# Fast-, Light-Cured Scintillating Plastic for 3D-Printing Applications
## 3 Results and Discussion
### Observational Characterization
#### 3.2.1 Purpling  
It was observed that some of the scintillators became purple upon exposure to the cure light. The purple color intensity and duration increased with the amount of PPO, as shown in Figure 7 of AFIT025 and AFIT026, which contained 20 wt% and 25 wt% PPO, respectively. This purple color did dissipate with time, with the decay time varying from being on the order of minutes to a few weeks. The decay time for the purpling to dissipate increased for increasing PPO concentration and increasing BPADMA concentration, as illustrated in Figure 7, where the purple color in AFIT113 (21 wt% BPADMA) can still be seen after 3 weeks.  
The direct cause of the purpling effect was not determined, but it appears to be related to an interaction between PPO and TPO. Methods to accelerate the dissipation were explored, the most promising being a heat treatment. To test this treatment, three scintillators were made with the same formulation as AFIT101. One was placed in a 4 \({}^{\circ}\)C refrigerator, one was left on the counter at room temperature, and another was placed in a 60 \({}^{\circ}\)C oven. After a couple of hours, the scintillator from the refrigerator and  
Figure 7: The intensity of the purple color observed increased with increased PPO concentration as illustrated by (**a**) AFIT025 (20 wt% PPO) and (**b**) AFIT026 (25 wt% PPO). The duration of the purpling, which varied based on the PPO and BPADMA concentration, faded away over time as illustrated in (**c**) AFIT113 (30 wt% PPO and 21 wt% BPADMA) shortly after being cured and (**d**) 3 weeks after being cured.  
room temperature maintained similar levels of purpling. However, the purple color in the scintillator from the oven had significantly faded, indicating that a post-cure heat treatment can be used to accelerate the dissipation of the purpling effect and obtain steady-state performance.  
The scintillator light yield was reduced while purpling was observed in the scintillator, even for the slight purpling shown in Figure 8. When there was still purpling observed in AFIT230A (30 wt% PPO, 0 wt% DIN), Figure 8a, the relative light yield was 67%. After the purpling faded away, Figure 8b, the relative light yield increased to 71%. The difference increased as the amount of purpling present increased.  
The light emission of AFIT131 was measured after the plastic was completely cured to better characterize the post-cure light emission. Measurements of the emission spectrum of the scintillator were taken for up to 7 h post-cure, while being excited at 350 and 380 nm, asynchronously, using a Horiba FluoroMax Plus. Figure 9a plots the emission spectrum of the plastic scintillator as a function of time after the curing process from an excitation wavelength of 380 nm. The light emission was fairly constant for the first 90 min post cure and then increased, as plotted in Figure 9b. This increase in the peak emission did not stop at 7 h post-cure. This is attributed to the dissipation of the purpling in the scintillator, and the timescales correlate well to significant observational reductions in scintillator purpling.  
Figure 8: Purpling observed in AFIT230A (**a**) 6 days after being produced but not at (**b**) 12 days.  
Figure 9: (**a**) Light emission spectra and (**b**) peak light emission from AFIT131 using a Horiba FluoroMax Plus with an excitation wavelength of 380 nm. There was no shift in the peak emission wavelength over the duration of the measurements.