Determination of the positron diffusion length in Kapton by analysing the positronium emission

Doppler profile spectroscopy and Compton-to-peak ratio analysis have been used to study the positronium (Ps) emission from the Kapton surface as a function of the positron implantation energy E. Two different positions for the sample have been performed in the experiment. In the first case the sample and the Ge-detector are perpendicular to the positron beam. With this geometry the emission of para-positronium (p-Ps) is detected as a narrow central peak. In the second case, by rotating the sample 45° with respect to the beam axis, the emission of p-Ps is detected as a blue-shifted fly away peak. The implantation of the positrons is described by the Makhov profile, where we used the modified median implantation for polymers as given by Algers et al. [J. Algers, P. Sperr, W. Egger, G. Kögel, F.H.J. Maurer, Phys. Rev. B 67 (2003) 125404]. Thermalised positrons can diffuse to the surface and may pick up an electron to be emitted as Ps. We found a thermal and or epithermal positron diffusion length L+ = 5.43 ± 0.71 nm and L+ = 5.51 ± 0.28 nm correspondingly for both cases, which is much more than the one found by Brusa et al. [R.S. Brusa, A. Dupasquier, E. Galvanetto, A. Zecca, Appl. Phys. A 54 (1992) 233]. The respective efficiency for the emission of Ps by picking up an electron from the surface is found to be fpu = 0.247 ± 0.012 and fpu = 0.156 ± 0.003.