Positron transmission into Kapton: The effective mass absorption coefficient of relevance to positron lifetime experiments

Abstract

The penetration of positrons from a Na22 source into Kapton foils sandwiched between different backscattering materials (Al, Cu, and Pt) has been studied for the geometry characteristic of positron annihilation techniques. Kapton has served as absorber and each of the selected metals as detector. The method, based on measuring the positron lifetimes, makes use of the difference in values of the fractions of positrons annihilating inside and outside the bulk matrix of the detector. A model on positron transmission and backscattering into 2π solid angle was constructed for the purpose. A correction term was added to the classical exponential form of the transmitted intensity, for absorber thickness z smaller than a critical thickness zc above which the transmitted intensity function falls exponentially. Numerical analysis of the data based on the model function has revealed the existence of two effective mass absorption coefficients pertaining, respectively, to the regions below and above zc. The coefficient αs+ for z<zc of relevance to positron lifetime experiments was found to be considerably smaller (by a factor 3) than the coefficient αl+ extracted by exponential regression measurements for sufficiently large absorber thicknesses. The need for new data on the coefficient for backscattering into 2π solid angle led to measuring the latter for the three used metals, yielding in the end the result αs+=14.12 cm2 g−1. The observed gap between αs+ and αl+ is qualitatively discussed.