# Fast-, Light-Cured Scintillating Plastic for 3D-Printing Applications
## 2 Materials and Methods
### Exploratory Formulation Studies  
A step-by-step, trial-and-error approach was taken to identify promising combinations to achieve a formulation that would create a mechanically hard, optically clear plastic with high light yield and neutron/gamma-ray discrimination. While not an exhaustive search of the parameter space, the series of small studies led to a promising general formulation that was then parametrically optimized as described in Section 2.4. All samples were approximately 10 g (\(\sim\)2.5 cm diameter and \(\sim\)1.6 cm thick), and the formulations, Table A1 and Table A2, are shown in Appendix A for the key steps and formulations described below. Figure 3: 2D PSD/energy histogram from exposing (**a**) AFIT101 and (**c**) EJ-276 to an AmBe source. A cut was made over the 450 and 550 keVee range, represented by the red lines, to generate a 1D AmBe PSD histogram, where a double Gaussian fit was used to calculate the PSD FoM according to Equation (2) for (**b**) AFIT101 and (**d**) EJ-276. The colorbar represents the number of counts. The first study explored the compatibility of different wave shifters with the 405 nm cure light and primary solvents. Three samples (AFIT002-004) were created with 0.2 wt% of Bis-MSB, Exalite 416, and DPS, respectively. The other chemicals in the scintillators were kept constant for all three. The DPS did not dissolve into the solution. The scintillator with Bis-MSB was solid throughout but softer in the center, while the Exalite 416 formulation was consistently hard throughout. It was determined that the Bis-MSB had a higher absorption at 405 nm, shown in Figure 4, preventing the light from penetrating the entire depth of the resin and obtaining a complete cure. Therefore, Exalite 416 was selected as the wave shifter for all future formulations. In the next step, the ratio of plastic-forming to light-yielding compounds was switched to 60/40 to increase the light yield of the scintillator. Reduced crosslinker concentration had previously been observed to result in soft, gummy-like cures, while high crosslinker concentrations led to cloudy scintillators. Therefore, the solid BPADMA was replaced with liquid HDDMA to explore formulations that lead to a harder, clearer plastic since the liquid HDDMA better dissolved and did not precipitate out of the solution. Three formulations with different IBOA/HDDMA ratios (70/30, 60/40, and 50/50) were explored. The formulations with the 60/40 and 50/50 IBOA/HDDMA ratios (AFIT013 and 014) yielded cloudy scintillators. The formulation with the 70/30 IBOA/HDDMA ratio (AFIT009) was clear but too soft, similar to hard rubber. PETA and DVB crosslinkers were also tried but did not provide better results than HDDMA or BPADMA. Additionally, all attempts to use DVB or any vinyl monomer significantly increased the required cure time. Therefore, an IBOA/HDDMA ratio of 70/30 was used for future formulations, and variations were made to improve the hardness and radiation detection performance. First, the amount of light-yielding compounds was reduced to improve the hardness of the plastic. Two formulations (AFIT016 and 017) were developed with 30 wt% (70 wt% IBOA and HDDMA) and 20 wt% (80 wt% IBOA and HDDMA) light-yielding compounds. The Exalite 416 and TPO were held the same at 0.2 wt% and 0.1 wt%, respectively. Second, the amount of Exalite 416 was doubled from 0.2 wt% (AFIT016) to 0.4 wt% (AFIT022). Third, an approximately 50/50 ratio of HDDMA and BPADMA (AFIT023) was tested as the crosslinker. Each of these was sequentially cured for 4-5 min in a Formlabs Form Cure, followed by 1 min in the Dymax BlueWave AX-550 VisiCure, and then sanded and polished. Figure 4: The light absorption of the secondary dyes, Exalite 416 (purple) and Bis-MSB (red), and of the photoinitiator, TPO (black), compared to the measured emission spectrum of the 405 nm cure light (blue).