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

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330 Charged Particle and Photon Interactions with Matter

words, a detailed mechanism through which physical and chemical features of ion beams lead to

these characteristics is not yet claried, and it is essential to deeply understand their basic phe-

nomena and their correlations.

13.1.3 hiStory: earlier StudieS and acceleratorS

The earliest study on water radiolysis with heavy ions was just after the discovery of radiation itself.

However, only alpha particles were available during the former half of the previous century. In the latter

half of the century, the development of ion accelerators and experimental techniques helped research-

ers use other ions and conduct more precise investigations. However, most of these investigations were

conducted with relatively lighter ions (H, D, and He ions) of lower energies (typically 10MeV/u or less)

under highly acidic conditions. Since the mechanisms for both the Fricke and cerium dosimeters were

well established with low-LET radiations, the combination of these two dosimeters could be used to

evaluate the yields of water decomposition products in ion beam radiolysis. Only 20–30 years ago,

eventually, ion beams of heavier ions of higher energies have become available and investigations with

these beams under neutral conditions are still on their way. Figure 13.2 shows a plot of research results

on water radiolysis with heavy ions under neutral conditions. Although not many studies have been

conducted under neutral conditions, much understanding has been derived over the past decade.

Only four groups conducted investigations on water radiolysis with steady-state ion beams:

those from the Brookehaven National Laboratory (BNL) (Appleby et al., 1985, 1986); the Notre

Dame Radiation Laboratory (NDRL) (LaVerne, 1989; LaVerne and Yoshida, 1993; Pastina and

LaVerne, 1999; LaVerne et al., 2005); Quantum Beam Sciences (QuBS), Japan Atomic Energy

Agency (JAEA) (Taguchi and Kojima, 2005, 2007; Taguchi et al., 2009); and the University

of Tokyo and Advanced Science Research Center (ASRC), JAEA (Yamashita et al. 2008a,b,c;

Baldacchino et al., 2009). Concerning pulse radiolysis studies, only some groups have succeeded

in heavy-ion pulse radiolysis until now, because of the constraints of accelerator specications and

experimental difculties, which are different from those of an electron beam, for example, shorter range

1000

100

1 10

FFs

1 mm H

1 cm H

10 cm H

Range in water

GCRs

Therapy

Atomic number of ion

Specific energy (MeV/u)

Figure 13.2 An overview of studies on radiolysis of neutral water with heavy ions. Circles, squares, tri-

angles, inverse triangles, and stars show beam conditions used in studies of Yamashita et al. and Baldacchino

et al. at the HIMAC facility (Yamashita et al., 2008a,b,c; Baldacchino et al., 2009), Baldacchino et al. and

Wasselin-Trupin et al. at the GANIL facility (Baldacchino et al. 1997, 1998a,b, 1999, 2001, 2003, 2004,

2006b; Wasselin-Trupin et al., 2000, 2002), LaVerne et al. at NDRL (LaVerne 1989; LaVerne et al. 2005),

Taguchi et al. at the TIARA facility (Taguchi and Kojima 2005, 2007; Taguchi et al., 2009), and Appleby et al.

at the BEVALAC facility (Appleby et al., 1985, 1986), respectively. Shaded areas correspond to typical energy

and atomic number of heavy ions contained in GCRs and FFs, and gray areas correspond to proton and carbon

ion beams used in actual cancer therapy. Solid, dashed, and dotted lines show energies for each ion to obtain

ranges

of 10, 1, and 0.1

in water, respectively, which are calculated with the SRIM code (Ziegler, 1998).

Radiation Chemistry of Liquid Water with Heavy Ions: Experimental Studies 331

than 1mm in water and lower yield of reactive species. Burns (Burns et al., 1981) and Sauer (Sauer

etal., 1977, 1983) had succeeded in the measurement with H

and He

in 1977. Since the 3MeV

ion that Burns et al. used has the shorter range in water, they performed the measurement using

a jet without a cell (without a glass window). Since then, no group has succeeded in such system

development. Research of heavy-ion pulse radiolysis, however, prospered from around 1997 due

to the development of the latest accelerator technology. The types of incident ions, energy, time

resolution, etc., of the newly developed system are summarized in Table 13.1. Katsumura et al.

succeeded in the measurement of

(SCN)

i−

with He ions with the low-energy linear accelerator

at the HIMAC facility (Chitose et al., 1999a,b, 2001). They took a Ti foil extraction window for

the beam as a wall of sample cell, and the horizontal beam was injected directly into the water

sample through the foil. On the other hand, Baldacchino et al. and Wasselin-Trupin et al. suc-

ceeded in the measurement of e

–

, H

, and

(SCN)

i−

using the high-energy heavy ion of GeV

class at the GANIL facility (Baldacchino et al., 1997, 1998a,b, 1999, 2001, 2003, 2004, 2006b;

Wasselin-Trupin et al., 2000, 2002; Baldacchino and Katsumura, 2010). They took advantage of

the high-energy heavy ion having a longer range in water and produced almost constant LET along

the probed sample, which produced results comparable to the Monte Carlo simulations. Moreover,

several years later, Taguchi et al. developed a spectroscopic system with superhigh sensitivity

under a pulsed heavy ion of several 100MeV injected into the water surface at the TIARA facil-

ity (Taguchi et al.). Yoshida et al. have developed a ns-time-resolved optical absorption measure-

ment system using the luminescence of a scintillator (Kondoh et al., 2008). Katsumura et al. used

a 6 MeV/u He ion with pulse widths of 5 and 10μs from a linear accelerator equipped in front of

the main synchrotron accelerators at the HIMAC facility. Moreover, Taguchi et al. used ions in the

range of 20 MeV/u H ions to 17.5MeV/u Ne ions in pulse widths of μs–ms from the AVF cyclotron

at the TIARA facility. On the other hand, Baldacchino et al. irradiated a GeV-class heavy ion with

a pulse width from 1ns to ms using the tandem-connected cyclotrons at the GANIL facility. Pulses

consisting of very short ne pulses in a continuous row were used for irradiation, except for the

1ns pulse used by Baldacchino et al. Although a short ne pulse irradiation can also be used as a

specication of the accelerator, the dose per pulse is too small due to such a short ne pulse, and

it is not easy to perform the transient absorption measurement. As mentioned above, if the lower

yields of the reactive species are considered, such as

•

OH and e

–

, in heavy-ion irradiation, as

compared with electron irradiation, 10–100Gy is required for the dose per pulse. The kHz repeti-

tion rate and million data averaging (Baldacchino et al., 2004) make the experiment possible even

with lower doses (a few cGy). On the other hand, if the behaviors of reactive species are followed by

table 13.1

list

of h

igh-energy

eavy-ion

Facilities and t

heir

pecications

institute (City/Country) Facility ions energy pulse width intensity

CEA/CNRS

(Caen/France) GANIL C–Kr 75–95

MeV/u 1ns–1ms

3–4μA

CNRS

(Nante/France) ARRONAX H, He 70

MeV 1 ns

100–750μA

CNRS

(Orléans/France) CEMTHI H, D, He 2.5–40

MeV

1μs−CW 15–40 μA

JAEA

(Takasaki/Japan) TIARA H–Au 2.5–27

MeV/u

1μs−1ms

3μA

NIRS

(Chiba/Japan) HIMAC He–Xe 6

MeV/u

5μs 0.1 μA

100–500MeV/u CW

≤1–4nA

NDRL

(Notre Dame/United

States)

FN Tandem

Van

de Graaff

H–C 15

MeV/u CW

∼1nA

GSI

(Darmstadt/Germany) FAIR H–U 2–10

GeV/u Pulse 10

–10

ions/pulse

BNL

(Brookehaven/

UnitedStates)

NSRL Ne–Au 0.3–1.5

GeV/u CW 0.7–27 nA

332 Charged Particle and Photon Interactions with Matter

the luminescence, measurement by an ns pulse with a low dose is easier than that by absorption

(Taguchi et al., 1997; Wasselin-Trupin et al., 2000, 2002).

Furthermore, whether irradiation can be done with a vertical or a horizontal beam can also be

an experimental constraint. For example, even low-energy ions of very short range in water can

be irradiated if an overhead irradiation system of a vertical beam is available. On the other hand,

if only a horizontal irradiation system is available, such low-energy ions cannot reach the sample

solution because there must be a wall of an irradiation vessel before the sample. A similar topic is

being discussed in cancer therapy. For example, there are three irradiation rooms equipped with a

vertical irradiation system, a horizontal one, or both of them for actual treatment at HIMAC. This

is because it is inevitably important to reduce exposure of normal cells, especially of radiosensitive

organs, such as blood-forming organs and gonad tissues, to ionizing radiations as well as to give suf-

cient dose to cancer. Some of the other therapeutic facilities have a gantry to adjust the irradiation

angle,

most appropriately, patient by patient, or irradiation by irradiation.

the following text, emphasis will be placed on advances in 5 years in experimental studies

on radiation chemistry with heavy ions. Especially, Section 13.2 for steady-state radiolysis stud-

ies and Section 13.3 for pulse radiolysis studies should be read complementarily because they

are strongly related to each other, although they are separately summarized. Studies on radiation

chemical effects and track structures of heavy ions until 2004 are well summarized in several

reviews, and it is recommended that readers refer to them as necessary (LaVerne, 1996, 2000,

2004; Pimblott and Mozumder, 2004). It is also recommended to refer to Chapter 14, which is

closely related to these studies.

13.2 steady-state radiolysis studies

13.2.1 Strategy and techniQueS

In steady-state radiolysis studies, scavengers have conventionally been used not only to convert

transient water radicals such as e

–

and

•

OH to easily detectable and stable species, but also to avoid

reactions between water radicals and relatively stable accumulated products such as H

and H

The

scavenging reaction with the bimolecular rate constant, k

(dm

/mol/s), is given as follows:

R S P k+ →

dm /mol/s

(13.6)

where R, S, and P are radical, scavenger, and product, respectively. As far as the dose and dose

rate are low enough, the concentration of the radical, [R] mol/dm

, is much lower than that of the

scavenger, [S] mol/dm

. Then, the scavenger concentration can be regarded as constant compared

to the radical concentration, which can drastically vary as a function of time. In most of the studies

using scavengers, it is assumed that the scavenging reaction can be regarded as a pseudo-rst-order

reaction

with a reaction rate equal to a product of k

and [S], as follows:

R P k→

−

S s[ ]

(13.7)

This reaction rate is called the scavenging capacity, and its inverse gives an approximate timescale

of the scavenging reaction. Then, by selecting a scavenger and its concentration, one can control

the scavenging timescale when transient water radicals are converted into products. It is noted

that, strictly speaking, the timescale should be carefully interpreted based on the Laplace transfor-

mation; however, the above-mentioned approximation is valid for a timescale normally later than

−8

s, because intra-spur and intra-track reactions are not so signicant at and after 10

−8

s (Pimblott

and Laverne, 1994; Yamashita et al., 2008b). In addition, this method is also applicable to stop

chemical reactions that may perturb the yield measurement. For example, H

can be destroyed

through

reactions with e

–

and

•

OH as follows:

Radiation Chemistry of Liquid Water with Heavy Ions: Experimental Studies 333

H O e OH OH dm /mol/s

aq2 2

10 3

1 3 10+ → + ×

− −i

(13.8)

H O H HO H dm /mol/s

2 22 2

10 3

1 0 10+ → + ×

i i

(13.9)

H O OH HO H O dm /mol/s

2 22 2

7 3

2 9 10+ → + ×

i i

(13.10)

scavenging both e

–

and

•

OH, such destructive reactions can be inhibited.

The production yield is briey dened as the number of products produced divided by the absorbed

energy. More strictly, the production yield of species X is represented by the following relationship:

G e N

( ) lim

( )

= ⋅ ⋅ ⋅ ⋅

→

100

∆

(13.11)

where

G(X) and C(X) are the production yield and the concentration of species X in units of (100eV)

−1

and

mol/dm

, respectively

ρ (kg/dm

), D (Gy), and N

(mol

−1

) are the density of irradiated matter, the absorbed dose, and

the

Avogadro number, respectively

Thus,

dosimetry as well as the quantication of product concentration is essential in determining

the product yield. Throughout this chapter, the radiation chemical yield is expressed in the form of a

G-value, which is dened as the number of produced or decomposed species per energy absorption of

100eV and can be converted into SI units by using the relationship G(100 eV)

−1

= G/(100eN

) mol/J =

1.036×10

−7

mol/J.

There are several methods used in dosimetry for heavy-ion irradiation. A Faraday cup and a

secondary electron monitor are normally used to measure the absolute and relative values of beam

current, respectively. If the energy deposition of the ion is known, the absorbed energy is obtainable

from the number of irradiated ions, which is obtained from the beam current. The absolute dose is

also measurable by an ionization chamber, which can count the number of ion pairs produced in

the chamber. With the W-value, which is dened as the averaged energy to create a single ion pair,

the dose is determined as a product of the W-value and the number of ionizations. Note that the

so-called recombination correction is normally necessary for a heavy ion. Track detectors such as

CR39 are also useful in determining the number of ions and their LET values based on the number

of holes and their sizes, respectively. In addition, chemical dosimeters are also useful if G-values

corresponding to the chemical change are known. The Fricke dosimeter focuses on the G-value of

a ferric ion (Fe

), which is produced through the oxidation of ferrous ions (Fe

) by oxidative water

decomposition products, such as

•

OH,

, and H

. It is remarkable that G(Fe

) in the radiolysis

of the Fricke dosimeter with a different ion beam has been intensively investigated by a group from

BNL (Appleby and Christman, 1974; Christman et al., 1981) and by a group from NDRL (LaVerne

and

Schuler, 1987, 1994; Pimblott and LaVerne, 2002).

13.2.2 track-SegMent, track-differential, and track-averaged yieldS

As described in Section 13.1.2.1, the energy deposition prole of a heavy ion varies with the energy

and the type of ions. As can be seen in Figure 13.2, a C ion of about 20MeV/u has a range of 1mm

in water. With much higher energy, the C ion can pass completely through a water sample of

1 mm thickness. In this case, the physical properties of the projectile ion are almost constant and

the measured yield is called the track-segment yield for an almost constant LET value. On the other

hand, with lower energy, the C ion cannot pass through and completely stops within the sample

solution. In other words, the Bragg peak appears within the sample and the physical properties of

334 Charged Particle and Photon Interactions with Matter

the ion drastically change in the sample solution. The simplest way to express the overall physical

properties of such low-energy ions is to show values averaged over their ranges. For example, the

track-averaged

LET is dened as follows:

LET

= =

∫

(13.12)

where

and x

are the penetration depth and the range of the projectile ion, respectively

is the kinetic energy of the ion at the entrance of the sample

Because there are not many ion accelerators capable of accelerating ions up to several hundred

MeV/u, radiation chemical yields for heavy-ion irradiation are commonly determined as track-averaged

yields for track-averaged LET or for other parameters represented as track-averaged values. It is

worth noting that LaVerne and Schuler developed a mathematical analysis to differentiate the

track-averaged yields for a variety of ion energies into track-differential yields, which are basically

equivalent to track-segment yields (LaVerne and Schuler, 1985). In addition, track-segment yields

including track-differential yields are important in developing a theoretical model that is reliable

enough to predict the track structure appropriately. Recently, radiation chemical studies with

heavy ions have been conducted to measure track-segment or track-differential yields, as opposed

earlier studies until the 1980s, in which track-averaged yields were measured.

13.2.3 priMary yieldS of Major productS related to track StructureS

13.2.3.1 primary yield and its signicance

The most intensively investigated water decomposition products are e

–

•

OH, H

, and H

. The for-

mer two radical products are initially produced during physical and physicochemical stages occurring

within 1ps, and their initial yields at 1ps are highest among all products. Based on the facts mentioned

above, the following three reactions are most signicant among all intra-spur and intra-track reactions:

e e H O H OH dm /m ol/s

aq aq 2

− − −

+ + → + ×( ) .2 2 5 5 10

9 3

(13.13)

e OH OH dm /mol/s

− −

+ → ×

2 5 10

10 3

. (13.14)

i i

OH OH H O dm /mol/s

+ → ×

9 3

6 0 10. (13.15)

In these reactions, the initially produced e

–

and

•

OH react with themselves, leading to the produc-

tion of H

and H

. As a result, the latter two molecular products are mainly produced from intra-

spur and intra-track reactions during the chemical stage occurring from 1ps to 100 ns. It is noted that

the term spur represents a relatively narrow region where a few ionizations and excitations occur in a

compact manner, while the term track represents a relatively wide region along the projectile trajec-

tory where a large number of spurs are conjugated. It is well known that intra-spur reactions almost

terminate by 100ns after the irradiation of low-LET radiations. Then, the product yield at 100 ns,

referred to as the primary yield, is valuable because it bridges the absorbed dose and the amount

of water decomposition products having escaped from the spur to the bulk. Although it is reported

that intra-track reactions for high-LET radiations do not terminate by 100ns (LaVerne et al., 2005),

the primary yield is still useful because it is an indicator inherently involving the information of the

track structure or the overlapping of spurs in the track.

Radiation Chemistry of Liquid Water with Heavy Ions: Experimental Studies 335

13.2.3.2 hydrated electron (e

–

)

A hydrated electron is the strongest reducing species among all water decomposition products and shows

strong optical absorption in the visible and near-IR region (λ

max

= 720nm, ε

max

= 18,500dm

/mol/cm).

This initiated intensive studies on the e

–

yield and its time prole as an anchor for understanding

radiation chemical phenomena. The e

–

is protonated under acidic conditions to convert into H

•

leading to lower and higher yields of e

–

and H

•

, respectively, than under neutral conditions. While

many works on e

–

produced in low-LET radiolysis have been accumulated, those with high-LET

radiations are still scarce. The rst reason is that the accessibility to ion accelerators is much more

limited than to electron accelerators. The second is low-LET radiation sources as well as that most

of the ion accelerators cannot provide an ultra-short pulse with a duration of few tens of ns. At pres-

ent, the heavy-ion beam pulse radiolysis with very high time resolution shorter than μs is available

only at the GANIL facility, and thus, the direct observation of e

–

is limited in the timescale

of >μs. Thus, the behavior of e

–

produced in steady-state heavy-ion radiolysis has been evaluated

with indirect measurements, in which e

–

is converted into stable, easily detectable, and accurately

measurable products through the scavenging reaction. Track-averaged yields of e

–

have been mea-

sured by Appleby and Schwarz for the rst time (Appleby and Schwarz, 1969), and there is a review

by LaVerne (2004). On the other hand, many measurements of track-segment or track-differential

yields of e

–

have been performed, and only few have been conducted under neutral conditions

(Appleby et al., 1986; LaVerne et al., 2005; Yamashita et al., 2008a, 2008c). Reported primary

yields of e

–

in these three studies are summarized in Figure 13.3. It should be noted that the results

of other studies mentioned above are not shown here because experimental conditions were highly

acidic or because yields were measured as track-averaged yields and cannot be directly compared.

It is also noted that H

•

yields measured by Schwarz et al. (1959) and by Anderson and Hart (1961)

would be summations of yields of H

•

and e

–

because these studies were performed in advance of

the

discovery of e

–

in 1962. There are three important ndings seen in this gure. First, it is clear

that the primary yield of e

–

decreases with increasing LET. This decrease is due to the increased

consumption of e

–

during intra-track reactions, such as reactions (13.13) and (13.14). Second, focus-

ing on data from each report separately, it is commonly seen that e

–

yields for radiolysis with dif-

ferent ions of comparable LET are different and lighter-ion irradiation tends to give smaller yields.

Although such a tendency was already observed in the radiolysis of the Fricke dosimeter, slower

reactions signicant at 100ns or longer can interfere with the correlation between the track struc-

ture and observed yields in this chemical system. Anyway, this tendency indicates that a smaller

ion generates a narrower and denser track than a heavier ion with the same LET value. Third, the

yields obtained by Yamashita et al. with He ions shown as open squares are slightly larger than

those obtained by LaVerne et al. with the same ions, shown as a dashed line. This disagreement can

be explained by the difference in scavenging timescales, or might be explained by the difference in

chemical and irradiation systems, or by the difference between track-segment and track-differential

yields. The scavenging timescale in the former study is 100ns while that in the latter is 230ns. The

consumption of e

–

would keep on going slowly but continuously during 100–230ns, as mentioned

by LaVerne et al. LaVerne and coworkers performed measurements with different scavenger con-

centrations, and e

–

yields at 23ns and 2.3μs are also determined, the former of which are shown

in Figure 13.4. As is clear from the gure, there is a consistent tendency that lighter-ion irradiation

results in a lower e

–

yield than heavier-ion irradiation of comparable LET. In addition, there is a

considerable difference between the yield for fast electrons and that for heavy ions of LET higher

than 100eV/nm even at only a few ns after irradiation. It is often assumed that the initial yields for

highly energetic charged particles are almost constant, because, in general, yields of ionizations and

excitations as a rst step of radiolysis do not depend on the radiation type, as well as because most of

the energy depositions occur through highly energetic secondary electrons. LaVerne et al. insisted

that the geminate recombination,

H O e H O

pre2 2

+ −

+ →

, before the thermalization and solvation

of e

pre

–

would be signicant for heavy ions of a few MeV/u or less, leading to smaller initial yields

336 Charged Particle and Photon Interactions with Matter

of e

–

and

•

OH and their consequent yields at later timescales (LaVerne et al., 2005). Note that

geminate recombination in water is much faster than in nonpolar liquids, and whether or not there

is geminate recombination in water and whether or not it can lead to a change in initial yields are

important

but still open questions. This will be a future subject.

13.2.3.3

hydroxyl

adical

(

•

oh)

A hydroxyl radical (

•

OH) is the strongest oxidizing species among all products in water radiolysis,

which is assumed to be responsible for lethal damage to DNA through indirect actions of ionizing

radiations. In spite of its importance,

•

OH is difcult to be observed directly by pulse radiolysis

because of weak absorption in the UV region (λ

max

∼ 240nm, ε

max

∼ 600dm

/mol/cm). Hence, con-

version with scavengers has been employed in

•

OH yield measurement both in pulse radiolysis and

3.0

2.0

1.0

0.0

3.0

2.0

1.0

0.0

1.0

0.5

0.0

–1

LET (eV/nm)

Track-segment primary G-value (100 eV)

–1

–

•

Figure 13.3 Primary yields of e

–

•

OH, and H

as a function of LET. Top, middle, and bottom panels

shows primary yields of e

–

•

OH, and H

, respectively. Solid lines on the left side of all the panels indi-

cate primary yields for low-LET radiations (Elliot, 1994). Open squares, circles, triangles, inverse triangles,

diamonds, and left triangles show data reported by Yamashita et al. with He, C, Ne, Si, Ar, and Fe ions

from HIMAC, respectively (Yamashita et al., 2008a,c). Solid circles and diamonds show data reported by

Baldacchino et al. with C and Ar ions from GANIL, respectively (Baldacchino et al., 1997, 1998a, 1999, 2003,

2004, 2006a,b). Squares and circles with a cross inside are data reported by Wasselin-Trupin et al. with He and

C ions from GANIL, respectively (Wasselin-Trupin et al., 2002). Dashed, dotted, and dash-dotted lines in the

top panel are track-differential e

–

yields for H, He, and C ions reported by a group from NDRL, respectively

(LaVerne et al., 2005). The dash-dotted line in the middle panel is the track-differential

•

OH yield with He ions

at NDRL (LaVerne, 1989). Dashed, dotted, and dash-dotted lines are track-differential

•

OH yields with He,

C, and Ne ions from the TIARA facility reported by Taguchi et al. (Taguchi and Kojima, 2005, 2007; Taguchi

etal., 2009), respectively. (From Yamashita, S. et al., Radiat. Phys. Chem., 77, 1224, 2008a. With permission.)

Radiation Chemistry of Liquid Water with Heavy Ions: Experimental Studies 337

in steady-state radiolysis. Among these studies, only ve studies have focused on track-segment or

track-differential yields of

•

OH. Appleby et al. and Yamashita et al. determined the track-segment

•

OH yields for C, Ne, and Ar ions at the BEVALAC facility and those for He, C, Ne, Si, Ar, and Fe

ions at the HIMAC facility, respectively (Appleby et al., 1985; Yamashita et al., 2008a,c). In these

studies, aerated aqueous solutions of formate and bromide anions were commonly used, and the dif-

ference in the production of H

in these solutions was taken as the

•

OH yield. Baldacchino et al.

used a uorescent probe to detect the nal product after the scavenging reaction for

•

OH produced

by the irradiation of C and Ar ions at the HIMAC facility (Baldacchino et al., 2009). Although this

method needs high performance liquid chromatography (HPLC) to separate the parent scavenger

and the nal uorescent product, its sensitivity is quite high and γ-irradiation of 10cGy or even

lower dose is detectable. Taking the high sensitivity as an advantage, they varied the scavenging tim-

escale from a few ns to μs. The track-differential

•

OH yield for irradiation of He ions was determined

by LaVerne at NDRL (LaVerne, 1989). Because formic acid is used as a scavenger for

•

OH, this is

not directly comparable to the

•

OH yield measure under neutral conditions. Since e

–

is protonated

into H

•

in acidic solutions, the consumption of

•

OH in intra-track reactions becomes minor because

the reaction between H

•

and

•

OH is slower than that between e

–

and

•

OH. Recently, Taguchi et al.

evaluated the track-differential

•

OH yields for the irradiation of He, C, and Ne ions under neutral

conditions at the TIARA facility (Taguchi and Kojima, 2005, 2007; Taguchi et al. 2009). In their

studies, phenol was taken as the scavenger for

•

OH and the production yields of the main products,

catechol, hydroquinone, and resorcinol, were measured. The primary yields of

•

OH in these studies

are summarized in Figure 13.3. Similarly as e

–

, there are three important ndings: in short, the

–1

LET (eV/nm)

G-value at a few ns (100 eV)

–1

1 ns

3 ns

–

1 ns

3 ns

γ-rays

Fast

electrons

•

Figure 13.4 Yields of e

–

and

•

OH at a few ns after irradiation obtained with steady-state beams. Top and

bottom panels show track-differential yields of e

–

and

•

OH, respectively. Solid lines on the left side of both pan-

els indicate yields at 1 and 3ns for low-LET radiations evaluated by LaVerne et al. (LaVerne, 1989; LaVerne et al.,

2005). Crosses, open squares, and open circles show data at 2.3ns obtained with H, He, and C ions by LaVerne

et al. (2005). Open squares in the bottom panel show data at 3ns obtained with He ions by LaVerne (1989), and

dashed, dotted, and dash-dotted lines show data at 1.5ns obtained with He, C, and Ne ions from the TIARA

facility by Taguchi et al. (Taguchi and Kojima, 2005, 2007; Taguchi et al., 2009), respectively.

338 Charged Particle and Photon Interactions with Matter

decrease of the

•

OH yield with increasing LET, smaller

•

OH yields for lighter ions than those for

heavier

ions of comparable LET, and slight differences between

•

OH yields in different studies.

The yields of

•

OH at a few ns have also been estimated by LaVerne (1989), Taguchi et al. (Taguchi

and Kojima, 2005, 2007; Taguchi et al., 2009), and Baldacchino et al. (2009), which are shown in Figure

13.4. There is a wide range of

•

OH yields at this timescale, that is, approximately from 3 to 0.2 (100eV)

−1

Repeatedly, it is also clearly shown that the irradiation of a lighter ion tends to lead to a smaller

•

yield than that of a heavier ion of comparable LET even at this timescale. These ndings indicate

that intra-track reactions take place signicantly within a few ns and that they are more signicant

for lower energy ions within a few ns, resulting in a wide range of

•

OH yields even at a few ns after

irradiation. In addition, although there is an overlapped LET region (20–70eV/nm) in the data series

for He ions reported by LaVerne and by Taguchi et al., the agreement between these two data series is

not always good. This would imply that the mathematical treatment employed in both studies might

lead to less reliable track-differential yields in the highest LET region or near the Bragg peaks.

In Figure 13.5, temporal information of the

•

OH yield for γ-rays and C and Ar ions from a few ns

to μs evaluated using a uorescent probe by Baldacchino et al. (2009) is summarized in comparison

to similar data for He ions reported by LaVerne (1989) and primary g-values reported byYamashita

et al. (2008a,c) as well as the results of the Monte Carlo simulation (Meesungnoen and Jay-Gerin,

2005a; Yamashita et al., 2008b). There is a perfect agreement between the

•

OH yields estimated by

Baldacchino et al., those by Yamashita et al., and the Monte Carlo simulation although the evalua-

tion with uorescence spectroscopy seems to result in relatively smaller

•

OH yields than the absorp-

tion analysis employed in the studies of Yamashita et al. This might be because of the error in the

conversion ratio from

•

OH to the nal uorescent product. Baldacchino et al. assumed that 5.6% of

the

scavenged amount of

•

OH leads to the production of the nal uorescent product.

13.2.3.4 hydrogen

eroxide

(

)

Hydrogen peroxide is a key species for understanding the entire concept of water radiolysis. It itself

is a reactive oxygen species (ROS) and can be a source of other ROS, such as

•

OH and

i−

. Because

γ-rays

C 11 eV/nm

Ar 90 eV/nm

He 100 eV/nm

Time or timescale (ns)

G (

OH) (100 eV)

–1

Figure 13.5 Time prole of

•

OH yield. Open squares with a plus inside and crosses show reported

•

yield for γ-rays (Draganic´ and Draganic´, 1973; Jonah and Miller, 1977). Pluses, solid circles, and diamonds

show experimentally determined

•

OH yields with γ-rays, 400MeV/u C ions, and 500MeV/u Ar ions from the

HIMAC facility, respectively (Baldacchino et al., 2009). Solid and dashed lines show Monte Carlo simulation

results for 400MeV/u C and 500MeV/u Ar ions, respectively (Meesungnoen and Jay-Gerin, 2005a; Yamashita

et al., 2008b). Open squares show experimentally reported yield for 1.1MeV/u He ions, the LET of which is

comparable to 500MeV/u Ar ions (LaVerne, 1989). (From Baldacchino, G. et al., Chem. Phys. Lett., 468, 275,

2009. With permission.)

Radiation Chemistry of Liquid Water with Heavy Ions: Experimental Studies 339

is not so reactive compared to e

–

and

•

OH, and is much more soluble in water than H

, it

can be determined rather easily. Only two groups have reported on track-segment yields of H

Yamashita et al. measured H

yields for He, C, Ne, Si, Ar, and Fe ions of several 100 MeV/u at

the HIMAC facility by applying the Ghormley tri-iodide method (Yamashita et al., 2008a,c). They

employed the conventional off-line absorption spectroscopy after the conversion of survived H

into I

–

for the quantication of produced H

. Wasselin-Trupin et al. applied emission spectros-

copy for irradiations of C and Ar ions of 95MeV/u at the GANIL facility, although a pulsed beam,

and not a steady-state beam, was used for irradiation. They detected chemiluminescence emitted

after luminol reactions for the quantication of H

(Wasselin-Trupin et al., 2002). Track-segment

primary

yields of H

reported in these studies are also shown in Figure 13.3.

Compared to radical products, H

yields are less labile to variation, and differences in

radiolysis with different ions are less clear. However, focusing on the two data series obtained

with C and Fe ions, the H

yields in Fe ion radiolysis seem to be shifted to a higher-LET

region from those in C ion radiolysis. For example, H

yields in C ion radiolysis at an LET

of about 20 eV/nm are about 0.8 (100 eV)

−1

, almost the same as that in Fe ion radiolysis at

an LET of about 200 eV/nm. In short, heavier-ion radiolysis of much higher LET can lead to

almost the same H

yield as obtained in lighter-ion radiolysis of much lower LET. This indi-

cates that an apparent radical density in the track can be regarded as equivalent for these two

ions in terms of H

production, which is consistent with the tendency mentioned in Sections

13.2.3.2 and 13.2.3.3 that a lighter ion tends to create a narrower and denser track structure. One

might think that the so-called higher-order reactions to consume already produced H

might

be effective in a heavier ion of higher LET; however, this speculation is inconsistent with the

above-mentioned nding seen in radical yields. Anyway, there are only few studies reporting on

track-segment or track-differential H

yields. Pastina and LaVerne measured H

yields for

H, He, and C ions of 2–15 MeV/u as track-averaged yields at neutral pH (Pastina and LaVerne,

1999). Of course, their results cannot be directly compared to track-segment yields in other

reports; however, it is worth noting that they found that H

yields for C ions monotonically

decrease with increasing LET from 629 to 787 eV/nm, while those for H and He ions monotoni-

cally increase with increasing LET from 10 to 34.8 and from 78 to 156 eV/nm, respectively.

This tendency is explained as follows: the importance of short-time reactions, which would be

higher-order reactions such as

H O e

2 2 aq

−

and H

•

OH, increases with increasing LET for

C ion radiolysis. A detailed mechanism for this tendency of the H

yield is not claried yet,

and another possible explanation is reported theoretically by incorporating multiple ionizations

(Meesungnoen and Jay-Gerin, 2005b).

13.2.3.5 hydrogen

olecule

(

)

Similar to H

, the hydrogen molecule is also a key species for understanding the entire concept

of water radiolysis. It is also important to investigate the initial processes induced in water radioly-

sis within 1ps. LaVerne and Pimblott proposed a new mechanism, so-called dissociative electron

attachment, for H

formation in water radiolysis (LaVerne and Pimblott, 2000). They insisted that

some precursors of

e e

aq pre

− −

( )

attach to the surrounding water molecule to produce a water radical

anion

•−

), leading to the production of H

in the following reactions:

e H O H O

pre 2 2

− −

+ →

(13.16)

H O H OH

i i− −

→ + (13.17)

H H O H OH

2 2

− −

+ → + (13.18)