Theoretical and Experimental Positron Annihilation and Scattering Cross Sections in Helium

Abstract

A comparison is made between the experimental results and results calculated from theoretical values of annihilation and scattering (momentum-transfer) cross sections on free-positron-annihilation lifetime spectra in helium gas at temperatures of 4.2, 77, and 300°K. It is found the annihilation cross section calculated by Drachman using Dalgarno and Lynn's complete first-order adiabatic correlation function, incorporated with the method of variational bounds, agrees with the experimental results. It is also found that both the momentum-transfer cross section calculated by Drachman with $\alpha =0\mathrm{}$ and $C=1\mathrm{}$ and the one by Massey et al. fit the experimental results well, although the one by Drachman seems to be better. The limitations of this method and the results using other values of cross sections are also discussed.