How can we design a solid-state plastic scintillator that remains operational in the extreme radiation field of a hot cell – and how can the same material expertise be used to build ultra-clean detectors for some of the rarest processes in particle and astroparticle physics?

At E66, we develop next-generation plastic scintillators from the molecular level upwards. Together with BGZ Gesellschaft für Zwischenlagerung mbH, Radiochemistry Munich (RCM) at TUM, and our national and international research partners, we are establishing a versatile platform for radiation-hard, high-efficiency, radiopure and functionalized solid-state polymer scintillators.Our research spans applications ranging from robust radiation detectors for highly radioactive materials, nuclear facilities and hot cells, through radiation-hard instrumentation for high-intensity accelerator and neutron beamlines, to ultra-low-background plastic scintillators for fundamental particle and astroparticle physics. Unlike liquid organic scintillators, our materials combine the optical and timing properties of organic scintillation with the mechanical robustness, handling advantages and geometric flexibility of a solid polymer detector.

The common principle behind all these activities is simple:

Control the chemistry, control the polymer – and tailor the plastic scintillator to the physics.

Plastic scintillators for extreme nuclear environments

The characterization of highly radioactive materials poses exceptional challenges for radiation detectors. Inside hot cells and nuclear facilities, detector materials may be exposed to intense gamma fields and significant neutron fluxes over extended periods. Conventional plastic scintillators can gradually lose optical transparency and scintillation light output as radiation-induced color centres and other defects accumulate in the polymer matrix.

Together with BGZ Gesellschaft für Zwischenlagerung mbH, we therefore develop radiation-hard plastic scintillators specifically for operation in these demanding environments. The objective is not merely to produce a detector with excellent initial performance. We want to develop solid polymer materials that retain their optical and scintillation properties even at very high accumulated radiation doses. This requires understanding radiation damage at its origin – in the chemistry and microscopic structure of the polymer itself.

Purity starts with the molecule

A central element of our approach is rigorous control over the complete production chain of the plastic scintillator.

Commercially available styrene contains polymerization inhibitors and other impurities. Even at very low concentrations, these compounds can affect the optical properties of the final polystyrene scintillator. We therefore purify the styrene monomer prior to polymerization using dedicated fractional vacuum distillation, drying and purification techniques. In our current process, this increases the attenuation length of the starting monomer at 430 nm from approximately 80 cm to about 10 metres, while the original polymerization inhibitor is no longer spectroscopically detectable. And our purification strategy does not stop with the monomer.

We are extending this approach towards the purification and careful selection of the other constituents of the plastic scintillator, including primary fluorescent molecules, wavelength shifters and functional additives. Together with the expertise of Radiochemistry Munich, this provides the foundation for controlling both chemical impurities and trace radioactive contaminants throughout the complete production chain. Purity is therefore not simply a prerequisite for optical transparency. It is one of the central technologies connecting our research on radiation hardness, high-performance plastic scintillators and radiopure detector materials.

Initiator-free polymerization for highly radiation-hard plastics

Our conventional polystyrene scintillators are produced using a carefully controlled pressurized cell-casting process. A key feature of this approach is that the purified styrene can be polymerized without the addition of conventional polymerization initiators. Polymer formation instead proceeds under carefully controlled conditions using a particularly gentle polymerization process. This minimizes the introduction of additional chemical species into the polymer and provides an exceptionally clean material system. For applications in nuclear facilities and at high-intensity accelerator beamlines, this approach is particularly promising. Chemical impurities, residual initiators and structural defects can influence the optical stability of polymer materials under intense irradiation.

We therefore investigate how the combination of

• high-purity starting materials
• initiator-free polymerization  
• controlled pressurized cell casting

can enable a new generation of plastic scintillators with exceptional resistance to radiation-induced ageing.

In close collaboration with Radiochemistry Munich (RCM), selected plastic scintillator formulations are subjected to systematic radiation-hardness studies reaching the several-hundred-kGy range. High-activity sources and hot-cell infrastructure at TUM provide access to radiation environments relevant to future applications at BGZ. After defined irradiation intervals, we characterize changes in optical transmission, scintillation light yield, emission spectra and scintillation kinetics. Our goal is not simply to determine when a plastic scintillator fails. We want to understand why radiation damage occurs – and engineer radiation hardness directly into the polymer.

Fast, bright and solid-state

Our first materials already demonstrate the potential of this approach. Using purified styrene and optimized combinations of p-terphenyl (pTP) and POPOP, we have produced highly transparent polystyrene-based plastic scintillators with light yields of approximately 13,500 photons/MeV and a fast dominant scintillation decay component of only 2.2 ns. The attenuation length of the resulting solid plastic scintillator reaches approximately 4.1 metres at 430 nm. Despite currently being produced only at laboratory scale, these materials already reach or exceed the performance of established commercial plastic scintillators. Their measured light yield is more than 50% higher than that of comparable commercial polystyrene-based reference materials investigated in our studies. The combination of high light yield, fast timing, mechanical robustness and radiation hardness makes solid-state plastic scintillators particularly attractive for detector systems operating in hot cells, nuclear facilities and intense particle beams.

Identifying neutrons in intense gamma backgrounds

Radiation hardness is only one part of the challenge. Measurements of highly radioactive materials can involve complex mixed radiation fields in which intense gamma backgrounds occur simultaneously with significant neutron fluxes. We therefore develop solid-state plastic scintillators with intrinsic Pulse Shape Discrimination (PSD).

Fast neutrons predominantly generate proton recoils in organic plastic scintillators, whereas gamma radiation primarily produces energetic electrons. These different excitation mechanisms can result in different temporal scintillation profiles. By controlling fluor composition, concentrations and molecular energy-transfer processes within the polymer, we aim to enhance these differences and develop radiation-hard plastic scintillators capable of event-by-event neutron–gamma discrimination.

Such a solid-state detector combines particle-identification capabilities traditionally associated with specialized organic scintillators with the robustness and simple handling of a plastic detector. The long-term goal is a solid, radiation-hard neutron detector capable of operating directly in intense gamma backgrounds.

From hot cells to high-intensity neutron beams

The fundamental response of our plastic scintillators is investigated under well-defined radiation conditions at the BELINA beamline of the Laboratori Nazionali di Legnaro (INFN-LNL), Italy. Pulsed quasi-monoenergetic neutron fields together with beam-correlated gamma radiation provide ideal conditions for studying neutron–gamma discrimination. Using Time-of-Flight (ToF), neutron- and gamma-induced interactions can be separated experimentally. In parallel, Time-Correlated Single Photon Counting (TCSPC) allows us to reconstruct temporal photon-emission profiles on the single-photon level with picosecond-scale resolution. These measurements provide direct access to the microscopic scintillation processes responsible for Pulse Shape Discrimination. Quasi-monoenergetic neutrons additionally allow us to investigate proton-recoil quenching and determine material-specific Birks parameters, providing essential input for realistic simulations of future neutron and gamma detector systems. For high-intensity beamlines, our plastic scintillators offer another important perspective: combining fast timing and high-rate capability with extreme resistance to accumulated radiation dose.

Functionalized plastic scintillators

Controlling the chemistry of the polymer allows us to go beyond conventional plastic scintillators. We investigate the incorporation of metals, selected isotopes and functional molecular complexes directly into the solid polymer matrix. For nuclear applications, high-Z elements such as bismuth or lead can increase the interaction probability for gamma radiation. Neutron-sensitive plastic scintillators can be produced by incorporating neutron-capturing isotopes or elements such as ⁶Li, ¹⁰B or gadolinium. Combined with Pulse Shape Discrimination, this opens a pathway towards compact multifunctional solid-state plastic scintillators capable of identifying and characterizing several components of complex radiation fields. Instead of surrounding a conventional detector with additional conversion materials, the desired interaction physics can therefore be incorporated directly into the active polymer.

From radiation hardness to radiopurity

At the opposite end of the experimental spectrum from a hot cell lies an ultra-low-background experiment. Instead of surviving an enormous radiation field, the detector must now contain extraordinarily little radioactivity itself. Experiments in particle and astroparticle physics searching for extremely rare processes can be limited by minute concentrations of naturally occurring radioactive contaminants in their detector materials.

The ability to control the complete plastic scintillator production chain therefore becomes equally important – but for a fundamentally different reason. At E66, we are extending our purification expertise towards the development of radiopure solid-state plastic scintillators produced from individually purified components. This includes not only dedicated purification of the styrene monomer, but also purification and careful selection of the primary fluors, wavelength shifters and other additives required to produce high-performance plastic scintillators.

Together with radiochemical expertise and material-screening techniques, our long-term objective is to control radioactive trace impurities throughout the complete production chain:

styrene monomer → primary fluor → wavelength shifter → functional additive → solid polymer scintillator

This approach gives us a degree of control that is difficult to achieve when relying on commercially available plastic scintillator materials with fixed compositions and production routes.

Radiopure plastic scintillators for astroparticle physics

This creates a direct connection between our materials programme and fundamental research in astroparticle physics.

For rare-event searches, detector performance is determined not only by light yield and energy resolution but by the radiopurity of every component from which the detector is made.

Producing the plastic scintillator ourselves allows us to select, purify and characterize these components before polymerization.

Our objective is to develop a new class of radiopure solid-state organic detectors combining:

• ultra-low intrinsic radioactive backgrounds
• high optical transparency
• high light yield
• long attenuation lengths
• controlled scintillation kinetics
• mechanical stability

Unlike liquid scintillator systems, these materials provide all of these properties in a self-supporting solid polymer matrix.

This makes radiopure plastic scintillators an intriguing platform for future low-background and rare-event experiments.

Double-beta isotopes inside a solid plastic detector

An especially exciting direction is the incorporation of double-beta-decay isotopes directly into radiopure plastic scintillators. In such a detector concept, the isotope under investigation is not introduced as a separate passive source. Instead, the isotope becomes part of the solid active detector material itself.

We are therefore interested in chemical approaches that allow suitable double-beta isotopes to be incorporated homogeneously into a polystyrene-based scintillator while maintaining optical transparency, scintillation efficiency and – critically – the radiopurity of the complete material. This is an interdisciplinary challenge at the interface of radiochemistry, coordination chemistry, polymer science, nuclear physics and detector physics. Suitable isotope-containing molecular complexes must be compatible with the polymer matrix and polymerization process. Their incorporation must not introduce excessive optical absorption or scintillation quenching, and all processing steps must be compatible with the stringent purity requirements of rare-event experiments.

The possibility to combine a radiopure solid-state plastic scintillator with a physics isotope embedded directly into its active volume opens an exciting research direction for double-beta-decay and other rare-event searches.

Active structural materials for rare-event detectors

Beyond incorporating double-beta-decay isotopes directly into the scintillator, we are exploring a second concept for radiopure plastic scintillators in rare-event experiments: turning passive structural components into active detector elements.

Low-background detectors such as high-purity germanium (HPGe) detectors and other detector systems for double-beta-decay searches require mechanical support structures in close proximity to the sensitive detector volume. Conventionally, these components are passive: an energy deposition inside a holder or structural element may therefore contribute to the experimental background without providing an independent signature that can be used to identify the event.

We aim to replace such passive components with radiopure scintillating polymer structures. A detector holder, support frame or other component manufactured from our plastic scintillator would retain its mechanical function while simultaneously acting as an active veto detector. Energy deposited in the support structure would produce scintillation light and could be detected in coincidence with the primary detector. Events associated with interactions in or close to the mechanical support could therefore be identified and rejected.

This concept is particularly attractive for experiments based on HPGe detectors and other highly sensitive rare-event detectors, where every material placed close to the active detector contributes to the overall background budget. Combining high radiopurity, scintillation capability and mechanical functionality in a single solid polymer material could reduce the amount of passive material surrounding the detector while adding an additional layer of background identification.

An important research direction will be the development of plastic scintillators that maintain their optical, scintillation and mechanical properties at cryogenic temperatures. If sufficiently radiopure and cryogenically stable materials can be realized, scintillating polymer structures could provide a powerful alternative to conventional passive metallic support components such as copper in selected detector concepts.

The ability to manufacture these materials through casting, machining and ultimately additive manufacturing provides an additional advantage: mechanically complex detector holders could be designed around the geometry of the experiment while simultaneously being optimized for efficient scintillation-light collection.

Our long-term vision is therefore to move beyond the distinction between detector and detector structure: the material holding the detector can itself become part of the detector.

Radiochemistry connects both extremes

Radiochemistry plays a central role across this research programme.

For our work with BGZ, Radiochemistry Munich provides expertise and infrastructure for exposing plastic scintillators to intense radiation fields and studying their behaviour under conditions relevant to hot cells and highly radioactive materials.

For our fundamental-physics programme, radiochemical expertise becomes equally important for the opposite challenge: purification, trace-contamination control, isotope chemistry and the development of radiopure plastic scintillators.

The same materials expertise therefore connects two extreme requirements:

A plastic scintillator stable enough to survive enormous radiation doses – and a plastic scintillator pure enough that its own radioactive background becomes vanishingly small.

3D-printed plastic scintillators: tailoring the geometry as well

The freedom to engineer the chemistry can be combined with another rapidly developing technology: additive manufacturing of plastic scintillators.

We are developing highly cross-linked scintillating polymer systems compatible with resin-based 3D printing.

This makes it possible to tailor not only the chemical composition but also the three-dimensional geometry of the active detector material.

For nuclear applications, customized geometries could allow solid plastic detectors to be integrated into constrained hot-cell environments or positioned around complex radioactive components.

For accelerator and particle-physics experiments, additive manufacturing could enable segmented, optically structured or application-specific active detector geometries that are difficult to realize using conventional casting and machining.

Ultimately, our aim is to design material composition, radiation response and detector geometry together.

One solid-state material platform – two extremes of radiation detection:

At first sight, a plastic scintillator operating next to highly radioactive material in a hot cell and an ultra-low-background detector searching for an extremely rare nuclear decay have little in common. For us, they represent two extremes of the same materials challenge.

• A detector for a hot cell must remain functional after an enormous amount of radiation exposure.
• A detector for rare-event physics must contain an extraordinarily small amount of unwanted radioactivity.

Both require exceptional control over the solid detector material.

For BGZ and nuclear applications, we focus on radiation hardness, long-term stability, mixed neutron/gamma fields and operation in hot-cell environments. For accelerator and beamline experiments, we combine radiation tolerance with fast timing, high light yield and high-rate capability. For particle and astroparticle physics, we develop radiopure plastic scintillators from purified starting components and explore metal- and isotope-loaded solid detector materials for rare-event searches.

From molecule to solid-state detector

• We purify the monomer.
• We purify the fluors and wavelength shifters.
• We control the polymerization.
• We tailor the scintillation chemistry.
• We incorporate selected metals and isotopes.
• We study radiation damage.
• We characterize the microscopic scintillation dynamics.
• And we design the plastic scintillator around the physics question.

Together with BGZ Gesellschaft für Zwischenlagerung mbH, Radiochemistry Munich (RCM) at TUM, INFN-LNL, and our other national and international collaborators, E66 is developing a new generation of tailored solid-state plastic scintillators spanning extreme nuclear radiation environments, accelerator physics and ultra-low-background fundamental research.

Radiation-hard when the radiation field is enormous. Radiopure when every single background event matters.

Involved Persons

• Dr. Hans Steiger
• Kilian Porth
• Konstantin Walter
• Maximilian Kolb