1. InvestigatingPeryleneasaSecondaryWavelength-shifter
forSNO+LiquidScintillator
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.