Median Implantation Depth And Implantation Profile Of 3-18 Kev Positrons In Amorphous Polymers

Most applications of positron beams require knowledge of the implantation characteristics for an appropriate interpretation of the experimental data. In this work, the median implantation depth as a function of implantation energy, z(1/2)(E), of 3-18 keV positrons and their implantation profile P(z,E) in a total of 13 thin films of atactic polystyrene, poly(styrene-co-acrylonitrile), and polymethylmethacrylate spin coated onto a silicon substrate were determined from positron lifetime measurements using a pulsed, low-energy positron beam. z(1/2)(E) and P(z,E) were determined from the measurement of the ortho-positronium yield obtained from the intensity I-3 of the long lifetime. z(1/2)(E) was parametrized with the commonly used power-law fit z(1/2)(E) = (A/rho)E-n, with rho and E in units of g cm(-3) and keV, respectively, yielding A = 2.81(+/-0.2) mug cm(-2) and n = 1.71(+/-0.05). Excellent agreement between amorphous polymer and literature data on Al and Cu suggests that the median implantation depth of positrons for low- to medium-Z materials in the studied energy range is independent of structure and only a function of mass density. Fitting of the Makhovian implantation profile to the experimental data suggested that the value of the parameter m varies between 1.7 and 2.3, systematically increasing with z at constant implantation energy, but is independent of the implantation energy. Using an equation proposed by Baker, the experimental data of 12 of the 13 studied polymer films could be described with a slightly better agreement than the Makhovian equation.