Elementary-particle detectors, 3D printed

In 2024, the T2K Collaboration started to collect new neutrino data following several upgrades to the experiment that included new types of detectors. One of these, called SuperFGD, has a mass of about 2 tons of sensitive volume and is made of approximately two million cubes. Each cube is made of plastic scintillator (PS) material that emits light when a charged particle passes through it. Neutrinos carry no charge, as their name indicates, but they sometimes interact with other particles, then producing electrons, protons, muons or pions that can be detected. Each PS cube is traversed by three orthogonal optical fibers that collect the scintillation light and guide it to 56’000 photodetectors. The data reveal three-dimensional (3D) particle tracks, which in turn allow researchers to learn more about neutrinos.

Detector upgrades of this kind are crucial to advance the discovery capabilities of large particle physics experiments, and yet it's fair to ask: what does it take to assemble, cube after cube and layer after layer, 2 million PS cubes into a functioning particle detector? Could large-scale detectors in high-energy physics be built differently? These are the questions that motivate one line of work of Professors Davide Sgalaberna and André Rubbia in the Institute for Particle Physics and Astrophysics. Together with colleagues from ETH Zurich, CERN, the HES-SO, the HEIG-VD, COMATEC-AddiPole and the Institute for Scintillation Materials in Ukraine, Sgalaberna and Rubbia have just published a research paper in the journal Communications Engineering where they present a fully additive-manufactured plastic scintillator detector for elementary particles. The authors are all part of the 3D printed DETector (3DET) Collaboration, which is led by Sgalaberna with the technical coordination of Dr Umut Kose. The team believes their demonstration to be a significant step in the direction of time- and cost-effective ways to build future large-scale particle detectors.

An engineering problem

PS detectors make it possible to track the paths and measure the energy loss of charged particles passing through the scintillator material with a fast temporal response. These characteristics have determined their growing success since they were proposed in the 1950s. In a PS, fluorescent emitters called fluors are introduced into a solid polymer matrix. A charged particle that propagates through the material excites the polymer matrix: a non-radiative dipole-dipole interaction transfers the excitation energy to the fluors, which de-excite by emitting near-ultraviolet light within few nanoseconds. A second type of fluor is often added to the polymer to shift the wavelength of the emitted light and avoid absorption in the scintillator material. Optical fibres collect the light produced by a PS by shifting its wavelength to the green part of the visible spectrum, making it possible to trap the emitted light and increase its attenuation length.

For optimal tracking of elementary particles, so-called granular 3D scintillating detectors have been assembled from many smaller volumes, such as the PS cubes in SuperFGD. In this scenario, it's crucial that the smaller units are optically isolated to track different charged particles independently. The 3DET Collaboration is familiar with these assembled detectors: Sgalaberna conceived SuperFGD and led its development and construction as a member of the T2K Collaboration. In the same way as the 2D screen of a laptop or smartphone is made of single fluorescing pixels, a granular 3D particle detector can be viewed as a collection of scintillating voxels. All voxels must work together to deliver high-quality data: each voxel is isolated but part of a bigger whole.

"This is really an engineering problem," says first author Tim Weber about the demonstration reported in the paper. Trained as a mechanical engineer at ETH Zurich, Weber joined the Exotic Matter and Neutrino Physics group in the Department of Physics and the 3DET Collaboration three years ago and brought in his diverse experience with additive manufacturing (AM), commonly known as 3D printing. He likes to take a pragmatic view on the matter: if the goal is to build ever larger particle detectors with excellent tracking resolution, the time and costs of production must be reduced. This calls for solutions that guarantee speed of production without compromising on the quality and performance of the particle detector.

The ideal production system can build thousands of scintillating voxels into a monolithic block. The 3DET Collaboration and others have already worked with AM for PS detector prototypes; some of the early challenges they encountered – especially in terms of detector performance – highlighted two crucial decision points: the choice of materials and the type of AM processes used to produce the detector. For example, AM is typically not great at handling multiple materials while achieving the material transparency needed for the scintillation light to not be reabsorbed by the PS. Additionally, not all AM processes can produce hollow structures. The latter issue often leads to subtractive interventions – drilling holes into the voxels for wavelength-shifting fibres, for instance – that make the fabrication procedure tricky to automate.