Generation of Positronium Annihilat ion Lifeime Specra in Polymerst

.

Abstract:

Positronium annihilation lifetime spectroscopy is a powerful prope that ia used in the study of free
volume in polymers. In this work we study the positronium annihilation lifetime spectra in polymers contain
free volume and in pure polymers. The spectra that generated contain single Gaussian component fixed at
400ps convoluted with three lifetime components (.1,.2 and .3) and considering .1 originated from annihilation
of para-positronium, .2 originated from annihilation of positrons in matter and .3 originated from annihilation
of ortho-positronium in free volume also the background fixed at 0.001% of peak. Also study the effect of
mean value of free volume size and its concentrations by study the effect of intensity of long-lived component
(I3) the theoretical spectra were compared with experimental spectra for polystyrene sample and we discussed
the differences. We notice that as the long-lived component increase the right hand side of the spectrum rise.
The difference between the experimental spectrum and the generated spectrum arises from that the background
and time-zero channel in the generated spectra considered to be constant, but in experimental spectra, these
parameter varied with thermal influences. Moreover the generated spectra described by single Gaussian
component. The separating of 0.1 ns difference in lifetime components give specific description for
positronium annihilation lifetime in polymers, also the difference of 5% in intensity give distinguished
separating in the spectra, and these give distinguished components which have small intensities in
experimental spectra . There are clear difference between positronium annihilation lifetime spectrum in pure
polymers and positronium annihilation lifetime in defective polymers (contain free volume), this due to the
effect of concentration and size of free volume on the spectrum. The small resolution (400ps) give a clear
separation in smallest lifetime components (.1 ,.2 ) and smallest intensities in the generated spectra.