Plastic scintillator is one of the most used active materials in high-energy physics. A charged particle crossing a plastic scintillator loses part of its initial energy and produces a quantity of scintillation light that depends on its energy and type. This property allows using this technology for distinguishing between protons, electrons, pions or muons. These detectors can be used to track particles [1] and reconstruct the momentum of stopping charged particles. Performant sampling calorimeters can be made by alternating thin plastic scintillator tiles or fibers with thin layers of lead/iron [2, 3].

Plastic scintillator detectors are widely used in neutrino long-baseline oscillation experiments, whose goal is to observe the violation of the Charge-Parity symmetry in the leptonic sector, providing a plausible explanation to the clearly-observed matter-antimatter imbalance in the universe. These experiments use an intense (anti-)neutrino beam, with energies of a few hundred MeV to a few GeV, generated by accelerated protons impinging on a target. The rate and the energy spectrum of neutrinos are measured close to the target by a ‘near detector’, before oscillations occur, and at a distance of several hundred kilometers by a ‘far detector’, where the oscillation probability is near maximal/reaches its maximum. Measurements of oscillation phenomena are carried out through comparison of the observations in both detectors, to cancel out key systematic errors [4]. Past neutrino experiments, like MINOS [5] and Minerva [6], instrumented their near or far detectors with plastic scintillators.

T2K, the current world-leading neutrino oscillation experiment, in Japan, is upgrading its near detector, ND280. Although, results from previous runs have provided first indications of substantial leptonic CP-violation [7], it is upgrading the near detector (ND280) to reduce the systematic uncertainties, dominated by the poor knowledge of the neutrino interaction cross section. Hence, T2K is planning for a second phase to be started in 2022 and completed around 2025 [8]. 

ND280 will adopt a plastic scintillator detector as active neutrino target, surrounded by Time Projection Chambers and Time-of-Flight detectors [9].A novel detector technology, the Super Fine Grain Detector (sFGD) has been adopted as the plastic scintillator (PS) active target [10]. It comprises a two-ton polystyrene-based PS detector segmented into 1x1x1 cm³ cubes, leading to a total of around two million sensitive elements. Each cube is surrounded by a reflector that traps the light. Wavelength-shifting (WLS) optical fibers inserted along three axes are used to convey the light to photodetectors that precisely measure the energy deposited. This detector geometry, shown in Fig. 1, provides isotropic 3D tracking and identification of charged particles, vastly improving on the state of the art. The fine granularity will allow to detect the low-energy protons and pions produced by neutrino interactions.

Figure 1: Example of a sFGD [10].

A detector technology analogous to sFGD is part of the conceptual baseline of the DUNE [11] ND. Made of 12M cubes, it will collect a high-statistic data sample and will precisely detect neutrons produced by neutrino interactions and precisely reconstruct their energy by measuring the time-of-flight. A 3D granularity finer than 1x1x1 cm³ would allow a more precise particle tracking and reduce the detection threshold for low-energy hadrons, particularly important for improving the modeling of neutrino interactions. However, the detector assembly would become much more difficult since the number of cubes would drastically increase. This will be a major step forward towards the world’s first statistically significant observation of neutrino CP-violation.

Future constructions of sFGD-like detectors will therefore require a step change in technology. The ideal solution would be to produce a ‘super-cube’, a single massive block of scintillator containing many optically independent cubes. A non-scintillator mockup example is shown in Fig. 2. Traditional technologies (e.g. extrusion and injection moulding) cannot achieve this goal with sufficient precision, but it is possible that a super-cube could be produced using 3D-printing technology.

In the last decade, the field of additive manufacturing (AM) has revolutionized production methods for many objects; almost any shape can be quickly obtained, using many different materials, and with precision of 0.1 mm or better. First tests of 3D-printing of PS have been performed by some research groups, with promising results, but not yet at the level of optimization needed for physics experiments [12]. However, the field of AM is rapidly maturing, and new techniques are continuously being developed and improved. The excellent precision of 3D printers will allow the construction of a plastic scintillator detector with very fine spatial granularity, further improving the tracking performance. Most of the current 3D printers are capable of producing objects of sizes up to 40x40x40 cm3 or bigger and are relatively fast (about one day).

Applying AM to PS will have a large number of applications across many other branches of high-energy physics, like reactor neutrinos (e.g. SoLid [13]) or fast detector prototyping. AM will also boost the performance of detectors used in astroparticle physics experiments that need full solid angle acceptance combined with high granularity.

Figure 2: Mockup of SuperCube for illustration purpose.

A collaboration that comprises CERN EP, the Haute Ecole d’Ingenierie et Gestion du Canton Vaud (HEIG-VD), the Institute for Scintillation Materials of the National Academy of Science of Ukraine (ISMA) aims to apply AM techniques to plastic scintillator with good detection performances to build the first 3D-printed plastic scintillator detector ever. An R&D agreement that describes the role of each partner has been defined: ISMA, leader in the scintillator development and production, will optimize the plastic scintillator composition, the Addipole laboratory at HEIG-VD, with strong expertise in additive manufacturing, will take care of the 3D-printing process of plastic scintillator, while CERN will test the scintillation light output and guide the collaboration toward the achievement of the required performance. If the R&D project is successful, it will represent a paradigm shift in the field and will certainly have a large number of applications across many branches of high-energy physics and outside the field.

A production chain aiming for R&D has been set for Fused Deposition Model (FDM) technique using polystyrene-based scintillator. The strategy is 3D-printing of plastic scintillator material widely used in particle physics, whose scintillation light and timing performances are good and very well known. A new scintillator composition would introduce more uncertainty in the understanding of the real performance. Moreover, for neutrino experiments it is very important to have carbon-based active material with very low contamination from other nuclei, to avoid introducing additional systematic uncertainties in the neutrino interaction modeling, and the presence of a large quantity of hydrogen that enhances the neutron detection capabilities. As shown in Fig. 3, a polystyrene-based scintillator filament has been produced, optimized for usage in FDM printers. Several plastic scintillator cubes of about one cubic cm have been 3D-printed.

Figure 3: Different steps of the FDM 3D-printing process of a polystyrene-based scintillator cubes is shown: filament (left), 3D-printing (middle), final scintillator cube (right).

Light output tests show good transparency and light yield, comparable with the more standard production techniques such as extrusion, injection moulding. A strontium-90 source was used. Electrons of about 1-2 MeV were detected by a 3D-printed cube. An optical connector maximized the coupling between the cube and the silicon photomultiplier (Hamamatsu MultiPixel PhotoCounter). This preliminary test gives the first proof of principle of 3D-printing polystyrene-based plastic scintillator.

Figure 4: ⁹⁰Sr spectrum (photodetector photoelectron) of  FDM-printed cube

The next steps will try to confirm the obtained results by collecting a relatively large cosmic data sample. In the future the R&D will move toward the first SuperCube production, i.e. 3D-printing the three holes and the optical reflector. Moreover, other additive manufacturing techniques will be tested and compared to FDM. If the R&D will be successful, the technology will be ready for the future neutrino oscillation experiments as well as applications outside high-energy physics.

The EP-Neutrino group is playing a leading role both in the T2K near detector upgrade as well as in the DUNE near detector design and has now started a vibrant R&D effort to demonstrate the possibility to apply additive manufacturing to plastic scintillator. If successful, it will provide the possibility of building the first 3D-printed particle detector for a real experiment.

Further reading

[1] [LHCb Tracker] C. Joram et al., \LHCb Scintillating Fibre Tracker Engineering Design Review Report: Fibres, Mats and Modules", CERN-LHCb-PUB-2015-008, 03/2015

[2] V. Andreev et al., “A high-granularity plastic scintillator tile hadronic calorimeter with APD readout for a linear collider detector”, Nucl.Instrum.Meth.A564:144-154,2006

[3] M. Adinolfi et al., “The KLOE  electromagnetic calorimeter” Nuclear Instruments and Methods in Physics Research A 482 (2002) 364–386

[4] K. Abe et al. (T2K Collaboration), “Combined Analysis of Neutrino and Antineutrino Oscillations at T2K”, Phys. Rev. Lett. 118, 151801

[5] D.G. Michael et al., "The magnetized steel and scintillator calorimeters of the MINOS experiment," Fermilab-Pub-08-126, Nucl.Instrum.Meth.A596:190-228(2008), Issue 2, 1 November 2008, arXiv:0805.3170.

[6] L. Aliaga et al., “Design, Calibration and Performance of the MINERvA Detector”, Nucl. Inst. and Meth. A743 (2014) 130

[7] K. Abe et al. (T2K Collaboration), “Search for CP violation in Neutrino and Antineutrino Oscillations by the T2K Experiment with 2.2x1021 Protons on Target”, Phys.Rev.Lett. 121 (2018) no.17, 171802

[8] K. Abe et al., “Proposal for an Extended Run of T2K to 2.2x1021 POT”, arXiv:1609.04111

[9] K. Abe et al. (T2K Collaboration), “T2K ND280 Upgrade – Technical Design Report”, CERN-SPSC-2019-001 (SPSC-TDR-006)

[10] A. Blondel et al., “A fully-active fine-grained detector with three readout views”, JINST 13 (2018) no.02, P02006

[11] R. Acciarri et al. (DUNE Collaboration), “Long-Baseline Neutrino Facility (LBNF) and Deep Underground Neutrino Experiment (DUNE) Conceptual Design Report, Volume 1: The LBNF and DUNE Projects”, FERMILAB-DESIGN-2016-01, arXiv:1601.05471

[12] Y. Mishnay, “Three-dimensional printing of scintillating materials”, Rev.Sci.Instrum. 85 (2014) 085102, arXiv:1406.4817

[13] Y. Abreu et al. (SoLID Collaboration), “A novel segmented-scintillator antineutrino detector”, 2017 JINST 12 P04024