Developing an Aqueous Scintillator

for Neutrino Detectors

Emily Baldwina, Stephen Wiggintona, Mr. Paul

Conrowa, and Professor Kevin McFarlandb

AbstractScintillation 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.

Source (Cs-137 or Co-60)

Scintillator chamber(holds 40 ml)

Wavelength shifting bars (BBQ) wrapped with reflective paper

PMT

Experimental Setup

M+

e-

M’

●●

●●

●●●

MM

M

M

M

M

MM

M+

M+

e-

A 662,000 eV -ray is sent into a scintillating substance with various molecules, M.

The 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

Scintillation Basics

S*F

uv photon emitted

SF*

The fluorophore emits a 4 eV photon.

The solvent transfers its excitation energy to the fluorophore, F.

A scintillating molecule, S, may interact with the electron. Such an interaction may promote the scintillator to an excited state, S*.

S

F

O

N

CH3

CH3

H3C

(Primary Fluorophore)

(Typical Scintillator)

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.

OH

H

OH

H

OH

H

OH

H

OH

H

OH

HO

H H

OH

H

OH

H

OHH

OHH

~25 nm

Surfactant molecules organize to form micelles.

Scintillator is driven tothe hydrocarbon core.

Micelles and Mixing

Secondary Fluorophore (POPOP)1,4-bis(5-phenyl-2-oxazolyl)benzene

Primary Fluorophore (PPO)2,5-diphenyloxazole

Surfactant (Igepal CO-630) nonylphenol ethoxylate

Primary Scintillator Toluene or 1,2,4-trimethylbenzene

CH3

CH3

H3C

CH2

H2C

O

O

~10

H

O

NN

O O

N

CH3

or

Scintillation Cocktail

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.

Typical Data Analysis of a Scintillation Run(Solid Standard with Cs-137 source)

-100

300

700

1100

1500

0 200 400 600 800 1000

Channel

Co

un

tDetermine an averaged maximum count (1217.5)

Find the channel at half the maximum count (channel 454)

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.

Effect of PPO on Scintillation of Toluene with Cs-137 source

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0

50

100

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200

250

300

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Channel

Co

un

tToluene

Toluene w/ PPO

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.

Scintillation Results from Early Gel and Organic Scintillator Studies (Cs-137 Source)

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300

500

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1100

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Channel

Co

un

tGel #1

Toluene with PPO

Solid Standard

Effect of POPOP on Scintillation of 5.0 g PPO/L Toluene with a Co-60 Source

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Channel

Co

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tno POPOP

0.05 g POPOP/L

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.

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.

Conclusion