Initial Characterisation of a Prototype TMR Assembly for an Electron-Driven CANS at CERN's CLEAR Facility

This paper presents the design, installation, and initial experimental results of a prototype target-moderator-reflector assembly for the VULCAN project at CERN's CLEAR facility, highlighting the successful detection of moderated neutron pulses while noting significant discrepancies between experimental and simulated energy spectra that require further investigation.

The Gist

The Big Picture: Building a "Neutron Factory" in a Suitcase

Imagine you want to study the microscopic structure of materials (like new medicines or stronger metals). Scientists usually use neutrons for this because they are like tiny, invisible X-rays that can see through heavy stuff and spot light elements easily.

However, the current "neutron factories" are massive, like entire cities dedicated to research. They are expensive, hard to get into, and there aren't enough of them for everyone who wants to use them.

The VULCAN project wants to build a "neutron factory" that fits in a single room (a Compact Accelerator-Driven Neutron Source, or CANS). Think of it as shrinking a nuclear power plant down to the size of a large refrigerator. To do this, they need a special machine called a TMR (Target-Moderator-Reflector).

The Recipe: How the TMR Works

The TMR is the heart of this mini-factory. Here is how it works, using a cooking analogy:

1. The Target (The Pan): A high-speed electron beam (like a super-fast stream of tiny bullets) hits a block of metal (Tungsten-Tantalum). This is like throwing a baseball at a wall; the impact creates a spray of high-energy photons (light particles).
2. The Pre-Moderator (The First Chill): These photons hit a block of plastic (High-Density Polyethylene). This slows the energy down a bit, like a speed bump.
3. The Moderator (The Ice Bath): The energy then hits a chamber filled with liquid methane (frozen natural gas) at -173°C. This is the most important part. The methane acts like a giant ice bath, slowing the neutrons down to the perfect "walking speed" (thermal neutrons) needed for scientific experiments.
4. The Reflector & Shielding (The Insulation): Surrounding everything are layers of lead and special plastic. These act like a cozy blanket, keeping the neutrons inside the system and bouncing them back toward the exit, while blocking anything that shouldn't be there.
5. The "Poison" (The Speed Bump): The team tested two versions: one with a special "poison" (Gadolinium foil) and one without. Think of the poison like a speed trap. It catches the slow neutrons that are lingering too long, forcing the system to release a sharper, faster "pulse" of neutrons. This is crucial for getting clear, sharp data.

The Experiment: A Test Drive at CERN

The team built a prototype of this TMR and took it to CERN's CLEAR facility (a research lab in Switzerland) to test it. They couldn't run it at full power yet (it wasn't cooled enough), so they ran it at a very low power, like testing a race car engine in a parking lot rather than on a racetrack.

They shot an electron beam at the TMR and used a special detector (a Helium-3 detector) to "listen" for the neutrons coming out. They measured:

• How many neutrons came out.
• How fast they were moving (their energy).
• How long the pulse lasted.

The Results: The "Plot Twist"

The experiment was a success in some ways, but a mystery in others.

• The Good News: The machine worked! They successfully detected neutrons coming out of the exit channel. About 95% of the signals they saw were real neutrons from the machine, not background noise. They proved the machine could be built, installed, and operated safely.
• The Bad News (The Discrepancy): The data didn't match the computer simulations.
  • The Expectation: The computer models predicted the neutrons would come out at a specific "speed" (energy peak around 15 meV).
  • The Reality: The actual neutrons came out much "faster" (energy peak around 65 meV).
  • The Mystery: Even when they warmed up the machine and let the liquid methane evaporate (so there was no "ice bath" at all), the neutrons were still faster than the computer predicted.

What Does This Mean?

The authors conclude that while they successfully built and tested the hardware, something is wrong with the math or the measurement tools, not necessarily the machine itself.

They suggest a few possibilities:

1. The Ruler is Wrong: The detector used to measure the neutrons might be slightly miscalibrated (like a speed gun that reads 60 mph when you are actually doing 30).
2. The Map is Wrong: The computer simulation might have the wrong settings for the materials or the temperature.
3. The Angle is Off: The detector might be slightly misaligned with the exit channel.

The Takeaway

This paper is essentially a "proof of concept" report. The team built a working prototype of a mini-neutron factory and proved it can be installed and run. However, the data they got didn't match their predictions, so they can't trust the numbers yet.

Next steps involve recalibrating the detectors, checking the computer models against known standards, and building a new version with better cooling to run at full power. They haven't solved the mystery of the energy mismatch yet, but they have cleared the path to solve it.

Technical Summary

Technical Summary: Initial Characterisation of a Prototype TMR Assembly for an Electron-Driven CANS at CERN's CLEAR Facility

Problem Statement
The Versatile ULtra-Compact Accelerator-based Neutron source (VULCAN) project aims to develop a compact accelerator-driven neutron source (CANS) optimized for neutron diffractometry in industrial and university settings. A critical component of this system is a Target–Moderator–Reflector (TMR) assembly designed to convert a 35 MeV pulsed electron beam into short neutron pulses (FWHM ≤20 µs) within the 1.5 Å to 3.5 Å wavelength range. While simulation-driven design frameworks exist, there is a need for experimental validation of these designs, particularly regarding the conversion efficiency of electron beams to thermal neutrons and the temporal pulse structure. This paper addresses the gap between simulation predictions and experimental reality by characterizing a prototype TMR assembly.

Methodology
The study involved the design, fabrication, installation, and testing of a prototype TMR at CERN's CLEAR (CERN Linear Electron Accelerator for Research) facility.

• Prototype Design: The TMR features a decoupled, cold liquid methane moderator. Key components include a cylindrical tungsten-tantalum (W10Ta90) target, a high-density polyethylene (HDPE) pre-moderator, a liquid methane chamber (operating at ~100 K), and a lead reflector. The design allows for two configurations: "poisoned" (using gadolinium foils to decouple the moderator and shorten pulse width) and "unpoisoned."
• Experimental Setup: The prototype was installed on CLEAR's in-air test stand. Due to the lack of active cooling in the prototype, it operated at a reduced beam power (1.6 W) compared to the final design target (>1 kW). The electron beam parameters were 40 MeV (due to facility constraints), with a transverse size of 4 mm and a repetition rate of 10 Hz.
• Detection and Measurement: Thermal neutron spectra were measured using a 3He Monoblock Aluminium Multitube (MAM) detector placed at flight paths of 1.6 m, 4.2 m, and 6.6 m. A borated polyethylene gate was used to quantify background noise.
• Simulation and Analysis: The experimental setup was modeled using the FLUKA Monte Carlo code with JEFF-3.3 cross-section libraries and S(α, β) corrections for liquid methane. Experimental data was processed into differential flux ($d\phi/dE$) and compared directly against simulations using recorded beam parameters as inputs.

Key Contributions

• Prototype Realization: The paper presents the first successful fabrication, commissioning, and operation of a decoupled, liquid-methane-based TMR prototype capable of operating in both poisoned and unpoisoned modes.
• Experimental Validation Framework: It establishes a methodology for benchmarking simulation-driven TMR designs against experimental data, including the integration of a cryogenic system with a pulsed electron beam in a research accelerator environment.
• Background Characterization: The study quantified the background signal, demonstrating that approximately 95% of detected events originated from the neutron extraction channel, validating the shielding and collimation design.

Results

• Neutron Detection: The experiment successfully detected moderated neutron pulses. The flux decay with distance was consistent between experiment and simulation, and spectral shapes remained consistent across all measurement positions, indicating that neutron transport beyond the TMR was well-modeled.
• Spectral Discrepancies: A significant discrepancy was observed between experimental and simulated energy spectra.
  • Poisoned/Unpoisoned (Liquid Methane): Experiments showed a peak energy of ~65 meV, whereas simulations predicted ~15 meV.
  • Vented (Room Temperature): When the methane was vented and the chamber warmed, the experimental peak shifted to ~85 meV, while the simulation predicted ~45 meV.
• Analysis of Discrepancies: The persistence of the spectral shift even when the moderator was vented suggests the cause is not solely due to moderator temperature, fill level, or thermal scattering model errors. The authors identify potential systematic errors in detector time-channel calibration, detector alignment relative to the extraction channel, or simulation setup specifications as likely contributors.

Significance and Claims
The paper modestly claims that while the prototype successfully validated the practical aspects of construction, installation, and operation, the significant spectral discrepancies highlight the necessity for further calibration and benchmarking. The study does not claim to have solved the spectral mismatch but rather identifies it as a critical area for future work.

The authors conclude that the prototype TMR was successfully operated, derisking the installation procedure at CLEAR and proving the facility's suitability for pulsed neutron characterization. However, they emphasize that future campaigns must include:

1. In-situ calibration of the 3He detector against a known neutron source.
2. Benchmarking of simulation models against reference measurements.
3. Direct monitoring of moderator temperature and fill volume.
4. Use of crystal diffractometers or chopper-based systems to accurately measure the initial pulse FWHM.

The work serves as a foundational step for the VULCAN project, confirming the feasibility of the hardware approach while underscoring the complexity of achieving precise spectral agreement between simulation and experiment in compact neutron source development.