Proposal for a new spectrometer at ESS: Njord and Remora

This paper proposes Njord and Remora, a paired spectrometer concept for the European Spallation Source (ESS), designed to overcome limitations in neutron flux and sample geometry for challenging scientific studies while simultaneously increasing available beamtime.

The Gist

Imagine the European Spallation Source (ESS) as the world's most powerful "neutron flashlight." Scientists use these invisible particles to peek inside materials, seeing how atoms vibrate and how magnets work. But right now, this flashlight has a problem: it's too bright and too wide for some very delicate jobs.

This paper proposes a brilliant solution: a two-part instrument team named Njord and Remora. Think of them as a high-performance symbiotic duo (like a shark and the little fish that cleans its teeth) designed to tackle two different problems at once.

Here is the breakdown in simple terms:

1. The Problem: "Too Big, Too Small, Too Hard"

Scientists are trying to study some of the most exciting materials on Earth, like:

• Tiny crystals that are too small to fit on current machines.
• Fragile organic materials (like the stuff in batteries or new drugs) that get destroyed by the beam.
• Materials under extreme pressure (squeezed like a stress ball) to see how they change.

Currently, the neutron beam is like a giant floodlight. If you try to shine it on a tiny speck of dust, most of the light misses the target and hits the background, creating "noise" that hides the answer. Also, there aren't enough hours in the day for all the scientists who want to use the machine.

2. The Solution: The Njord & Remora Duo

Njord: The "Laser Pointer" for Tiny Samples

Njord is the specialist for the hard stuff.

• The Analogy: Imagine trying to read the text on a grain of sand. A normal flashlight washes it out. Njord is like a super-focused laser pointer that concentrates all the light onto that single grain of sand.
• How it works: It uses a special technology called "Nested Mirror Optics" (think of it like a set of Russian nesting dolls made of mirrors) to squeeze the wide neutron beam into a tiny, intense spot (3mm x 3mm).
• What it achieves: It allows scientists to study tiny samples (like a speck of dust) and materials under extreme pressure (inside a diamond press) without the signal getting lost in the noise. It's the only way to see the "breathing" of metal-organic frameworks or the secrets of quantum magnets that are currently invisible to us.

Remora: The "Leftover Catcher"

Remora is the partner that makes sure nothing goes to waste.

• The Analogy: If Njord is the laser pointer, it only uses a specific color of light. But the ESS flashlight emits a rainbow of colors. Remora is like a sponge that soaks up all the "leftover" colors that Njord doesn't need.
• How it works: It sits right in front of Njord on the same beamline. It grabs the neutrons that Njord ignores, shapes them, and sends them to a second detector.
• What it achieves: It creates extra capacity. It gives scientists a second machine to use without building a whole new facility. It's like adding a second lane to a highway to stop traffic jams. It handles standard experiments that don't need the extreme focus of Njord, ensuring more scientists get their turn.

3. Why This Matters (The "So What?")

This partnership unlocks three major doors:

1. The "Tiny Sample" Revolution:

   • Analogy: Before, you needed a whole apple to study its core. Now, with Njord, you can study a single seed.
   • Real-world impact: We can finally study Metal-Organic Frameworks (MOFs). These are sponge-like materials that could revolutionize how we store hydrogen fuel or clean the air. They are hard to grow in big chunks, but Njord can study the tiny ones we can make.

2. The "Extreme Conditions" Lab:

   • Analogy: Imagine trying to study how a car engine works while it's being crushed by a hydraulic press. You need a tiny engine and a focused camera.
   • Real-world impact: We can study materials under high pressure (like the ice on Neptune) or in strong magnetic fields. This helps us understand superconductors (materials that conduct electricity with zero loss) and new types of magnets for quantum computers.

3. The "More Time" Bonus:

   • Analogy: A popular restaurant is always fully booked. Remora is like adding a second dining room to the same kitchen.
   • Real-world impact: It doubles the number of experiments scientists can run. This means faster discoveries in energy, medicine, and computing.

Summary

Njord is the scalpel: precise, intense, and capable of cutting into the tiniest, most difficult scientific mysteries.
Remora is the net: it catches the rest of the resources to ensure no one is left waiting in line.

Together, they turn the European Spallation Source into a machine that can see the invisible, handle the impossible, and keep the scientific community moving forward.

Technical Summary

Based on the proposal paper "Njord and Remora: Spectrometers in Symbiosis," here is a detailed technical summary covering the problem, methodology, key contributions, results, and significance.

1. Problem Statement

The European Spallation Source (ESS) faces two critical limitations in its current and planned instrument suite regarding cold neutron spectroscopy:

• Flux and Sample Size Constraints: Many scientifically vital systems (e.g., Metal-Organic Frameworks (MOFs), organic superconductors, quantum magnets, and high-pressure samples) are either too small (sub-mm³), require extreme environments (high pressure, high magnetic fields), or produce weak signals. Existing instruments, even at ESS, lack the necessary flux density to measure collective excitations on these tiny samples within a reasonable timeframe.
• Beamtime Capacity: The demand for neutron beamtime significantly exceeds supply (oversubscription factors >4 at similar facilities). The community requires more instruments to accommodate diverse research needs and train the next generation of scientists.
• Geometry Limitations: Current indirect-geometry spectrometers often lack sufficient out-of-plane coverage, making them unsuitable for powder samples or studies requiring large solid angles (e.g., Debye-Scherrer cone integration).

2. Methodology: The Symbiotic Instrument Concept

The proposal introduces a paired instrument concept, Njord and Remora, sharing a single beamport at the ESS. This "symbiotic" approach leverages the unique long-pulse nature of the ESS source to split the neutron spectrum efficiently.

Njord: The Extreme-Flux Spectrometer

• Type: Indirect-geometry Time-of-Flight (ToF) spectrometer.
• Core Technology: Utilizes Nested-Mirror Optics (NMO). This technology employs a stack of short, elliptically shaped supermirrors to focus the neutron beam from a virtual source onto a tiny sample spot (3 mm × 3 mm) without sacrificing intensity.
• Optical Strategy: Njord accepts a continuous bandwidth (1.7 Å, $\lambda < 4.7$ Å) from the ESS pulse. The NMO compresses the beam to achieve extreme flux density.
• Analyzer System: Employs a MUSHROOM-type crystal-analyser array (inspired by ISIS). This design provides massive out-of-plane coverage (vertical angles from +15° to -2.3°) and large in-plane coverage, allowing the measurement of full 4D ($Q, E$) volumes in a single setting.
• Sample Environment: Designed specifically to accommodate extreme conditions, including high-pressure cells (diamond anvil, sintered diamond) and horizontal magnetic field magnets, by minimizing background through tight beam definition.

Remora: The Capacity Spectrometer

• Type: Direct-geometry Time-of-Flight (ToF) spectrometer.
• Core Technology: Positioned upstream of Njord, Remora extracts the "excess" neutron spectrum that Njord does not use. It utilizes a Highly Oriented Pyrolytic Graphite (HOPG) monochromator to select specific wavelengths via Bragg reflections (002, 004, 006).
• Repetition Rate Multiplication: By exploiting the long ESS pulse, Remora simultaneously utilizes multiple harmonics (e.g., 4.8 Å and 2.4 Å) from a single pulse. This expands the dynamic range (0.1 meV < $\hbar\omega$ < 50 meV) and increases efficiency.
• Optical Strategy: Uses a compact secondary flight path (2.5 m) and a Fermi chopper to control illumination time. It employs vertical focusing via the monochromator and horizontal focusing via a compact NMO.
• Role: Acts as a standard, high-throughput spectrometer for experiments that do not require the extreme flux of Njord but need high signal-to-noise ratios and broad accessibility.

3. Key Contributions

• Novel Optical Implementation: The proposal introduces the first application of Nested-Mirror Optics (NMO) for a ToF spectrometer at ESS, enabling a 3–4-fold increase in flux density compared to existing instruments like BIFROST on the same extraction system.
• Symbiotic Beamport Design: It demonstrates a viable method to host two distinct spectrometers (one indirect, one direct) on a single beamline by exploiting the temporal separation of the long ESS pulse and the spectral transparency of the HOPG monochromator.
• Expansion of Scientific Scope: The design specifically targets "impossible" experiments:
  • Sub-mm³ Samples: Enabling studies on MOFs and organic superconductors where crystals are too small for current facilities.
  • Extreme Conditions: Facilitating high-pressure (up to GPa range) and high-field (horizontal) studies on small samples.
  • Powder Spectroscopy: The large out-of-plane coverage allows efficient integration over the Debye-Scherrer cone, making small-sample powder studies feasible.

4. Results (Simulated Performance)

Preliminary McStas simulations and analytical calculations yield the following specifications:

Njord Performance:

• Flux: Exceeds $2.5 \times 10^{10}$ n/s/cm² on a 3 mm × 3 mm spot (at 2 MW ESS power).
• Resolution:
  • Energy Resolution: ~190 µeV (Pulse Shaping Chopper parked) to ~42 µeV (0.5 ms opening).
  • $Q$-Resolution: Optimized for low-resolution, wide-range studies; compatible with continuum excitations in quantum magnets.
• Coverage: 1.4 Sr solid angle; vertical coverage from +15° to -2.3°.

Remora Performance:

• Flux: $3 \times 10^5$ n/s/cm² at 4.8 Å and $9 \times 10^4$ n/s/cm² at 2.4 Å.
• Resolution: Elastic resolution of ~150 µeV (4.8 Å) and ~380 µeV (2.4 Å) depending on chopper frequency.
• Dynamic Range: Covers a broad energy transfer range (0.1 meV to 50 meV) via multi-wavelength operation.

5. Significance

• Scientific Breakthroughs: Njord will unlock the study of collective excitations in Metal-Organic Frameworks (MOFs) (e.g., breathing modes in MIL-53), organic superconductors (e.g., $\kappa$-(BEDT-TTF)$_2$X), and quantum magnets (e.g., spin supersolids in MnCr$_2$S$_4$ and Kitaev spin liquids in Na$_3$Co$_2$SbO$_6$). These materials are currently inaccessible to neutron spectroscopy due to sample size or flux limitations.
• Community Impact: Remora addresses the critical shortage of beamtime, providing a high-throughput instrument comparable to world-class facilities like LET or IN5. This ensures the sustainability of the user community and supports training for early-career scientists.
• Strategic Positioning: Together, the duo ensures ESS remains the global leader in neutron scattering. Njord pushes the boundaries of what can be measured (extreme flux/small samples), while Remora maximizes how much science can be done (capacity/access).
• Technological Legacy: The successful implementation of NMO and the symbiotic beamport concept sets a new standard for instrument design at pulsed neutron sources, potentially influencing future generations of spallation source instrumentation.