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.

M+

e-

M’

●

●

●●

●●●

S*F

uv photon emitted

MM

M

M

M

M

M M

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.

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*.

Interaction site

SF

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.

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

Source (Cs-137 or Co-60)

Scintillator chamber(holds 40 ml)

Wavelength shifting bars (BBQ) wrapped with reflective paper

PMT

Developing an Aqueous Scintillator for Neutrino DetectorsEmily Baldwina, Stephen Wiggintona, Mr. Paul Conrowa, and Professor Kevin McFarlandb

Scintillation Basics

Abstract Experimental Setup

Micelles and Mixing

Scintillation Cocktail

Experimental ResultsScintillation 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. 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.

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

-50

0

50

100

150

200

250

300

0 200 400 600 800 1000

Channel

Co

un

t

Toluene

Toluene w/ PPO

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

-100

0

100

200

300

400

500

0 200 400 600 800 1000

Channel

Co

un

t

no POPOP

0.05 g POPOP/L

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

-100

100

300

500

700

900

1100

0 200 400 600 800 1000

Channel

Co

un

t

Gel #1

Toluene with PPO

Solid Standard

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

t

Determine an averaged maximum count (1217.5)

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

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.

Conclusion

More Results

Emily and Stephen at work, taking measurements with the BETTY Daq.

b

a