# Fast-, Light-Cured Scintillating Plastic for 3D-Printing Applications
## 1 Introduction  
Plastic scintillators are among the most common radiation detection materials used due to their low costs, ease of fabrication, fast response time, and ability to distinguish between neutron and gamma-ray interactions [1]. Plastic scintillators are useful for detector applications relating to national security, radiation therapy, and basic science [2, 3]. The application of additive manufacturing techniques to plastic scintillators offers the potential for new and exciting uses. For instance, 3D printing allows for the fabrication of complex geometries and multi-material composites that are either impossible or cost prohibitive to produce using conventional methods.  
Successful 3D printing of plastic scintillators requires the development of a printing process as well as suitable feedstock material that is similar in cost to a conventional scintillator and has mechanical, chemical, and optical properties suitable for the 3D-printing process. The combination of the feedstock and printing process will need to result in printed parts having mechanical, optical, and scintillation (light yield and pulse shape discrimination, or PSD) performance near or exceeding that of conventional plastic scintillators.  
There is a wide range of 3D printing methods available, including filament-based methods, such as fusion deposition modeling (also called fused filament fabrication), powder-based laser sintering, and light-based resin curing [4]. Of the common 3D printing methods, light-based methods--that is, methods based on photopolymerization of a liquidresin--such as stereolithography (SLA) and digital light processing (DLP) offer the best optical clarity [5]. A few researchers have developed formulations of scintillating resins that either have or can be used in light-based 3D printers, but the light yield of these scintillators was low (\(\sim\)35% of commercially available plastic scintillators) and PSD was not reported [6, 7, 8]. Other recent research reported a DLP 3D-printed scintillator with a light yield of up to 67% of BC-408 plastic scintillators. This formulation used bisphenol A ethoxylate dimethacrylate (BPA(EO)\({}_{1}\)5 DMA) as the primary solvent, 1-methyl-naphthalene as the secondary solvent, 2,5-diphenyloxazole (PPO) as the primary dye, 1,4-bis(9-ethyl-3-carbazo-vinylene)-9,9-dihexyl-fluorene (ADS086 BE) as the wavelength shifter, and diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) as the photoinitiator [9, 10]. However, like the other light-based, 3D-printable formulations, PSD performance was not reported. Recent research was successful in producing a DLP 3D-printed plastic scintillator sample with a reported PSD FoM of 1.73 \(\pm\) 0.02 from 300-1000 keVee using 30% PPO [11]. Another novel, light-cured, PSD plastic scintillator was recently reported in [12], but the required cure time of tens of minutes makes its use in 3D printing impractical, where cure times of tens of seconds or less are required for rapid prototyping and manufacturing.  
The goal of this work was to create a mechanically hard, optically clear, 3D-printable (i.e., DLP or SLA), PSD plastic scintillator with a light yield comparable to commercially available PSD plastic scintillators, i.e., EJ-276 (8600 photons per 1 MeVee) [13]. This resulted in the development of a light-based, PSD-capable plastic scintillator formulation that cures in seconds using industry-standard 405 nm light and is suitable for the 3D printing of radiation detectors. In particular, this work reports the development of the fast-curing formulation, the material properties and radiation detection performance of the resulting light-cured scintillators, a parametrically optimized formulation, and an initial aging study.