Fluorescence in the heavy ion radiolysis of benzene

Fluorescence emission has been measured in the radiolysis of neat liquid benzene with 1-15 MeV protons, 5-20 MeV helium ions, 5-30 MeV lithium ions, and 10-30 MeV carbon ions using single photon counting techniques. Companion studies were performed with Sr-90-Y-90 beta-particles with an average energy of about 1.66 MeV. Within 30-50 ns following the passage of the heavy particles the fluorescence intensity decreases rapidly and then levels off to a much slower decay rate which is very nearly the same for beta-particles and high-energy protons and helium ions. The lifetimes of both the fast and slow components of the fluorescence decrease with increasing linear energy transfer (LET), and the magnitude of the fast component increases with respect to the slower one. Quenching of the fluorescence by radicals or other transient species produced in the particle track are thought to be responsible for the observations. At low LET the relative yield of the fluorescence decreases with increasing LET, presumably due to the quenching. However, at higher LET the fluorescence yield increases with increasing LET. Self-annihilation reactions of the triplet states to give singlet states may be the source of these observations. These experiments are the first in which the temporal variation of fluorescence emission has been determined in a neat hydrocarbon liquid irradiated with heavy ions.