Effects of 3D printed capsule material on activation thin foil irradiation and counting for fusion neutron yield measurements

This study evaluates activation foils and capsule materials for fusion neutron yield measurements, demonstrating that aluminum and copper foils are suitable for multi-foil configurations, 3D-printed thermoplastic capsules introduce negligible measurement bias despite slight count reductions, and lanthanum-based detectors offer a viable alternative to high-purity germanium spectrometers.

The Gist

Imagine you are trying to count how many raindrops hit a specific spot during a massive storm. In the world of fusion energy (the same process that powers the sun), scientists need to count "neutron raindrops" to know how much energy their reactors are producing.

This paper is about building a better bucket to catch those raindrops and a better sieve to count them, specifically for a future super-powerful reactor called SPARC.

Here is the story of their experiment, broken down into simple concepts:

1. The Problem: Catching Invisible Raindrops

Neutrons are tiny, invisible particles. You can't see them, and they don't leave a wet spot. To count them, scientists use "activation foils." Think of these foils as specialized sponges.

• When a neutron hits the sponge, the sponge gets "charged up" (activated).
• After the storm (the neutron burst) is over, the sponge starts glowing with a faint light (gamma rays) as it "relaxes."
• By measuring how much light the sponge glows, scientists can calculate exactly how many neutrons hit it.

2. The Challenge: The Delivery Box

You can't just leave these glowing sponges out in the open. They need to be moved from the dangerous, radioactive reactor core to a safe counting room. They have to travel inside a protective capsule (like a lunchbox).

The scientists had two big worries about this "lunchbox":

1. The Shield: Does the box block too many neutrons from hitting the sponge? (Like putting a thick blanket over a rain gauge).
2. The Filter: Does the box block the light (gamma rays) coming out of the sponge when we try to count it? (Like trying to read a glowing sign through a dirty window).

They wanted to know: Can we use 3D-printed plastic boxes for this job? 3D printing is cheap and easy, but would the plastic mess up the measurements?

3. The Experiment: The "Plastic Box" Test

The team at MIT and Commonwealth Fusion Systems decided to test this in the lab using a small neutron generator (a mini-storm).

• The Sponges (Foil Materials): They tested two types of metal sponges: Aluminum and Copper.
  • Analogy: Imagine Aluminum is a sponge that glows for a short time, and Copper is one that glows longer. They found both worked well, but Copper was much "louder" (glowed brighter), making it easier to count, though a bit trickier to analyze.
• The Boxes (Capsule Materials): They 3D-printed boxes out of three different plastics: PLA (like a biodegradable coffee cup), PETG (like a water bottle), and Polycarbonate (like a bulletproof window).
• The Counters (Detectors): Usually, scientists use a super-expensive, super-sensitive camera (Germanium detector) to see the glow. But these are expensive and need to be kept freezing cold. The team tested cheaper, room-temperature cameras made of Lanthanum (LaBr3 and LaCl3).
  • Analogy: The Germanium camera is like a high-end DSLR that sees every tiny detail. The Lanthanum camera is like a smartphone camera—it's cheaper and easier to use, but the photos are a little blurrier.

4. The Results: What Did They Find?

The Plastic Boxes are Fine!
This was the big news. They found that the 3D-printed plastic boxes blocked almost nothing.

• The Neutrons: The plastic was so thin and light that the neutrons passed right through it like ghosts.
• The Light: The plastic blocked less than 2% of the light coming out.
• The Verdict: It's like trying to measure rain through a thin sheet of plastic wrap. It doesn't matter. This means they can use cheap, 3D-printed capsules for the SPARC reactor, which is huge for saving money and time.

The "Blurry" Cameras Work Too!
The cheaper Lanthanum cameras worked surprisingly well.

• They couldn't see the tiny details as clearly as the expensive Germanium camera (the photos were a bit blurry), but they were bright enough to get the job done.
• The Verdict: For the SPARC reactor, these cheaper cameras are a viable alternative. They are like using a smartphone to take a photo of a bright firework; you don't need a $10,000 camera to see the explosion.

Stacking Sponges Works
They tried stacking different sponges (Aluminum and Copper) together in one box. It worked perfectly! This is important because it lets them learn more about the storm (the neutron energy) than just counting the total drops.

5. Why Does This Matter?

The SPARC reactor is being built to prove that we can create clean, limitless energy. To do that, they need to know exactly how much energy is being made.

This paper says: "Don't worry about the delivery box or the camera cost."

• We can use cheap 3D-printed plastic boxes to move the sensors.
• We can use cheaper, room-temperature cameras to count the results.

This removes two major hurdles, making the design of the SPARC reactor simpler, cheaper, and more reliable. It's a small step in the lab that helps build a giant leap for fusion energy.

Technical Summary

Here is a detailed technical summary of the paper "Effects of 3D printed capsule material on activation thin foil irradiation and counting for fusion neutron yield measurements."

1. Problem Statement

Activation foils are a critical diagnostic tool for measuring time-integrated neutron yield and total fusion energy in both inertial and magnetic confinement fusion devices (e.g., ITER, SPARC). The process involves placing foils near a neutron source, irradiating them to induce radioactivity, and then remotely transporting them to a gamma-ray spectrometer for counting.

The core challenges addressed in this paper are:

• Capsule Interference: Foils must be housed in capsules for remote transport. The capsule material can attenuate both the incoming neutron flux (during irradiation) and the outgoing gamma rays (during counting), potentially skewing yield measurements.
• Material Selection: There is a need to select optimal foil materials (Al, Cu, etc.) that provide sufficient signal within the constraints of transport time and counting duration, while balancing half-life and cross-sections.
• Detector Alternatives: High-Purity Germanium (HPGe) detectors are the standard but are expensive and require cryogenic cooling. The paper investigates whether cheaper, room-temperature lanthanum-based scintillators (LaBr$_3$ and LaCl$_3$) are viable alternatives for future fusion systems like SPARC.
• 3D Printing: The feasibility of using 3D-printed thermoplastic capsules (PETG, PLA, PC) for remote transport systems needs validation regarding their impact on measurement accuracy.

2. Methodology

The study employed a combination of neutronics simulations and physical experiments:

• Simulations (FISPACT-II):

  • Simulated irradiation conditions for the SPARC tokamak (Primary Reference Discharge) and the MIT Vault Lab (DT neutron generator).
  • Evaluated 16 different elements to determine optimal foil materials based on half-life (sufficient for transport but short enough for weekly reuse), activation cross-sections at 14.1 MeV, and gamma-ray yield.
  • Modeled gamma-ray attenuation through various capsule materials.

• Experimental Setup:

  • Source: A Deuterium-Tritium (D-T) neutron generator producing $\approx 10^8$ n/s.
  • Foil Materials: Natural Aluminum (Al) and Copper (Cu) foils were selected based on simulation results.
  • Capsules: 3D-printed capsules made of three thermoplastics: PETG, PLA, and Polycarbonate (PC).
  • Detectors: LaBr$_3$ (Lanthanum Bromide) and LaCl$_3$ (Lanthanum Chloride) scintillators were used for counting. HPGe was not used in the physical experiments but was used as a benchmark for resolution in simulations.
  • Procedure: Foils were irradiated for 1 to 1.5 hours. After a transfer time of ~120 seconds, they were counted for 1 hour.
  • Configurations: Experiments included single-foil runs, multi-foil runs (stacked Al/Cu), and "swap" runs (cross-checking detector assignments). Gamma attenuation was also tested independently using $^{137}$Cs and $^{22}$Na sources.

• Data Analysis:

  • Gamma spectra were analyzed using a custom Python library ("Daisy") to fit peaks (Gaussian) and subtract backgrounds.
  • Results were normalized by the measured neutron rate to account for generator instability.
  • Statistical and systematic uncertainties (Poisson statistics, solid angle, fitting errors, time delays) were rigorously calculated.

3. Key Contributions

• Validation of 3D-Printed Capsules: The study provides empirical data confirming that 3D-printed thermoplastic capsules (PETG, PLA, PC) introduce negligible attenuation (<1.7% for 662 keV gammas) and do not significantly affect neutron flux, making them suitable for remote transport systems in fusion reactors.
• Foil Material Optimization: Confirmed that Al and Cu are optimal for multi-foil configurations in the 14.1 MeV energy range, offering a balance of high cross-sections and manageable half-lives for laboratory and reactor conditions.
• Detector Viability Assessment: Demonstrated that LaBr$_3$ scintillators can serve as viable, cost-effective alternatives to HPGe detectors for fusion diagnostics, offering higher efficiency and adequate resolution for specific foil combinations, despite lower energy resolution than HPGe.
• Multi-Foil Configuration: Validated that stacking different foil materials (Al and Cu) in a single holder does not compromise measurement accuracy, enabling simultaneous measurement of different neutron energy components.

4. Key Results

• Capsule Attenuation:
  • Gamma attenuation coefficients for PETG, PLA, and PC were measured at 1.67%, 1.33%, and 1.32% respectively for 662 keV photons.
  • The difference in attenuation between materials is smaller than the experimental uncertainty, suggesting any of the three plastics are acceptable.
  • Neutron attenuation by the capsules was found to be well below the systematic uncertainties of the experimental setup.
• Foil Performance:
  • Copper: Produced significantly higher gamma counts (approx. 4x more photons per foil, 8x more per unit mass) compared to Aluminum. However, Cu emits 511 keV gammas via two different reaction pathways, complicating spectral analysis.
  • Aluminum: Provided a cleaner spectral signature for 14.1 MeV neutrons but with lower statistics.
  • Multi-Foil: Stacked Al/Cu experiments yielded results consistent with single-foil runs, validating the multi-foil approach for SPARC.
• Detector Comparison:
  • LaBr$_3$ vs. LaCl$_3$: LaBr$_3$ demonstrated superior performance due to higher detection efficiency and better energy resolution (3.2% vs 4.5% at 662 keV). It successfully resolved the 1369 keV Al peak from the intrinsic 1465 keV background of the LaBr$_3$ crystal.
  • Resolution Limits: While LaBr$_3$ is an improvement over LaCl$_3$, it still struggles to resolve peaks that are very close in energy (e.g., overlapping peaks from different foil isotopes) compared to HPGe.
• Uncertainty Analysis:
  • The total relative uncertainty in the current MIT lab setup was found to be >10% (dominated by systematic errors in geometry and fitting).
  • This level of uncertainty is too high for the SPARC goal of 10% total uncertainty on fusion energy ($E_{fus}$), indicating that future SPARC systems require optimized foil mass, positioning, and counting times to reduce statistical and systematic errors.

5. Significance

This work is a critical step toward the deployment of activation foil diagnostics on the SPARC tokamak.

• Design Guidance: It provides the engineering data necessary to design the automated pneumatic transport system for SPARC, confirming that 3D-printed thermoplastic capsules are safe and effective.
• Cost-Effective Diagnostics: By validating LaBr$_3$ detectors, the paper suggests a pathway to reduce the cost and complexity of fusion diagnostics (removing the need for liquid nitrogen cooling) while maintaining sufficient accuracy for neutron yield measurements.
• Data Reliability: The rigorous uncertainty analysis and multi-foil validation ensure that future fusion yield measurements will be robust, allowing for precise reconstruction of total fusion energy and fuel ion ratios.
• Future Directions: The study highlights the need for further Monte Carlo simulations to refine absolute attenuation coefficients and suggests that future experiments could incorporate Iron (Fe) foils to better characterize the neutron energy spectrum between 5 and 12 MeV.