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In 2024, the T2K Collaboration commenced gathering fresh neutrino data, leveraging advancements made to the experiment that included the introduction of new detector technologies. Among these innovations is the SuperFGD, a detector weighing approximately 2 tons, constructed from nearly two million plastic scintillator (PS) cubes. Each of these cubes generates light when charged particles traverse them, although neutrinos themselves are neutral and do not directly induce this light. Nevertheless, when neutrinos interact with other particles, they can produce detectable outputs such as electrons, protons, muons, or pions. The structure of each PS cube includes three orthogonal optical fibers that channel the scintillation light to 56,000 photodetectors, thus enabling the reconstruction of three-dimensional (3D) particle trajectories, providing researchers with vital insights into neutrino behavior.
Upgrading detectors is imperative for enhancing the capabilities of large-scale particle physics experiments. This raises pertinent questions: What processes are involved in constructing two million PS cubes into a functional detector? Is there an alternative approach for creating large-scale detectors in high-energy physics? These inquiries have driven Professors Davide Sgalaberna and André Rubbia from the Institute for Particle Physics and Astrophysics. Collaborating with colleagues from ETH Zurich, CERN, HES-SO, HEIG-VD, COMATEC-AddiPole, and the Institute for Scintillation Materials in Ukraine, they recently published a paper in the journal Communications Engineering, where they unveil a fully additive-manufactured plastic scintillator detector designed for tracking elementary particles. This team is part of the 3D printed DETector (3DET) Collaboration, led by Sgalaberna and coordinated technically by Dr. Umut Kose. They assert that their findings represent a significant leap toward developing more time- and cost-efficient methods for manufacturing future large-scale particle detectors.
An engineering problem
Plastic scintillator detectors hold the capability of effectively tracking the paths of charged particles and measuring energy loss, a feature that has propelled their popularity since their initial proposal in the 1950s. Fluorescent compounds, known as fluors, are embedded within a polymer matrix in these detectors. When charged particles travel through the scintillator, they excite the polymer, resulting in a rapid transfer of energy to the fluors, which then emit near-ultraviolet light in a matter of nanoseconds. To enhance the detection efficiency, an additional fluor is commonly used to shift the emitted light’s wavelength, preventing its absorption within the scintillator. The optical fibers facilitate this process by converting the emitted light to wavelengths within the green spectrum of visible light, thereby maximizing its transmission and reducing absorption losses.
To optimize the tracking capabilities of elementary particles, granular 3D scintillating detectors are developed using numerous smaller units, such as the PS cubes in SuperFGD. In this setup, ensuring optical isolation between the smaller components is essential, allowing for the independent tracking of different charged particles. The 3DET Collaboration is well-acquainted with this approach, as Sgalaberna played a pivotal role in the design and construction of SuperFGD as part of the T2K Collaboration. Just as a smartphone screen is made up of individual fluorescing pixels, a granular 3D particle detector consists of a multitude of scintillating voxels working in unison to produce high-quality data, with each voxel remaining distinct yet integral to the overall system.
Tim Weber, the lead author of the paper, describes this endeavor as fundamentally an engineering challenge. With a background in mechanical engineering from ETH Zurich, Weber has applied his expertise in additive manufacturing (AM), also known as 3D printing, since joining the Exotic Matter and Neutrino Physics group and the 3DET Collaboration. He emphasizes the practical need for reduced time and costs in constructing increasingly larger particle detectors with enhanced tracking resolution. This pursuit necessitates efficient production solutions that balance speed with the quality of the detectors.
An ideal manufacturing framework would facilitate the creation of thousands of scintillating voxels as a cohesive block. The 3DET team has previously experimented with AM techniques for PS detector prototypes; however, they encountered notable challenges that underscored key decisions regarding material selection and the AM processes employed. For instance, common AM techniques often struggle with manipulating multiple materials while maintaining the transparency essential for the scintillation light. Furthermore, many AM methods lack the capacity to create hollow structures, necessitating additional procedures—such as drilling holes in the voxels to accommodate wavelength-shifting fibers—that complicate automation.
Custom-made solutions
Recognizing the need for a tailored AM solution, Weber, Sgalaberna, and their team developed a new manufacturing technique known as fused injection modeling (FIM), which combines elements of both fused deposition modeling (FDM) and injection molding. The FIM process unfolds in three phases: initially, a 5 × 5 array of optically reflective frames is produced using FDM, forming a mold for the PS with 25 open-top white-coated cubes, including holes for optical fibers. Following this, metal rods are positioned in the holes to accommodate the fibers. Subsequently, the FDM system is swapped out for an elongated nozzle that injects scintillator material into the mold, allowing the molten substance to fill the cubes evenly from the bottom up. In the final step, a heated punch flattens the top surface, preparing it for the next layer.
This method yielded what the researchers have termed a SuperCube, consisting of 125 optically isolated voxels arranged in a 5 × 5 × 5 grid with dimensions of 59 mm by 59 mm by 57.2 mm. Each voxel utilizes two orthogonal wavelength-shifting fibers for readout, with a production time for a single voxel estimated at approximately six minutes. This timeframe is anticipated to decrease with further automation of the newly designed 3D printing apparatus.
The prototype’s performance was evaluated using cosmic particle data, focusing on the light yield and crosstalk between the voxels. The SuperCube’s performance was benchmarked against a similar system built using conventional casting techniques, revealing no significant differences. The crosstalk, which reflects the optical isolation success of each voxel, was slightly higher in the FIM-produced system but remained within an acceptable range for 3D particle tracking. “This marks the first instance of a 3D printed scintillator detector successfully detecting charged particles from cosmic rays and test beams at CERN, enabling the reconstruction of both their trajectories and energy loss,” Sgalaberna stated.
The research team continues to refine their prototypes, aiming to enhance the optical isolation among the detector’s voxels. Concurrently, Weber is working on a complete redesign of the production system to develop an automated printer that can scale the fabrication for larger detector volumes. Sgalaberna highlighted the significant impact of such advancements, noting that expanding from a detector with 2 million voxels to one with 10 million would vastly improve data collection for experiments like T2K. Thus, solutions derived from 3D printing are poised to empower particle physics researchers to envision and construct larger, more effective detectors.
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