Hydrogen peroxide production in the radiolysis of water with heavy ions

Article abstract of DOI:10.1021/jp984433o

The radiation chemical yields of hydrogen peroxide in water at neutral pH have been determined with protons, helium ions, and carbon ions at energies of a few to 30 MeV. The long-time yields of hydrogen peroxide increase with increasing linear energy transfer, LET, for protons and helium ions, but it decreases for carbon ions due to higher order reactions within the particle track. However, the maximum increase in hydrogen peroxide yields is only about 50% from γ rays (LET = 0.2 eV/nm) to helium ions (LET = 156 eV/nm). Methanol was used as an OH radical scavenger in order to probe the temporal dependence of hydrogen peroxide formation. The differences in the time dependence of the formation of hydrogen peroxide with various ions are discussed and compared to that observed with γ rays.

Full text of DOI:10.1021/jp984433o

1592
J. Phys. Chem. A 1999, 103, 1592-1597
Hydrogen Peroxide Production in the Radiolysis of Water with Heavy Ions
Barbara Pastina†,‡ and Jay A. LaVerne*,†
Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and Commissariat a` l’Energie
Atomique DSM/DRECAM/SCM Baˆtiment 125 CE/Saclay 91191 Gif sur YVette Cedex, France
ReceiVed: NoVember 13, 1998; In Final Form: January 19, 1999
The radiation chemical yields of hydrogen peroxide in water at neutral pH have been determined with protons,
helium ions, and carbon ions at energies of a few to 30 MeV. The long-time yields of hydrogen peroxide
increase with increasing linear energy transfer, LET, for protons and helium ions, but it decreases for carbon
ions due to higher order reactions within the particle track. However, the maximum increase in hydrogen
peroxide yields is only about 50% from γ rays (LET ) 0.2 eV/nm) to helium ions (LET ) 156 eV/nm).
Methanol was used as an OH radical scavenger in order to probe the temporal dependence of hydrogen
peroxide formation. The differences in the time dependence of the formation of hydrogen peroxide with
various ions are discussed and compared to that observed with γ rays.
TABLE 1: Characteristics of the Particles Used in This
Worka
Introduction
Hydrogen peroxide is the main oxidizing molecular product
formed during the radiolysis of water. It is formed primarily
by combination reactions of OH radicals produced in the
radiolytic decomposition of water. Therefore, its yield and
chemistry are central to understanding the fundamental processes
of the OH radical and other oxidizing species. On a more
practical aspect, hydrogen peroxide produced in water radiolysis
is involved in oxidation damage in almost every domain of
nuclear technology where high linear energy transfer (LET )
stopping power) radiation is involved. For instance, hydrogen
peroxide scavenging of H atoms has a major role in the water
cooling circuit of nuclear reactors.1 Despite its importance, the
few published reports on the production of hydrogen peroxide
are somewhat inconsistent and there is virtually no information
on the temporal variation of hydrogen peroxide with high LET
heavy particles.
The effect of track structure on radiolytic yields with high
LET particles has been investigated in aqueous solutions.2-13
Most of the results concern the yields of molecular hydrogen
and of the radicals formed by water radiolysis (H, OH, HO2).
No experimental data are available on the yields of hydrogen
peroxide obtained with heavy ions as a function of the
concentration of added scavengers, and there are few studies at
neutral pH. It is known that the increased energy deposition
density by high LET particles leads to an increase in hydrogen
peroxide production.14 However, the exact correlation between
the effects of track structure on OH radicals, hydrogen peroxide,
and other possible oxidizing species has not been established,
in part because of lack of information on the hydrogen peroxide
yield. It is especially important to know the temporal variation
in the hydrogen peroxide yield in the particle track. The addition
of a scavenger for the OH radical enables one to gain knowledge
on the time scale of hydrogen peroxide formation and on the
relationship between OH radicals and hydrogen peroxide. This
information will help elucidate the role of oxidizing species in
water radiolysis and on the general decomposition of water with
high LET radiation.
energy
(MeV)
track average LET
in water (eV/nm)
range in water
(mg/cm2)
particle
1H
2
5
10
15
34.8
20.7
13.8
10.3
7.31
35
119
246
4He
12C
5
15
20
156
92.1
78
3.56
21.1
34.7
10
20
30
787
703
629
1.35
2.99
5.06
a The track average LET is the stopping power averaged over the
entire range of the particle.24
In this work, hydrogen peroxide yields were determined in
the radiolysis of water at neutral pH with protons, helium ions,
and carbon ions of different energies. The characteristics of the
particles used are listed in Table 1. Some of these particles are
particularly important for nuclear technology. For instance, 5
MeV helium ions are equivalent in energy to most natural R
particles. The temporal variation of hydrogen peroxide formation
was examined by the addition of various concentrations of
methanol, an efficient OH radical scavenger. The experimental
techniques are presented next, followed by the results and their
discussion.
Experimental Section
The heavy ion experiments were performed using the facilities
of the Nuclear Structure Laboratory of the University of Notre
Dame Physics Department. The particles were produced and
accelerated using a FN Tandem Van de Graaff. After accelera-
tion, the ions were energy and charge-state selected magnetically
with a resulting energy resolution of less than 0.1%. The window
assembly and the irradiation procedure were the same as
reported earlier.10,11 The energy of the particles after passing
through all windows was determined from the TRIM stopping
power compilation.15 The solutions were irradiated with com-
pletely stripped particles at beam currents of about 5 charge
† University of Notre Dame.
‡ Commissariat a` l’Energie Atomique.
10.1021/jp984433o CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/27/1999

Hydrogen Peroxide Production
J. Phys. Chem. A, Vol. 103, No. 11, 1999 1593
nA. Absolute dosimetry was obtained from the product of the
integrated beam current and the particle energy. The doses given
were about 70 krad (700 Gy) in 22 mL samples.
γ ray radiolysis was performed using a Gammacell 220 60Co
γ ray source at the Radiation Laboratory of the University of
Notre Dame. The dose rate was 1.6 krad/min (16 Gy/min).
Samples were irradiated to total a dose of 70 krad (700 Gy).
The Fricke dosimeter was used to determine dose rate.14
All solutions were prepared using water from a Millipore
Milli-Q UV system. The samples consisted of N2-saturated
aqueous solutions containing 25 mM of sodium nitrate (reagent
grade, Aldrich) and various concentrations of methanol (reagent
grade, Fischer Scientific). The pH of the solutions was near
neutral (pH ≈ 6.5) and did not change during the course of the
experiments. The doses at the highest methanol concentrations
(1 and 10 M) have been corrected according to the fractional
electron density of the water. The hydrogen peroxide concentra-
tions were measured using the Ghormley method, in which I-
is oxidized to I3- by the hydrogen peroxide.16,17 The absorbance
of the solution was measured at 350 nm using a diode array
spectrophotometer (Hewlett-Packard HP8453). The molar ex-
Figure 1. Hydrogen peroxide yields in γ radiolysis as a function of
the scavenging capacity for OH radicals: (b) methanol and 25 mM
nitrate, this work; (4) ethanol and sodium nitrate, ref 21; (0) ethanol
and acetone, ref 22; (]) aerated bromide, ref 20.
the hydrogen peroxide from further reaction with the hydrated
electron, (5) and (6), and with H atom, (7) and (8).
-
tinction coefficient for the I3 was measured at 350 nm and
OH + CH3OH f CH2OH + H2O;
k4 ) 9.7 × 108 M-1 s-1 (4)
eaq- + H2O2 f OH + OH-; k5 ) 1.1 × 1010 M-1 s-1 (5)
calibrated with a standard solution of H2O2 (Fischer Scientific).
It was found that ꢀ350 ) 25850 M-1 cm-1, in good agreement
-
with previous data.17 The molar extinction coefficient of I3
changes slightly in mixtures of water and methanol, and for
the 10 M methanol solution it was found to be 26000 M-1 cm-1
.
eaq- + NO3- f NO3
;
k6 ) 9.7 × 109 M-1 s-1 (6)
2-
The results using 10 M methanol give a minimum estimation
of the yield because of the secondary reaction between the
radical of the methyl alcohol and the hydrogen peroxide.18
However, the agreement of the present results with those
obtained in γ radiolysis with other chemical systems (see below)
suggests that the contribution of this secondary reaction is small.
H + H2O2 f OH + H2O; k7 ) 9.0 × 107 M-1 s-1 (7)
-
H + NO3- f HNO3 ; k8 ) 1.4 × 106 M-1 s-1 (8)
By varying the concentration of methanol, one can probe the
chemistry in the track using the competition between reactions
1 and 4. The resulting dependence of hydrogen peroxide yields
on methanol concentration can be used to obtain information
on its time scale of formation.
Results and Discussion
The energy deposited by all ionizing radiation is localized in
nonhomogeneous regions along their path as they are stopped
in the bulk of the water medium. Hydrogen peroxide, H2O2, is
formed within these tracks mainly by the combination of OH
radicals formed by the decomposition of water molecules,
reaction 1. At very high LET, reactions of the hydroperoxyl
radical, HO2, may also lead to hydrogen peroxide formation,
reactions 2 and 3.
The chemical system (NaNO3, CH3OH, and the Ghormley
analysis method) was first verified by comparing the G(H2O2)
obtained with γ rays to those values published in the literature
for water at neutral pH.20-22 (Radiation chemical yields are
expressed as G values in units of molecules/100 eV of total
energy deposition.) Figure 1 shows the hydrogen peroxide yields
as a function of the scavenging capacity for OH radicals. The
scavenging capacity is the product of the rate coefficient and
the scavenger concentration and it is equivalent to the pseudo-
first-order rate constant for the scavenging reaction. The inverse
of the scavenging capacity gives the lifetime of the radical with
respect to that reaction. It is seen in Figure 1 that there is good
agreement between all the sets of data over the entire scavenging
capacity range. Low scavenger concentrations correspond to
long times in the track, and the limiting yield represents the
amount of product that escapes the track into the bulk medium,
the escape yield. The escape yield of hydrogen peroxide with
γ rays is found to be 0.7 molecules/ 100 eV. Experiments show
this value is linear with dose up to about 150 krad (1500 Gy).
This linearity suggests that possible side effects due to the
reactions of nitrate or its products are negligible.
OH + OH f H2O2; 2k1 ) 1.1 × 1010 M-1 s-1 (1)
HO2 + HO2 f H2O2 + O2; k2 ) 9.8 × 105 M-1 s-1 (2)
HO2 + O2- f HO2- + O2; k3 ) 9.7 × 107 M-1 s-1 (3)
All rate constants are from the compilation of Buxton et al.19
unless otherwise stated specifically in the text. Hydroperoxyl
radical yields are usually considered to be negligible, but they
increase with increasing LET.14 The maximum yield of hydro-
peroxyl radicals with low-energy carbon ions has been measured
by LaVerne and Schuler to be approximately 0.25 molecules/
100 eV.11 This relatively low yield and the small rate coefficients
suggest that for the present purposes the contributions of
reactions 2 and 3 to the formation of hydrogen peroxide can be
neglected. However, these reactions must be included in any
refined track structure calculation.
Nitrate was added to the system to protect the hydrogen
peroxide from attack by the hydrated electron and H atom,
reactions 5-7. However, the addition of nitrate can also present
problems. A high concentration of nitrate can scavenge the
hydrated electron before the OH radical is scavenged. Since
In this work, methanol has been used as an OH radical
scavenger, (4), to compete with the production of hydrogen
peroxide in reaction 1. Sodium nitrate was also added to protect

1594 J. Phys. Chem. A, Vol. 103, No. 11, 1999
Pastina and LaVerne
variations in the experimental conditions. Many of the previous
experiments were performed in acidic conditions (0.4 M sulfuric
acid). With the same particle, the G values measured in this
work at lower LET are 10-16% lower than those measured in
acidic conditions. For example, the data of Burns and Sims6
are about 15% higher than expected for the 20 MeV He ions
examined here. At high LET, the pH effects are more striking.
The data of LaVerne12 for 10 MeV carbon ions are 50% higher
than those found here. Apparently, the effect due to pH also
depends on the LET. The lifetime of the hydrated electron is
much shorter in acidic solutions than in neutral solutions because
of its reaction with the hydrated proton. Hydrated electrons
normally react with part of the OH radicals that would otherwise
lead to the formation of H2O2, reaction 1. Removal of the
hydrated electrons results in an increase in OH radical yields
and in the subsequent hydrogen peroxide yields. The yields of
hydrogen peroxide in a neutral solution are expected to be lower
than in 0.4 M acid solution, which is observed. Note that in the
present work a hydrated electron and H atom scavenger were
used to eliminate their reactions with hydrogen peroxide,
reactions 6 and 8. If the hydrated electron scavenger is not
added, then one observes a decrease in hydrogen peroxide yields
with decreasing pH.25
Figure 2. Production of hydrogen peroxide [G0E0(H2O2)] as a function
of the initial particle energy, E0: (9)1H. (1)4He, (b)12C, this work
2
(10-5 M methanol, 25 mM nitrate); (O) 12C, ref 12; (0) H and (3)
4He, ref 7; The dashed line (---) gives the yield with fast electrons
(0.7), ref 23, and the dotted line (‚‚‚) with fission fragments (0.96), ref
5.
one of the primary radical reactions is between the OH radical
and the hydrated electron, removal of one species will raise the
apparent yield of the other. At the lowest concentrations of
methanol used here, the concentration of the nitrate should be
about 2.5 mM and not the 25 mM used in these experiments in
order not to interfere in OH radical chemistry. However, it has
been shown that the increase in OH yields with γ radiolysis is
only about 5% over these nitrate concentrations.22 This increase
is within experimental error, and the higher nitrate concentration
is better for minimizing effects due to H atoms, which may be
important at the higher LET. The products of reactions 6 and 8
involve radical species that may scavenge primary radicals from
the water decomposition or that interfere with the analysis of
hydrogen peroxide. However, the data shown in Figure 1 involve
several different radical scavenging systems and they give
similar results. Any interference in the radiation chemistry or
the analysis by the addition of nitrate is expected to be
negligible.
The production of hydrogen peroxide, G0E0(H2O2) (molecules/
100 particles), by the proton, helium ion, and carbon ion
irradiation of deaerated 10-5 M methanol with 25 mM sodium
nitrate aqueous solutions is shown in Figure 2 as a function of
the initial particle energy, E0. Here, G0 is the observed radiation
chemical yield, which is the integral, or track averaged, yield
since the particles are stopped in the solution. Also shown are
the yields found with 12C, ref 10; 2H (plotted at one-half energy)
and 4He, ref 7; fast electrons, ref 23; and fission fragments, ref
5. The later experiments were performed at low scavenging
capacity of OH radicals comparable to the present work. The
studies with the light ions are the few for which hydrogen
peroxide yields were examined over a substantial energy range.
The near linearity of the lines for a given set of experiments
with a particular particle shows that the integral radiation
chemical yield is nearly independent of the particle energy and
that track segment, or differential, yields are virtually equivalent
to the integral yields. Therefore, to a first approximation, one
can probe the effects of track structure by examining the integral
or track averaged yields as a function of the track average LET.24
As with γ rays, experiments show the production of hydrogen
peroxide is linear with dose up to about 150 krad (1500 Gy).
This linearity suggests that possible side effects due to the
reactions of nitrate or its products are negligible.
The results obtained by Schwarz et al. in neutral solutions
are 11% higher than the present values.2 They measured the
hydrogen peroxide yield using an air-saturated potassium
bromide solution. The G value was determined by correcting
the measured hydrogen peroxide yield by the estimated con-
tribution of hydroperoxyl radicals. This correction may be the
source of the small differences in the data. The yields of
hydrogen peroxide obtained with helium and carbon ions are
very similar to those obtained by Bibler with fission fragments
(G ) 0.96 molecules/100 eV), although those measurement were
made in 0.4 M sulfuric acid.5 The striking feature of Figure 2
is that the G value for the formation of hydrogen peroxide is
nearly independent of the nature of the particle or its energy.
This result is also confirmed by deterministic track calculations
that predict an increase of about 50% from γ rays to 1000 eV/
nm particles, which is similar to the LET of carbon ions.12 The
calculations suggest that competing reactions between several
oxidizing species are responsible for determining the hydrogen
peroxide yield. These reactions become more significant with
increasing LET due to the buildup of H2O2 and HO2 to high
concentrations in the track at short times. These products are
then capable of scavenging radicals that would otherwise have
reacted with each other. The near constancy of the hydrogen
peroxide yields suggests that it would make a practical chemical
dosimeter. An average G value of 0.88 is reasonably (within
about (20%) independent of particle type or LET.
The track of a low LET fast electron in water mainly consists
of isolated spurs containing two to three ion pairs.26 The OH
radicals produced from the decomposition of water molecules
can combine with sibling radicals or diffuse into the bulk
medium. Reaction of OH radicals with other OH radicals
obviously leads to the production of hydrogen peroxide. With
increasing LET, the ionization events in water become, on
average, more closely spaced from one another. At sufficiently
high LET, the individual spurs coalesce into a track that is often
depicted like a cylinder surrounded by spurs formed by the
secondary ionization processes (δ rays). Increasing the density
of radicals leads to an increase in molecular products. In Figure
3 are shown the radiation chemical yields of hydrogen peroxide
obtained in the present work with protons, helium ions, and
carbon ions as a function of their integral or track average LET.
There are noticeable differences between the yields obtained
with the same type of particles that can be explained by

Hydrogen Peroxide Production
J. Phys. Chem. A, Vol. 103, No. 11, 1999 1595
Figure 3. Hydrogen peroxide yields as a function of the LET for:
Figure 4. Hydrogen peroxide yields as a function of the scavenging
capacity of methanol (25 mM nitrate) with protons of different
energies: (b) 2 MeV; (2) 10 MeV; (1) 15 MeV. The results with γ
rays are (0).
1
(9) H, (2)4He, and (b) 12C (10-5 M methanol, 25 mM nitrate), this
work; (0) 2H and (4) 4He, ref 7; (]) 1H and (×) 7Li, ref 8. The error
bars represent (5%. The dotted line represents the limiting yield with
γ rays.
radicals in the track.
The data are determined with 10-5 M methanol concentration
and represent the yields at about 100 µs when most of the track
structure has collapsed. Also shown in Figure 3 are the data of
Anderson and Hart7 and Elliot et al.8 at nearly the same OH
radical scavenging capacity. It can be seen that for near neutral
solutions the results agree. The data of Anderson and Hart at
the lowest particle energies appear to be slightly high, but that
could easily be due to straggling effects. As the LET of protons
increases, only a small increase of G(H2O2) is observed and
the yields are very close to that obtained with γ rays. This result
is expected since high-energy protons produce tracks of (on
average) well separated spurs. Only at the lowest proton energies
would one expect a cylindrical track with a resultant increase
in hydrogen peroxide yields.
HO2 + OH f H2O + O2;
k10 ) 1 × 1010 M-1 s-1 (ref 27) (10)
reactions 9 and 10 could be competing with reaction 1 causing
a decrease in the observed hydrogen peroxide yield. The exact
nature and extent of the reactions leading to a decrease in
hydrogen peroxide yields at high LET will probably only be
fully explained by extensive track model calculations.
Figure 4 shows the influence of the OH radical scavenger
concentration on the hydrogen peroxide yields for protons of
different energy. It can be seen that the results are comparable
to those found with γ rays, suggesting that the spatial distribution
of the primary species is not very different for protons and for
γ rays. This result is expected considering the coalescing of
the spurs considered above. Cobalt γ rays consist of two photons
of average energy 1.25 MeV.28 These photons undergo Compton
scattering to give fast electrons with an average LET of about
0.36 eV/nm.28 The average energy loss event by electrons and,
to a first approximation, protons involves about 60 eV.29 Energy
loss events are stochastic in nature, but on the average the spurs
formed by these events are separated by about 170 nm. The
typical spur has a Gaussian distribution of OH radicals with σ
of only about 1 nm, and therefore spurs are well separated in γ
radiolysis.30 Only at an LET greater than about 15 eV/nm (60
eV/4σ) can one expect sufficient spur overlap along the main
particle track that OH radicals in one spur can react to a
significant extent with those from a different spur to increase
the hydrogen peroxide yield. From the data in Table 1, low-
energy protons are expected to have a track structure, and
subsequent chemistry, much different than that of γ rays.
The hydrogen peroxide yield decreases with increasing
methanol concentration for the same reasons given above for γ
rays. Significant (>10%) scavenging of the OH radicals occurs
at scavenging capacity greater than about 108 s-1. This scaveng-
ing capacity suggests that the tracks (spurs) produced by γ rays
and protons are very nearly dissipated by about 10 ns. Although
the time scale may be similar for both types of particles, the
difference in the yields in the limiting plateau region suggests
that there are small variations in the distributions of radicals in
the tracks.
Irradiation with helium ions leads to an increase in hydrogen
peroxide yields over that observed with γ rays. The LET with
helium ions is more than 2 orders of magnitude greater than
that for γ rays (0.2 eV/nm), but the yield of hydrogen peroxide
is only greater by about 50%. The yield of hydrogen peroxide
with carbon ions is similar to that obtained with helium ions.
However, the yield of hydrogen peroxide increases with
increasing helium ion LET and it decreases with increasing
carbon ion LET. It is possible that some unknown experimental
artifact is giving too low of a yield for the highest LET carbon
ion and that the hydrogen peroxide yields are constant with
carbon ions in this energy range. Nevertheless, it is readily
apparent from the data that at least the yields of hydrogen
peroxide are not increasing steadily with increasing LET. Such
a scenario, as possibly suggested by the early helium ion data,
is expected if the hydrogen peroxide chemistry is almost
exclusively due to the combination of two OH radicals. Any
leveling or drop in the hydrogen peroxide yield with increasing
LET to the G values found here indicates that other, as yet
unknown, reactions are depleting it. These reactions can become
significant with relatively small increases in LET.
Burns and Sims6 suggest that the concentration of hydrogen
peroxide formed within the track can be considerable at very
high LET and it may react with the other primary species,
reactions 5, 7, and 9.
H2O2 + OH f HO2 + H2O; k9 ) 2.7 × 107 M-1 s-1 (9)
Another possible explanation for the decreased yield of hydrogen
peroxide may be due to the increased yield of the hydroperoxyl
radical, HO2, at high LET. This radical can also deplete OH
The results of Figure 3 indicate that at higher LET the yields
in the plateau region are dependent on particle type as is
observed in Figure 5. This figure shows the dependence of

1596 J. Phys. Chem. A, Vol. 103, No. 11, 1999
Pastina and LaVerne
for heavy ions with an accuracy of (20%. This information is
useful for reactor technology since roughly the same yield can
be used for a wide variety of different particles, including fission
fragments, fast neutrons, and R particles.
Hydrogen peroxide yields show very similar OH radical
scavenging capacity dependencies for the different types of
particles examined here. Although the results would suggest
similar distributions of transient species in the tracks, extreme
caution must be used. The limiting yields at low scavenger
capacities clearly show increased competition in the track at
high LET. The increased importance of short-time reactions
which decrease hydrogen peroxide yields may coincidentally
give similar yields as particles with lower LET even though
they have very different track structure. Detailed track model
calculations in conjunction with results on other water species
produced at similar LET will help elucidate the correct track
structures and their effects on radiation chemical processes.
Figure 5. Hydrogen peroxide yields as a function of the scavenging
capacity of methanol (25 mM nitrate) for: (0) γ rays; (b) 2 MeV 1H;
4
(1) 5 MeV He; (2) 10 MeV 12C.
G(H2O2) on the OH radical scavenging capacity for γ rays, 2
MeV protons, 5 MeV helium ions, and 10 MeV carbon ions.
The 5 MeV helium ions give the highest hydrogen peroxide
yields. At high concentrations of OH scavenger, the curves for
helium ions and carbon ions coalesce, which superficially could
be interpreted as the probing of track regions with similar
characteristics. This assumption is clearly wrong since the long-
time yields are different. At high scavenging capacity, OH
radicals are being scavenged at short times where transient
concentrations can be high. The reaction of hydrogen peroxide
with these transients may lead to an apparent lower yield. Further
studies giving a direct comparison of the temporal variation of
hydrogen peroxide with that for OH radicals will be very
illuminating on the track processes at high LET.
Acknowledgment. We thank Professor J. J. Kolata for
making the facilities of the Notre Dame Nuclear Structure
Laboratory available. The latter is funded by the National
Science Foundation. B.P. has been supported by the Commis-
sariat a` l′Energie Atomique (DSM/DRECAM/SCM). The
research described herein was supported by the Office of Basic
Energy Sciences of the Department of Energy. This contribution
is NDRL-4093 from the Notre Dame Radiation Laboratory.
References and Notes
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Phys. Chem. 1996, 100, 9014-9020.
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The relative invariance of the hydrogen peroxide yield at
scavenging capacities below about 108 s-1 appears to be in
disagreement with the temporal evolution of the particle track
as predicted with previous studies on OH radicals31 and hydrated
electrons.32 Scavenger studies on the radicals suggest that
considerable track chemistry can occur on the time scales of
10-9 to 10-6 s depending on the LET. However, a constant yield
of hydrogen peroxide at low scavenger capacities indicates that
the track has completely relaxed spatially and one is measuring
the escape or long-time yield. Calculations have shown that the
molecular yields, H2O2 and H2, are almost always less sensitive
to track conditions at long times relative to the radical yields,
-
eaq and OH.33 Molecular yields are generally smaller than
radical yields, so it is difficult to observe small relative changes.
Furthermore, two radicals are involved in the production of one
molecular product, so the net change of radicals is greater. The
observed temporal variation in radical yields strongly suggests
that the tracks of particles in the LET range of 10-100 eV/nm
are still evolving on the microsecond time scale. It is important
to have reliable data for both radicals and molecular products
to properly assess the effects of track structure on the radiation
chemistry of water.
(19) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J.
Phys. Chem. Ref. Data 1988, 17, 513-886.
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Conclusion
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57.
The yields of hydrogen peroxide in water at neutral pH have
been measured in solutions of sodium nitrate and methanol. It
has been observed that the yields are nearly constant with the
energy of the particle, suggesting that track averaged yields are
virtually the same as track segment yields. Furthermore, the
hydrogen peroxide yields depend very little on the nature of
the particle over a wide range of LET. This result suggests that
a hydrogen peroxide yield of 0.88 can be used as a dosimeter
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Eo
(24) Track average LET is defined by LET ) 1/Eo ∫0 (dE/dx) dE.
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