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Developing an Aqueous Scintillator for Neutrino Detectors Emily Baldwin a , Stephen Wigginton a , Mr. Paul Conrow a , and Professor Kevin McFarland b. Abstract.
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Developing an Aqueous Scintillator for Neutrino Detectors Emily Baldwina, Stephen Wiggintona, Mr. Paul Conrowa, and Professor Kevin McFarlandb
Abstract Scintillation occurs when a gamma ray interacts with a molecule and through a series of steps produces light. The ultimate goal of this project is to make a water-based scintillator that can be produced on a metric ton scale, at a reasonable price. This scintillator will be used in a detector in future high-energy particle experiments. To make a water-based scintillator, a surfactant is needed to keep hydrophobic scintillator in solution with water. The surfactant forms micelles that keep water and scintillator apart while in solution. Organic liquid scintillators and a commercial solid scintillator have produced good preliminary results. Organic scintillators xylene, toluene, and 1,2,4 trimethylbenzene were used with PPO and POPOP in solution. The only water-based solution produced currently is a gel. The aqueous gel was made without POPOP and has given encouraging results. Future efforts will be concentrated on making better aqueous solutions.
Wavelength shifting bars (BBQ) wrapped with reflective paper PMT g Source (Cs-137 or Co-60) Scintillator chamber (holds 40 ml) Experimental Setup
Scintillation Basics M M+ M M g’ M M g M+ e- M M M M A 662,000 eV g-ray is sent into a scintillating substance with various molecules, M. The g-ray Compton scatters and ionizes one molecule before exiting the scintillator. An electron is ejected from the molecule with < 478,000 eV. The electron interacts with thousands of molecules in a few mm of sample. At each interaction, a molecule is excited with 5-10 eV as the electron loses an equivalent amount of its kinetic energy. Interaction site M+ ● ● e- ● ● ● ● ●
A scintillating molecule, S, may interact with the electron. Such an interaction may promote the scintillator to an excited state, S*. S* F (Typical Scintillator) The solvent transfers its excitation energy to the fluorophore, F. S F* (Primary Fluorophore) The fluorophore emits a 4 eV photon. S F uv photon emitted
Micelles and Mixing Micelles are dynamic molecular assemblies that we use to solubilize organic scintillator within an aqueous environment. The surfactant molecules that make micelles have polar heads that interact favorably with water. Surfactant molecules have nonpolar tails that form the middle of a micelle. The micelle interior is where organic scintillator is driven and made soluble.
Scintillation Cocktail or Primary Scintillator Toluene or 1,2,4-trimethylbenzene Surfactant (Igepal CO-630) nonylphenol ethoxylate Primary Fluorophore (PPO) 2,5-diphenyloxazole Secondary Fluorophore (POPOP) 1,4-bis(5-phenyl-2-oxazolyl)benzene
Experimental Results A method was developed to reliably compare results from one sample to another. The peak maximum is too broad and noisy to use. Instead, the channel at half the peak maximum is used. This channel, along the Compton edge, is effected by the energy of the gamma source, the voltage of the across the PMT, and the components of the scintillating mixture.
An early experiment tested the effect of PPO on the signal. PPO is the primary fluorophore in our liquid scintillation experiments. With PPO, there is sn increase in signal and the Compton edge is shifted to the right.
A transparent gel made of 50% water, 25% surfactant, and 25% scintillator, by volume, was studied. The gel gives a weak signal, compared to both toluene and the solid standard. However, we are in the early phases of making an aqueous scintillator. Better performance is a goal of the future.
POPOP shifts the light that is emitted by PPO, the primary fluorophore. With POPOP added in small amounts, the signal dramatically shifts to a broader spectrum with a higher channel Compton edge.
Emily and Stephen at work, taking measurements with the BETTY Daq.
Conclusion We have established the primary components that must be included in the scintillating mixture. Qualitative results have convinced us that PPO, POPOP, and a suitable scintillator are needed. POPOP is difficult to dissolve and a derivative may be substituted. Surfactants have been used to solubilize the organic scintillator molecules in a water-based gel. Future work will focus on development of a high performing aqueous liquid or gel that will scintillate and remain stable for weeks at a time.
