Track effects in radiation chemistry: core processes in heavy-particle tracks as manifest by the hydrogen yield in benzene radiolysis

Abstract

The energy dependences of the yields of H/sub 2/ produced in the radiolysis of liquid benzene by /sup 9/Be, /sup 11/B, and /sup 12/C ions 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(H/sub 2/) = 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/Angstrom. The total amount of hydrogen produced by these particles in excess of that expected for fast electrons is estimated as 86,000, 141,000, and 190,000 molecules/particle, respectively, with most of this excess being produced at LETs well above 10 eV/Angstrom. The results presented here, together with previous data on /sup 1/H, /sup 2/D, /sup 4/He, and /sup 7/Li, give a comprehensive picture of the dependence of more processes in heavy-particle tracks on particle charge and energy. At a given LET the differential yields decrease by 20-40% for each unit increase in charge on the irradiating particle. This decrease cannotmore » be explained by delta-ray effects but rather reflects the increased diameter of the track core produced by the core highly charged ions. The local density of energy deposition and not the LET per se controls the H/sub 2/ yield. The authors conclude that the H/sub 2/ 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. 7 figures, 2 tables.« less