16034
J. Phys. Chem. 1996, 100, 16034-16040
Radiolysis of the Fricke Dosimeter with Linear Energy Transfer
58Ni
and
238U
Ions: Response for Particles of High
Jay A. LaVerne* and Robert H. Schuler Radiation Laboratory and Department of Chemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: May 22, 1996; In Final Form: July 5, 1996X
The radiation chemical yields of the Fricke dosimeter have been determined for incident nickel and uranium ions with energies of up to 470 and 3780 MeV, respectively. Track segment yields have been determined from the observed energy dependences. Experiments utilizing both aerated and deaerated solutions indicate that at the very high values of linear energy transfer (LET) exhibited by these ions in water (a maximum of 5000 and 17 000 eV/nm, respectively) a significant fraction of radicals escape from the track. The radiation chemical yield of H atoms for both ions is found to be ∼10% of the value with fast electrons. The data also suggest that there is a substantial yield of an oxidizing species produced by particles of very high LET. A comparison of the results with particles of lower LET is made.
Introduction Some of the earliest studies on the radiolysis of water with positive ions found that the yield of radical species was very dependent on the linear energy transfer (LET) (i.e., the stopping power, -dE/dx) of the ionizing particle.1 With increasing LET radical yields decrease because of the increased importance of intratrack reactions between radicals as compared to their diffusion into the bulk medium. Much of the radiation chemistry literature on water and aqueous solutions is devoted to the measurement of the radical yields produced in water and aqueous solutions by fast electrons, or γ-rays, where the LET is ∼0.2 eV/nm. While there is a substantial amount of information with positive ions that have LETs in the range 1-1000 eV/nm, there is currently little information on the yields induced by the passage of higher LET radiation.2 The purpose of this investigation was to examine the radiolysis of water with accelerated nickel and uranium ions which have an order of magnitude higher LET. It has generally been assumed that the concentrations of radicals in the tracks of very high LET particles are so great that few, if any, survive sufficiently long (>µs) that they can escape into the bulk medium.3,4 Indeed, Bibler5 used a combination of solutes and concluded that the yields of both OH radicals and H atoms which escape the tracks of fission fragments (LET of ∼4000 eV/nm)6 were zero. The assumption that very high LET particles have no escape radical yields can have a profound implication on the interpretation of other radiation studies. For instance, the radiolysis of mammalian and yeast cells shows significant inactivation even when irradiated with very high LET uranium ions.7,8 If the radical yields are actually zero, then other processes must be responsible for the radiation damage. However, some of the solutions Bibler used are known to be problematical in that the measured yields are sensitive to factors that are difficult to control. 9 Escape radical yields in water may not be zero with very high LET particles. It is important to understand the basic radiation chemistry of water with particles of very high LET in order to elucidate these and other radiolytic mechanisms. Studies of the oxidation of ferrous ions in the Fricke dosimeter have been found to give a good overview of the track structure of high LET particles.10-12 A comparison of the yields in X
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01482-7 CCC: $12.00
aerated and deaerated solutions is particularly well suited to give the H atom yield in acidic water. The present paper reports a study of the Fricke dosimeter and its deaerated counterpart with nickel ions having energies in the range 50-470 MeV (LETs of 5100-3300 eV/nm) performed using the ATLAS linear accelerator of Argonne National Laboratory. Additional experiments were performed with uranium ions having energies in the range 1250-3780 MeV (LETs of 17 000-15 000 eV/ nm) using the K1200 cyclotron of the Michigan State University National Superconductor Cyclotron Laboratory (NSCL). These studies represent the highest LET for which radiation chemical yields have been determined. Experimental Section Solutions and Absorption Measurements. Fricke dosimeter solutions were 0.01 M in ferrous ammonium sulfate and 0.8 N in sulfuric acid with no added chloride. Solutions were either air-saturated or deaerated in the radiolysis cell by purging with helium before and during the radiolysis. Water was obtained from a conventional radiation chemical triple still employing alkaline permanganate and acidic dichromate or from a Millipore Milli Q UV-Plus system. The feedstock to both devices was from a Millipore Milli Q system. No difference in the response of the Fricke dosimeter was observed using water from these different sources. Solutions prepared from this water were sufficiently stable to enable measurements over the few days necessary to complete a set of experiments. Absorption measurements were made at 304 nm with a Beckman DU2 spectrophotometer and were generally reproducible to an absorbance of ∼0.001. The ferric ion extinction coefficient at 304 nm was taken as 2174 M-1 cm-1 at 23.7 °C, and corrections were applied for variations in the analysis temperature.13 Irradiation Cells. The irradiations were carried out in cells similar to those used in previous heavy ion studies at Notre Dame.10,11 These cells had a 1 cm optical cuvette and a magnetically driven pump which rapidly circulated the solution through the irradiation zone and cuvette. The irradiating particles entered the cell through a ∼6 mg/cm2 titanium window into an irradiation zone ∼2 mm deep which was sufficient to completely stop all incident particles. Total volume of solution was 32 mL. Previous experiments with lighter ions used mica windows, however, mica was found to be severely distorted after only a few irradiations with nickel or uranium ion beams. © 1996 American Chemical Society
Radiolysis of the Fricke Dosimeter with
58Ni
and
238U
Several experiments comparing the results between mica and titanium windows showed that the latter were inert toward the Fricke solution. Irradiations at the ATLAS with 58Ni. The measurements at the ATLAS were made with 58Ni ions having energies of 660, 649, and 500 MeV before passing through the window system. Aluminum absorbers were used to obtain energies below these values. Energies of the fully accelerated ions were determined from time-of-flight measurements in the accelerator vacuum system and were known to better than 0.5%. Magnetic analysis found that the particles were stripped to the 20+ charge state. Energy loss to the windows and the absorbers was determined using stopping power tables14 and is discussed further in the next section. The uncertainty in particle energy incident to the sample is estimated to correspond to (0.5 mg/ cm2 of its range. This uncertainty corresponds to a final energy uncertainty of about 2% for a 660 MeV nickel ion passing through all of the windows. The targeting assembly at the ATLAS was the same as previously used in studies of HO2 production with nickel ions at that facility.15 It consisted of a 0.32 cm2 circular beam collimator, a magnetic electron suppression region, and an exit window of 3.6 mg/cm2 titanium. The beam current was collected from the sample and exit window, which together effectively acted as a Faraday cup. Current on the suppression electrode was 15% of the magnitude (with an opposite sign) of the measured beam current. The total number of particles irradiating the solution was determined by integrating the beam current as previously described10,11 and applying the correction for high-energy electrons backscattered from the accelerator exit window. Subsequent experiments showed that with highly charged ions the current collected by the electron suppression electrode included electrons scattered from the collimator. The effect is to underestimate the dose and as a result to overestimate the radiation chemical yield. Since this effect was not recognized at the time of the Fricke experiments with nickel ions, the contribution of forward scattered electrons was not determined or eliminated. Later experiments with 129Xe ions under similar conditions suggest that the correction for forward scattered electrons in the nickel ion experiments should be 20%. The uncertainty in this correction is ∼5% and is well within other error limits and does not warrant the long delay involved in repeating these experiments. Beam currents in these studies were 0.5-1.0 nA. The particle flux, spread reasonably uniformly over 0.32 cm2, was (0.51.0) × 109 particles cm-2 s-1. A 660 MeV nickel ion is attenuated to 465 MeV in passing through the windows. With a residual range of 0.12 mm the dose rate of these ions in the irradiation zone was about 4 × 1019 eV g-1 s-1 (i.e., ∼6 × 105 rad s-1). Extremely vigorous stirring was applied to the solutions, and no evidence for sample depletion was found in spite of the small volume in the irradiation zone. Samples of ∼32 mL were given total doses of ∼7 × 1017 eV g-1 (∼1 × 104 rad). With repeated irradiations the ferric ion concentration was found to increase linearly with total dose at all nickel ion energies. Irradiations at the NSCL with 238U. Experiments at the NSCL were performed with 238U ions accelerated to 4760 MeV ( 0.5% as determined by magnetic analysis at the exit of the cyclotron. Particle charge incident to the targeting assembly was 35+ as verified with magnetic selection immediately before the target area. Energy loss to the windows and the absorbers was determined using stopping power tables14 and is further discussed in the next section. The uncertainty in particle energy
J. Phys. Chem., Vol. 100, No. 39, 1996 16035
Figure 1. Heavy ion beam targeting system. Beam current was collected from the cell and accelerator exit window. Backscattered electrons were collected on suppresser S2 while electrons forward scattered from the collimator were collected on suppresser S1; both suppressers had a 2 kG magnetic field applied.
incident to the sample is estimated to correspond to (0.5 mg/ cm2 of its range. The target system and beam monitoring for the experiments at the NSCL were the same as at the ATLAS except that, as indicated in Figure 1, an additional electron suppression electrode (S1) was placed upstream of the original one (S2). Using this configuration, it was possible to determine the amount of electrons forward scattered from the collimator. With these highly charged uranium ions the current due to the forward scattered electrons from an unfocused beam was of the same order of magnitude as the incident particle beam. During the actual irradiations the collimator and the electron suppresser immediately behind it were electrically connected together, and the total current was monitored. Further, the beam was focused sufficiently well that the current from the collimator as measured on S2 was less than 1% of the sum of the currents on the exit window and sample cell. The correction from electrons backscattered from the exit window as monitored on S2 amounted to 68% of the total beam current. Beam currents were ∼0.1 nA (∼2 × 107 particles/s) with a particle flux of ∼6 × 107 particles cm-2 s-1. With no added absorber the maximum particle energy incident to the sample was 3780 MeV, and the total residual range of these ions was 0.25 mm. Total doses were typically 3 × 1019 eV in 32 mL of solution (∼1.5 × 104 rad). Local doses were as high as 8 × 1018 eV g-1 s-1 (∼1.4 × 104 rad s-1). The solutions were stirred rapidly throughout the radiolysis, and ferric ion production was found to be linear with dose. Results and Discussion Radiation Response and Energy Determination. These experiments give an absolute value for the number of ferric ions produced per irradiating particle, which is expressed as G0E0 in units of molecules/100 particles (Go is the radiation chemical yield16 averaged over the particle track, and Eo is the incident particle energy in electronvolts). This quantity is known to the accuracy of the chemical and integrated current measurements and should be good to a few percent. Uncertainties in E0 do not contribute to the error limits assigned to the measurements of GoEo. This independence is very important because one of the problems in the radiolysis with heavy particles is the determination of the value of Eo. The particles used here lose a considerable fraction of energy in passing through windows and added absorbers so an accurate method for the determination of Eo is necessary. Stopping power tabulations are normally used for this purpose, but their accuracy must be ascertained. The rate of energy loss of a particle is strongly dependent on its charge, which constantly decreases as the particle slows. To correctly estimate the velocity dependence of the charge state
16036 J. Phys. Chem., Vol. 100, No. 39, 1996
LaVerne and Schuler
Figure 2. Production of ferric ions (G0E0, molecules/100 particles) as a function of added aluminum absorber for aerated solution irradiated with nickel ions of 500 (2) or 649 MeV (9) and uranium ions of 4760 MeV (b). The vertical lines are the maximum ranges predicted by ref 14.
of highly charged particles is difficult. A simple way to determine the accuracy of the tabulations is to find how well they predict the total range of the particles in the appropriate targeting assembly. Knowledge of the actual charge state of the particle in the solution is not needed as that information is incorporated into the stopping power. The values of GoEo are shown in Figure 2 as a function of added aluminum absorber for 500 and 649 MeV nickel ions and 4760 MeV uranium ions incident to the targeting assembly and sample cell. All three sets of data show a near to linear decrease with increasing absorber thickness. Such a decrease is expected if the radiation chemical yield, Go, is essentially constant over the particle range which should be the case for heavy particles having energies near to the Bragg peak. The vertical solid lines show the respective maximum ranges as calculated using the stopping power tables of Hubert et al.14 which were especially designed for very high LET particles. Agreement between the expected and the measured ranges is excellent. In all of the following discussion the energy loss of the incident particles in passing through the windows and any added aluminum absorbers was determined using these stopping power tables. The estimated uncertainty in range determination is about (0.5 mg/cm2. For uranium ions this uncertainty amounts to an energy error of less than 3% for experiments with added aluminum absorber of 22 mg/cm2 or less. Energy straggling, uncertainties in absorber thickness, and other things make energy determination unreliable at high absorber thickness. Ferric ion yields determined for particles with less than 20% of their initial energy may have considerable error and are not considered in the discussion of radiation chemical yields. Track-Averaged Fe3+ Yields. The track-averaged radiation chemical yields of ferric ions for nickel and uranium ions are given in Figure 3 as a function of incident particle energy for both aerated and deaerated Fricke solutions. Also included in the figure are previously reported results for protons,17 helium ions,10,11 and carbon ions.11,12 For aerated solutions the ferric ion yield varies from 15.45 with fast electrons17 to 2.8 with 1250 MeV uranium ions. The results with uranium ions agree favorably with the previously reported values of 3.35 and 3.06 found with fission fragments. The large variation in the oxidation of ferrous ions reflects the effect of track structure on radiation chemical processes. Aerated solutions are especially sensitive to H atoms, the yield of which decreases with decreasing particle energy (increasing LET). Ferric ion yields in deaerated solutions vary from 8.22 with fast electrons18 to
Figure 3. Track-averaged ferric ion yields (Go, molecules/100 eV) as a function of initial particle energy, Eo, for aerated (top) and deaerated (bottom) solution. The symbols are for nickel ions (9), this work; uranium ions ([), this work; protons (+), refs 17 and18; helium ions (2), refs 10 and 11; and carbon ions (b), refs 10-12. The dashed lines are the fast electron limits (G(Fe3+) ) 15.45 in aerated solution, ref 17, and 8.22 in deaerated solution, ref 18). The solid lines were obtained by the use of eq 3 with the appropriate values of Table 1.
2.45 with 1250 MeV uranium ions. In this case the narrower range of ferric ion yields is mainly due to the decreased sensitivity to H atom yields. There appears to be a lower limit to ferric ion production which in aerated solutions is about 2.6 and in deaerated solutions about 2.2 ions/100 eV. The stoichiometric equations for the oxidation of ferrous ions in aerated and deaerated solutions are normally written as follows.1
G0(Fe3+)aerated ) 3G(H) + 3G(HO2) + G(OH) + 2G(H2O2) (1) G0(Fe3+)deaerated ) G(H) + 3G(HO2) + G(OH) + 2G(H2O2) (2) The H atom yield decreases with decreasing particle energy, but the contribution to ferric ion production by the oxidizing species (given by the last three elements) remains essentially constant. A lower limit to ferric ion yields is not too surprising if one considers the chemical mechanisms occurring. For example, the main intratrack reaction of OH radicals is to give H2O2.3 Two OH radicals are necessary for this reaction, and yet two ferrous ions are oxidized by H2O2 so the same equivalents of ferric ions will be produced regardless of the
Radiolysis of the Fricke Dosimeter with
58Ni
and
238U
J. Phys. Chem., Vol. 100, No. 39, 1996 16037
TABLE 1: Parameters Used in the Analytical Equations EB particle (MeV) 1H 4
He
12C 58Ni 238
U
0.1 1.0 3.0 40.0 1100.0
aerated
deaerated
GB
a
m
GB
a
m
5.00 3.80 2.65 3.10 2.80
0.222 3 0.049 69 0.025 78 0.020 35 0.018 76
0.9037 0.7516 0.5201 0.2 0.1
4.00 3.20 2.60 3.00 2.20
0.100 2 0.059 93 0.032 37 0.010 05 0.010 77
1.077 0.6225 0.4802 0.2 0.1
extent to which this reaction occurs in the particle track. To a first approximation, similar arguments can be made for the HO2. It should be noted that the yields with low-energy carbon ions appear to be less than those found with nickel and uranium ions of considerably higher LET. Such a result is due to subtle effects of the local track structure as further discussed below. The solid curves in Figure 3 represent the energy dependences given by the empirical expression12
G0(Fe3+) ) GB + (G∞ - GB)(1 - EB/Eo)F
(3)
where F is a dimensionless factor given by
F ) a(Eo - EB)m/(1 + a(Eo - EB)m)
Figure 4. Track-averaged hydrogen atom yields (molecules/100 eV) as a function of initial particle energy, Eo. The symbols are for nickel ions (9), this work; uranium ions ([), this work; protons (+), refs 17 and 18; helium ions (2), refs 10 and 11; and carbon ions (b), refs 10-12. The dashed line is the fast electron limit (3.62). The solid lines were obtained by the use of eq 3 with the appropriate values of Table 1.
(3a)
and GB is the yield at the energy corresponding to the Bragg peak, EB, and G∞ is the limiting yield at high energies which is taken to be that observed with fast electrons. The empirical parameters a and m given in Table 1 have been used to fit the specific set of data for a given particle. These equations were derived previously12 on basis of the sigmoidal character of the energy dependences of the radiation chemical yields. Obviously, the processes occurring in the particle track are quite complicated, and these equations only represent an empirical description of the observed dependences. However, these equations are useful for extracting dosimetric data and other parameters for which no understanding of the underlying mechanism is necessary. In order to describe the correct energy dependence of GoEo, they were fit as a function of Eo using nonlinear regression techniques.19 The parameters assumed or extracted from these fittings are given in Table 1. For the lighter ions the values of a and m were allowed to vary. However, a sufficient energy range does not exist with nickel and uranium ions to perform a meaningful fit to the data. With these ions the value of m approached zero, implying that the track segment yields, discussed below, are independent of energy. Therefore, the values of m for these two ions were assigned, and only the parameter a was fit to the data. The fits were not very dependent on the estimated values of m as long as they were small. In all cases it is seen that the empirical fits describe the observed energy dependences very well. The Yield of H Atoms. A simple manipulation of the stoichiometric eqs 1 and 2 for the production of ferric ions shows that one-half of the difference of the yield in aerated to deaerated solution provides a measure of the H atom yield. The track averaged H atom yields corresponding to the data of Figure 3 are given in Figure 4. The solid lines in Figure 4 are calculated from the appropriate differences in aerated and deaerated solutions a predicted by eq 3. As expected, H atom yields increase with increasing particle energy and are very dependent on the type of particle. It is seen that for nickel and uranium ions the H atom yield is very small, ∼0.2-0.5. These values are about 10% of that found with fast electrons, but they are not zero. The fact that radicals can escape from very high LET tracks can have significant consequences, particularly in radia-
tion biology. Conventional theories of track processes do not adequately describe the processes occurring in very high LET tracks. Not only do H atoms escape the track of a uranium ion, they do so with a yield that appears to be greater than that observed with low-energy carbon ions. Low-energy uranium ions have LETs which approach 17 000 eV/nm, whereas low-energy carbon ions have a maximum LET of only about 1000 eV/nm. The reason for the apparent discrepancy in ferric ion yields is due to the local track structure. Even at energies corresponding to the Bragg peak uranium ions have a reduced energy of about 5 MeV/amu as compared to the value of 0.5 for carbon ions. The reduced energy is proportional to the square of the velocity of the particle. Since the maximum energy loss to a secondary electron is twice its mass times the square of the velocity of the incident particle (2 mV2), the reduced energy gives a simple indication of the contribution of secondary electrons to the track structure. Of course, the actual track structure is much more complicated since electrons are ejected at a variety of angles and undergo a number of elastic collisions as they traverse the medium. Nevertheless, the track of a 0.5 MeV/amu particle is expected to be as much as an order of magnitude smaller in diameter than that of a 5 MeV/amu particle. The local track structure of a 3 MeV carbon ion apparently has a higher density of reactive species than that of a 1100 MeV uranium ion even though the LET of the latter is much greater. It is interesting that a carbon ion with a reduced energy of 5 MeV/amu has a yield of 0.6 for H atom production. Even a 5 MeV/amu helium ion is expected to have a yield of about 0.5 for H atom production. Both of these yields are very similar to those found here with nickel and with uranium ions of comparable velocity. The velocity of the particle clearly plays an important role in determining the track structure. It is possible that the tracks of very highly charged particles such as nickel or uranium ions are slightly enlarged by other processes not normally encountered in the radiolysis with lighter ions. Repulsion of the presumably large number of cations initially produced along the paths of highly charged ions can lead to an increase in their apparent track diameter. This “Coulomb explosion” model has been used to explain observed defects in solid nuclear track detectors.20 Similar effects are probably impossible to observe directly in liquid targets.
16038 J. Phys. Chem., Vol. 100, No. 39, 1996
LaVerne and Schuler
Certainly, if any such process does occur in water, it is not extensive since the H atom yields are only slightly larger with uranium ions than with carbon ions even though the maximum LET is about 17 times greater. Net Water Decomposition and O2 Production. The net water decomposition is a measure of the total radiolytic decomposition induced by the passage of ionizing radiation. It is also quite useful for determining the overall material balance in these systems and for comparing the results from a number of different systems. An analysis of experiments on the radiolysis of the Fricke dosimeter with the series of particles from protons to carbon ions at energies corresponding to the Bragg peaks, suggested that the limiting net water decomposition was ∼2.8.10 Imamura and co-workers estimated the limiting net water decomposition to be 2.9 by observing the response of the Fricke dosimeter to carbon and nitrogen ions.21 However, Imamura et al. based their conclusions on aerated solutions only and did not consider the formation of H atoms to be significant. Neither of these works considered the contribution of HO2 to the net water decomposition. The traditional methods for writing the stoichiometry involved in the net water decomposition, including the contribution of HO2, are
G0(-H2O) ) 2G(H2) + G(H) - G(HO2)
(4)
G0(-H2O) ) 2G(H2O2) + G(OH) + 2G(HO2)
(5)
G0(-H2O) ) G0(Fe3+)deaerated - G(H) - G(HO2) (5a) where the contributions of O atoms or H atoms to the products are appropriately summed. Substitution of eq 2 into eq 5 leads to eq 5a, which suggests that the yield for net water decomposition is given by the yield of ferric ions in the deaerated solutions less the sum of the H atom and HO2 yields. For fast electrons the net water decomposition yield is about 4.6 molecules/100 eV.17,18 Most of the radicals produced with these particles do not recombine in the track since only about 5.5 molecules of water are initially decomposed per 100 eV deposited.22 All of the data with heavy ions give water decomposition yields that are appreciably less than for fast electrons. These results indicate that there is considerable recombination of the radicals within their tracks. Ignoring the contribution by HO2, the lowest net water decomposition yield calculated for uranium ions is less than 1.8. If the standard models of radiation chemistry formulated for low LET radiation are simply applied to cylindrical tracks of infinite length, they predict the minimum net water decomposition to be 3.6.22 All of the nickel and uranium ion results are below this value. Clearly, radiation chemical models developed for low LET radiation are inadequate to describe the chemistry in the tracks of very high LET particles. It is possible that the initial yield of water decomposition for heavy ions is somewhat lower than that for fast electrons. A large concentration of cations and electrons in the particle track could enhance the probability for neutralization processes to occur before fragmentation to give radical species. Certainly, such a mechanism would nullify the possibility of “thermal spikes” being produced in the tracks of very highly charged ions.23,24 These models predict a significant increase in the bulk water temperature because of the large deposition of energy within a small volume. Any such increase in the temperature is expected to lead to an increase in water decomposition not a decrease.25 An increase in water temperature would also increase the radical yield.26 Although radical yields with nickel
and uranium ions are greater than expected, they are not high enough to indicate local heating much beyond ambient temperature. Energy deposition densities in the tracks produced by the passage of nickel and uranium ions in water could be huge. Unfortunately, very little is known about the physical dimensions of these tracks or the distribution of energy deposition within them. Energy deposited in the tracks of very high LET particles may be “wasted” if two or more energy loss events occur on the same molecule. A simple calculation assuming a cylindrical track of 5.0 nm would predict that at 104 eV/nm the number of initial energy deposition events would be equivalent to ∼1/3 of the total water molecules within this volume. Multiple energy loss events on one water molecule could lead to the same products at a greater expense in energy. Alternately, the distribution of electronic states of the water molecule may be shifted to higher energies, resulting in new modes of decomposition or products normally considered to be produced in insignificant yields becoming important. Previous measurements reported the yield of HO2 to be about 0.5 with nickel ions of the same energy as used here.15 Using this value, one would predict the net water decomposition to be as low as 1.3 with uranium ions. On the other hand, eq 4 can also be used to predict net water decomposition from the yields of reducing species. If the net water decomposition is greatly suppressed by unknown mechanisms in the track, then the yield of reducing species must also be low. However, experiments have shown that the yield of H2 is about 2.1 with fission fragments,5,6 and it is not expected to be significantly different with nickel or uranium ions. One would then calculate a net water decomposition of about 4.0 for very highly charged ions. This value is considerably larger than obtained by a summation of the oxidizing species and suggests that some oxidizing species is being overlooked. The most likely candidate is molecular oxygen. Its formation in the tracks of highly charged particles has been proposed by several studies.5,27 The more complete material balance for the net water decomposition is then given by the following.
G0(-H2O) ) 2G(H2O2) + G(OH) + 2G(HO2) + 4G(O2) (6) G0(-H2O) ) G0(Fe3+)deaerated - G(H) - G(HO2) + 4G(O2) (6a) Of course, the material balance does not specify the source of molecular oxygen, and the literature is not very clear either. It is quite certain that the production of oxygen in the tracks of highly charged particles can have significant biological consequences.28,29 The maximum yield of excess molecular oxygen is determined to be about 0.6 for highly charged particles. Previous measurements reported a yield of 0.5 for the production of HO2 with nickel ions.13 However, those measurements really determined the sum of HO2 and O2 production. Not only does this sum increase with increasing particle charge, the mixture becomes richer in O2. Bibler5 found that the O2 yields could be as high a 0.8 with fission fragments. Those measurements support the proposal of a substantial production of O2 in the tracks of highly charged particles. Experiments on the production of O2 and H2 in the radiolysis of water with uranium ions would clarify if the net water decomposition is greatly decreased or if a significant yield of an oxidizing species is produced by particles of very high LET. Track Segment Yields of Fe3+. The ranges of the particles studied here are so short that the particles are completely stopped
Radiolysis of the Fricke Dosimeter with
58Ni
and
238U
Figure 5. Differential ferric ion yield (Gi, molecules/100 eV) as a function of particle energy for aerated (top) and deaerated (bottom) solution. The dashed lines are the same as in Figure 3. The solid lines were obtained by the use of eq 7 with the appropriate values of Table 1.
in the solution so that all experiments determine the trackaveraged ferric ion yield. While these measurements are very important for dosimetry studies, differential or track segment yields (Gi ) d(G0Eo)/dEo) are better for the comparison between different types of particles and with track calculations. Because it is difficult to determine differences directly with reasonable accuracy, the energy dependences of the differential yields in terms of the empirical dependence are given by eq 7.
Gi(Fe3+) ) GB + (G∞ - GB)[(1 + m)a(Eo - EB)m + (a(Eo - EB)m)2]/(1 + a(Eo - EB)m)2 (7) which is consistent with track-averaged yield described by eq 3.12 The parameters used are as given in Table 1. The differential ferric ion yields for protons, helium ions, carbon ions, nickel ions, and uranium ions are shown in Figure 5 as a function of particle energy. Included in this figure are the results for both aerated and deaerated solution. The combination of all the data gives ferric ions yields over 5 decades of energy and 3 decades of nuclear charge of the particle. No other radiation chemical system has been studied over such a wide range of different particles and track structures. The general trends in the differential ferric ion yields with particle energy are similar to that found with track average yields, except that the magnitude of the yield for a given ion at a particular energy may be up to 25% greater than the track average yield.12 Differential ferric ion yields are shown in Figure 6 as a function of particle LET for protons, helium ions, carbon ions, nickel ions, and uranium ions in aerated and deaerated solution.
J. Phys. Chem., Vol. 100, No. 39, 1996 16039
Figure 6. Differential ferric ion yield (Gi, molecules/100 eV) as a function of particle LET for aerated (top) and deaerated (bottom) solution. The dashed lines and the solid lines are the same as in Figure 5. The solid point is for fission fragments, ref 5.
As previously discussed many times, the LET dependence of the differential ferric ion yields can be considerably different for two ions of the same LET. These differences are quite obvious for the lesser charged ions at high energies (low LET). The local track structure determines product yields, and this quality is not solely determined by LET. At a given LET the lesser charged particles have a higher velocity which results in an increased track diameter and a greater escape yield of radicals. The same trends may hold for the very high LET particles studied here, unless the tracks become so similar that the effects on the chemistry are beyond the range of detectability. Experiments with higher energy uranium and nickel ions may help elucidate these problems. Unfortunately, nuclear interactions leading to fragmentation of the irradiating particle into smaller fragments complicates the interpretation of such data. Figure 6 shows that the differences in ferric ion yields between the different types of ions begins to decrease with decreasing LET. The data suggest that if the radiation chemical yields with very high velocity heavy particles eventually reach the limiting fast electron yield, they do so at about the same LET. It appears that at high LET there is no unique limiting yield; rather, the yield is also dependent on particle velocity. At very high velocities the track structure may appear to be independent of particle type. Experimental verification of this conjecture may be impossible because the particles tend to fragment making interpretation of the experiments difficult. However, there remains the basic question as to whether an individual water molecule reacts differently when the same amount of energy is transferred to it by particles of vastly different charge.
16040 J. Phys. Chem., Vol. 100, No. 39, 1996 In summary, the tracks of very high LET nickel and uranium ions are somewhat less dense than expected from other studies with lighter ions, and a nonnegligible yield of radicals can escape to induce radiation damage. It appears that the escape yield of radicals for low-energy ions is more dependent on the square of the particle velocity than its LET. There seems to be an excess production of an oxidizing species, probably O2, that is not observed with lower LET particles. This excess occurs without an increase in net water decomposition so the types of radical species formed or their kinetics is not completely understood in the radiolysis of water. The presence of radicals and O2 can have profound effects in radiation biology and other applications. Acknowledgment. We thank Argonne National Laboratory for the use of the ATLAS facility and the National Superconducting Cyclotron Laboratory at Michigan State University for the use of their facilities. The ATLAS is funded by the U.S. Department of Energy, and the NSCL is funded by the National Science Foundation. We thank Dr. B. Glagola of the Argonne staff and Dr. R. Ronningen of the NSCL staff for their assistance with accelerator setup and beam transport. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This contribution is NDRL-3937 from the Notre Dame Radiation Laboratory. References and Notes (1) Allen, A. O. The Radiation Chemistry of Water and Aqueous Solutions; Van Nostrand: New York, 1961. (2) LaVerne, J. A.; Schuler, R. H.; Ross, A. B.; Helman, W. P. Radiat. Phys. Chem. 1981, 17, 5. (3) Pimblott, S. M.; LaVerne, J. A. J. Phys. Chem. 1994, 98, 6136. (4) Reference 1, p 50. (5) Bibler, N. E. J. Phys. Chem. 1975, 79, 1991.
LaVerne and Schuler (6) Ehrenberg, L.; Saeland, E. Joint Establishment for Nuclear Energy Research; JENER Publications No. 8: Kjeller, Norway, 1954. (7) Wulf, H.; Kraft-Weyrather, W.; Miltenburger, H. G.; Blakely, E. A.; Tobias, C. A.; Kraft, G. Radiat. Res. 1985, 104, S-134. (8) Scho¨pfer, F.; Kiefer, J.; Schneider, E.; Kraft, G. Radiat. Res. 1980, 82, 235. (9) Collinson, E.; Dainton, F. S.; Kroh, J. Proc. R. Soc. London 1962, A265, 407. (10) LaVerne, J. A.; Schuler, R. H. J. Phys. Chem. 1983, 87, 4564. (11) LaVerne, J. A.; Schuler, R. H. J. Phys. Chem. 1987, 91, 5770. (12) LaVerne, J. A.; Schuler, R. H. J. Phys. Chem. 1994, 98, 4043. (13) Schuler, R. H.; Allen, A. O. J. Chem. Phys. 1956, 24, 56. (14) Hubert, F.; Fleury, A.; Bimbot, R.; Gardes, D. Ann. Phys. Suppl. 1980, 5, 1. (15) LaVerne, J. A.; Schuler, R. H. J. Phys. Chem. 1987, 91, 6560. (16) Radiation chemical yields, G values, are given in units of molecules per 100 eV. For consistency, the values of the total amount of product G0E0 are given in units of molecules per 100 particles so that the radiation chemical yield can be obtained by dividing by the particle energy in electronvolts. (17) Schuler, R. H.; Allen, A. O. J. Am. Chem. Soc. 1957, 79, 1565. (18) Barr, N. F.; Schuler, R. H. J. Phys. Chem. 1959, 63, 808. (19) Bevington, P. R. Data Reduction and Error Analysis for Physical Sciences; McGraw-Hill: New York, 1969. (20) Kraft, G.; Kra¨mer, M.; Scholz, M. Radiat. EnViron. Biophys. 1992, 31, 161. (21) Imamura, M.; Matsui, M.; Karasawa, T. Bull. Chem. Soc. Jpn. 1970, 43, 2745. (22) LaVerne, J. A.; Pimblott, S. M. J. Phys. Chem. 1991, 95, 3196. (23) Norman, A. Radiat. Res. Suppl. 1967, 7, 33. (24) Apfel, R. E.; Sun, Y. Y.; Nath, R. Radiat. Res. 1992, 131, 124. (25) Burns, W. G.; Marsh, W. R. J. Chem. Soc., Faraday Trans. 1 1981, 77, 197. (26) LaVerne, J. A.; Pimblott, S. M. J. Phys. Chem. 1993, 97, 3291. (27) Burns, W. G.; May, R.; Baverstock, K. F. Radiat. Res. 1981, 86, 1. (28) Scho¨pfer, F.; Schneider, E.; Rase, S.; Kiefer, J.; Kraft, G.; Liesem, H. Int. J. Radiat. Biol. 1984, 46, 305. (29) Baverstock, K. F.; Burns, W. G. Radiat. Res. 1981, 86, 20.
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