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J . Phys. Chem. 1984,88, 1200-1205
Track Effects in Radiation Chemistry: Core Processes in Heavy-Particle Tracks as Manifest by the H2 Yield in Benzene Radiolysis' Jay A. LaVerne and Robert H. Schuler* Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: May 31, 1983)
The energy dependences of the yields of H2 produced in the radiolysis of liquid benzene by 9Be, I l B , and 12Cions have been examined over the particle energy range of 8-32 MeV. With increasing energy the differential yields rapidly decrease from maximum values of 0.52,0.60, and 0.70 molecules/100 eV found at low energies. Extrapolation of the observed dependences indicates that the yields will approach that observed for fast electrons (G(H2) = 0.038) in the region of 50-100 MeV even though the LET of the particle at these energies is still considerably greater than a few eV/A. The total amount of hydrogen produced by these particles in excess of that expected for fast electrons is estimated as 86000, 141000, and 190000 rnolecules/particle, respectively,with most of this excess being produced at LETS well above 10 eV/A. The results presented here, together with previous data on 'H, 2D, 4He, and 'Li, give a comprehensive picture of the dependence of core processes in heavy-particle tracks on particle charge and energy. In particular, it is noted that at a given LET the differential yields decrease by 20-40% for each unit increase in charge on the irradiating particle. This decrease cannot be explained by &ray effects but rather reflects the increased diameter of the track core produced by the more highly charged ions. It is the local density of energy deposition and not the LET per se that controls the H2 yield. We conclude that the H2 is produced almost entirely from high-order processes that occur within the track core as the result of events in very close proximity and at very early times.
Introduction
TABLE I: Characteristics of Available Heavy Ionsa
Because of their high local energy deposition, heavy ions in the few megaelectronvolt energy region are expected to produce rather different radiation chemical effects than fast electrons or y-rays. In water this difference is very well understood in terms of the increased importance of radical combination processes within the particle track in competition with diffusion of the radicals out of the track? Radiation chemical effects produced by heavy particles in other media are much less well documented and essentially not understood in any significant detaiL3v4 The principal studies on an organic system have been carried out on liquid benzene5-13 where it has been shown that the H2 yield produced by low-energy heavy ions is considerably greater than the very low yield observed in electron radiolysis (G(H2) = 0.038). The low background of the latter makes it possible to use this system to focus on the importance of core processes in studies of heavy-particle radiolysis. Systematic studies have been carried out with proton^,^,^ deuterons,6v9 helium ions:," and, very recently, lithium ions13 which show that the H2 yields increase markedly as the particle loses energy and approaches the end of its track. It is found that even at modest energies (e.g. 10 MeV for helium ions) the differential yields are only slightly above those observed with fast electrons. It is clear from this observation that the excess H2 results mainly from processes which occur in the core of the track, where the
max max max residual tot range residual range energy, EIM, in Al, energy,b in benzene,b particle MeV MeV/amu mg/cm2 MeV mg/cm2 'H 18 18.0 473 18 35 1 4He 21 6.8 87 25 41 'Li 38 5.4 48 32 19 Be 46 5.1 32 35 11 I' B 56 5.1 26 38 7 "C 65 5.4 22 40 5 14N 74 5.3 19 38 4 l60 83 5.2 17 36 3 a From FN Tandem Van de Graaff of the Notre Dame Nuclear Structure Laboratory. After passing through all windows, calculated by assuming a window system equivalent to 10 mg/cm2 of Al.
(1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2455 from the Notre Dame Radiation Laboratory. (2) A. 0. Allen, "The Radiation Chemistry of Water and Aqueous Solutions", Van Nostrand, Princeton, NJ, 1961. (3) W. G. Burns and R. Barker, "Aspects of Hydrocarbon Radiolysis", T. Gaumann and J. Hoigne, Eds., Academic Press, New York, 1968, p 33. (4) See J. A. Laverne, R. H. Schuler, A. B. Ross, and W. P. Helman, Radiat. Phys. Chem., 17, 5 (1981) for a summary of references to about 100 papers on heavy-particle studies in nonaqueous liquids. (5) T. Gaumann and R. H. Schuler, J . Phys. Chem., 65, 703 (1961). (6) W. G. Burns, Trans. Faraday SOC.,58, 961 (1962). (7) W. G. Burns and C. R. V. Reed, Trans. Faraday Soc., 59, 101 (1963). (8) A. W. Boyd and H. W. J. Connor, Can. J . Chem., 42, 1418 (1964). (9) R. H. Schuler, Trans. Faraday Soc., 61, 100 (1965). (10) J. Y. Yang, J. D. Strong, and J. G. Burr, J . Phys. Chem., 69, 1157 (1965). (11) W. G. Burns and W. R. Marsh, Trans. Faraday Soc., 64, 2375 (1968). (12) W. G. Burns, W. R. Marsh, and C. R. V. Reed, Nature (London), 218, 867 (1968). (13) J. A. LaVerne and R. H. Schuler, J. Phys. Chem., 86, 2282 (1982).
0022-3654/84/2088-1200$01.50/0
density of energy deposition is very high and second-order intratrack processes can play a dominant role. Various suggestions have been offered as to the chemical source of this hydrogen which include intratrack reactions of hydrogen atoms or of excited states, but the mechanistic details are still a matter of speculation. In spite of this the H2 yield does provide a measure of the importance of core processes against which to test track theories. Most theories advanced to date have attempted to correlate observed effects with stopping power (-dE/dx; commonly referred to by radiation chemists as linear energy transfer or LET), but it is recognized that differences in track diameters and in the contributions from &rays will make the chemical processes produced by different particles with the same LET appear to be somewhat different. The relative contribution made by intratrack reactions will, of course, depend on the nature of the processes which compete as the track evolves in time. Studies of water radiolysis by heavy particles provide relatively little information on the initial characteristics of the track core because the yields are controlled by competition between intratrack reactions of radicals and their diffusion out of the track. In water the latter predominates, and differences in the initial radial distributions of the intermediates are to a large extent lost. We have now extended our studies of benzene radiolysis9J3 to irradiations with 9Be, IlB, and 12Cions and have obtained results which show very clearly that the intratrack processes are very much more sensitive to the initial track 0 1984 American Chemical Society
Track Effects in Radiation Chemistry structure. These results imply that the processes responsible for H2 formation occur to a large extent at very short times and, therefore, provide a somewhat earlier probe of the core than is available from studies of water radiolysis. The rather comprehensive data obtained in this study are summarized below and are given in more detail numerically in supplementary Tables SI-SIV. Emphasis has been on obtaining information on the energy dependence of the differential yield (sometimes called instantaneous yield, Gi = d(GoEo)/dEo,where Go is the integral yield observed at energy Eo, since it is ultimately this quantity that must be correlated with the theoretical models. Experimental Section Irradiations were carried out at the heavy-ion facility of the Nuclear Structure Laboratory of the Notre Dame Physics Department (9-MV FN Tandem Van de Graaff with a 2.2-MV Van de Graaff injector).14 The characteristics of available particles are given in Table I. After acceleration the ions were energy and charge state selected magnetically. A feedback signal from the magnetic analysis was used to control the initial particle energy to better than 0.01 MeV. It is this high degree of energy control which makes the Van de Graaff a particularly good instrument with which to carry out the type of studies reported here. In order to avoid any ambiguity in the current measurements, only fully stripped ions were used in the irradiations. The vacuum in the beam-handling system (
