Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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350 Charged Particle and Photon Interactions with Matter

13.4 summary: Current subJeCts and Future perspeCtives

In this chapter, the studies that have been experimentally claried recently for water radiolysis

with positively charged atomic heavy ions are described. Due to the recent studies, track-seg-

ment and track-differential yields of water decomposition products are being accumulated day

by day, while earlier studies developed the information of track-averaged yields for low-energy

ions. In terms of timescale, the focus is now shifting to the ns region or earlier from the μs region

or later. Theoretical studies are also being improved, as described in Chapter 14, owing to the

improvement in the numeric capacity of computers. These improvements commonly attempt to

clarify what occurs in the track at a very early timescale, what is the track structure like, and

what is the origin of distinctive irradiation effects of ion beams in the subsequent chemical and

biological stages.

For both steady-state and pulse radiolysis studies, the limitation of the ion accelerator is the high-

est barrier for such improvements. The heavy-ion pulse radiolysis systems presently under operation

are at the GANIL and TIARA facilities, and these facilities are planning the advancement of the

pulse properties. Simultaneously, a synchrotron is reconstructed at the HIMAC facility to succeed

in the drawing of a nanosecond pulse (Noda et al., 2005). Constructions of the new pulse radiolysis

system at a worldwide scale are foreseen. With the increase of high-energy ion sources, it is desired

to clarify the contributions of fragmentation reactions in more detail. However, there is a lack of

cross sections for nuclear interaction even for a high-energy proton impact, and the situation is much

severe for heavier-ion irradiations.

Although

the behaviors of major products have been claried, those of minor products are still

not claried. For example, yields of

i−

and O

are strongly related to multiple ionizations in physi-

cal interactions. However, their yields at neutral pH are not known well and only discussed based on

experimental evidence in highly acidic conditions or based on theoretical calculations. These minor

products are key species in radiation biology as well. The oxygen-in-track hypothesis is one possible

explanation

of the low-OER property of heavy ions.

Demands

on interdisciplinary studies with physics, biology, engineering, and medicine are

increasing and pervasive effects to the elds are expected and desired by understanding in more

detail

radiation chemical effects of heavy ions.

aCknowledgments

The authors thank a number of collaborators, particularly, T. Maeyama, D. Hiroishi, C. Matsuura

(University of Tokyo), Dr. Murakami, and operators at HIMAC (NIRS) for their support for

heavy-ion irradiation at HIMAC, NIRS; Dr. S. Kurashima for his excellent contribution for

the pulse radiolysis system at the TIARA facility (JAEA); and Dr. B. Hickel for his valuable

advices and discussions, and for the management of the rst studies in France with pulsed

heavy ions. They also thank J. Meesungnoen and J.-P. Jay-Gerin (Université de Sherbrooke),

who are authors of Chapter 14, for kindly allowing one of the authors (Shinichi Yamashita)

to use the (unpublished) simulated track structures for 400 MeV/u C and 500 MeV/u Fe ions

shown in Figure 13.1. The studies in France were performed at GANIL in collaboration with

Dr. S. Bouffard (Commissariat à l’Énergie Atomique et aux Énergies Alternatives, Caen). The

authors would like to thank him for sparing his valuable time and for sharing his great knowl-

edge of the beam control. They also acknowledge the GANIL staff and the colleagues in the

Laboratoire de Radiolyse at Saclay for their great help during the runs: Dr. S. Pin, G. Vigneron,

Dr. J. P. Renault, Dr. S. Le Caër, Dr. S. Pommeret, and Dr. J.-C. Mialocq. The rst experi-

ments at the GANIL facility were also performed in collaboration with Prof. M. Gardès-Albert

Laboratory (Paris 5 University), and Prof. S. Deycard (Caen University). The authors are

thankful for their help and discussion.

Radiation Chemistry of Liquid Water with Heavy Ions: Experimental Studies 351

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355

Radiation Chemistry of Liquid

Water

with Heavy Ions: Monte

Carlo

Simulation Studies

Jintana Meesungnoen

Université de Sherbrooke

Sherbrooke,

Québec, Canada

Jean-Paul Jay-Gerin

Université de Sherbrooke

Sherbrooke,

Québec, Canada

Contents

14.1 On the Importance of Aqueous Radiation Chemistry.......................................................... 356

14.2 Energy

Deposition Events and Creation of Tracks by Charged Particles ............................ 357

14.2.1

Interaction

of Ionizing Radiation with Matter.......................................................... 357

14.2.2

Linear Energy Transfer and Interaction Cross Sections

forHeavyChargedParticles...................................................................................359

14.3 Structure of Charged-Particle Tracks in Liquid Water......................................................... 361

14.3.1 Track

Structure in Radiation Chemistry and Radiobiology..................................... 361

14.3.2

Spatial

Aspects of Track Structures ......................................................................... 361

14.3.2.1

Low-LET

Radiation and Track Entities..................................................... 361

14.3.2.2

Structure

of High-LET Radiation Tracks ..................................................363

14.4

Radiolysis of Liquid Water and Aqueous Solutions .............................................................364

14.4.1 Radiolysis

of Water at Low LET ..............................................................................364

14.4.2

Time

Scale of Events and Formation of Primary Free-Radical and Molecular

Products

in Neutral Water Radiolysis.......................................................................365

14.4.2.1

The

“Physical” Stage .................................................................................365

14.4.2.2

The “Physicochemical” Stage....................................................................366

14.4.2.3 The

“Nonhomogeneous Chemical” Stage .................................................368

14.4.3

Controversial

Issues: Primary Yields of

HO O

i i

−

and H

versus LET

andthe

Production of O

in the Heavy-Ion Radiolysis of Water at High LET......... 370

14.4.4 Hypothesis of Multiple Ionization of Water in Single Collisions

underHeavy-Ion

Bombardment .............................................................................. 371

14.5

Monte

Carlo Simulations of High-LET Water Radiolysis.................................................... 372

14.5.1

The

IONLYS-IRT Simulation Code......................................................................... 373

14.5.2

Multiple Ionization Cross Sections and the Fate of Multiply Ionized Water

Molecules.................................................................................................................. 375

14.5.2.1 Cross

Sections for Multiple Ionization of Water Molecules...................... 375

14.5.2.2

Fate

of Multicharged Water Cations .......................................................... 377

14.5.3

Yields

HO /O

2 2

i i−

Radicals....................................................................................380

14.5.4 Yields

of H

..........................................................................................................382

14.5.5 Production

of O

and the “Oxygen-in-the-Track” Hypothesis ................................. 385

356 Charged Particle and Photon Interactions with Matter

14.1 on the importanCe oF aQueous radiation Chemistry

A large fraction of radiation chemistry studies have been concerned with the studies of water and

aqueous solutions because of the unique importance of water in biological systems and because of

its relevance to a number of practical applications, including (1) various areas of nuclear science

and technology such as water-cooled nuclear power reactors (where the radiolytic processes need to

be carefully controlled to avoid deleterious effects), (2) radiation effects in space, (3) radiotherapy

and diagnostic radiology, and (4) the environmental management of radioactive waste materials

(LaVerne, 2004; Garrett et al., 2005). Water also serves as the standard reference material for clini-

cal radiation therapy because its absorption properties for ionizing radiation are similar to those of

biological tissue (Medin et al., 2006). The study of the radiolysis* of water has been actively exam-

ined for more than a century. In fact, Curie and Debierne (1901), Giesel (1902, 1903), and Ramsay

and Soddy (1903) were the rst to observe that dissolved radium salts decompose aqueous solutions

by liberating hydrogen and oxygen gases continuously (due principally to the release of α-particles

from the radium). Excellent accounts of the history of aqueous radiation chemistry have been given

by Kroh (1989), Jonah (1995), Ferradini and Jay-Gerin (1999), LaVerne (2000), Zimbrick (2002),

and

Buxton (2004).

All

biological systems are damaged by ionizing radiation. Since living cells and tissue consist

mainly of water (∼70%–85% by weight), the knowledge of the radiation chemistry of aqueous solu-

tions is critical to our understanding of early stages in the complicated chain of radiobiological

events that follows the passage of radiation. In this context, reactive species produced by water radi-

olysis in the cellular environment are likely to be major contributors to the induction of chemical

modications and changes in cells that may subsequently act as triggers of signaling or damaging

effects (Muroya et al., 2006), and ultimately lead to the observation of a biological response (Hall

and Giaccia, 2006; von Sonntag, 2006; Tubiana, 2008). Moreover, cellular radiobiological phenom-

ena depend not only on the absorbed dose but also on the quality of radiation employed, a measure

of which is given by the “linear energy transfer” (LET) that represents, to a rst approximation,

the nonhomogeneity of the energy deposition on a submicroscopic scale. For example, for similar

absorbed doses of different types of radiation, a less uniform geometric distribution of absorbed

energy produces biological changes more efciently (e.g., Barendsen, 1968; Tsuruoka et al., 2005).

The nding of a relationship between the “relative biological effectiveness” (RBE), dened in radio-

biology to describe these differences,

†

and radiation quality (LET) emphasizes the importance of

the initial spatial distribution of reactants—commonly referred to as the “track structure”—in the

response

of irradiated cells.

From

the viewpoint of pure aqueous radiation chemistry, the successful prediction of the effects

of radiation type and energy in radiolysis requires not only a realistic description of the early physi-

cal aspects of the radiation track structure, but also an accurate modeling of the temporal, three-

dimensional development of the track, in which the various water decomposition products are

specied and allowed to diffuse and react with one another or with the milieu (Muroya et al., 2006).

* The term radiolysis refers to any chemical changes induced by ionizing radiation, and includes synthesis as well as

degradation

(Hart and Platzman, 1961).

†

The RBE of some test radiation Y is dened as the ratio of the absorbed dose of a “standard radiation” (customarily

250 kV x-rays) to that of the radiation Y required to produce the same specic biological effect (Hall and Giaccia, 2006).

The RBE applies to all ionizing radiation, whether photon or particle, and depends on several factors such as the biologi-

cal system or end point studied, the absorbed dose, the irradiation parameters (e g., type of radiation, its energy and dose

rate),

and the environmental conditions (e.g., the level of oxygenation).

14.6 Concluding Remarks ............................................................................................................390

Acknowledgments..........................................................................................................................392

References...................................................................................................................................... 392

Radiation Chemistry of Liquid Water with Heavy Ions: Monte Carlo Simulation Studies 357

Therefore, it is critical to understand how the beam quality and the irradiation conditions affect the

subsequent water decomposition products and their space distribution. Finally, it is also important

to know how the initial, spatially nonhomogeneous distribution of reactive species relaxes in time

toward

a homogeneous distribution.

14.2 energy deposition events andCreationoFtraCks

Charged p

artiCles

14.2.1 interaction of ionizing radiation with Matter

Ionizing radiations are dened as those types of energetic particle and electromagnetic radiations

that, either directly or indirectly, cause ionization of a medium, i.e., the removal of a bound orbital

electron from an atom or a molecule and, thereby, the production of a residual positive ion. Some

molecules, instead of being ionized, may also be excited to upper electronic states (e.g., Evans, 1955;

Anderson, 1984; Mozumder, 1999; Toburen, 2004). Directly ionizing radiations are fast-moving

charged particles (e.g., electrons, protons, α-particles, stripped nuclei, or ssion fragments) that

produce ionizations through direct Coulomb interactions. In this case, note that particle–particle

contact is not necessary since the Coulomb force acts at a distance. Indirectly ionizing radiations

are energetic electromagnetic radiations (like x- or γ-ray photons) or neutrons that can also liberate

bound orbital electrons, but secondarily to a preliminary interaction. For photons, this interaction

is predominantly via the production of Compton electrons and photoelectrons (and, if the incident

photon energy is greater than 1.02MeV, the production of electron–positron pairs). Neutrons inter-

act with matter through elastic nuclear scattering, resulting in the production of energetic recoil

positively charged nuclei (ions), protons and oxygen ions in water, which can go on to generate ion-

ized and excited molecules along their paths. Regardless of the type of ionizing radiation, the nal

common result in all the modes of the absorption of ionizing radiation is thus the formation of tracks

of energy-loss events in the form of ionization and excitation processes and in a geometrical pattern

that

depends on the type of radiation involved.

Generally,

the electrons ejected in the ionization events may themselves have sufcient energy to

ionize one or more other molecules of the medium. In this way, the primary high-energy electron

can produce a large number (∼4 × 10

by a 1MeV particle) of secondary or higher-order generation

electrons* along its track as it gradually slows down (ICRU Report 31, 1979). From atomic phys-

ics, it is known that most energy-loss events by fast electrons involve small transfers of energy. In

fact, the probability of a given energy transfer Q varies inversely with the square of that energy loss

(Evans, 1955). “Distant” or “soft” collisions, in which the energy loss is small, are therefore strongly

favored over “close” or “hard” collisions, in which the energy loss is large (Mozumder, 1999). The

vast majority of these secondary electrons have low initial kinetic energies, with a distribution that

lies essentially below 100eV, and a most probable energy around 9–10eV (LaVerne and Pimblott,

1995; Cobut et al., 1998; Pimblott and LaVerne, 2007). In most cases, they lose all their excess

energy by multiple quasi-elastic (i.e., elastic plus phonon excitations) and inelastic interactions with

their environment, including ionizations and/or excitations of electronic, intramolecular vibrational

or rotational modes of the target molecules (Michaud et al., 2003), and quickly reach thermal equi-

librium (i.e., they are “thermalized”). Determining exactly which of these competing interaction

types will take place is a complex function of the target medium and the energy range of the incident

electron. By denition, a measure of the probability that any particular one of these interactions will

occur is called the “cross section” (expressed in units of area) for that particular interaction type.

The total interaction cross section σ, summed over all considered individual processes i, is used

to determine the distance to the next interaction, and the relative contributions σ

to σ are used to

* It is customary to refer to all electrons that are not primary as “secondary.”

358 Charged Particle and Photon Interactions with Matter

determine the type of interaction. Actually, the mean distance between two consecutive interactions

“mean free path” λ is dened by

, (14.1)

where

N is the number of atoms or molecules per unit volume, and

σ σ=

∑

. (14.2)

In a dilute aqueous environment, thermalized electrons undergo trapping and hydration in quick

succession (within ∼10

−12

s) as a result of the water electric dipoles rotating under the inuence of

the negative charge (Bernas et al., 1996). Some electrons that have kinetic energies lower than the

rst electronic excitation threshold of the medium, the so-called subexcitation electrons (Platzman,

1955), may also undergo, prior to thermalization, prompt geminate ion recombination (Freeman,

1987) or induce the production of energetic (∼1–5eV) anion fragments via formation of dissociative

negative ion states (resonances) (i.e., dissociative electron attachment, DEA) (Christophorou et al.,

1984; Bass and Sanche, 2003). As a consequence of the energy gained by the medium, a sequence

of very fast reactions and molecular rearrangements leads to the formation of new, highly nonho-

mogeneously distributed chemical species in the system, such as charged and/or neutral molecular

fragments, reactive free radicals, and other excited chemical intermediates. The trail of the initial

physical events, along with the chemical species, is generally referred to as the “track” of a charged

particle, and its overall detailed spatial distribution, including contributions from secondary elec-

trons, is commonly known as “track structure” (e.g., Magee and Chatterjee, 1987; Paretzke, 1987;

Kraft and Krämer, 1993; Paretzke et al., 1995; Mozumder, 1999; LaVerne, 2000a, 2004; Dingfelder,

2006;

Muroya et al., 2006).

charged particle is called “heavy” if its rest mass is large compared with that of an electron.

Thus, protons (

), α-particles (

) and their near relatives (

), higher-charged par-

ticles (i.e.,

10+

26+

, etc.), and ssion fragments are all heavy-charged particles. As

most radiation chemical studies involve ions with initial energy of a few MeV per nucleon or more

depending on the source, the main interaction responsible for energy loss at these energies is the

Coulomb force between the charge of the incident heavy ion and that of the bound electrons of the

medium. The tracks of heavy ions can thus be explained in basically the same manner as that for fast

electrons. A heavy-charged particle traversing matter loses energy primarily through the ionization

and excitation of target atoms or molecules. This proceeds until the ions are so much slowed down

that they may be neutralized by electron capture, whereby their travel is stopped. For sufciently

fast ions and electrons, there are, however, some obvious differences due almost entirely to the small

mass of the electron. For example, a (nonrelativistic) heavy ion (which has mass M, velocity v

, and

kinetic energy

E Mv

i i

) is restricted to a maximum allowed energy loss (Q

max

) to an initially free

stationary

atomic electron (which has mass m

) given by (Evans, 1955; Anderson, 1984)

Q E E m v M m

imax

4 ),≈

( )













≈













i i

M m

 (14.3)

whereas a fast incident electron can transfer all its initial kinetic energy in a single collision

with a bound electron. As seen in Equation 14.3, the energy that a heavy-charged particle loses

in each event and the corresponding change in its momentum amount to only a small fraction

(proportional to m

/M) of its total energy and momentum. Because the heavier mass of fast ions

prevents them from being scattered as much as electrons, heavy ions move in a relatively straight

Radiation Chemistry of Liquid Water with Heavy Ions: Monte Carlo Simulation Studies 359

line (their deections in collisions are negligible) until very near the end of their range. By con-

trast, electrons can undergo large-angle scattering in collisions with medium electrons, leading to

extremely torturous paths.

14.2.2 linear energy tranSfer and interaction croSSSectionS

for heavy charged particleS

The earliest experiments on the radiation–chemical effects in liquid water showed that there were

differences in the yields of radiolytic products for various types of radiation (Kernbaum, 1910;

Usher, 1911; Duane and Scheuer, 1913; Debierne, 1914). The amounts of changes observed in the

quantities and proportions of the chemical species produced depend both on the total energy depo-

sition and the spatial distribution of the energy. The LET, rst introduced by Zirkle et al. (1952),

is a measure of the rate of energy deposition along and within the track of a penetrating charged

particle. This quantity is especially important in evaluating the overall chemical effect in the appli-

cations of radiation to biological systems because the concentration of the deposited energy is sig-

nicant

in those cases. LET is dened as

LET = −

, (14.4)

where dE is the average energy locally (i.e., in the vicinity of the particle track) imparted to the

medium by the particle in traversing a distance dx (ICRU Report 16, 1970). In this way, the (some-

times termed “energy-unrestricted,” i.e., when all permissible energy transfers are included) LET

is equivalent to the “stopping power” (which is commonly used in the domain of radiation physics)

of the medium traversed (Anderson, 1984; Mozumder, 1999). Usually, LET values are in units of

keV/μm

(the conversion to SI units is: 1

keV/μm

≈ 1.602 × 10

−19

J/nm).

The Bethe theory of stopping power describes the average energy loss due to the electromagnetic

interactions between fast charged particles and the electrons in absorber atoms. For ions whose

kinetic energies are small compared to their rest mass energy, the nonrelativistic stopping power

formula

of Bethe (Bethe, 1930; Bethe and Ashkin, 1953) is given by (in SI units)

− =

























Z e

m V

πε

(14.5)

where

is the charge on the incident ion

V is the ion velocity

is the mass of an electron

is the number of electrons per cubic meter of the absorbing medium

is the mean of all the ionization and excitation potentials of the bound electrons in the absorber

In most cases, I is determined by tting the theory to experimental results related to the stopping

power. For liquid water, I = 79.7 ± 0.5eV (Bichsel and Hiraoka, 1992). Based on the lowest-order

(or rst “Born”) approximation in the electromagnetic interaction between the incident particle and

the atomic electrons, the Bethe equation has a wide range of validity except for slow, highly charged

heavy

particles such as ssion fragments.

we can see from Equation 14.5, for any incident particle, the LET depends on the particle’s

kinetic energy only through its velocity. It is approximately inversely proportional to V

, but the

mass of the fast-moving particle does not appear. Hence, there are really no isotope effects, provided

the ions have the same velocity. For example, protons (

) and deuterons (

) of equal velocity