InvestigatingPeryleneasaSecondaryWavelength-shifterforSNO+LiquidScintillator

Jennifer MauelQueen’s University

Department of Physics, Engineering Physics and Astronomy

IntroductionThe Experiment

SNO+ is a kilo-tonne scale liquid scintillator experimentlocated 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 lightemitted 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 130Tecombined with a surfactant, a primary wavelength shifterPPO and a secondary wavelength shifter.

Perylene and bis-MSB are the mains candidates for asecondary wavelength-shifter. The goal of this study wasto measure the optical absorption and emission propertiesof perylene to predict its performance in the SNO+ cock-tail.

Experimental ProcedureSamples of perylene mixed in optically inactive liquids

(dodecane and cyclohexane) were prepared for concentra-tions from 1g/L to 1mg/L. Emission spectra were obtainedfor each concentration using a PTI steady state fluores-cence spectrometer. One measures the emission spectrumof a sample by selecting an excitation wavelength, thewavelength of light absorbed by the sample, and the inten-sity of fluorescence at each emission wavelength is countedby 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 SpectrumFig.1 presents the emission spectra produced by pery-

lene concentrations from 1g/L to 1mg/L, and for excitationwavelengths between 313nm up to 450nm. The excitationwavelengths correspond to those in the PPO emission spec-trum, which naturally overlaps with perylene’s absorptionspectrum.

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 spectralshape is constant for each concentration. In fact, for pery-lene and most fluorophores fluorescence intensity scaleswith 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 moleculesin 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 electronsdispersed uniformly throughout the AV. We compare thenumber of PMT hits (nhits) per MeV electron using an oldperylene emission spectrum and the new emission data col-lected. Using the new emission spectrum shifts the meanby ∼10 nhits/MeV. This result is likely due to the fact thatthe new emission spectrum is positioned at shorter wave-lengths where the PMTs have a higher quantum efficiency.

ConclusionsPrecisely calibrated measurements of the emission spec-

trum can now be used in simulations to predict the lightoutput of SNO+ liquid scintillator cocktail with peryleneas 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 thereemission probability which was calculated from the data.

Despite high uncertainty in extinction measurements,simulations predict that absorption from perylene in thisregion has a limited impact on scintillator light output.

Perylene at Long Wavelengths

Two-photon Absorption and Re-emissionProbability 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. Singlephotons are too low-energy at these wavelengths to excitefluorescence in perylene, so the most likely source of fluo-rescence is two-photon absorption (2PA).

Figure 2: Since the normalized emission spectra have the sameshape, the emission must be due to fluorescence rather thaninelastic Raman scattering.

2PA is a non-linear optical process where two pho-tons are simultaneously absorbed by the fluorophoremolecule. As a result, the reemission probability will havea wavelength-dependence [2].

Optical Absorption at Long Wavelengths

To investigate the impact of perylene absorption atlong 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 peryleneabsorption at long wavelengths. Again we measure theshift in the mean nhits/MeV, which would indicate thatincreased absorption from perylene is diminishing the lightyield of the scintillator.

Simulation Results: When we increase absorption fromperylene at long wavelengths, this appears to have only avery small impact on the mean nhits/MeV until the largestscaling factors ( 1

40 − 1100 ). Therefore there is rather wide

berth for error in these measurements at long wavelengths.