Commissioning and Full Realization of the PLASEN System at BRIF

The PLASEN system at the Beijing Radioactive Ion-beam Facility has been fully commissioned, successfully utilizing an RFQ cooler-buncher to deliver high-quality bunched beams for high-resolution and high-sensitivity collinear resonance ionization spectroscopy of exotic nuclei.

The Gist

Imagine you are trying to listen to a specific whisper in a crowded, noisy stadium. That's essentially what scientists are doing when they study the tiniest building blocks of the universe: atomic nuclei.

This paper reports the successful "opening night" of a brand-new, high-tech listening device called PLASEN at a facility in Beijing called BRIF. Here is the story of how they built it, how it works, and why it matters, explained in everyday terms.

The Problem: The "Noisy Stadium"

For decades, scientists have wanted to study unstable, short-lived atoms (like radioactive Rubidium) to understand how the universe is built. To do this, they need to shine a very specific laser light on these atoms to see how they "sing" (their energy levels).

However, the atoms coming out of the BRIF facility were like a chaotic crowd running through a hallway.

• The Issue: The atoms were moving at wildly different speeds and were scattered all over the place.
• The Result: If you tried to shine a laser on them, the "whisper" of the atom was drowned out by the noise of their chaotic movement. The signal was blurry, and the scientists couldn't hear the details they needed.

The Solution: The "Traffic Cop and the Bus"

To fix this, the team built a system with two main parts, which they call PLASEN.

1. The RFQ-cb (The Traffic Cop & The Bus)
Think of the incoming stream of atoms as a chaotic traffic jam. The first part of the machine, the RFQ-cb, acts like a super-efficient traffic cop and a bus station combined.

• Cooling: It uses a gas (like helium) to slow the atoms down, calming them down from a sprint to a walk.
• Bunching: Instead of letting them run free, it packs them into tight, neat groups (bunches), like passengers boarding a bus at specific times.
• The Magic: Even though the original traffic was a mess, the "bus" (the bunched beam) leaves the station perfectly organized. This solves the problem of the atoms moving at different speeds.

2. The Laser System (The Tuning Fork)
Once the atoms are in neat "buses," they enter a long, quiet hallway (the beamline). Here, they meet a team of three lasers working together like a relay race.

• Step 1: The first laser gives the atoms a gentle nudge.
• Step 2: The second laser gives them a bigger push.
• Step 3: The third laser gives them a final kick that turns them back into ions (charged particles).
• The Detection: A detector counts how many atoms got the final kick. By slowly changing the "pitch" (frequency) of the lasers, the scientists can find the exact note the atom likes to sing. This creates a high-definition "spectrum" (a fingerprint) of the atom.

The Big Test: Did It Work?

The team put this new system to the test using two types of Rubidium:

1. Stable Rubidium: The "practice run."
2. Unstable (Radioactive) Rubidium: The real challenge. These atoms are like fragile glass; they break apart (decay) in seconds or milliseconds.

The Results:

• The Noise Was Gone: Even though the BRIF facility was still producing a chaotic beam, the "Traffic Cop" (RFQ-cb) successfully organized the radioactive atoms. The "whisper" became clear.
• High Definition: They achieved a resolution so sharp they could see details as small as 100 MHz (imagine distinguishing two musical notes that are incredibly close together).
• High Sensitivity: They were able to detect these fragile atoms with amazing efficiency. For every 200 atoms they started with, they successfully measured one. This is a huge deal because these radioactive atoms are incredibly rare and hard to catch.

Why Should You Care?

This isn't just about Rubidium. This system is a new, powerful microscope for the universe.

• Unlocking Secrets of the Stars: By studying these unstable atoms, scientists can understand how heavy elements (like gold and uranium) are forged in exploding stars.
• Testing the Rules of Physics: These experiments help test the fundamental laws of nature. Do the rules of physics change inside a weird, unstable atom?
• Future Tech: This setup allows scientists to study things that were previously impossible to measure, potentially leading to new discoveries in nuclear energy, medicine, and our understanding of time and space.

In a nutshell: The scientists built a "sorting machine" that tamed a chaotic stream of radioactive atoms, allowing them to take a crystal-clear photo of the atom's internal structure for the first time at this facility. It's a major leap forward in our ability to listen to the whispers of the universe.

Technical Summary

Here is a detailed technical summary of the paper "Commissioning and Full Realization of the PLASEN System at BRIF."

1. Problem Statement

The Beijing Radioactive Ion-beam Facility (BRIF) is a critical resource for producing low-energy radioactive ion beams (RIBs) in China. However, the facility faces a significant technical limitation: the continuous ion beams extracted from BRIF exhibit a large energy spread (estimated at $\sim$22 eV FWHM at 30 keV). This instability is primarily caused by fluctuations in the high-voltage (HV) platform induced by intense ionizing radiation during proton irradiation of the target.

This large energy spread results in severe Doppler broadening of spectral lines, which degrades the spectral resolution of laser spectroscopy experiments. Consequently, it limits the precision with which fundamental nuclear properties (such as spins, electromagnetic moments, and charge radii) can be extracted from unstable nuclei. Previous attempts at BRIF using Collinear Laser Spectroscopy (CLS) with Laser-Induced Fluorescence (LIF) achieved limited efficiency and resolution due to these beam quality issues.

2. Methodology

To overcome these limitations, the authors developed and commissioned the PLASEN (Precision LAser Spectroscopy for Exotic Nuclei) system at BRIF. The system integrates two main components:

• Radio-Frequency Quadrupole Cooler-Buncher (RFQ-cb):
  • A linear Paul trap floated on a 30-kV HV platform.
  • Function: It decelerates the continuous 30 keV ion beam, cools it using a helium buffer gas (99.999% purity) to reduce transverse emittance, and accumulates ions.
  • Bunching: Using a high-voltage switch on the extraction cap, ions are released in short pulses (bunched beams) rather than continuously. This process effectively resets the energy spread of the beam.
• Collinear Resonance Ionization Spectroscopy (CRIS) Setup:
  • Neutralization: The bunched ion beam is guided to a Charge-Exchange Cell (CEC) containing dense sodium vapor, where ions are neutralized into atoms.
  • Laser System: A three-step pulsed laser system is used for resonance ionization.
    • Step 1: Injection-locked Ti:Sa laser (5s $^2S_{1/2} \to$ 5p $^2P^{\circ}_{3/2}$).
    • Step 2: Pulsed OPO laser (5p $^2P^{\circ}_{3/2} \to$ 6d $^2D_{5/2}$).
    • Step 3: Nd:YAG laser (ionization).
  • Detection: Re-ionized ions are detected by a MagneTOF ion detector. The system utilizes a fully remote-controlled EPICS-based DAQ system with machine-learning-assisted beam tuning.

Experimental Validation:
The team performed online commissioning using both stable ($^{85,87}$Rb) and radioactive ($^{92,95}$Rb) Rubidium isotopes. They specifically compared beam performance with and without proton irradiation on the BRIF target to quantify the RFQ-cb's ability to suppress energy spread.

3. Key Contributions

• Full System Commissioning: Successfully installed and operated the complete PLASEN system (RFQ-cb + CRIS) at a radioactive ion beam facility for the first time.
• Energy Spread Mitigation: Demonstrated that the RFQ-cb can effectively decouple the high spectral resolution of laser spectroscopy from the inherent instability of the BRIF ion source.
• High Sensitivity & Resolution: Achieved a state-of-the-art balance between spectral resolution and detection efficiency for short-lived isotopes.
• First Online CRIS at BRIF: Conducted the first successful collinear resonance ionization spectroscopy experiment on unstable nuclei at this facility.

4. Results

• Beam Energy Stability:
  • Direct monitoring of the BRIF HV platform showed that proton irradiation increases voltage fluctuations by a factor of ~40 (from 0.38 V to 14.69 V FWHM).
  • This corresponds to an ion beam energy spread of $\sim$22.04 eV, which would theoretically cause a Doppler broadening of $\sim$121 MHz.
• RFQ-cb Performance:
  • Despite the large input energy spread, the RFQ-cb produced bunched beams with a temporal width of $\sim$2 $\mu$s.
  • The measured Hyperfine Structure (HFS) spectra of stable $^{87}$Rb showed a Full Width at Half Maximum (FWHM) of $\sim$100 MHz both with and without proton irradiation.
  • This confirms that the RFQ-cb successfully suppressed the beam energy spread, maintaining high resolution regardless of the source instability.
• Spectroscopy of Unstable Isotopes:
  • Successfully measured high-resolution HFS spectra for short-lived isotopes $^{92}$Rb ($T_{1/2} = 4.48$ s) and $^{95}$Rb ($T_{1/2} = 377.7$ ms).
  • Efficiency: Achieved a total detection efficiency of 1:200 (ion counts to detected resonance) at 100 MHz resolution. When accounting for mass separation to the RFQ-cb, the overall efficiency was $\sim$1:666.
  • Data Quality: Extracted magnetic-dipole ($A$) and electric-quadrupole ($B$) hyperfine constants and isotope shifts for $^{87}$Rb and $^{95}$Rb. These values showed excellent agreement with literature data, validating the system's accuracy despite observed spectral asymmetries (attributed to buffer gas density effects).

5. Significance

• New Capabilities for BRIF: The PLASEN system transforms BRIF into a premier facility for precision nuclear physics, enabling the study of neutron-rich isotopes with production yields as low as $\sim$100 particles per second (pps).
• Nuclear Structure Physics: It opens the door to investigating exotic structural phenomena in the medium-mass region (e.g., shape coexistence near $Z=40, N=60$) and nuclei along the astrophysical r-process path.
• Fundamental Symmetries: The system provides a platform for:
  • Parity Non-Conservation (PNC): Studying unstable species with enhanced PNC effects.
  • Electric Dipole Moments (EDM): Facilitating the formation and spectroscopy of radioactive molecules (e.g., RaF, RaOH) to search for CP violation.
• Benchmarking: The performance of PLASEN at BRIF is now comparable to the state-of-the-art CRIS setups at CERN-ISOLDE, establishing a robust, independent platform for global nuclear physics research.