Initial results of the TRIUMF ultracold advanced neutron source

The TRIUMF collaboration reports initial successful production of ultracold neutrons from a new spallation-driven superfluid helium-4 source, with results aligning with simulations and indicating the facility is on track to achieve its ultimate goals for a high-precision measurement of the neutron electric dipole moment once the cold moderator system is completed.

The Gist

The Big Picture: Hunting for a Tiny "Tilt" in the Universe

Imagine the universe is built on a set of perfect, symmetrical rules. If you took a particle and its "anti-particle" (its evil twin) and swapped them, the laws of physics should work exactly the same way. This is called symmetry.

However, physicists suspect there might be a tiny, invisible "tilt" in the universe that breaks this symmetry. They are looking for a specific particle called the neutron to see if it has a tiny electric "tilt" (called an Electric Dipole Moment or nEDM).

• The Analogy: Think of a perfectly round ball. If you spin it, it looks the same from every angle. But if the ball is slightly egg-shaped, it has a "tilt." Finding this tilt in a neutron would be a massive discovery. It would explain why the universe is made of matter (us) instead of being empty, and it would point to "New Physics" beyond our current understanding.

To find this tiny tilt, you need to catch the neutrons and hold them very still for a long time. To do that, you need Ultracold Neutrons (UCNs).

The Machine: The "Super-Cold Ice Cream Factory"

The paper describes a new machine built at TRIUMF (Canada's particle accelerator center) called TUCAN. Its job is to make these Ultracold Neutrons.

Here is how it works, using a kitchen analogy:

1. The Proton Beam (The Blender): The machine shoots a high-speed beam of protons (like tiny, fast bullets) at a heavy metal target. This is like turning on a high-powered blender.
2. The Explosion (Spallation): When the bullets hit the target, they shatter the atoms, creating a chaotic spray of neutrons. This is the "spallation."
3. The Cooling System (The Freezer): These neutrons are flying around way too fast to be useful. They need to be slowed down to a near-stop. The machine uses a special substance called Superfluid Helium-4.
   • The Metaphor: Imagine the neutrons are hot, energetic bees buzzing around a hive. The Superfluid Helium is like a magical, frictionless ice bath. When the bees fly into the ice bath, they instantly lose their energy and freeze in place, becoming "ultracold."
4. The Collection (The Jar): Once the neutrons are cold enough, they are guided out of the ice bath into a detector to be counted.

The Results: First Taste of Success

The paper reports the first results from this new machine. Before this, they had an older, smaller version (a "vertical" source). The new one is a "horizontal" source, which is much bigger and more powerful.

• The Test: They ran the machine for 60 seconds with a specific beam strength.
• The Catch: They successfully caught 930,000 ultracold neutrons.
• The Verdict: This is a huge success! It matches what their computer simulations predicted. It proves the machine works.

The Surprise: The Engine Runs Cooler Than Expected

Here is the most exciting part of the paper.

• The Expectation: The scientists thought the machine would get very hot very quickly. They worried that the heat from the "blender" (the proton beam) would warm up the "ice bath" (the helium) too much, causing the neutrons to bounce around and escape before they could be counted. They expected the machine to hit a "speed limit" (saturation) where adding more power wouldn't help.
• The Reality: The machine didn't hit that speed limit. Even as they cranked up the power, the number of neutrons kept going up in a straight line.
• The Metaphor: It's like driving a car. You expected that if you pressed the gas pedal past a certain point, the engine would overheat and the car would stop getting faster. Instead, you pressed the pedal, and the car kept accelerating smoothly. This suggests the machine is better at handling heat than anyone thought.

What's Next? The "Full Tank" Upgrade

Right now, the machine is running with a "partial tank." They are missing a crucial ingredient: Liquid Deuterium.

• The Current State: The machine is like a car running on a small gas tank. It works, but it's not at full speed.
• The Future: Once they install the Liquid Deuterium system (which acts like a super-efficient "turbocharger" for the neutrons), the scientists predict the output will jump by a factor of 61.
• The Goal: Instead of catching 930,000 neutrons, they expect to catch 57 million.

Why Does This Matter?

If they get those 57 million neutrons, they can perform the experiment to measure the neutron's "tilt" with incredible precision.

• The Stakes: If they find a non-zero tilt, it's a Nobel Prize-winning discovery that changes our understanding of the universe. If they don't find it, they prove that our current theories are even more robust than we thought, but they also rule out many theories about "New Physics."

Summary in One Sentence

The TUCAN team at TRIUMF has successfully built a new, high-tech "neutron freezer" that is working even better than expected, and once they turn on the full turbo-charger, it could help solve one of the biggest mysteries in physics: why our universe exists at all.

Technical Summary

1. Problem and Context

The primary goal of the TRIUMF UltraCold Advanced Neutron (TUCAN) collaboration is to perform a high-precision measurement of the neutron electric dipole moment (nEDM). A non-zero nEDM would violate time-reversal (T) and charge-parity (CP) symmetries, providing crucial evidence for physics beyond the Standard Model, such as new sources of CP violation, baryogenesis scenarios, and solutions to the strong CP problem.

Current nEDM experiments rely on Ultracold Neutrons (UCNs) stored in measurement chambers. The sensitivity of these experiments is directly proportional to the number of UCNs available. While previous experiments (e.g., at PSI) have set upper bounds on the nEDM, next-generation experiments aim to improve sensitivity by an order of magnitude or more. This requires a significant increase in UCN flux.

The TUCAN project utilizes a spallation-driven superfluid $^4$He (He-II) source. Unlike solid ortho-deuterium ($sD_2$) sources which suffer from high upscattering losses and short lifetimes, He-II offers potentially longer neutron storage lifetimes (limited primarily by phonon upscattering, scaling as $T^7$). However, previous prototypes faced limitations in heat removal and production volume. The challenge addressed in this paper is the validation of a new, significantly upgraded horizontal He-II source at TRIUMF, specifically testing its ability to handle high heat loads and produce UCNs at rates sufficient for future nEDM measurements.

2. Methodology

The paper reports on the first operational results of the new TUCAN source, which replaced a previous vertical prototype.

• Source Design:

  • Geometry: A horizontal source with a 27 L production volume (significantly larger than the previous 8 L vertical source).
  • Cooling: Equipped with a new $^3$He refrigerator and a larger helium pumping system, capable of handling heat loads up to 10 W (compared to 300 mW for the previous source).
  • Moderation: The system uses a heavy water ($D_2O$) moderator and is designed to eventually incorporate a liquid deuterium ($LD_2$) cold moderator to maximize cold neutron flux.
  • Beam: Driven by the 483 MeV proton beam from the TRIUMF cyclotron, directed at a tungsten spallation target.

• Experimental Setup:

  • Irradiation: Proton beams were directed at the target with currents up to 36.7 $\mu$A (aiming for 40 $\mu$A) for 60 seconds.
  • Detection: UCNs were extracted horizontally and guided downward into a detector located 1.11 m below the source exit.
    • Primary detector: Lithium-loaded scintillating glass (90% efficiency).
    • Secondary detector: $^3$He proportional counter for systematic checks.
  • Cycle: A typical cycle involved irradiation (60 s), followed by opening a gate valve to count UCNs for 120 s, and then closing the valve to measure background.
  • Variables: Experiments varied beam current and tested the effect of a 100 $\mu$m Aluminum foil (simulating a future polarizer) placed between the source and detector.

• Simulations:

  • MCNP: Used to model neutron transport, target interactions, and moderator efficiency.
  • PENTrack: Used to simulate UCN transport and losses.
  • Thermal Modeling: Used to predict temperature profiles in the He-II based on the Gorter-Mellink regime of quantum-turbulent heat conduction.

3. Key Contributions

• First Operational Data: This is the first report of UCN production from the new, large-volume, high-power horizontal TUCAN source.
• Cryogenic Validation: Demonstrated the ability to maintain a base temperature of 0.8 K in the $^3$He pot with a resting heat load of 2 W, and successfully managed beam-induced heat loads up to ~5 W without clogging or superleak issues.
• Performance Benchmarking: Provided the first empirical data comparing actual UCN yields against detailed Monte Carlo simulations for a spallation-driven He-II source of this scale.
• Systematic Analysis: Detailed the systematic uncertainties, particularly regarding background subtraction (dominated by gamma rays from activated components) and the impact of the Al foil.

4. Key Results

• UCN Yield:
  • At a proton beam current of 36.7 $\mu$A (approx. 37 $\mu$A) and a 60 s irradiation time, the source produced $(9.3 \pm 0.8) \times 10^5$ UCNs.
  • The production rate was found to be linear with beam current: $(2.52 \pm 0.02) \times 10^4$ UCNs/$\mu$A.
• Comparison with Simulation:
  • The experimental data lies somewhat below the simulated predictions (by a factor of roughly 2–3 depending on the thermal model used), but the agreement is considered fair given the complexity of the system.
  • Crucial Deviation: Simulations predicted that UCN production would saturate at higher beam currents due to rising temperatures in the He-II (which increases upscattering losses). The experimental data showed no saturation; the yield remained linear up to the maximum tested current.
• Storage Lifetime:
  • The measured storage lifetime of UCNs in the source volume was 23–27 seconds.
  • Contrary to thermal model predictions (which suggested lifetime would drop at higher currents due to heating), the lifetime was independent of beam current. This suggests that wall losses are currently the dominant loss mechanism rather than phonon upscattering.
• Foil Impact: The presence of a 100 $\mu$m Al foil reduced the UCN count to 63% of the no-foil value, consistent with expectations.

5. Significance and Future Prospects

• Path to Ultimate Goals: The results indicate that the source is on track to meet its ultimate production goals once the liquid deuterium ($LD_2$) cold moderator is installed and the system is fully filled.
  • Simulations predict that with full $LD_2$, $D_2O$, and He-II levels, the source will produce $5.7 \times 10^7$ UCNs in a 60 s irradiation (a 61-fold increase over current partial-fill results).
  • This would deliver $1.4 \times 10^6$ UCNs to each of two nEDM measurement cells.
• Physics Impact: With this flux, the TUCAN experiment aims to achieve a statistical uncertainty of $1 \times 10^{-27}$ e cm on the neutron EDM in 280 days of running, representing a significant leap in sensitivity.
• Scientific Surprise: The lack of observed saturation and the linear scaling with beam current suggest that the source may be capable of handling higher power and fluxes than originally anticipated. This challenges the current understanding of heat conduction limits in He-II (Gorter-Mellink regime) and opens the possibility for even more intense UCN sources in the future.
• Conclusion: The TUCAN source has successfully transitioned from a prototype to a functional, high-yield facility. The initial results validate the design and suggest that the technical hurdles for world-leading nEDM sensitivity have been overcome.