Universidad Nacional de La Plata, Argentina; 2 Instituto de Física La Plata (IFLP)-CONICET; 3 Comisión de Investigaciones Científicas- Provincia de Buenos Aires, Argentina. Positron Annihilation Spectroscopy (PAS) is a branch of γ- ray spectroscopy that gives information of the positron lifetime and the kinetic energy spectrum of the electrons in the material under study. The physical basis of positron annihilation resides in its capability to sense electron density in the bulk of the material. Several complementary methods are used to characterize porous materials using PAS. Among these techniques, the most common are Positron Annihilation Lifetime Spectroscopy (PALS), Doppler-broadening Spectroscopy (DBS) and Angular Correlation of Annihilation Radiation (ACAR).
PALS gives account of the characteristic positron lifetimes in the material of study, fact that is associated with the size and relative fraction of the nano- free volumes or nanopores present in the sample. In DBS the Doppler broadening of the annihilation radiation is measured while ACAR focuses in the relative angle between the two 511 keV gamma rays that result from the annihilation. DBS and ACAR are designed to be sensitive to the momentum of the electron-positron pair at the moment of the annihilation. This momentum can probe the electronic and physical environment in which the positrons are implanted. In addition, the combination of PALS and DBS provides a powerful tool for the identification of defects.
In spite of the success of PAS in probing atomic and molecular defects in solids and semiconductors, the technique has been scarcely applied in life science research. Meanwhile, in the last two decades, this spectroscopy has been used to study free volume properties and free radical status in a variety of chemical and biological systems including polymers, contact lenses and biomembranes. Among the biological field, it is worth mentioning the investigations on protein dynamics and tumoral tissues. A brief discussion of the mentioned topics and the advantages and disadvantages of these types of studies is presented.
Keywords: polymers, contact lenses, proteins, tissues