Chemical Processes in Heavy Ion Tracks

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Chapter 8
Chemical Processes in Heavy Ion Tracks
Gérard Baldacchino* and Yosuke Katsumura†
1. Introduction
In radiation chemistry, “heavy ion” is usually the name given to
particles different from electrons and
γ
-rays. There is no relation-
ship with the mass or the charge of ions as they can be protons or
uranium nuclei. The chemistry as a consequence of irradiation with
heavy ions has been studied for a long time in several ways, for
example, by using solution of radio-elements or by external irradiation
of pure solution. All these aspects have been summarized in recent
reviews for a large range of particles.1–4 During these last 10 years,
heavy ion irradiation researches have concerned many materials and
applications from the nuclear fuel cycle5,6 to the life science7,8 and
more recently the cancer-therapy.9,10 Recently new fields and new
objects using the specific characteristic of the interaction of heavy
ions in matter have emerged such as the synthesis of nano-wires11
231
* CEA, IRAMIS, SIS2M, Laboratoire de Radiolyse, F-91191 Gif-sur-Yvette Cedex,
France. E-mail: gerard.baldacchino@cea.fr
†Department of Nuclear Engineering and Management, School of Engineering,
The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan.

and nano-clusters,12 or by using the sputtering effect, the produc-
tion of membranes13 for filtering and analysis, or the interest of
understanding the processes in organic materials like benzene,14
polymers15 or scintillators.16
Even if irradiation with heavy ions concerns evidently many mate-
rials such as solids or liquids and gases, this chapter will deal with the
effect of high-energy heavy particles in liquid water. Actually, chemi-
cal mechanisms in pure water have been depicted a few decades ago
and this medium is undoubtedly the one in the nature, more gener-
ally in the living systems, and also in the industry to be extensively
used. Therefore this chapter tries to give the new trends of the
research on water radiolysis with heavy ion beams, the methods
developed with their associated problems and the facilities used for
these studies.
As radiolytic yields remain the main parameter to quantify the
evolution of the chemistry after the ionization step, we attempt to
depict the evolution of the localization of the reactions during the
diffusion stage. It is simply a description of time- and spatially-
resolved events with various initial track structures which gives
sensitively various results in terms of yield.
At the same time, a critical approach of problems such as the
dosimetry for high-energy particles, the fragmentation of water mol-
ecules and the determination of radiolytical yields will be attempted.
As it is not fully satisfactory to have only experimental data, a few
words will be written about the modelization and particularly the
recent Monte Carlo simulations. Even if many studies have been
developed during these last decades, finally this field of radiation
chemistry is probably at the beginning and one can imagine that
part of the conventional radiation chemistry studies (i.e.
γ
and elec-
tron beam radiolysis) will be partly transferred to ion beams.
Unfortunately, the difficulties of access to cyclotrons will remain for a
long time and the experiments as interesting they are, cannot be
quickly achieved. That is why a brief future map of this discipline
will be depicted along with the current facilities used for ion beam
irradiations and some known future projects.
232 G. Baldacchino and Y. Katsumura

2. Summary of the Specific Interaction
of High Energy Ions with Water
The interaction of an accelerated heavy nucleus, which has a charge Z
and an energy Ebetween a few MeV to a few GeV, and a liquid-state
water molecule is mainly a Coulombic interaction. The energy
exchange is then purely electronic and no nuclear effect is observed in
water. The theory describing this interaction is a few decades old. In
most cases the Bethe equation allows the calculation of the fraction of
energy deposited dEalong a segment dxof the propagation axis
of the particle. The electronic linear energy transfer (LET) is then
given by,
(1)
where eand mare the charge and mass of the electron, Zand Vthe
charge and velocity of the projectile, and Nand Ithe electron density
and mean excitation potential of the medium.2At higher energy level,
one can expect a fragmentation process of both the swift nucleus and
the water molecule. It can cause some difficulties of interpretation in
the experiments with the highest energy ions, particularly in the
evaluation of the deposited dose. We try to explain how to understand
the effect of this process in the next few paragraphs. For intermediate
values of energy, LET and ranges can then be calculated for many
projectiles and homogeneous targets (see the example of Carbon ions
in water presented in Fig. 1).
If the number of positive charges brought by the nucleus of the
projectile is elevated, the electric field in the surrounding of its pro-
pagation axis becomes huge. The water molecules in this region are
affected by ionizations and electronic excitations. The excess of
energy deposited in these molecules in a few attoseconds is the start
point of a long story depicted in Fig. 2.
The first 10−15–10−12 s are not accessible to time-resolved heavy ion
experiments but they are available from laser femtochemistry.18 In the
-=
d
d
E
x
eZN
mV
mV
I
42
42
2
2
pln冢冣
Chemical Processes in Heavy Ion Tracks 233

case of heavy ion irradiation, this step is the most important because
high local concentrations (localized in space due to the value of LET)
of species make the chemistry very efficient in this range of time.
The radicals and molecules that escaped from the track recombi-
nation can diffuse in the bulk by making the chemical system
homogeneous where the slowest reactions can take place. At the
234 G. Baldacchino and Y. Katsumura
Ion energy (MeV)
0 200 400 600 800 1000
LET (eV/nm)
0
200
400
600
800
1000
Range (mm)
0
5
10
15
Range (mm)
024681012141618
LET (eV/nm)
0
100
200
300
400
500
900 MeV
700 MeV
500 MeV
300 MeV
Fig. 1. Carbon ions in water. Ranges and LET as a function of its energy
(determined from SRIM2006, Ref. 17).
H2O, H2O+, H2O*,e−
s
H2O, H2O+, H2O*,e−
th
OH•, H3O+, OH−, H•, H2, e−
aq
H•, OH•, HO2
•, e−
aq, H3O+, OH−, H2, H2O2
0
10−15
10−12
10−9
10−6
0
10−15
10−12
10−9
10−6
Physical energy deposition:
Ionizations, Excitations
Thermalization of electrons
Chemical bond breaking in water:
formation of H, OH, H2, H3O+ and OH−
Solvaion : e−
aq
Chemical reactions
in high concentration species area
Formation of H2, H2O2
Following interactions in the tracks
Still heterogeneous system
stochastic simulation
Homogeneous system
Deterministic simulation
U
α
γ
235
Ionization track
of He2+ 20MeV
1 µm of water
Time (s) Localization
of events
(
m
)
Local events
heterogeneous chemistry
Diffusion
Homogeneous
solution
LET(
e
V/
nm
)
Spurs formation
with a 10 MeV electron
in 1 µm of water
Spatial structure
of the energy
deposition
eaq
2.7 10
7
mol/J
OH•
2.7 10
7
mol/J
H•
0.6 10
7
mol/J
HO2
•
0 mol/J
H2
0.45 10
7
mol/J
H2O2
0.7 10
7
mol/J
< 2.7 10
7
mol/J
< 2.7 10
7
mol/J
< 0.6 10
7
mol/J
> 0 mol/J
> 0.45 10
7
mol/J
> 0.7 10
7
mol/J
Radiolytic yields
e−,
few MeV 5MeV
0.27 130
30
, 20MeV
C6+, 1GeV
250
Ar18+
2GeV
γα α
Fig. 2. Description of events after the ionization step due to radiation on water
molecules. 3D presentation with time, space and LET.

microsecond scale the distribution of chemical species is given by the
radiolytic yields. From this time onwards the evolution of the species
concentrations is determined by the resolution of the same differential
equations system that is used for
γ
or high-energy electron radiolysis.
In other terms, the chemical system reactivity becomes independent
from the LET value. If one takes a picture at this moment, the distri-
bution of the species is different from the case of heavy ion irradiation
from a
γ
radiolysis. The radicals have been massively recombined in
the earliest steps to produce stable molecules i.e. the molecular hydro-
gen and the hydrogen peroxide. As a con-sequence, the yield of the
latter increases dramatically. That is the main reason why the high
LET radiolysis of water is a key research area for the safety in nuclear
industry (power plant and waste storage).
3. Determination of Doses, Concentrations and Yields
3.1. Some comments concerning the G-value
and track segment G-value
The radiolytic yield, named G-value, is essential to predict the long
term chemistry. For example, a deterministic program which solves a
set of differential equations uses it to quickly obtain the amount of
molecular hydrogen produced during the exploitation of a water
cooling system. The yield determination needs the measurement of
the concentration of the concerned chemical species produced or
consumed during irradiation and also the dose delivered to the
solution. Both of them are difficult to obtain in the case of heavy
ion irradiation.
3.2. Concentration measurement methods
Actually the concentrations of interesting radicals in water radiolysis
are about 10−8–10−9M and it is necessary that they are measured using
sensitive methods. The direct absorption spectroscopy is limited by
the saturation and the linearity of the detector that must receive a
high flux of light to detect a small amount of absorbed light. The
emission spectroscopy is more convenient and more sensitive but
Chemical Processes in Heavy Ion Tracks 235

often the chemical system response is not easy to deconvolute.14,18
This is also often a relative measure needing a calibration. The
scavenging method is also limited by the highest concentrations
necessary to measure the earliest G-values in the track-core. Chemical
systems used must be checked for their possible interference of “high
local-concentration” reactions.
3.3. Dose evaluation
Dosimetry with high-energy particles is a sensitive point because there
are not enough experimental data for each type and energy of ion
beams and the calculated yields depend strongly on the dose. The
evaluation of the dose cannot be as accurate as for
γ
or high energy
electron beams for which a few secondary dosimeters have been
determined such as Fricke dosimeter, thiocyanate and ceric systems,
for example.
Nevertheless, the physical approach consisting in counting the
number of ion seems to be currently the best method because it is
based on the wide-range data base and Monte Carlo program devel-
oped by IBM, SRIM.17 But the accuracy of this program of about 10%
will never give an absolute value of the dose. As an example of a
chemical system extensively used for the dosimetry in liquid, a com-
pilation of the radiolytic yields of the common Fricke dosimeter for
various and large range of ions is presented in Fig. 3. Other presenta-
tions exist by considering MZ 2/Einstead of LET.19 As it is shown, the
radiolytic yield of the Fricke dosimeter changes too much with the
type and the energy of the particle to be reliable enough and used in
experiments. It remains a good topic of experimental research and cal-
culations.19,21 Unfortunately the structure dependence of the energy
deposition, the change of energy in the experiment with low energy
particles (moreover sometimes integrating the Bragg peak) then con-
sequently cannot give a stable and trusty radiolytical yield for dose
determination. The chemical dosimetry is rather difficult because it
needs also a primary G-value deduced from a primary dosimeter
(calorimeter for example). So the accuracy of the measure becomes
uncertain. Some attempts were also made with the thiocyanate system
236 G. Baldacchino and Y. Katsumura

which is a well-known dosimeter in pulse radiolysis with high energy
electrons.22 This system requires a pulse beam and currently the lack
of knowledge of the OH “story” with ion irradiation cannot make it
a good dosimeter yet.
As one can notice in the Fig. 4 the signal obtained from a
microsecond pulse of 1-GeV carbon ions has a rather good signal-to-
noise ratio and could be useful as it is in the electron pulse radiolysis.23
However, this newly developed method for ion currently suffers from
a lack of data and it is not yet so easy-to-use to be widely exploited.
Then the current determinations of the dose in high LET exper-
iments are mainly performed by counting the number of ions
delivered to the sample. This is a physical determination using the X-
ray emission or secondary-electron emission in thin metallic materials.
This method is indirect and needs a calibration with a direct counting
method using a Faraday cup. For very low dose (for example by using
the short-duration pulses), the sensitivity of the detector is the limit-
ing factor, unlike at higher dose or very high LET particles the
linearity of the detectors is also limiting. In the future, great effort is
Chemical Processes in Heavy Ion Tracks 237
LET (keV/µm)
0.1 1 10 100 1000 10000
Gaerated(Fe3+) (10−7
mol.J−1)
0
5
10
15
20
Fig. 3. Compilation of radiolytic yields of the Fricke dosimeter in aerated condi-
tions for various ions and energy. The squares are unpublished results obtained from
34 MeV protons, 1 GeV carbon ions and 2 GeV argon ions.

expected in the dosimetry of high LET particle because the accuracy
of G-value simply depends on the dose measurement.
3.4. Fragmentation for very high energy particles
Nuclear fragmentation of the projectile is expected for very high
energy (E>GeV) of particles because nuclear interaction cross section
becomes no more negligible between atom-nuclei of water and the
projectile.24–29 The nuclear physics can provide information for each
projectile and target.17 Nevertheless what is the chemical consequence
of fragmentation? As the energy deposition depends mainly on the
charge and velocity of the projectile and as the charge and velocity can
only decrease during this physical processes, the value of LET for each
fragment should be lower and lower. The ranges of the new projec-
tiles are also reduced. As a result the radiolysis along the track can
change dramatically from a mixture of high and low LET radiolysis to
238 G. Baldacchino and Y. Katsumura
Time after the ion pulse (10-3s)
0.0 0.5 1.0 1.5
Absorbance
0.000
0.002
0.004
0.006
0.008
0.010
Fig. 4. Absorption kinetics over the time range of 1.7 ms of (SCN)2
•− at 515 nm
in an aqueous solution of 10−3M of thiocyanate saturated with nitrous oxide after a
5-µs pulse of 12C6+. The dose was 4 Gy/pulse and the number of accumulations was
3899 with a repetition rate of 20 Hz.23

a high LET radiolysis. Then a part of the energy deposition can
influence the fate of the subsequent chemistry in the proportion of
the final molecules. This effect could explain some possible abnor-
mal values of yields in some peculiar cases. A clear experimental
determination of the chemical effects should be very useful.
4. Time Dependence, Comments About
the Homogeneous Distribution and the
Scavenging Time
G-values are mandatory for deterministic simulations of long-term
effects of radiation. But these values must correspond to a homoge-
neous distribution of chemical species in the solution. The highly-
structured ionization track, unlike the spurs in
γ
-irradiation, makes
the diffusion of chemical species very long. Which time after ioniza-
tion to choose, 1 µs or more? Sometimes a criterion of homogeneity
is essential. On the other hand, in fundamental studies, the scav-
enging effect can occur at very different time-ranges and by this way
the G-value can change a lot. Attention must be paid to the scav-
enging method because several reactions in the tracks occur at the
same time.
4.1. Track average yields
Many experimental data come from low-energy ion beams and con-
sequently the range of the ions is very short (less than 1 mm). The
yield determinations in this case are the result of the integration of the
energy deposited along the track (from the penetration of the ion in
the solution to the end of the Bragg peak). A model is needed to
obtain the differential yield relevant from a segment of the track then
to a given track structure.
4.2. Track segment yields
The track segment yield (also named differential yield) describes the
chemistry within a track segment in which heavy ion characteristics
Chemical Processes in Heavy Ion Tracks 239

such as energy and LET remain constant (as far as possible and easily
achievable with high energy particles). This quantity is directly calcu-
lated by the Monte Carlo simulations, but some experiments
especially those which were performed with high energy ions can also
produce this kind of yield. In the other cases, the product G
×
Eis
relevant to the evolution of the yield with the energy of the incident
particle and many determinations as a function of energy are neces-
sary. Generally, track segment and track average yields must give the
same value when the ion energy approaches zero (in the Bragg peak)
and when the yield becomes independent of energy (this state is
expected at very short time, t<ns, in the track core).
4.3. G-value dependence of LET and MZ 2/E
The radiolytic yields do not seem to be uniquely determined by the
LET value. That means many attempts have been made to find an
empirical relationship between the G-values and LET. If it was right
for one type of ion, it does not fit with others with other energies and
charges, even with the same LET because two types of heavy ions of
different velocities can have the same LET. Significant differences in
the track structure, due essentially to the spatial distributions of the
ejected secondary electrons, are the origin of this behavior. It has
been suggested that the parameter MZ 2/E is a better indicator than
LET (see Table 1 for some values) for describing the long time yields
in heavy ion radiolysis, where M, Z, and E are the mass, charge, and
energy of the incident ion, respectively.30–32
240 G. Baldacchino and Y. Katsumura
Table 1. A collection of radiolytic yields in molecule/100 eV for four orders of
magnitude of LET and at various time ranges.
O2
•− OH•H2O2e−
aq
Type of radiation LET (eV/nm) µs–ms ns µsµsnsµs
γ
/fast electrons 0.27 0.02 3.2 2.7 0.72 3.8 2.7
12C6+30 0.02 1.5 <1 0.96 3.5–4.5 1
36S16+, 40Ar18+250–280 0.06–0.05 0.4 <0.4 0.9 0.06 —

5. Experimental Results with High LET Particles
Many of the results from the last decade were obtained by scavenging
method but some of them were from direct detection with pulsed
beam. We summarize here a selection of important papers concerning
the hydrated electron, the hydroxyl radical, the superoxide radical, the
molecular product, the hydrogen peroxide, and molecular hydrogen.
5.1. The hydrated electron
The hydrated electron is commonly detected by pulse radiolysis with
electron beams. In the case of highly-structured track, this very reduc-
ing species reacts easily with oxidant like OH•radical in its vicinity at
earliest time after the ionization track is formed. That is the reason
why it is a real challenge to detect this species yet with heavy ion irra-
diation. Giving a G-value remains delicate because the concentrations
are lower than 10−7M and dose must be measured with a high accu-
racy. As it is shown in Fig. 5, the time dependence of hydrated
electron is typical of a track structure in space and time: in the first
100 ns the concentration of initial hydrated electron is at least divided
Chemical Processes in Heavy Ion Tracks 241
Time after pulse (ns)
0 20406080100
Absorbance
0.00000
0.00002
0.00004
0.00006
0.00008
0.00010
0.00012
Fig. 5. Absorbance kinetics of the hydrated electron after a 1-ns pulse of 12C6+of
1 GeV in water [result published in Ref. 34].

by a factor of two. Afterward the concentration seems to begin a
slower decay. This evolution is well depicted in Fig. 6 where, in a log-
representation, the Monte Carlo simulation reproduces the time
dependence. The comparison of G-value is not reliable but is not
so bad. To detect very low concentrations at short time with the
absorption spectroscopy, one needs to optimize all elements in the
acquisition chain: the light source, the irradiation cell and the optics
for transmission, the detector and the signal acquisition. The
improvement of the first and the last one has contributed to detect-
ing almost 1 nM of hydrated electron. The acquisition system allowed
an averaging of about one million of kinetics with 1 kHz of repetition
rate during the flow of the solution.33–35
LaVerne et al. has discussed recently the results of Baldacchino
et al. by pointing out the high value of yield at earliest times.30 This
242 G. Baldacchino and Y. Katsumura
Time (s)
10-10 10-9 10-8 10-7 10-6 10-5
Radiolytic yields (10-7 mole/J)
0
1
2
3
4
5
Fig. 6. Measured and calculated time-dependence of the hydrated electron decay.
Dark small symbols are from experiment with 1-ns pulses of 12C6+of 1 GeV in water,
red open circles are from experiments with 2-MeV ns-pulse of protons and line with
corresponding color are simulations of track-segment yields.30,34,35

value is actually greater than 4 molecule/100 eV which is a common
admitted value deduced from the ionization potential of water. As
discussed before, many reasons can be reported for explaining
this experimental yield: the dosimetry needing also high stability of
the pulse, the detection accuracy for very low level of absorbance, and
so on. This result is also the first result for this kind of ion and for a
track segment as well. Therefore one cannot exclude other unknown
effects in the core-tracks. Other track-average yields were reported in
the 1980s with low-energy protons (2 MeV) including the Bragg
peak energy deposition.
But the comparison is still not easy because there are not enough
comparable results that are obtained with the same method (pulse
radiolysis) and similar conditions (track segment). Most of the other
studies on hydrated electron under heavy ion irradiation were
performed by the scavenging method by using the glycylglycin
species30 or S2O8
2−.36,37
The plots of G-value as a function of the scavenging capacity
(product of the rate constant and the concentration of scavenger)
must be discussed in several factors by considering the energy and the
range of the ions in order to compare with high energy track segment
results. In every case, the tendency of higher yields at earliest times is
respected. Discussing the absolute G-value needs much more results.
5.2. The hydroxyl radical
The hydroxyl radical is visible only in the deep UV with a low absorp-
tion coefficient. It is almost only detected by using scavengers.
Recently Taguchi and co-workers have used the phenol molecule and
detected the stable products of the reaction by using a powerful
HPLC method in order to distinguish the different adducts formed
in the track of low energy carbon ions.38,39 Other methods using pulse
radiolysis have given interesting results by detecting transient species.
They showed that the scavenger molecule can be subject to multiple
possible reactions in the track of heavy ions. Nevertheless other results
tend to show that the increase of OH•-yield, with the increase of the
scavenging capacity, can saturate or even decrease after a maximum
Chemical Processes in Heavy Ion Tracks 243

value of the yield. This effect has been noticed in two papers con-
cerning the same scavenger species, SCN−and similar ions (carbon
and helium ions).23,40 Non-linear effect due to interfering reactions at
high concentration can become efficient with some track structures.
The size of the track core can actually play an important role in the
shape of the scavenging plot as we can see in Fig. 7 with carbon and
argon ions.
5.3. The superoxide radical
The superoxide radical (HO2
•/O•
2−) is a peculiar case in water radi-
olysis. Its yield of production increases with LET which is completely
contrary to the recombination rule in dense ionization tracks.
Actually, the general trend is that these radical recombinations
increase the production of molecular species (H2and H2O2). The
low reactivity of this radical in pure water essentially due to its
244 G. Baldacchino and Y. Katsumura
Time
(
s
)
10-10 10-9 10-8 10-7
G-value (10-7 mol J-1)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Fig. 7. Measured OH yields by using the thiocyanate solutions as scavenger and
µs-pulses of 12C6+of 1 GeV (circles) and 36Ar18+of 2 GeV (triangles) in water [from
Ref. 23].

disproportionation (reaction 1) allows it to escape the track in the
diffusion step.
(2)
This has several potential consequences in biology and medicine.
Actually the hadrontherapy uses heavy ion beam for the local energy
deposition in the Bragg peak region.
Nevertheless, even if the main therapeutic gain is brought by the
precise localization of the energy deposition, the generation of molec-
ular oxygen in hypoxic tumors is known to enhance the sensitization
of the cells. But the production of molecular oxygen in the track is
still questionable. Some studies have showed the possible direct
detection by the pulse radiolysis method and have tried to explain
how superoxide radical is formed in the tracks of heavy ions and many
mechanisms have been suggested in the past.41
Multiple-ionizations model is one of the most probable models
because the huge energy deposited in the medium is considerably
greater than the total energy needed to ionize the total number of
water molecules along the ion track.42 This has been recently
exploited in Monte Carlo simulations (see Fig. 8).43,44 This model
seems to be in good agreement with the few experimental results
concerning HO2
•/O2
•− and H2O2obtained for low LET.
With a low-energy ion beam, the ions generally stop in the
volume of the analyzed solution and the whole energy beam is
deposited including the Bragg region which is characterized by a
huge energy deposition. Effects and ionization structures in the
Bragg peak cannot be currently taken into account separately due to
the lack of knowledge and the unavailable simulation about this part of
the track. Determination of track segment yields of HO2
•/O2
•− and O2is
expected in the near future in order to clarify the situation.
5.4. Molecular species: H2O2and H2
Molecular and stable species like molecular hydrogen and hydrogen
peroxide are the result and consequence of the short story of the
HOO HOOOH Ms
22
HO 22 2 1
2
∑∑- - --
+æÆææ + + = ¥k97 10
71
.
Chemical Processes in Heavy Ion Tracks 245

track reactions. They are less reactive but are the source of main prob-
lems in nuclear industry such as corrosion (H2O2) and safety (H2). As
their yield is increased due to the intra-track recombination, they
become a serious potential danger (see for instance the variation of
H2O2-yield as a function of LET in Fig. 9).
From a fundamental point of view the origins of H2O2and H2
take place at the earliest step after ionization: basically the recombi-
nation of OH forms H2O2,45 and the recombination of hydrated
electron forms H2and a “non-scavengeable” part of H2directly orig-
inates from the ionization of water molecule46 and then enters the
reaction mechanism of water radiolysis very early.
246 G. Baldacchino and Y. Katsumura
LET (eV nm-1)
0.1 100 1000 10000
G-value (10-7 mole J-1)
0.0
0.1
0.2
0.3
0.4
0.5
Fig. 8. Superoxide radical G-values as a function of LET. The square symbol is
the common yield accepted with
γ
-rays and high energy electron beam irradiation.
Plain lines are from scavenging experiments with C, Ne and Ni ion beam by Laverne
and Schuler [Cited in Refs. 41 and 42]. The circle and triangle symbols are
from track-segment pulse radiolysis experiments.41 Dash line is from Monte Carlo
simulation.43

6. Simulation
Several methods have been published to simulate the time-evolution
of an ionization track in water. Monte Carlo (with the IRT method
or step-by-step) and deterministic programs including spur diffusion
are the main approaches. With the large memory and powerful
computer now available, simulation has become more efficient. The
modeling of a track structure and reactivity is more and more precise
and concepts can now be embedded in complex simulation programs.
Therefore corrections of rate constants with high concentrations of
solutes in the tracks and the concept of multiple ionizations have
improved the calculation of G-values and their dependence on time.
The range of energy and LET value to consider through the
simulation is also wider because the large number of ionizations at
elevated LET is directly related to the memory space in the computer.
Chemical Processes in Heavy Ion Tracks 247
Fig. 9. Hydrogen peroxide yield compilation as a function of LET: (dark squares)
137Cs
γ
-rays, 30 MeV protons, 1140 MeV carbon ions, (open squares) 60Co γ-rays;
10 MeV protons; 35, 18.5, 6.4 MeV helium ions; 46 MeV nitrogen ions; 30 MeV
neon ions, (triangles) 60Co
γ
-rays; 18 MeV deuterons; 32, 18 MeV helium ions, (dark
circles) 15, 10, 5, 2 MeV proton; 15, 10, 5 MeV helium ions; 30, 20, 10 MeV carbon
ions, (open circles) rapid neutron; 26 MeV deuterons. [Data from Ref. 45]

A virtual limit of 1 keV/nm seems to be reached but it depends on
the distance of the selected track segment.47–50 Because simulations
are generally performed with a segment of track where the energy of
the particle does not change too much, they do not take into account
the end of the track where the Bragg peak energy deposition is. This
should be a large field of investigation for future simulations and
experiments.
Simulations can be used to calculate the radiolytical yields of any
existing species in water radiolysis, at any time after an ion crosses the
bulk of water, but only a few can represent the experimental results.
In these cases the comparison is possible in order to adjust some
parameters, e.g. all the earliest processes like the distance of thermal-
ization of electrons or the branching ratio for the multiple-ionization
model.
7. Future
7.1. Heavy ion picosecond pulse radiolysis
In the near future, the ion-beam radiation-chemist community will
probably understand earlier processes in the track of heavy ions. This
supposes two things: accelerators must deliver shorter pulses than
those used currently and the detection must be more sensitive and
highly time-resolved. That means a picosecond pulse radiolysis
research with heavy ions. That is not a foolish project because new
designs of accelerators for proton and heavier ions have already
started.51,52 Physics of plasma has made recent progress51,52 and prob-
ably in the next year will appear the first results in radiation chemistry
with protons with at least a picosecond time resolution. A few inten-
tions have already been published.53
7.2. Influence of chemical and thermodynamic
parameters
The need of basic research in the field of nuclear industry and the
fundamental interest to understand the reactivity in the tracks will
248 G. Baldacchino and Y. Katsumura

partly motivate the next experiments in this field. The coupled influ-
ence of LET and high temperature or high pressure should bring new
approaches to the understanding of the track structures. After solving
the technical challenges the expected studies could be done by the
scavenging method as well as the pulse radiolysis. Moreover, as it was
investigated in the picosecond time-range, the effect of temperature
in the tracks should be coupled to the new generation of particle
accelerators which will deliver picosecond high energy heavy ions.
Here also the energy of available particle bunches is only about a
few MeV and is still incompatible with high-pressure and high-
temperature cells. High values of pH or solute concentrations are
also a large domain of investigation that can bring new interesting
behaviors in the intense chemical-competition in the track cores.
8. A Non-exhaustive List of Facilities Devoted
to Radiation Chemistry with Heavy Ion
Europe
•GANIL/Caen/France
•CERI/Orléans/France
•GSI/Darmstadt/Germany
Japan
•HIMAC/NIRS/Chiba/Japan
•TIARA/JAEA/Takasaki/Japan
United States
•FN Tandem Van de Graff of Notre Dame Nuclear Structure
Laboratory
•ATLAS/Linear Accelerator of Argonne National Laboratory
•K1200 cyclotron of the Michigan State University/National
Superconductor Cyclotron Laboratory (NSCL).
Acknowledgment
The authors would like to thank the staff of GANIL and HIMAC
cyclotrons used for the radiation chemistry experiments related in this
Chemical Processes in Heavy Ion Tracks 249

chapter. All participants of these difficult experiments are greatly
acknowledged: G. Vigneron, S. Pin, E. Ballanzat, JC Mialocq, JP
Renault, S. Le Caer, S. Pommeret, M. Taguchi and S. Yamashita. Special
thanks go to the instigators of the pulse radiolysis with high energy ions:
B. Hickel, M. Gardes-Albert and S. Bouffard. We have also appreciated
the fruitful discussions and help of B. Gervais and JP Jay-Gerin.
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