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13
Radiation Chemistry of
Liquid
Water with Heavy
Ions:
Steady-State and
Pulse
Radiolysis Studies
Shinichi Yamashita
Japan Atomic Energy Agency
Tokai,
Japan
Mitsumasa Taguchi
Japan Atomic Energy Agency
Takasaki,
Japan
Gérard Baldacchino
Commissariat à I’ Énergie Atomique et aux Énergies Alternatives
Saclay,
France
and
Centre
National de la Recherche Scientique
Gif-sur-Yvette, France
Yosuke Katsumura
The University of Tokyo
Tokyo,
Japan
and
Japan
Atomic Energy Agency
Tokai, Japan
Contents
13.1 Introduction ..........................................................................................................................326
13.1.1 Importance
of Heavy-Ion Beams..............................................................................326
13.1.2
Irradiation
Effects of Heavy-Ion Beams...................................................................327
13.1.2.1
Physics:
Energy Deposition and Appearance of Track.............................. 327
13.1.2.2
Chemistry:
Intra-Track Reactions and Product Yields ..............................328
13.1.2.3
Biology: RBE, OER, etc. ...........................................................................329
13.1.3 History:
Earlier Studies and Accelerators ................................................................330
13.2
Steady-State
Radiolysis Studies............................................................................................ 332
13.2.1
Strategy
and Techniques........................................................................................... 332
13.2.2
Track-Segment,
Track-Differential, and Track-Averaged Yields............................... 333
13.2.3
Primary Yields of Major Products Related to Track Structures............................... 334
13.2.3.1 Primary
Yield and Its Signicance............................................................ 334
13.2.3.2
Hydrated
Electron (e
aq
−
)............................................................................. 335
326 Charged Particle and Photon Interactions with Matter
13.1 introduCtion
13.1.1 iMportance of heavy-ion beaMS
The term ion beam involves, in a broad sense, beams of not only positively charged atomic ions but
also of positively/negatively charged molecular and cluster ions. In this chapter, this term is used to
collectively designate beams of positively charged atomic ions. It is phenomenologically well known
that irradiation of fast heavy ions leads to much different results compared to γ-rays, x-rays, and fast
electrons, which are categorized as low-LET radiations. Heavy ions deposit their energies much more
densely than low-LET radiations, and are categorized as high-LET radiations. Here, LET is the acro-
nym for linear energy transfer, and is practically used as an indicator of 1D energy deposition density
along the radiation trajectory, similarly as stopping power (−dE/dx), although their strict denitions
are different. It is worth noting that one of the signicant criteria between high- and low-LET radia-
tions can be estimated by considering the initial size of spur at 1ps and the diffusion distance during
intra-spur reactions terminating by about 100ns. The former is 3nm or less in deviation, and the latter
is briey estimated to be 16–34nm by using the diffusion coefcient of the water molecule, 2nm
2
/ns,
and that of H
3
O
+
, 9nm
2
/ns. Then, spurs located within 40nm would overlap each other during intra-
spur processes. In addition, the average energy necessary to produce a single spur is about 60eV.
Thus, one of the signicant criteria is 60eV/40nm = 1.5eV/nm, at which or higher LET the intra-spur
processes are affected by neighboring spurs. Due to their high-LET feature, heavy ions induce much
denser ionizations and excitations in matter than low-LET radiations, leading to distinctive irradiation
effects, such as high efciency in induction of lethal damage to cells and so on. In this chapter, radiation
chemical effects of heavy ions onto water are focused on not only because water is one of the simplest
and ubiquitous molecules on Earth, but also because it is one of the main components of our bodies.
Such distinctiveness is now utilized in many applications (see Chapters 20, 21, and 24), for example,
proton and C ions of several hundreds of MeV per nucleon (MeV/u) are used in the therapy of cancer
typically located at deep positions inside human bodies. After intensive clinical studies and conrma-
tion of the powerfulness and safety or risk of the application, the actual utilization of heavy-ion therapy
has begun; however, a detailed mechanism that would lead to distinctive biological effects suitable
for
therapy is not yet known well. Human bodies would be exposed to heavy-ion beams during space
activities, which are increasing day by day. Examples of such heavy-ion beams are space radiations
such as solar particle events (SPEs) and galactic cosmic rays (GCRs), which mainly consist of a wide
variety of atomic ions. It is inevitably important to assess the exposed dose of astronauts during their
space missions and to maintain risk to the lowest possible level. Due to the wide variety of ion types
13.2.3.3 Hydroxyl Radical (
•
OH)............................................................................. 336
13.2.3.4 Hydrogen
Peroxide (H
2
O
2
)......................................................................... 338
13.2.3.5 Hydrogen
Molecule (H
2
)............................................................................ 339
13.2.4 A
Parameter Alternative to LET, (Z
eff
/β)
2
..................................................................340
13.2.5 Intra-Track
Dynamics............................................................................................... 341
13.3
Pulse
Radiolysis Studies.......................................................................................................344
13.3.1
Strategy.....................................................................................................................344
13.3.2 Detection
Method and Measurement System...........................................................344
13.3.3
Water Radical Kinetics.............................................................................................346
13.3.3.1 Applicable
Scope of Direct Observation ...................................................346
13.3.3.2
Hydrated
Electron (e
aq
−
) .............................................................................346
13.3.3.3 Hydroxyl
Radical (
•
OH).............................................................................347
13.3.3.4 Superoxide
Radicals (HO
2
•
/O
2
•−
) ................................................................348
13.4 Summary:
Current Subjects and Future Perspectives .......................................................... 350
Acknowledgments.......................................................................................................................... 350
References...................................................................................................................................... 351
Radiation Chemistry of Liquid Water with Heavy Ions: Experimental Studies 327
and energy involved in space radiations, it is desired to establish and sophisticate a universal theory
that can be applied to a wide range of ion beams. The distinctiveness of ion beams is also utilized in
analytical techniques such as particle-induced x-ray emission (PIXE), in surface processing such as
focused ion beam (FIB) (see Chapter 26), and in inducing mutation in plant breeding to give a wide
variety of plants (see Chapter 34). In addition, understanding irradiation effects induced by ion beams
is important in nuclear engineering (see Chapter 35). For example, very heavy ions of a few mega elec-
tron volts per nucleon are produced as ssion fragments (FFs), and ejected alpha particles as He ions
of a few mega electron volts. Ejected fast neutrons are also associated with ion beams because they
lose their energies mainly through elastic collision with hydrogen nuclei of water molecules, result-
ing in the generation of recoil protons possessing half of the initial neutron energy at an average. It is
recommended that the chapters of this book indicated in parentheses above be referred appropriately.
Note that understanding irradiation effects of atomic ions also helps understand those of molecular and
cluster ions, because these basically convert into atomic ions after initial collision processes.
The energy unit of kinetic energy divided by the mass number of ions, that is, MeV/u, is very
useful and widely employed in ion beam research, because the same value gives the same velocity
even for a different kind of ion.
13.1.2 irradiation effectS of heavy-ion beaMS
13.1.2.1 physics: energy deposition and appearance of track
The above-mentioned applications utilize, more or less, distinctive features of heavy-ion beams
in physics, chemistry, and biology. When a fast charged particle has a large amount of energy, it
deposits its energy through Coulombic interactions with matter. These consist of electronic and
nuclear interactions, which are dened as interactions with bond electrons and atomic nuclei in
matter, respectively. For a high-energy heavy ion, the former interaction is predominant and results
in ejection of high-energy secondary electrons. These secondary electrons also deposit their energy
through Coulombic interactions, and tertiary electrons are produced; however, electrons of all gen-
erations produced in these processes are collectively referred to as secondary electrons. Energy
depositions from both the primary projectile ion and secondary electrons lead to the production of
electrons and holes as well as excited molecules, which participate in consequent chemical reac-
tions and would be responsible for subsequent biological responses. Thus, the patterns of energy
deposition and distributions of chemical species are required to comprehend consequent chemistry,
biology, and so on, and are called track or track structure. In order to compass the whole picture
of energy deposition processes initiated by the fast heavy ion, cross sections for energy transfer
between secondary electrons and bond electrons as well as between the heavy ion and electrons are
necessary. These cross sections have intensively been investigated, and several reviews on them and
theories to correlate them to the stopping power have been proposed (see Chapters 2 through 5 and
13
(Mozumder, 2004; Toburen, 2004) and references therein).
The
stopping power or LET is the most widely and conventionally used parameter to indicate how
densely the ionizing radiation deposits its energy. The most important and basic physical parameters
that represent particle radiations are kinetic energy (E), velocity (β), and effective charge (Z
eff
),
which are used in the estimation of stopping power. These parameters are related to each other by
the
following equations:
β = −
+
1
2
2
2
Mc
Mc E
(13.1)
Z Z Z
eff
= − −
−
1 125
2 3
exp( )
( / )
β
(13.2)
328 Charged Particle and Photon Interactions with Matter
where
M
and Z are the mass and the atomic number of the projectile ion, respectively
c is the light velocity in vacuum
A
few theories have been put forward to determine the effective charge. Note that Equation 13.2
is given by the Barkas theory (Barkas, 1963), and that high-energy ions of 0.4MeV/u in β or of
85MeV/u in E are fully stripped (Z
eff
/Z is higher than 0.9988) based on this equation if Z is 20 or
smaller. Neglecting several assumptions, stopping powers for a proton and ions can be represented
in
a compact form by the Bethe equations (Berger et al., 1993):
−
=
dE
dx
e
mc
N Z
Z mc
I
ion
tar tar
eff
4 2
4
2
2
2
2 2
π
β
β
( ) ln (13.3)
−
=
dE
dx
e
mc
N Z
mc
I
proton
tar tar
4 1 2
4
2 2
2 2
π
β
β
( ) ln (13.4)
where
m
and e are the mass and the charge of the electron, respectively
N
tar
and Z
tar
are the number density and the atomic number of atoms in the target material,
respectively
I
is the mean excitation potential of the target material
Thus,
the ratio of the stopping power for ions to that for the proton at the same velocity is given by
the square of effective charge:
−
−
=
dE
dx
dE
dx
Z
ion proton
eff
2
(13.5)
In fact, effective charge is originally dened by this equation to extrapolate the stopping power
for a proton to that for heavier ions, and can be different from the average charge, which is simply
dened as the average of the charge. Going back to Equation 13.3, the stopping power of ions is
approximately proportional to the inverse of the square of β as long as β is not extremely small.
Thus, the stopping power is low when the ion energy is high and vice versa. In other words, just
after penetrating into matter, the stopping power becomes relatively low and the projectile ion loses
its energy slowly. With decreasing velocity, the stopping power increases, which accelerates the
increase of the stopping power itself as well as the decrease of the velocity. As a result, the heavy
ion shows a drastic decline in the velocity as well as a drastic rise in the stopping power just before
the position where it stops, which is referred to as the Bragg peak. For example, stopping powers for
C ions of 100, 10, and 1MeV/u in water are calculated as 25.9, 175, and 745 eV/nm, respectively,
by the SRIM code (Ziegler, 1998). In cancer therapy with ion beams, irradiation is conducted with
putting the Bragg peak on cancer, leading to the desired dose distribution in human body. In surface
processing, relatively low-energy ions are used and their penetration depths are controlled by con-
trolling the ion energy.
13.1.2.2 Chemistry:
i
ntra-track
r
eactions
and p
roduct
y
ields
After energy deposition events, physicochemical processes follow, in which, for example, a second-
ary electron is thermalized, trapped, and hydrated, and thus it converts into a hydrated electron (e
aq
–
).
Hole of water (H
2
O
•+
) gives protons (H
+
) to surrounding water molecules and becomes a hydroxyl
radical (
•
OH), and thus H
3
O
+
is produced. Excited water molecules (H
2
O*) dissociate into hydro-
gen atoms (H
•
) and
•
OH or into smaller amounts of hydrogen molecules (H
2
) and oxygen atoms(O).
Radiation Chemistry of Liquid Water with Heavy Ions: Experimental Studies 329
These processes, having completed within 1 ps, are categorized as physicochemical stages and
thought to be common for high-energy heavy ions and electrons, because most of the energy is com-
monly deposited via secondary electrons. However, the spatial distribution of these initial products
must strongly depend on the type and the velocity of the projectile ion, which determine the fre-
quency of energy deposition as well as ejection angles and energies of secondary electrons. Note
that the positions of the initial products can be slightly different from the positions where energy
depositions occur, because their ights can occur during the physicochemical stage. Figure13.1
shows two examples of track structures for C and Fe ions of 290 and 500MeV/u, respectively, at
1ps after their passages were obtained by the Monte Carlo simulation with the IONLYS-IRT code
(see Chapter 15 and Meesungnoen and Jay-Gerin (2005a)). As seen in this gure, the track of the
Fe ion is much wider and denser than that of the C ion, and it is clear that the track structure is
strongly related to radiation quality. After the physicochemical stage, water decomposition products
begin to react with each other parallel to their diffusions from the positions they are produced. Such
reactions are called intra-track reactions. The denser the initial distribution, the more frequently
the intra-track reactions occur. Thus, chemical yields after or during intra-track reactions depend
strongly on the type and the energy of heavy ions as well. In general, yields of molecular products,
such as H
2
and hydrogen peroxide (H
2
O
2
), increase with increasing LET, while radical yields except
for
those of
HO
2
i
and
O
2
i−
decrease.
13.1.2.3 biology:
rbe
,
oer
,
etc.
Although
it is phenomenologically well known that the distinctiveness of heavy ions in energy
deposition and chemical yields leads to different biological effects, these effects are not explained
well in terms of such distinctiveness. For example, some special cancers are radio resistant against
low-LET radiations but not against heavy ions. Relative biological effectiveness (RBE) is a com-
monly used barometer to represent biological features of radiation, and is dened as the dose for
250 kV x-rays divided by the dose for the radiation of interest to attain the same biological effects
such as cell death, mutation, and strand break. A higher value of RBE indicates that a smaller dose
is required as compared with the dose for the 250 kV x-rays. Oxygen enhancement ratio (OER) is
also a commonly used barometer, which indicates how much a certain biological effect is increased
by the presence of oxygen. C ion irradiation gives the maximum value of RBE at an LET of about
200 eV/nm, which is almost at the end of the range of the ion, and OER decreases with increasing
LET and reaches the minimum value of 1 at almost the same LET. These biological characteristics
have signicant advantages in the treatment of cancer because the concentration of O
2
in tumors is
very low, as given in Chapter 24, and are well established, but only phenomenologically. Inother
1 μm
1 μm
290 MeV/u
500 MeV/u
OH, etc.
OH, etc.
200 nm
200 nm
e
aq
–
e
aq
–
12
C
6+
56
Fe
26+
•
•
Figure 13.1 Track structures simulated with the code named IONLYS-IRT (Meesungnoen and Jay-
Gerin, 2005a). Left and right panels show track structures at 1 ps after passages of C and Fe ions of 290 and
500MeV/u, respectively, and insets in the panels are blowups of corresponding regions surrounded by squares.