Angular correlation of annihilation photons were measured for H2O, D2O, 1.74 M PPS/H2O and 1.74 M PPS/D2O solutions of Cl−, Br− and I−. The three components of the angular correlation spectra for D2O and H2O were nearly identical in shape. The positronium (Ps) yields for the H2O and D2O solutions, extracted from the angular correlation spectra, were somewhat different from those extracted previously from the lifetime spectra. Positrons form bound states [X−, e+] with Cl−, Br− and I−, but not with acetate, oxalate, SO2−4, OH− and F− ions, in water. The percentage of positron bound state formation before annihilation (lifetime 400 ps) was determined for the three halides in the four solvents. Simple kinetic equations (“trapping model”) with time dependent rate constant, solved analytically, could explain the [X−, e+] formation in H2O fairly well for concentrations below 0.03 M X−, if a diffusion controlled reaction with positron diffusion constant D = 5 × 10−5 cm2/s and reaction radius R = 1 nm were assumed. The three halides gave roughly identical [X−, e+] formation below 0.03 M X−. The difference between the four solutions could be explained partly only in terms of viscosity change for the model used. The effective rate constant decreased strongly for increasing X− concentration above 0.03 M X−. Ion association may influence the e+aq reaction. The high concentration results are very difficult to explain.
International Journal of Radiation Applications and Instrumentation. Part C: Radiation Physics and Chemistry, 1986, Vol 28, Issue 1, p. 33-48