Initial Performance of the TUCAN Magnetically Shielded Room

The TUCAN collaboration has successfully commissioned a multi-layer magnetically shielded room that achieves a quasi-static shielding factor of approximately $3.25 \times 10^4$ within the TRIUMF cyclotron's ambient field, demonstrating residual field conditions sufficient for a future neutron electric dipole moment search targeting a precision of $10^{-27}~e\mathrm{cm}$.

The Gist

Imagine you are trying to listen to a single, incredibly quiet whisper in the middle of a roaring stadium. That is essentially what the TUCAN collaboration is trying to do. They are hunting for a tiny, almost invisible property of the neutron called the neutron electric dipole moment (nEDM). Finding this "whisper" could help us understand why the universe is made of matter instead of just disappearing into energy.

But there's a problem: The "stadium" is the TRIUMF particle accelerator in Canada, which is incredibly noisy with magnetic fields. To hear the whisper, they need to build a room so quiet magnetically that it's like a soundproof booth in the middle of a hurricane.

Here is the story of how they built that booth, the TUCAN Magnetically Shielded Room (MSR), and how it's performing.

1. The Goal: A "Silent" Room for Neutrons

Neutrons are tiny particles. To measure their secret "dipole moment," scientists trap them in a bottle and watch them spin. But if the magnetic environment around them wiggles even a tiny bit, the measurement gets ruined.

They needed a room where the magnetic field is:

• Stable: No shaking.
• Quiet: No outside noise getting in.
• Uniform: The field is the same everywhere inside, like a flat, calm lake.

2. The Construction: A Russian Nest of Blankets

To build this room, they didn't just use one wall. They built a six-layered "Russian nesting doll" structure.

• The Inner Layers (The Blankets): Five layers are made of a special, super-spongy metal called MuMetal. Think of MuMetal like a magnetic sponge. When outside magnetic waves hit it, the sponge soaks them up and redirects them around the room, leaving the inside dry and calm.
• The Copper Layer (The Raincoat): One layer is made of pure copper. While the MuMetal handles slow, steady magnetic changes, the copper acts like a raincoat for fast, sudden electrical storms (high-frequency noise), stopping them from getting through.
• The Size: The room is a cube, about 3.5 meters wide on the outside and 2.25 meters on the inside. It's big enough to hold the experiment but small enough to fit in the crowded lab.

The Door Challenge:
Building a door for a magnetic room is like trying to close a zipper on a suit of armor without letting any gaps in. If there's a gap, the "noise" leaks in. They built a massive, double-axis door that slides away and then swings open, using air-pressure "bladders" (like inflatable seals) to press the metal layers tight against each other, ensuring no magnetic "drafts" sneak in.

3. The Environment: The "Elephant in the Room"

Here is the tricky part. This quiet room is being built right next to the TRIUMF cyclotron, a giant particle accelerator. The cyclotron creates a massive magnetic field (about 370 microtesla) that is constantly trying to push its way into the room.

It's like trying to build a soundproof recording studio right next to a jet engine. The jet engine is so loud that even the best soundproofing struggles.

4. The Results: How Well Does It Work?

The team tested the room by shaking it with magnetic waves and seeing how much of that shaking got inside.

• The Shielding Factor: They measured how much the room dampened the outside noise.

  • The Goal: They hoped for a factor of 100,000 (reducing noise by 100,000 times).
  • The Reality: With the jet engine (cyclotron) running, they achieved a factor of about 32,500. Without the jet engine, it jumped to 37,500.
  • The Analogy: If the outside noise was a loud shout, the room turned it into a very faint murmur. It's not quite as quiet as the absolute best rooms in the world (which have 8 layers), but it's incredibly impressive for a 6-layer room in such a noisy environment.

• The "Residual" Field (The Background Hum): Even with the room closed, there is a tiny bit of magnetic "hum" left inside.

  • They measured this hum to be about 1.8 nanotesla. That is incredibly small—imagine the difference between a mountain and a grain of sand.
  • However, the "slope" of the magnetic field (how much it changes as you move from one side of the room to the other) was a bit steeper than they wanted. It's like having a floor that is mostly flat but has a slight tilt.

5. The Fix: Tuning the Instrument

The paper admits the room isn't perfect yet. The "tilt" in the magnetic floor and the slight background hum need to be fixed.

The team has a plan:

1. Re-magnetizing (Degaussing): They will run a special "reset" procedure through the metal layers, like shaking a snow globe and letting the snow settle perfectly flat, to remove any leftover magnetic "memory" in the walls.
2. Active Compensation: They plan to install giant coils around the outside of the room to actively push back against the cyclotron's magnetic field, effectively canceling it out.
3. Internal Shimming: They will add small magnets inside the room to fine-tune the field, making the "floor" perfectly flat.

The Bottom Line

The TUCAN team successfully built a massive, high-tech magnetic fortress. While it's currently fighting a tough battle against the nearby particle accelerator, it has already proven it can silence the universe's magnetic noise enough to hear the faintest whispers of the neutron.

With a little more tuning (like a musician tightening their strings), this room is expected to be the perfect stage for a historic discovery that could rewrite our understanding of the universe.

Technical Summary

1. Problem Statement

The TRIUMF Ultracold Advanced Neutron (TUCAN) collaboration aims to measure the neutron electric dipole moment (nEDM) with a precision of $10^{-27} \text{ ecm}$, an order of magnitude improvement over current limits. This measurement relies on Ramsey's method of separated oscillating fields, requiring ultra-stable, uniform magnetic fields within the neutron storage cell.

The primary challenge is the experimental environment: the facility is located near the TRIUMF cyclotron, which generates a large, static vertical ambient magnetic field of approximately 370 µT. This background field is strong enough to partially saturate standard magnetic shielding materials, potentially degrading performance. The collaboration requires a Magnetically Shielded Room (MSR) that meets four strict criteria within this challenging environment:

1. Shielding Factor: $> 10^5$ at 0.01 Hz (to reduce external 100 nT fluctuations to <1 pT).
2. Residual Field: $< 1 \text{ nT}$ in the central volume.
3. Field Stability: Stability on the pT level over minutes.
4. Field Gradients: First-order gradients $< 100 \text{ pT/m}$.

2. Methodology and Design

The paper details the design, construction, and commissioning of a custom 6-layer MSR.

• Structure and Materials:

  • Geometry: A cubic room with outer dimensions of 3.5 m and inner dimensions of 2.25 m.
  • Layers: The shield consists of five layers of annealed MuMetal (high-permeability alloy for DC/low-frequency shielding) and one layer of pure copper (for AC/high-frequency shielding via eddy currents).
  • Configuration: Layers are concentric, electrically isolated (except for a single ground point), and supported by non-magnetic aluminum and plastic.
  • Door: A unique dual-axis door (North face) moves longitudinally and transversely to fit spatial constraints, utilizing pneumatic locks and inflatable bladders to ensure tight magnetic contact.
  • Penetrations: 88 access ports are mirrored on opposing walls to minimize field gradients.

• Construction and Assembly:

  • Construction occurred between 2023 and 2024.
  • A critical design modification occurred late in the process: after the initial 5-layer assembly, the shielding margin under the 370 µT cyclotron field was deemed insufficient. An additional inner MuMetal layer (Layer 6) was installed, bringing the total to six layers.

• Measurement Techniques:

  • Shielding Factor: Measured by applying sinusoidal external fields (0.01 Hz to 1000 Hz) using a large external coil and measuring the attenuation inside the room.
    • Sensors: A Bartington fluxgate magnetometer was used for Layers 1–5. A QuSpin Zero-Field Magnetometer (QZFM) was used for Layer 6 due to its lower noise floor, allowing measurement of higher shielding factors.
  • Residual Field & Gradients: Mapped using the QZFM mounted on a rod inserted through a port array. Measurements were taken with the cyclotron magnet energized (simulating final operating conditions).
  • Idealization (Degaussing): Each ferromagnetic layer was equipped with internal "L" or toroidal coils. A controlled, slowly decaying AC current was applied to each layer to demagnetize the material and minimize remnant magnetization.

3. Key Contributions

• Commissioning in a High-Field Environment: This is one of the first reports of a large-volume MSR commissioned specifically to operate within the strong static field of a major particle accelerator (cyclotron), rather than in a low-background environment.
• Iterative Design Improvement: The paper documents the decision to add a sixth layer mid-construction to compensate for the saturation effects of the ambient cyclotron field, demonstrating a flexible engineering approach.
• Comprehensive Characterization: The study provides a complete dataset on shielding factors across frequencies, residual field maps, and gradient measurements under realistic operating conditions (cyclotron ON).
• Active Compensation Strategy: The design includes provisions for large Helmholtz coils (to cancel the 370 µT background) and internal shim coils, outlining a path to meet final specifications.

4. Key Results

The paper reports the "initial performance" of the room, measured with the cyclotron magnet ON (370 µT background) and a relatively simple degaussing procedure.

• Shielding Factor:

  • At 0.01 Hz with a 2 µT peak-to-peak external perturbation:
    • With Cyclotron ON: $3.25(2) \times 10^4$.
    • With Cyclotron OFF: $3.75(4) \times 10^4$.
  • The presence of the cyclotron field reduces the shielding factor by approximately 13–23% depending on the perturbation amplitude.
  • Each additional MuMetal layer improved the shielding factor by a factor of 5–7.
  • Note: The requirement was $>10^5$. The current result is roughly $3 \times 10^4$, but the authors note that active compensation coils are expected to recover the lost performance.

• Residual Field:

  • At the room center, the absolute residual field was measured at $B = 1.8(2) \text{ nT}$.
  • This meets the $<1 \text{ nT}$ requirement for the vertical component ($B_z$) but shows larger horizontal components ($B_y$), likely due to the specific degaussing sequence used.

• Field Gradients:

  • The vertical gradient ($dB_z/dz$) at the center was $-279(64) \text{ pT/m}$.
  • This exceeds the $100 \text{ pT/m}$ requirement by a factor of ~3. The authors attribute this to the "final few moments" of the degaussing process not being perfectly optimized.

• Comparison to Peers:

  • The TUCAN MSR has fewer MuMetal layers (5 vs. 8 in the PTB BMSR-2) but operates in a much harsher magnetic environment.
  • The shielding factor is comparable to the PTB room ($7.5 \times 10^4$) but lower than the PSI room ($10^5$), which is expected given the environmental constraints and layer count.

5. Significance and Future Outlook

The results demonstrate that the TUCAN MSR is a functional and robust shield, but it is not yet fully optimized for the $10^{-27} \text{ ecm}$ goal. The initial performance is considered a baseline rather than a final limit.

Path to Full Performance:
The authors outline three specific strategies to bridge the gap between current results and requirements:

1. Optimized Idealization: Refining the degaussing sequence to reduce residual fields and gradients below 1 nT and 100 pT/m.
2. Active Compensation: Installing the planned Helmholtz coils to cancel the 370 µT cyclotron field. The paper estimates this will improve the shielding factor by 15–30%, effectively recovering the loss caused by the ambient field.
3. Internal Shimming: Installing an array of 54 internal shim coils to actively correct remaining field inhomogeneities.

Conclusion:
The paper establishes that a multi-layer MuMetal/Copper shield can be successfully constructed and characterized in a high-field accelerator environment. While the initial passive performance falls short of the $10^5$ shielding factor target, the combination of an additional inner layer, improved degaussing, and active field compensation is projected to make the room fully adequate for the TUCAN nEDM search. This work provides a critical blueprint for future high-precision fundamental physics experiments located near large magnetic infrastructure.