Positronium annihilation lifetime spectroscopy (PALS)


Positronium annihilation lifetime spectroscopy (PALS) is a technique used in the characterization of porosity in nanostructured materials. Positrons, the antiparticle to the electron, enter a sample and either immediately annihilate with an electron or they can capture an electron to form Positronium (Ps), the hydrogen-like bound state. Ps then annihilates with a lifetime corresponding to the pore size. This annihilation converts the total mass of the positron and electron into high-energy photons. These annihilations are detected to measure Positronium lifetime. SBA-15, a mesoporous silica that consists of a two dimensional hexagonal array of cylindrical mesopores with interconnecting micropores, is a system of interest as a catalysis support and low- dielectric constant material. Nitrogen adsorption, x-ray di raction, and small angle neutron scattering (SANS) have been used to characterize the pore structure in SBA- 15, but none of these techniques have been able to yield a complete picture. PALS is a technique that has the potential to yield insight of the pore structure in SBA-15.