The effects of track structure on the luminescence decays in cyclohexane and in 2,2,4-trimethylpentane (isooctane) solutions of 2,5-diphenyloxazole (PPO) have been determined in the presence and in the absence of an external magnetic field. Irradiations were performed with protons of 1-15 MeV and with helium ions of 2-20 MeV energy. Companion studies were performed with 90 Sr-90 Y -radiolysis. The magnetic field effect is due to the hyperfine interaction of nuclear spins in the geminate pair of solvent radical ions produced. In both solvents, the effect of the magnetic field on luminescence decreases with increasing linear energy transfer (LET) from about 40% for -particles to only a few percent with helium ions. Magnetic field effects with protons decrease in time whereas they are constant with -particles. This result is attributed to the overlap of initially isolated spurs during the evolution of the proton track; the probability of nongeminate recombination increases with the number of neighboring ion pairs. The total luminescence intensity per incident particle remains constant with proton energy but increases slightly with increasing helium ion energy. At a given particle energy, the intensity is greater in cyclohexane than in isooctane. The pulse shapes of the luminescence decays reflect the distributions in ion recombination times, and very little variation in luminescence decay rates is observed with increasing LET.
IntroductionEnergy deposited by the passage of ionizing radiation in hydrocarbons leads to a series of clusters of ions and excited states (spurs) stochastically placed along the particle path. 1 The spatial distribution of these species and the radicals that they form make up the particle track structure, and it is largely responsible for the observed yields of final products. Increasing the linear energy transfer (LET ) stopping power, -dE/dx) as in the case of proton, helium ion, or other heavy ion radiation increases the concentration of reactive species in the particle track, and intratrack radical-radical reactions can be preferred over radical diffusion into the bulk medium. For instance, in the fast electron radiolysis of cyclohexane almost 80% of the cyclohexyl radicals produced react in the bulk medium whereas less than 10% escape the tracks of low-energy carbon ions. 2 Almost all of the studies on the chemical effects of track structure have been made from observations of the final product formation. 3 Only a few studies have attempted to observe the formation of ions and excited states in the heavy ion radiolysis of hydrocarbons, 4-11 but it is well-known that scintillation yields decrease markedly with increasing LET. 12 Luminescence from aromatic compounds added to hydrocarbons has long been used to examine the radiolytic production of ions and excited states. 5,13 The initial ionization and direct excitation of the solvent and the subsequent reactions can be represented by RH -Df RH + + e -or RH* (R1)RH* + S f RH + S* (R3)where RH is the solvent and S is the solute. When moderat...