Development and Application of an eV Neutron Polarization for Parity Violation Studies at CSNS Back-n Beamline

Researchers at the China Spallation Neutron Source successfully developed and validated an eV-range polarized neutron system using an in-situ $^3$He Spin-Exchange Optical Pumping filter and solenoidal guide field, achieving a measured parity violation asymmetry of $7.8 \pm 2.4$% at the 0.747 eV resonance of $^{139}$La to support future Time-Reversal Invariance Violation studies.

The Gist

The Big Picture: Why Are We Doing This?

Imagine the universe is a giant party that started with the Big Bang. In a perfect world, the party should have had an equal number of "matter" guests and "anti-matter" guests. If that were true, they would have all hugged each other and vanished in a flash of light, leaving an empty universe.

But here we are! The universe is full of matter (stars, planets, you, me). This means something went wrong at the party: the anti-matter guests left early, or the matter guests got a special VIP pass. Scientists call this the Baryon Asymmetry.

To figure out why the universe is full of matter, we need to find a rule that treats "left" and "right" differently, or "forward" and "backward" differently. This paper is about building a super-precise machine to catch a tiny, almost invisible violation of these rules using neutrons.

The Setup: The "Neutron Super-Highway"

The experiment takes place at the China Spallation Neutron Source (CSNS). Think of this facility as a massive particle accelerator that smashes protons into a target to create a flood of neutrons.

• The Back-n Beamline: This is a specific "lane" on the highway. Unlike other lanes that slow neutrons down (like a speed bump), this lane keeps them fast and energetic (in the "eV" range).
• The Flight Path: The neutrons travel about 76 meters (roughly the length of a football field) before hitting the target. This long distance is crucial. It acts like a high-speed camera shutter; because the neutrons travel at different speeds, the long path lets scientists separate them by energy very precisely, like sorting marbles by size as they roll down a long ramp.

The Problem: The Neutrons Are "Spin-Blind"

Neutrons have a property called spin. You can think of spin like a tiny arrow sticking out of the neutron, pointing either "Up" or "Down."

To test the laws of physics, we need to shoot a stream of neutrons where all the arrows point "Up." If we just shoot a random mix of Up and Down neutrons, the effects we are looking for get canceled out, like trying to hear a whisper in a noisy crowd.

The Solution: The 3He Filter (The "Spin-Selective Door")
The team built a special door made of a gas called Helium-3 (3He).

• Imagine the Helium-3 atoms are like bouncers at a club.
• The scientists use lasers to tell the bouncers: "Only let the neutrons with 'Up' arrows in. If a neutron has a 'Down' arrow, the bouncer eats it."
• This creates a clean, polarized beam of neutrons where everyone is marching in step.

The Challenge: The "Spin Flipper"

To prove that the universe treats "Up" and "Down" differently, scientists need to switch the neutrons' arrows from "Up" to "Down" and back again very quickly.

• The Old Way: Usually, scientists use radio waves to flip spins, but for these fast, energetic neutrons, radio waves are like trying to stop a speeding train with a feather. It requires too much power and creates too much noise.
• The New Way (Adiabatic Spin Flipper): The team built a custom magnetic machine. Imagine a hallway lined with magnets that slowly rotate the direction of the magnetic field.
  • As the neutron runs through this hallway, its "arrow" is gently guided to turn around 180 degrees.
  • It's like a dancer being spun around by a partner; if the partner spins slowly enough, the dancer stays perfectly balanced.
  • This machine can flip the spins in less than half a second, allowing scientists to compare "Up" vs. "Down" instantly, canceling out any errors caused by the equipment heating up or drifting.

The Experiment: The "La" Target

The polarized neutrons are fired at a target made of Lanthanum (La).

• The Resonance: At a very specific energy (0.747 eV), the Lanthanum atoms act like a tuning fork. They "sing" (resonate) when hit by neutrons of that exact speed.
• The Test: When the neutrons hit this tuning fork, the scientists look for a tiny difference in how many neutrons get absorbed when they are "Up" versus when they are "Down."

If the laws of physics were perfectly symmetrical, the number would be exactly the same. But if there is a violation (a hint of why the universe exists), one direction will be absorbed slightly more than the other.

The Results: Catching the Ghost

The team measured the difference and found an asymmetry of about 7.8%.

• What this means: This isn't a huge number, but in the world of subatomic physics, it's a massive, clear signal. It proves their machine works perfectly.
• The "System Check": They also tested the machine at other energies where they know the answer should be zero. It was zero. This proves their machine isn't lying to them due to glitches or magnetic interference.
• The Goal: Now that they have proven they can measure this "Parity Violation" (the left/right difference) so accurately, they can use this same setup to hunt for Time-Reversal Violation. If they find that, it could finally explain why the universe is full of matter and not empty space.

Summary Analogy

Imagine you are trying to detect a tiny, invisible wind that only blows on your left ear.

1. The Neutron Source: A giant fan blowing air.
2. The Polarizer: A filter that only lets air molecules spin clockwise.
3. The Spin Flipper: A machine that instantly switches the air molecules to spin counter-clockwise.
4. The Target: Your ear.
5. The Result: You measure if your ear gets wetter when the air spins clockwise vs. counter-clockwise.

This paper says: "We built a super-quiet room, a perfect filter, and a lightning-fast switch. We tested it, and it works. Now we are ready to find that invisible wind that explains why we are here."

Technical Summary

1. Problem Statement and Motivation

The origin of the Baryon Asymmetry of the Universe (BAU) remains a fundamental unsolved problem in cosmology. The Standard Model (SM) cannot account for the observed matter-antimatter imbalance, necessitating the search for Beyond the Standard Model (BSM) sources of CP violation. Under CPT conservation, this is equivalent to searching for Time-Reversal Invariance Violation (TRIV).

• The Challenge: Direct detection of TRIV is extremely difficult due to the minute magnitude of the effect.
• The Solution: The Neutron Optics Parity and Time Reversal Experiment (NOPTREX) utilizes the dynamic enhancement of symmetry-breaking effects in neutron-nucleus resonances. Specifically, the mixing of $s$-wave and $p$-wave resonances via weak interactions amplifies Parity Violation (PV) asymmetries by factors of $10^4$ to $10^6$.
• The Requirement: To search for TRIV, one must first precisely measure PV asymmetries to calibrate nuclear amplification factors. This requires a high-intensity, epithermal (eV-energy) polarized neutron beam with high Time-of-Flight (TOF) resolution to isolate specific resonances (e.g., the 0.747 eV resonance in $^{139}\text{La}$) from background.

2. Methodology and Experimental Setup

The authors developed a specialized polarized neutron system at the Back-n white neutron beamline of the China Spallation Neutron Source (CSNS). The system is designed to generate, manipulate, and transport polarized neutrons in the eV energy range.

A. The Neutron Source (Back-n)

• Advantages: Unlike moderated sources (like J-PARC/ANNRI), Back-n is an unmoderated source. This eliminates emission tails caused by scattering, providing a "pristine" time structure and superior TOF resolution.
• Geometry: The experiment utilizes the ES#2 station with a flight path of ~76 m, significantly longer than other facilities, enhancing energy resolution for resolving complex resonance structures.

B. Polarization Generation (In-situ $^3$He)

• Technique: Spin-Exchange Optical Pumping (SEOP) is used to polarize a $^3$He gas cell, which acts as a neutron spin filter (NSF).
• Performance: The system achieved a $^3$He polarization of 70.33%, resulting in a neutron polarization of ~30.3% at the 0.734 eV resonance.
• Placement: The polarizer is located between End Station 1 (ES#1) and ES#2 to minimize background scattering near the Gamma Total Absorption Facility (GTAF).

C. Spin Manipulation (Adiabatic Spin Flipper)

• Challenge: Conventional Radio Frequency (RF) flippers are impractical for eV neutrons due to excessive power requirements and electromagnetic interference.
• Innovation: A custom adiabatic spin flipper was designed. It uses a spatially rotating magnetic field to flip spins when "ON" and a magnetic zero-crossing to transmit spins when "OFF."
• Design:
  • Length: 1.75 m; Width: 1.27 m.
  • Uses longitudinal gradient coils and transverse rotating coils.
  • Operates with a rapid switching cycle (0.4 s) to cancel detector gain drifts and systematic errors.
  • Efficiency: Simulations and measurements confirmed high efficiency for eV neutrons, with specific optimization for the 0.7–10 eV range.

D. Polarization Transport

• System: A 6 m vacuum transport system equipped with solenoidal guide fields connects the flipper to the detector.
• Purpose: Prevents depolarization due to the Earth's magnetic field and air scattering.
• Configuration: Segmented solenoids (three inside vacuum, one outside) maintaining a minimum guide field of 12 G.

E. Detection

• Detector: The Gamma Total Absorption Facility (GTAF) consists of 40 BaF$_2$ crystals arranged in a spherical shell, covering 95.2% of the solid angle.
• Method: Measures $(n, \gamma)$ capture events. This method offers a sensitivity enhancement of two orders of magnitude over transmission measurements and is less susceptible to scattering backgrounds.

3. Key Contributions

1. First eV Polarized Neutron System at CSNS: Successfully established a complete polarized neutron beamline at the Back-n facility, specifically tailored for the NOPTREX collaboration.
2. Custom Adiabatic Spin Flipper: Developed a novel flipper design capable of manipulating eV neutrons without the instability and shielding issues associated with RF flippers. It features a unique switching sequence to cancel first-order systematic errors.
3. Integrated System Characterization: Provided a comprehensive calibration of the entire chain (polarizer $\to$ flipper $\to$ transport $\to$ detector), including the measurement of flipper efficiencies and the impact of guide field discontinuities.
4. Validation of NOPTREX Capability: Demonstrated that the CSNS Back-n source, combined with the new polarization system, can resolve fine resonance structures and measure PV asymmetries with high precision.

4. Experimental Results

• Target: Natural Lanthanum ($^{139}\text{La}$) at the 0.747 eV p-wave resonance.
• Measured Asymmetry: The experiment measured a raw asymmetry of $4.67 \pm 1.19\%$.
• Corrected Asymmetry: After applying corrections for spin transport efficiency (flipper-on: 90%, flipper-off: 70%) and systematic effects, the final parity-violating asymmetry was determined to be:
  $$A_{PV} = 7.8 \pm 2.4 (\text{stat.}) \pm 0.3 (\text{sys.}) \%$$
• Systematic Checks:
  • Measurements at the 70 eV resonance (where polarization is near zero) yielded an asymmetry consistent with zero ($0.05 \pm 0.34\%$), confirming the absence of electronic cross-talk or magnetic field artifacts.
  • Analysis of $s$-wave resonances (3.0 eV and 4.0 eV) also yielded zero asymmetry, validating that systematic errors related to polarized neutron interactions in the target are negligible.
• Flipper Efficiency: The flipper-on efficiency was measured at 90%, and flip-off efficiency at 70% at the resonance energy. The drop in flip-off efficiency at higher energies was attributed to off-axis neutrons encountering non-zero fields at the flipper center.

5. Significance and Future Outlook

• Scientific Impact: This work validates the CSNS Back-n beamline as a premier facility for precision nuclear physics. The successful measurement of PV asymmetry in $^{139}\text{La}$ confirms the system's readiness for the ultimate goal: a sensitive search for Time-Reversal Invariance Violation (TRIV).
• Methodological Advance: The paper demonstrates that unmoderated spallation sources combined with adiabatic spin flippers offer superior resolution and stability for eV neutron studies compared to moderated sources.
• Future Plans:
  • Statistical Improvement: Enlarge the beam spot diameter to increase flux.
  • Efficiency Optimization: Refine the spin-flip efficiency and upgrade the $^3$He spin filter (e.g., optimizing pressure and length).
  • Energy Range Expansion: Extend capabilities to the thermal energy range and integrate a vertical cryostat to cool samples below 15 K for future experiments.

In conclusion, the paper presents a robust, high-precision experimental platform that has successfully measured parity violation in the eV energy regime, laying the critical groundwork for discovering new physics related to the matter-antimatter asymmetry of the universe.