Investigating Perylene as a Secondary Wavelength-shifter for SNO+ Liquid Scintillator Jennifer Mauel Queen’s University Department of Physics, Engineering Physics and Astronomy Introduction The Experiment SNO+ is a kilo-tonne scale liquid scintillator experiment located at the Sudbury Neutrino Observatory (SNOLAB) just over 2km underground at in Vale’s Creighton Mine. The primary goal of SNO+ will be to search for neutrino- less double beta decay. The detector is a 12m diameter acrylic sphere (AV) con- tained within a steel PMT support sphere (PSUP). Ap- proximately 10 000 PMTs surround the AV to capture light emitted by particle interactions in the liquid scintillator. A Secondary Wavelength-shifter in the SNO+ Cocktail During the search for neutrinoless double beta decay, the AV will be loaded with a cocktail of linear alkylben- zene (LAB) liquid scintillator, a 0νββ decay isotope 130 Te combined with a surfactant, a primary wavelength shifter PPO and a secondary wavelength shifter. Perylene and bis-MSB are the mains candidates for a secondary wavelength-shifter. The goal of this study was to measure the optical absorption and emission properties of perylene to predict its performance in the SNO+ cock- tail. Experimental Procedure Samples of perylene mixed in optically inactive liquids (dodecane and cyclohexane) were prepared for concentra- tions from 1g/L to 1mg/L. Emission spectra were obtained for each concentration using a PTI steady state fluores- cence spectrometer. One measures the emission spectrum of a sample by selecting an excitation wavelength, the wavelength of light absorbed by the sample, and the inten- sity of fluorescence at each emission wavelength is counted by a PMT [1]. References [1] Photon Technology International. Quantamaster 300: Phospho- rescence/fluorescence spectrophotometer. 2014. [2] B. Valeur. Molecular fluorescence: Principles and applications. Wiley-VCH Verlag GmbH, 2001. [3] M. Johnson. Scintillator purification and study of light propaga- tion in a large scale liquid scintillation detector. June. The Perylene Emission Spectrum Fig.1 presents the emission spectra produced by pery- lene concentrations from 1g/L to 1mg/L, and for excitation wavelengths between 313nm up to 450nm. The excitation wavelengths correspond to those in the PPO emission spec- trum, which naturally overlaps with perylene’s absorption spectrum. Figure 1 The Impact of the Excitation Wavelength Emission scans demonstrate that the excitation wave- length impacts only the fluorescence intensity (the inte- grated spectral intensity). This is clear when we normal- ize the emission spectra, where we find that the spectral shape is constant for each concentration. In fact, for pery- lene and most fluorophores fluorescence intensity scales with the absorption probability for the particular excita- tion wavelength [2]. The Effect of Sample Concentration The concentration of the solution impacts both the in- tensity of fluorescence and the shape of the emission spec- trum. The intensity of fluorescence scales with the concen- tration, proportional to the number of perylene molecules in the sample. The changing shape of the emission spec- trum is due to self-absorption of fluorescence and non- uniform absorption of the incident beam at higher con- centrations [3]. Performance in Simulations To measure the impact of perylene on the light out- put of the liquid scintillator, we simulate 1 MeV electrons dispersed uniformly throughout the AV. We compare the number of PMT hits (nhits) per MeV electron using an old perylene emission spectrum and the new emission data col- lected. Using the new emission spectrum shifts the mean by ∼10 nhits/MeV. This result is likely due to the fact that the new emission spectrum is positioned at shorter wave- lengths where the PMTs have a higher quantum efficiency. Conclusions Precisely calibrated measurements of the emission spec- trum can now be used in simulations to predict the light output of SNO+ liquid scintillator cocktail with perylene as a secondary fluor. Emission scans at long excitation wavelengths demon- strate that perylene may have a significant 2PA cross- section. This results in a wavelength dependence of the reemission probability which was calculated from the data. Despite high uncertainty in extinction measurements, simulations predict that absorption from perylene in this region has a limited impact on scintillator light output. Perylene at Long Wavelengths Two-photon Absorption and Re-emission Probability at Long Wavelengths Emission scans of perylene at long excitation wave- lengths ≥440nm demonstrate that perylene emits signif- icant fluorescence when excited at low energies. Single photons are too low-energy at these wavelengths to excite fluorescence in perylene, so the most likely source of fluo- rescence is two-photon absorption (2PA). Figure 2: Since the normalized emission spectra have the same shape, the emission must be due to fluorescence rather than inelastic Raman scattering. 2PA is a non-linear optical process where two pho- tons are simultaneously absorbed by the fluorophore molecule. As a result, the reemission probability will have a wavelength-dependence [2]. Optical Absorption at Long Wavelengths To investigate the impact of perylene absorption at long wavelengths on the scintillator light yield, we simu- late 1 MeV electrons in the AV loaded with perylene liq- uid scintillator with various increasing levels of perylene absorption at long wavelengths. Again we measure the shift in the mean nhits/MeV, which would indicate that increased absorption from perylene is diminishing the light yield of the scintillator. Simulation Results: When we increase absorption from perylene at long wavelengths, this appears to have only a very small impact on the mean nhits/MeV until the largest scaling factors ( 1 40 - 1 100 ). Therefore there is rather wide berth for error in these measurements at long wavelengths.