MEASUREMENT OF TRITIATED WATER

WITH HIGH EFFICIENCY

BY USING LSC

WITH PLASTIC SCINTILLATOR

Etsuko FURUTA, Ryu-ichiro OHYAMA , Shigeaki YOKOTA, Toshiya NAKAJO,

Yuka YAMADA, Takao KAWANO, Tatsuhiko UDA and Yasuo WATANABE

Ochanomizu University

Japan

Tritiated Water；HTO

Tritium exists in our environment.

Tritium is mainly produced by cosmic rays.

Also, for a development of the nuclear fusion in future, it is necessary to check a leak of the tritiated water from a reactor.

When we measure a concentration of tritium in an environmental sample, the sample is combusted, and each compound of tritium changes its chemical structure into tritiated water.

So measurement of tritiated water is required.

The measurement of tritiated water in environment is already carried out now. However, a long time measurement with a low-back LSC is necessary because the concentration of tritiated water in the environment is very low.

So a detection method with a high measurement efficiency is necessary for tritiated water.

The measurement of tritiated water with a LSC using liquid scintillator is the most effective.

Though a liquid scintillator becomes the radioactive waste fluid in each measurement, there is no alternative method for measurement tritiated water by a LSC with liquid scintillator.

A measurement method with high efficiency for tritiated water which does not produce the radioactive waste fluid of liquid scintillator is necessary.

Environmental load !!

Difference of the measurement efficiencies between PS and liquid scintillator by LSC.

The case of low energy beta emitters of 14C and 3H, which are used frequently as tracers. 14C (Emax=156 keV); approximately the same. 3H (Emax=18.6 keV); the efficiency of the PS was not more than the half of the liquid scintillator’s.

Though the efficiency of 3H is low, to use the PS is good because of no radioactive waste fluid.

However, this PS sheet method cannot use for measurement of tritiated water.

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Liquid-3H Liquid-14C P-D-3H P-D-14C

The solution of methionine was put on a PS sheet, dried and covered with another PS sheet.

LSC-2008; plastic scintillator

PS sheet method

What is the method to measure tritiated water in high efficiency by using plastic scintillator with LSC without being dried ?

1. We tried to increase the contact area of PS for

tritiated water. So the solution volume maybe

increase.

→ Not a PS sheet, but a cylindrical

PS with many holes.

Unfortunately, this method did not work well because

the solution (water) was working to decrease the

fluorescence emitted.

100%> approx. 35% (14C; Glysine)

What is the method to measure tritiated water in high efficiency by using plastic scintillator with LSC without being dried ?

2. The tritium solution was modified as if it was liquid scintillator.

→ to add a small amount of hand-soap

→ to add acid or alkaline

→ to add a small amount of alcohol

As a result of these methods, the measurement efficiency increased a little.

However, it was not enough to measure the environmental tritiated water.

Characteristic of plastic

The feature is that the plastic is hydrophobic.

Processing to hydrophilize of plastic is necessary.

To make an oxidation layer is common method for

hydrophilizing the surface of the target.

ex. KMnO4, TiO2, etc.

However, when the oxidation layer is prepared onto

the PS surface, they absorb the beta-ray energy of 3H

and tritium can not be measured by this method.

Purposes of this study

To propose a method for getting a high

measurement efficiency for tritiated water by

using plastic scintillator.

To clarify the effect of the method.

To achieve using little amount of liquid scintillator

Surface-treatment for plastic scintillator

Atmospheric Pressure Glow Plasma Processing

A high-frequency impulse voltage source

(Haidenlaboratory, PHF-2K-20)

The peak applied voltage：10 kV

High frequency：10 kHz

The Ar gas flow rate：4 L min-1

The contact area of the liquid

sample was spread more than

4 times on the PS treated

with the plasma method.

Grounded ring electrode (width 10 mm)

Gap 1 mm

Silica glass tube

Ar gas

Room air

HV source

60 mm

(The plasma method)

Schematics of experimental set-up and optical

image of plasma jet.

Gas flow tube combined with discharge electrode

5mm The surface is

treated by

the plasma

method.

No surface

treatment.

J. Phys. D: Appl. Phys. 42

(2009) 105203 (4pp)

PS sheet

Variation of the surface of the PS

by the plasma method

AFM condition; from 300 nm height

BG; Untreated surface of PS Treated surface of PS;1min

Effect of the plasma method of PS

to tritiated water measurement

P-D-;

plasma double

(sandwiched PS)

P-S-;

Plasma single

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Measurment efficiency of tritiated water by LSC

Liquid No-Single No-Double P-S-3min P-S-2min

P-S-1min P-D-3min P-D-2min P-D-1min

The amount of each sample was 5 μL.

Schematic diagram of treatment of

PS with plasma

1 min; Less exposure time is not enough to treat the surface with plasma.

2 min; Just right time exposure makes the best surface irregularities.

3 min; Too much exposure is no good for treatment the surface with plasma.

Treatment of PS with plasma

Before

1 min

2 min

3 min

Relationship between radioactivity and count rate on the plasma treated PS

The quantifiability by the plasma method is confirmed.

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Co

un

ts p

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Activity (Bq)

Detection limits by the plasma method

where k is a coverage factor (3; 99.7%), nB is the background counts of PS (8.77counts)

and the ts and tB are measurement time of the sample and background, respectively. (2 min)

nD = k2/2[1/tS+{1/ tS2+4nB/k2(1/ tS +1/tB)}1/2]

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Bq

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Measurement time of sample (min)

20μL loaded

2μL loaded

5μL loaded

20 μL tritiated water

15.6 Bq mL-1;2min

Results

Beta-ray measurement with a high efficiency by LSC

was possible with plastic scintillator by treating the

atmospheric pressure glow plasma processing (the

plasma method) for tritiated water.

Radioactive waste fluid hardly produces by the plasma

method. Therefore, we can save our expense and time

which is using for disposal of radioactive waste fluid.

The plastic scintillator does not become hazardous

wastes for the human body and is low environmental

load.

Summary

Measurement of tritiated water with high efficiency by normal plastic scintillator (PS) by using LSC is difficult because of hydrophobic surface of the PS.

Surface treatment of PS with Atmospheric Pressure Glow Plasma Processing (the plasma method) is useful for increasing contact area of samples to the PS.

When the plasma was exposed for the surface of PS in 2 min, the tritiated water of 5 μL was put on them, then covered with another PS treated, the measurement efficiency of tritiated water was almost same as that of using liquid scintillator; approx. 50%.

The detection limit of PS treated by the plasma method was 15.6 Bq mL-1 in 2 min measurement by LSC, when 20 μL of tritiated water was applied to the PS.

A short measurement time is available by the plasma method. For example, 2 min measurement is enough.

Using PS with the plasma method to measure beta-emitters is eco-friendly because of little radioactive liquid waste fluid.

Thank you for your attention !!