Symposium abstracts 283

Another example of the excellent parallelism

between Ps formation and fluorescence is

shown in Fig. 1 in which I3 and the

fluorescence intensity L (one component,

C6F6 , of the binary mixture Is fluorescent)

are seen to follow the same trend.

Ps formation in the mixture shows a peculiar

S-shaped dependence on the composition, and

we found that the fluorescence of C F also 66

shows a similar S-shape with the maxima and

minima very close to those of Ps formation.

It has been deduced from such parallelism

that some of the Ps formatfan takes place via

electron abstraction from excited molecular

state C6Ft: a new mechanism of Ps formation.

Further results of Ps formation studies

have been presented, and emphasis was made

on the importance of the study not only

from a scientific viewpoint but also for

potential applications as a tool of

studying ultra-fast (sub-picosecond)

processes in radiation spur.

Fig.1 Comparison of fluorescence yield, L,

and Ps intensity, I 3’

as a function of

volume fraction of C F in cyclohexane. 66

44. BEHAVIOR OF POLYMERIZED LIQUID CRYSTAL PHASES STUDIED BY POSITRON ANNIHILATIO#

RAFAEL LOPEZ, SONIA MILLAN. JUAN CARLOS ALVAREZ AND LUZ ALICIA FUCUGAUCHI

lnstituto National de lnvestigaciones Nucleares. Unidad de lnvestigacidn Aplicada

Ave. lnsurgentes Sur 1073, 03720, N6xico. D. F.

In earlier work the technique of positron

annhilation was applied to a study of the

polymerization of lyotropic liquid crystals.

In this study, the same technique has been

used to detect phase transitions in the

liquid crystal system sodium ID-undecenate-

water. The sharp changes of the positron

annihilation parameters as a function of

temperature indicate structural chanqes in

this system. Photomicrographs of these

samples show the presence of nematic,

hexagonal, laminar and isotropic phases.