JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

4-30
Observation of Heavy Ion Induced Transient Species in
Water by Spectroscopic Technique
M. Taguchi a) , Y. Sugo b) , S. Kurashima c) , A. Kimura a) and K. Hirota a)
a) Environment and Industrial Materials Research Division, QuBS, JAEA,
b) Division of Fuels and Materials Engineering, NSED, JAEA,
c) Department of Advanced Radiation Technology, TARRI, JAEA
Introduction
High energy heavy ions induce unique irradiation effects,
which are differ from those induced by low LET radiations,
such as -rays or electron beam. These effects are induced
by reactions of reactive species generated heterogeneously
and densely around its trajectory in medium, and used as a
new tool for the various basic and application studies for
material and biological sciences. We decide water samples
as a first target because it is the main component of living
body, and radiation-induced reactions in water are well
understood under low LET radiation. Hydroxyl(OH)
radical is the most important species for reactions in water
samples because of its high reactivity and formation yield.
1, 2)
In a previous paper , we decided experimentally the
formation yield of the OH radicals depending on the mass
and energy of incident ions, and elapsed time just after
irradiation. However, in order to understand the chemical
reactions in the track in more detail, a time resolved
spectroscopy is a good approach for observing radical
behaviors. We constructed the highly sensitive transient
absorption measurement system using pulsed heavy ions
3)
from AVF cyclotron . The reactions caused by the OH
radical were observed by using this measurement system.
Experimental
The aqueous sample solution was poured into the
metallic cell and irradiated with the pulsed heavy ions in the
atmosphere, and optical absorbance was measured in online.
The number of heavy ions included in the pulse was
evaluated by reading the electric charge received by Faraday
cup that had been set in the vacuum in the upper-stream of
the irradiation cell. The fine structure of the pulse was also
evaluated by measuring the luminescence from a scintillator
set at the sample position. The semiconductor light source
or Xe lamp was used as a probe light source. The probe
light passed the sample cell twice at the angle of 20 degree
toward the beam axis by mirrors above and under the sample
cell, and then was detected with Si photodiode. The cell
has the thickness of 2 mm and 50-m glass windows on the
top and bottom for preventing energy loss of the heavy ions
and optical measurement.
Results and discussion
The absorption spectrum, which has a peak in the visible
region by the heavy ion irradiation to aqueous potassium
thiocyanate (KSCN) solution, was observed. This
absorbance was assigned to (SCN) 2 - from spectrum structure.
The well-known reaction between the OH radical and SCN -
JAEA-Review 2010-065
Absorbance
- 154 -
LET eV/nm
20
10
20
10
0
0
0 1 2 3 4 5
Depth / mm
0.0004
0.0003
0.0002
0.0001
0
Energy / MeV
0 1000 2000 3000 4000
Time / s s s
Fig. 1 Depth curves of energy and LET value of H ion in
water(upper), and time profiles of absorbance at 447 nm
for aqueous KSCN solution at each depth(lower).
under low LET radiation is also observed by the heavy ion
irradiation. In order to investigate energy or LET effects
on radical behaviors, the incident energy was controlled by
putting thin aluminum foils on the irradiation cell. Since H
ion has 4 mm range in water, the absorbance measurements
could be carried out for reactions occurred in the
distinguished region, for example, plateau (○) and Bragg
peak regions (◇) as shown in Fig. 1. The yield of the OH
radical was estimated from the absorbance peak of (SCN) 2 -
just after the pulsed heavy ion irradiation. The yield in the
Bragg peak region was a fraction of that in the plateau
region, and this LET dependence could be explained in
terms of the track structure theory.
References
1) M. Taguchi et al., Radiat. Res. 171 (2009) 254-263.
2) G. Baldacchino et al., Chem. Phys. Lett. 468 (2009)
275-279.
3) M. Taguchi et al., Radiat. Phys. Chem. 78 (2009)
1169-1174.