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J. Phys. Chem. 1985,89, 242-243
Production of HO, wlthin the Track Core in the Heavy Partkie Radiolysis of Water' Jay A. Laverne, W. G. Bums, and Robert H. Schuler* Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556, and The United Kingdom Atomic Energy Authority, Harwell, England (Received: July 30, 1984) The radiolysis of the ferrous sulfate-cupric sulfate system with 12Cions has been examined to provide a measure of H 0 2 production within heavy particle tracks. The H 0 2 yield decreases rapidly with increased I2C ion energy and at a given LET is considerably less than found for 4He ions. We conclude that H 0 2 production primarily reflects the local density of energy deposition within the track core and not the partition of energy between the core and 8 rays. Substantial revision of currently existing models is required to take into account the increase in core size with increased particle Z. It is known that an appreciably greater yield of H02 is produced from primary intermediates in the radiolysis of water with heavy ions than occurs with fast electron^.^-^ The additional H 0 2 observed is clearly a manifestation of intratrack processes, which for a given heavy particle become increasingly important as the particle is slowed. Radiation chemical yields of up to 0.08 have been observed for low-energy helium ions2s3,0.25 for 30-MeV w e ion^,^.^ and as high as 0.5 for fission fragments8 These values can be compared to a yield of only 0.025 for fast electron^.^-^ Because the latter yield is small the observed increases provide a good measure of the importance of intratrack processes in heavy particle radiolysis. We are currently carrying out a detailed study of H 0 2 formation in the tracks of low-energy 4He, 'Li, 9Be, llB, and 12Cions, taking advantage of the well-characterized particles available at the heavy-ion facility of the Notre Dame Nuclear Structure Laboratory. The yield of O2produced from a deaerated M ferrous sulfate-10-2 M cupric sulfate solution at pH 2 is taken as a measure of H 0 2 p r o d ~ c t i o n . ~In, ~our initial studies with 10-35-MeV carbon ions, which we report here, we find that the energy dependence of the yield parallels, to a surprising extent, the yield of H2 observed in benzene radiolysis.1° The observed differential yield rapidly decreases from a value -0.25 at 12 MeV, where the LET is -80 eV/& to -0.10 at 30 MeV where the LET is still -50 eV/A. The yield can be expected to approach that of fast electrons at LETs well above a few eV/A. These results show rather dramatically that the H02 is produced almost entirely within the core of the carbon ion track. However, by comparison with the conclusions from our detailed study on benzene radiolysis by heavy particles,1° it becomes clear that the yield is predominantly controlled by processes which depend more than linearly on the local density of energy deposition within the track core. The observed yields only indirectly reflect the LET of the radiation. Currently existing models" have largely been developed to explain the qualitative differences between radiation chemistry with heavy ions and with fast electrons and to a large extent emphasize the partition of energy between the track core and the 6 rays. The conclusions from this present study make it clear that these models will have to be revised substantially to take into account more adequately the density with which energy is deposited within the track core. Experimental Section Irradiations were carried out with 4He and 12C ion beams from a 9-MV FN Tandem Van de Graaff with a 2.5-MV Van de Graaff (1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2618 from the Notre Dame Radiation Laboratory. (2) W. G. Burns, R. May, and K. F. Baverstock, Radiat. Res., 86, 1 (1981). (3) K. F. Baverstock and W. G.Burns, Nature (London),260, 316 (1976). (4) E. J. Hart, Radiat. Res., 2, 33 (1955). (5) E. Bjergbakke and E. J. Hart, Radiat. Res., 45, 261 (1971). (6) W. G.Brown and E. J. Hart, Radiat. Res., 51, 249 (1972). (7) D. M. Donaldson and N. Miller, Tram. Faraday Soc.,52,652 (1956). (8) N . E. Bibler, J. Phys. Chem., 79, 1991 (1975). (9) A. Appleby and H. A. Schwarz, J . Phys. Chem., 73, 1937 (1969). (10) J. A. LaVerne and R. H. Schuler, J. Phys. Chem., 88, 1200 (1984). (1 1 ) J. L. Magee and A. Chatterjee, J. Phys. Chem., 84,3529 (1980); A. Chatterjee and J. L. Magee, J. Phys. Chem. 84, 3537 (1980).
0022-3654/85/2089-0242$01 S O / O
injector. The ion characteristics, window assembly, and irradiation techniques are the same as in previous experiments.1° 4He ions up to 25 MeV and ions up to 35 MeV after passing the window system were available. The radiolysis cell was of Pyrex (- 30 cm3) with a mica window (- 5 mg/cm2) and contained a glass-enclosed magnetic stirring bar. Beam currents were -1 X A. Oxygen was determined as previously described2by continuously bubbling oxygen-free helium through the radiolysis cell and then through an electrolytic oxygen detector (Hersch ce11).2v12 The signal from the latter is proportional to the oxygen concentration in the helium stream. An electrolysis cell, inserted into the gas train, was used for absolute calibration. The probable error in oxygen determination is estimated at &4%. The solutions were 1 mM ferrous ammonium sulfate and 10 mM cupric sulfate (reagent grades, G. Frederick Smith Chemical Co.) in 10 mN sulfuric acid (reagent grade, Fisher Scientific). Water was triply distilled from acid permanganate and alkaline dichromate solutions and stored in quartz until use. Results and Discussion We have measured the total molecular oxygen (G,&o (02)) produced by 4He ions to provide a check against previous studies with cyclotron radiations carried out at Harwel12 and at Brookhaven.9 The radiation chemistry of this system has been thoroughly studied2-* and it is known that molecular oxygen is produced primarily by the oxidation of H02 by the cupric ion. The dependence on initial particle energy (Eo) is shown in Figure 1. The agreement between the results from the three different laboratories is seen to be excellent. In very early studies a yield of 0.23 was reported for zlOPoa-particles' but this value is about twice as great as can be accounted for by the more recent measurements. It is seen in Figure l that above about 8 MeV the differential yield (Gi = d(G&)/dEo) for helium ions is very similar to the yield with fast electrons. It is only at the very low energies, Le., where the particle approaches the end of its track, that the differential yield rises significantly above the fast electron value. The surprising thing is that the differential yield is still low at LETs as high as -5 eV/A. The data for 12Cions show a large yield at low energies (0.25 at 12 MeV) which decreases significantly as the energy increases. The yields found here are comparable to values reported by Burns2" for low-energy I4N and 2oNeions. Bibler's studies with fission fragments,8 where a yield of 0.5 was found, indicate that the limiting yield for high Z particles is even higher. These huge increases in H 0 2 production compared to fast electron radiolysis can, of course, only be explained by intratrack processes. More importantly it is seen in Figure 1 that the differential yield observed for 12Cions decreases appreciably with particle ener y and is only -0.10 at 30 MeV where the LET is still -50 eV/%. well above the B r a g maximum of 23 eV/%, for 4He ions. The phenomenology closely parallels that observed for production of H2 in benzene radiolysis.'o Indeed the solid lines in Figure 1 represent the dependences found in the radiolysis of benzene by 4He and I2C ions after appropriate scaling. The forms of the (12) P. Hersch, Instrum. Pruct., 11, 817, 937 (1957).
0 1985 American Chemical Society
Heavy Particle Radiolysis of Water
6.0
-
b 1:
--
5.0-
0
8t B-p
4.0-
3.0-
0
W
eo
2.0
-
Initial
Enargy (MOW
Figure 1. H 0 2 produced (G&, molecules/particle) as a function of initial ion energy (Eo) for (a) (0)4He, this work; (0)4He, (A) w e , ref 2, 3; ( 0 ) 4He, ref 9. Error limits indicated for the present study correspond to an uncertainty in particle range of *0.3 mg/cm2 and in oxygen analysis of 14%. The dashed line gives the H 0 2 production expected for fast electrons (G(H0,) = 0.025), ref 3-5. The solid curves represent the energy dependence for H2 production in benzene radiolysis (ref 10) scaled by factors of 0.36 for carbon ions and 0.58 for helium ions. The corresponding factor for fast electrons is 0.66.
'v,
dependences are clearly similar and the relative yields comparable (88% for 4He and 55% for 12C,as compared to the fast electron value). One does not, of course, expect these vastly different chemical systems to be so similar in their response, so that this comparison shows that there is something intrinsic to the way energy is deposited that gives rise to the observed similarity. In both cases the product being examined results from processes occurring within the high-energy density region of the track core. Although the exact mechanism for H 0 2formation in water (see below) and H2formation in benzene are not yet known they are presumed to be due to high order processes, although obviously with different precursors. The energy deposition density within the core will determine precursor concentrations and, therefore, effect the competition between product formation and diffusion of the precursors out of the core. Since the track structure of a heavy ion is to a large extent independent of the medium, one can therefore expect a qualitative similarity in the local concentrations of precursors to H 0 2 in water and H2 in benzene as a function of ion energy. The absolute variation in differential yields are due to different diffusion and reaction rates and to a lesser extent the minor differences in the track structure in each medium. It is very important to note that the yields of H 0 2 are not uniquely determined by the core LET as would be expected from the results of previous calculations." Rather, the dependence of the yield of H02on the energy deposition density within the track core should be similar to that found in the H2 formation from benzene.I0 The radius of the track core (the maximum distance at which energy can be directly transferred from the ionizing particle to the medium) can be estimated from Bohr's adiabatic ~ r i t e r i o n and ' ~ is found to be proportional to the velocity of the incident particle divided by the minimum excitation energy of the (13) A. Mozumder, J . Chem. Phys., 60,1145 (1974).
The Journal of Physical Chemistry, Vol. 89, No. 2, 1985 243 medium (6.5 eV for water). At very low velocities this would give unrealistic values since it is assumed that the energy will be quickly delocalized over the entire water molecule. In this work the minimum core radius is taken as one molecular diameter (- 3 A in water) which is slightly smaller than Mozumder's limit of the penetration of lowest energy 6 rays (-10 A in water).I3 If we assume all the energy is deposited within the track core we find that a 12-MeV carbon ion and a 1.2-MeV helium ion both produce a track with an energy density of -0.51 ev/A'. As seen in Figure 1 the differential H 0 2 yields are similar. However, this model would predict a differential H 0 2 yield of 0.06 for a 30-MeV carbon ion which is about 40% too low. The most obvious correction to the model would be to take into account the energy loss to 6 rays which Mozumder14 estimates may be up to about 30% of the total. Nevertheless, we may generalize that, at a given LET, a 12Cion will have a considerably larger core size than that for a 4He ion and, therefore, a lower-energy density and lower resultant H02yield. In an opposite sense this effect also shows up as a significant yield of H atoms escaping from the track of 20-MeV ions, as determined in the Fricke system,lS even though the LET exceeds 60 eV/A. It is obvious that more data are needed, particularly with different ions of similar LETS. Improved models for processes dominated by reactions within the track core must be developed to take into account decreases in the energy densities deposited with increases in particle 2. The mechanism for formation of H 0 2 in aqueous solutions is still uncertain. Previous studies have in many cases assumed that it arises from a secondary reaction of hydroxyl radicals with hydrogen peroxide in the tracks of high LET particles. However, the rate constant for this reaction is extremely low and according to diffusion models9J6 would account for only a small fraction of the observed yield. Burns2 has suggested that O2 formed from two excited water molecules is actually the precursor to H02. We suggest here, as an alternative, the production of oxygen atoms followed by their intratrack reaction with OH. The 0 atom has often been suggested as a primary product in water radiolysis6J7 and it is well-known as a product in the photodissociation of water vapor.'* With low LET the most probable fate of the 0 atom is to react with water to give two hydroxyl radicals. However, as the local concentration of hydroxyl radicals increases, as in these heavy particle tracks, intratrack reactions between 0 and O H can compete to give H02. Hart6 attempted to scavenge the 0 atom in aqueous solutions and determined a minimum yield of 0.008. Clearly, further work is obviously necessary to elucidate the mechanism for H 0 2production, particularly since the presence of oxygen species in heavy ion tracks can have important biological conseq~ences.~J~ We are currently extending these studies to the other radiations available to us to provide more detailed information on this important intratrack process. Acknowledgment. We thank Dr. C. P. Browne of the Notre Dame Physics Department Nuclear Structure Laboratory for making the accelerator facilities available to us. The Nuclear Structure Laboratory is supported by the National Science Foundation. We also thank Dr. E. D. Berners for his assistance with the accelerator operations. Registry No. H02, 3170-83-0; He, 7440-59-7; C, 7440-44-0; ferrous ammonium sulfate, 10045-89-3; cupric sulfate, 7758-98-7; sulfuric acid, 7664-93-9; water, 7732-18-5. (14) A. Mozumder, A. Chatterjee, and J. L. Magee, Adv. Chem. Ser., No. 81, 27 (1968). (15) J. A. LaVerne and R. H. Schuler, J . Phys. Chem., 87,4564 (1983). (16) H. A. Schwarz, J. Phys. Chem. 66, 255 (1962). (17) A. 0. Allen, Radiar. Res. Suppl., 4, 54 (1964). (18) T. G. Slanger and G. Black, J . Chem. Phys., 77, 2432 (1982). (19) K. F. Baverstock and W. G. Burns, Radiui. Res., 86, 20 (1981).
