April 13, 2024

‘Frankenstein design’ enables 3D-printed neutron collimator

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Credit: Oak Ridge National Laboratory

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Credit: Oak Ridge National Laboratory

The time-tested strategy of “divide and conquer” took on a new, high-tech meaning during neutron experiments carried out by scientists at the Department of Energy’s Oak Ridge National Laboratory. They discovered that the problems they faced when trying to 3D print a one-piece collimator could be solved by developing a “Frankenstein design” involving multiple body parts – and some pretty obvious scars.

The team’s article is published in the journal Nuclear instruments and methods in physics research Section A: accelerators, spectrometers, detectors and associated equipment.

Collimators are important components used in neutron scattering. Similar to X-rays, neutrons are used to study energy and matter on an atomic scale. Neutron collimators can be thought of as funnels that help guide neutrons toward a detector after interacting with experimental sample materials. These funnels primarily serve to reduce the number of stray neutrons that interfere with data collection, for example, neutrons that scatter from sample holders or from other apparatus used in the experiment, such as high-pressure cells.

During this process, most of the unwanted neutrons, those that scatter from features other than the sample, enter the channels inside the collimators at odd angles and are absorbed by the channel walls, also known as foils. The blades act like chutes on a bowling alley, which catch bowling balls that aren’t heading toward the pins.

“The trend in research to use smaller samples of materials in more complex environments results in a greater number of neutrons that have not interacted with the sample and are not spreading throughout the sample,” said Fahima Islam, lead author of the study and a neutron scientist at the ORNL Spallation Neutron Source, or SNS.

“These unwanted neutrons produce undesirable signatures in the data, which is why we were working to produce a 3D-printed collimator that could be custom designed to filter out these undesirable background features during different types of scattering experiments.” neutrons.”


Images of the 3D-printed “Frankenstein design” collimator show the “scars” where the individual parts are joined, which are clearly visible on the right. Credit: Genevieve Martin/ORNL, US Department of Energy

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Images of the 3D-printed “Frankenstein design” collimator show the “scars” where the individual parts are joined, which are clearly visible on the right. Credit: Genevieve Martin/ORNL, US Department of Energy

The team collaborated with experts at ORNL’s Manufacturing Demonstration Center, or MDF, to use a 3D printing method called Binder Jetting. This additive manufacturing process builds parts and tools from powdered materials. Similar to printing on paper, the precision process builds the part layer by layer, based on a digital drawing, until the object is complete.

One obstacle the team faced involved increasing the size of the printed collimator while maintaining the accuracy of the finished product. A large collimator was needed to capture a greater number of neutrons scattered throughout the sample and the complex pressure cell chosen for the test. In a pressurized environment, the sample is enclosed in a non-transparent sample container, which causes a significant number of unwanted neutrons to scatter strongly in a way that can dominate the weaker data signal that scientists are looking for.

“To demonstrate the feasibility of using 3D-printed custom collimators, we decided to use a very small sample contained in a diamond anvil – a high-pressure chamber that uses diamonds to squeeze materials. Some of these cells are so complex and strong that they are capable to produce pressures close to those at the center of the Earth,” said Bianca Haberl, corresponding author of the study and neutron scattering scientist at SNS.

“In fact, high-pressure cells are some of the most complex environments used in neutron experiments, so it is a real challenge to filter out the enormous amount of unwanted cell scattering they produce.”

The scientific principles for designing collimators are generally well understood, so the team’s first attempt at 3D printing a collimator for such a small sample involved simply increasing the size of the printed part while retaining the continuous front-to-back blades that formed the channels. . The Binder Jet 3D printer allowed us to print the single-piece version in dimensions of about 12 by 9 by 9 inches, which maximized the ability to direct neutrons to the detector while still fitting into the instrument.

Unfortunately, the complexities in scaling up the 3D printing process hampered the accuracy of the printed part to such an extent that it was not suitable for beamline use.


The team that developed the 3D-printed collimator included, from left, Fahima Islam, Bianca Haberl and Garrett Granroth. Credit: Genevieve Martin/ORNL, US Department of Energy

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The team that developed the 3D-printed collimator included, from left, Fahima Islam, Bianca Haberl and Garrett Granroth. Credit: Genevieve Martin/ORNL, US Department of Energy

“Simply scaling up the print as one large part with continuous sheets was clearly not feasible without further optimizing the printing process,” said Garrett Granroth, co-author and SNS neutron scattering scientist. “A new concept was subsequently developed to print multiple smaller parts and then manually assemble them in a complete collimator. The main reason for using smaller parts is that the cracking observed in the single-part design was primarily due to variations in the contraction rate of the material during the curing and cooling process. By reducing their overall size, the individual parts cooled more evenly.”

Instead, an alternative blade design was used with progressively tighter blades from the sample-facing end to the detector-facing end. This configuration allowed for higher blade density with reduced channel sizes and avoided some size-related 3D printing limitations. By ensuring that the blades did not cross the boundaries between individual parts, the design was less sensitive to misalignment between parts during assembly.

Employing this approach, the team optimized collimator performance by simulating the entire experiment using advanced computational methods developed for the project. The simulation produced a design that could go directly to production without additional engineering.

The 3D-printed reciprocating blade collimator was evaluated for performance at SNAP, the Spallation Neutron and Pressure beamline, a dedicated high-pressure neutron diffractometer. The experiments revealed extreme sensitivity to collimator alignment, emphasizing the need for ultra-high precision in fabrication and positioning of the collimator in the beamline.

Once precisely aligned, the collimator enabled the desired increase in the sample’s relative signal over cell scatter, proving the concept. The scientists also identified areas for future refinement, including further improvements through tighter manufacturing quality control and better alignment. By combining modeling and advanced manufacturing, the study identified a new way to customize neutron scattering instrumentation and advance neutron science.

More information:
Fahima Islam et al, Advanced Manufacturing of Custom 3D Boron Carbide Collimators Designed for Complex Environments for Neutron Scattering, Nuclear instruments and methods in physics research Section A: accelerators, spectrometers, detectors and associated equipment (2024). DOI: 10.1016/j.nima.2024.169165

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