Commissioning the Resonance Ionization Spectroscopy Experiment at FRIB

This paper reports the successful commissioning of the Resonance Ionization Spectroscopy Experiment (RISE) at FRIB's BECOLA facility, which integrates new beamline components and laser systems to enable sensitive measurements of isotope shifts and hyperfine structure in short-lived isotopes, as demonstrated by initial tests on stable $^{27}$Al.

The Gist

Imagine the atomic nucleus as the heart of a tiny, chaotic city. Scientists want to understand the "architecture" of these cities—how big they are, how they spin, and what makes them unique. But many of these atomic cities are incredibly short-lived; they pop into existence for a fraction of a second and then vanish. Studying them is like trying to take a high-definition photo of a firefly that only blinks once a year.

This paper is about the "commissioning" (the final testing and tuning) of a new, super-powerful camera called RISE (Resonance Ionization Spectroscopy Experiment). This camera is installed at FRIB (Facility for Rare Isotope Beams), a massive particle accelerator in Michigan that creates these fleeting atomic cities.

Here is the story of how they built and tested this new tool, explained simply:

1. The Problem: The "Flashlight" Was Too Weak

Before RISE, scientists used a method called "fluorescence detection." Imagine trying to find a specific person in a dark stadium by shining a flashlight and waiting for them to wave back.

• The Issue: The "waving" (light emission) is very faint. Most of the light gets lost, and the background noise (other lights in the stadium) is blindingly bright. If the person you are looking for only stays for a split second, you might miss them entirely.

2. The Solution: The "Magnetic Net" (RISE)

The new RISE instrument changes the game. Instead of waiting for the atom to glow, it uses a clever trick called Resonance Ionization.

• The Analogy: Imagine the atoms are like specific keys. You have a laser that acts like a master key. You tune the laser to fit only the key you want.
• The Trick: When the laser hits the right atom, it doesn't just make it glow; it gives the atom a "kick" that turns it into a charged particle (an ion).
• The Catch: Once the atom is charged, it can be caught in a magnetic net (a detector) with incredible efficiency. It's like switching from trying to catch a ghost with a net (fluorescence) to catching a magnetized ball with a magnet (RISE). This method is much quieter (less background noise) and catches almost everything.

3. The Setup: The Atomic Highway

To make this work, the scientists had to build a special highway for the atoms:

• The Starting Line: They create a beam of atoms (in this test, they used Aluminum, a stable metal, to practice).
• The Cooling Zone: The atoms are moving too fast to be measured, so they are slowed down and "bunched" together, like cars merging onto a highway in a tight group.
• The Neutralizer: The atoms are usually charged. To interact with the laser, they need to be neutral (like a calm person). They pass through a cloud of sodium vapor that strips away their charge, turning them into neutral atoms.
• The Laser Dance: This is the core of the experiment.
  • Step 1: A precise laser hits the atoms, exciting them to a higher energy level (like lifting a ball up a step).
  • Step 2: A second, powerful laser hits them immediately, knocking them off the step entirely (ionizing them).
  • The Result: Only the atoms that were perfectly tuned to the first laser get knocked off. The rest stay put.
• The Finish Line: The "knocked off" atoms are steered into a super-fast detector (the MagneTOF) that counts them one by one.

4. The Test Drive: Proving It Works

To make sure the new camera worked, the team didn't use the dangerous, short-lived radioactive atoms yet. They used Aluminum-27, a stable, common metal.

• The Challenge: They needed to measure the "hyperfine structure." Think of the nucleus as a spinning top. The electrons orbiting it feel the spin of the top. This interaction splits the energy levels slightly, like a musical note splitting into two very close harmonies.
• The Result: The team successfully measured these tiny splits in the Aluminum atoms. They proved that:
  1. The machine is stable enough to run for days without drifting.
  2. The background noise is incredibly low (the "stadium" is much darker now).
  3. They can measure the "beat" of the atom with extreme precision.

5. Why This Matters

Now that RISE is "commissioned" (tested and approved), it is ready for the real job.

• The Future: Scientists will use this to study the most exotic, unstable isotopes that FRIB creates. These are the "fireflies" that blink for less than a second.
• The Goal: By measuring these short-lived atoms, scientists can answer big questions about the universe: How do stars make heavy elements? What is the limit of how big an atomic nucleus can get before it falls apart?

In Summary:
The scientists built a new, ultra-sensitive "magnet net" (RISE) to catch and count single atoms. They tested it on a common metal (Aluminum) to prove it works perfectly. Now, they are ready to use it to catch the rarest, most fleeting atoms in the universe, helping us understand the fundamental building blocks of matter.

Technical Summary

1. Problem Statement

The Facility for Rare Isotope Beams (FRIB) aims to explore exotic nuclei at the limits of stability. However, these nuclei often have extremely short lifetimes (sub-second) and are produced at very low rates (a few ions per second). Traditional collinear laser spectroscopy techniques, specifically those relying on fluorescence detection, face significant limitations in this regime:

• Low Efficiency: Fluorescence detection is limited by the small solid angle of photon collection and the low quantum efficiency of photomultiplier tubes (typically <25%).
• High Background: The signal-to-background ratio is degraded by scattered laser light, which can be orders of magnitude higher than the signal.
• Sensitivity Threshold: These factors make it difficult to study isotopes with production yields below a certain threshold or to resolve hyperfine structures in low-yield beams.

There is a critical need for a more sensitive detection method capable of measuring the electromagnetic properties (charge radii, spins, moments) of these rare isotopes with high precision.

2. Methodology

The authors describe the commissioning of a new instrument, the Resonance Ionization Spectroscopy Experiment (RISE), integrated into the existing BECOLA (Beam COoler and LAser spectroscopy) facility at FRIB.

Instrument Design and Layout:

• Beamline Extension: The existing BECOLA beamline was extended to include an electrostatic ion-beam bender and an ultra-high vacuum (UHV) interaction region.
• Ion Source & Cooling: Ions (stable or radioactive) are extracted from a Penning ionization gauge (PIG) source or FRIB gas-stopping cells, cooled in a Radiofrequency Quadrupole Cooler Buncher (RFQCB), and neutralized in-flight using a charge-exchange cell (CEC) with sodium vapor.
• Laser System:
  • Resonant Excitation: A continuous-wave (CW) Ti:Sapphire laser (seeded) is injection-seeded into a pulsed Ti:Sapphire cavity to produce narrow-linewidth (~20 MHz) pulses. This drives the first transition (e.g., $2P_{1/2} \to 2S_{1/2}$ in Al).
  • Selective Ionization: A pulsed Nd:YAG laser (Quantel Merion) provides the energy for the final ionization step, selectively ionizing only atoms that were resonantly excited in the previous step.
• Detection Scheme: Instead of detecting photons, RISE detects the ions created by the laser. The re-ionized beam is steered by a 30° electrostatic bender onto a MagneTOF particle detector.
  • The MagneTOF offers >80% detection efficiency and sub-nanosecond timing resolution.
  • It operates with extremely low intrinsic background (<20 counts/min).
• Decay Station: The beamline terminates in a $\Delta E-E$ $\beta$-telescope, allowing for $\beta$-tagging to further suppress background from stable contaminants and enabling nuclear decay spectroscopy.

Commissioning Procedure:
The team commissioned the system using stable $^{27}$Al produced by the offline PIG source. They utilized a two-step resonance ionization scheme:

1. Excitation: $3s^23p \ ^2P_{1/2} \to 3s^25s \ ^2S_{1/2}$ (via frequency-tripled laser light at ~265 nm).
2. Ionization: Non-resonant ionization from the excited state using a 532 nm laser pulse.
3. Beam Energy Calibration: To achieve Doppler-free precision, they performed both collinear and anticollinear measurements. By comparing the Doppler-shifted centroids in both directions, they could calculate the absolute transition frequency and correct for beam energy drifts.

3. Key Contributions

• Implementation of RISE: Successfully integrated a high-efficiency resonance ionization detection scheme into the BECOLA facility, transitioning from fluorescence to direct particle counting.
• System Characterization: Demonstrated the ability to perform MHz-level resolution spectroscopy on stable isotopes, validating the laser systems, beam transport, and detection electronics.
• Background Suppression: Showed that the RISE detection scheme reduces background noise by a factor of ~6 compared to fluorescence detection for the tested transitions, resulting in a 10-fold improvement in signal-to-background ratio.
• Precision Measurement: Established a robust method for determining absolute transition frequencies and hyperfine constants by combining collinear/anticollinear data with linear beam energy corrections.

4. Results

• Hyperfine Structure: The team successfully resolved the four distinct hyperfine transitions of $^{27}$Al ($I=5/2$) arising from the splitting of the $^2P_{1/2}$ and $^2S_{1/2}$ states.
• Precision Metrics:
  • Centroid Frequency ($\nu_0$): Measured as $1,129,899.838(2)_{\text{stat}}(39)_{\text{sys}}$ GHz. This agrees well with literature values.
  • Hyperfine Constant ($A_{2P_{1/2}}$): Determined to be $502.93(13)_{\text{stat}}(21)_{\text{sys}}$ MHz.
  • Stability: Over a 90-hour period, the system demonstrated long-term stability. After applying a linear correction for beam energy drift (observed at 0.0036 eV/hr), the residual standard deviation in the transition centroid was $\pm 1.5$ MHz, and in the hyperfine constant was $\pm 0.6$ MHz.
• Efficiency: While absolute efficiency could not be calculated due to unknown beam purity, a lower bound of $\eta \ge 10^{-5}$ was established. More importantly, the comparison with fluorescence detection confirmed that RISE is significantly more sensitive for low-yield beams.

5. Significance

• Enabling Rare Isotope Studies: The RISE instrument extends the sensitivity of the BECOLA facility, making it possible to study isotopes produced at rates as low as a few ions per second. This opens up previously inaccessible regions of the nuclear chart.
• Nuclear Structure Physics: The high precision and low background allow for the extraction of nuclear charge radii, spins, magnetic dipole moments, and electric quadrupole moments for exotic nuclei. This data is crucial for testing nuclear models and understanding the equation of state for dense nuclear matter.
• Future Applications: The facility is now operational for a series of approved experiments targeting neutron-deficient Al, Ni, and Zr isotopes, as well as neutron-rich Si, O, Fr, and Th isotopes. The integration of $\beta$-decay spectroscopy further allows for the study of isomers and nuclear decay properties with high initial state purity.

In conclusion, the commissioning of RISE represents a major technological advancement in collinear laser spectroscopy, providing a powerful tool to probe the fundamental properties of the most exotic nuclei available at FRIB.