and background $<$20 counts/s (S/B ≈10–15), substantially outpacing conventional fluorescence S/B ($\lesssim 1.5$), where background was dominated by scattered laser light ($\sim$100 counts/s). Thus, RISE realizes an order-of-magnitude improvement in S/B for comparable signal rates (Brinson et al., 12 Nov 2025).

5. Operational Capabilities and Future Prospects

With the RISE integration, BECOLA enables:

• MHz-scale resolution in hyperfine and isotope-shift measurements on isotopes produced at rates of just a few ions/s,
• Implementation of $\beta$-decay tagging for isomer-selective decay spectroscopy,
• Highly selective multi-step ionization schemes involving Rydberg or auto-ionizing levels,
• Doppler-free absolute frequency determination via collinear/anti-collinear measurements.

Approved experimental programs target neutron-deficient Al, Ni, Zr, neutron-rich Si, O, and heavy elements such as Fr and Th. In conjunction with ab initio and density-functional-theory calculations, RISE at BECOLA is positioned to deliver precise charge-radius and electromagnetic-moment data across new nuclear regions, including short-lived isomeric systems significant for studies of fundamental symmetries and nuclear-structure benchmarks (Brinson et al., 12 Nov 2025).