# Fast-, Light-Cured Scintillating Plastic for 3D-Printing Applications
## 2 Materials and Methods  
### Scintillator Development  
The formulation of the resin was based on five basic components; a monomer, a crosslinker, a photo-initiator to induce photopolymerization, and primary and secondary fluorescent dyes to produce light from radiation excitation. Several compounds were evaluated as part of the formulation development for each component. In developing a 3D printable scintillator resin, the monomer forms the base of the resin and is the largest component by mass. The monomer needs to have good optical clarity at the scintillation wavelength, exhibit high solubility for fluorescent dyes, and not lead to the quenching of the scintillation light. Monomers, such as vinyltoluene or styrene, which are primarily used in plastic scintillators [14, 15], were found to exhibit low reactivity during photopolymerization [12, 16, 17]. Attempts to use these types of monomers resulted in cure times of \(>\)30 min to multiple hours, which is not acceptable for 3D-printing applications. Methacrylate and acrylate compounds exhibited much higher reactivity, on the order of 10s of seconds, which is suitable for 3D-printing applications. Attempts to find an off-the-shelf aromatic methacrylate or acrylate were unsuccessful. Compounds such as ethylene glycol phenyl ether acrylate, phenyl acrylate, or benzyl methacrylate resulted in either poor hardness, making mechanical polishing difficult, or poor light yield, as was the case for benzyl methacrylate which was \(<\)1000 photons per MeVee [18]. In lieu of a suitable aromatic methacrylate, or acrylate monomer, the compound isobornyl acrylate (IBOA) was chosen due to its optical clarity, low cost, high boiling point, and high glass transition temperature. For this work, \(\geq\)85% technical-grade IBOA from Sigma Aldrich with 200 ppm monomethyl ether hydroquinone inhibitor was used. Crosslinkers are needed to increase hardness and decrease the gel time of the photocured resins. Multiple crosslinkers were tested during this work, such as difunctionized 1,6-hexanediol dimethacrylate (HDDMA), bisphenol-A dimethacrylate (BPADMA), divinylbenzene (DVB), and tetrafunctionized crosslinkers, such as pentaerythnitol tetraacrylate (PETA). The HDDMA was \(\geq\)90% pure and contained 100 ppm hydroquinone as an inhibitor, BPADMA was \(>\)98% pure with no inhibitor, PETA contained 350 ppm monomethyl ether hydroquinone as the inhibitor, and DVB was 80% pure. The photo-initiator used to start the polymerization from incident 405 nm light was TPO from Sigma Aldrich, which was 97% pure.All compounds were used as received. For the production of light yield in the plastic scintillators, the solvents and fluors tested were EJ-309, diisopropyl naphthalene (DIN) from Eljen Technology (Sweetwater, TX, USA) (base EJ-309 solvent with no fluors), and PPO from Sigma Aldrich, which was of scintillation grade and 99% pure.1,4-Bis(2-methylstyryl)benzene (Bis-MSB) from Sigma Aldrich (St. Louis, MO, USA), diphenyl tsilbene (DPS) from Luxotica Exciton (Lockbourne, OH, USA), and Exalite 416 from Luxottica Exciton were evaluated as wavelength shifters. The compounds for the desired formulation to be tested were weighed and placed into a 28 \(\times\) 57 mm (20 mL) reaction vial. The compounds were thoroughly mixed together using an alternating combination of an ultrasonic bath and hot plate to gently warm the mixture. To reduce oxygen-induced quenching, the reaction vial was transferred to a glove box pressurized with a nitrogen atmosphere (\(<\)0.1 ppm O\({}_{2}\) and \(<\)1 ppm H\({}_{2}\)O). Dissolved oxygen was removed by sparging with nitrogen for 10 min, and the reaction vial was sealed before removal from the glove box. During this process, care was taken to minimize exposure to ambient light, as the resin polymerizes with light.