Positron transmission and effective mass absorption coefficient in nickel

The penetration of positrons from a²²Na source into Ni foils sandwiched between different backscattering materials (Kapton, NaCl, Ni, and Mo) has been studied for the geometry commonly used in PAT experiments. Transmitted intensities and effective mass absorption coefficients have been found and the dependence on the backscattering material is discussed. General formulas for the effective mass absorption coefficients are derived. The large geometrical effects are stressed. Suggestions for source corrections are supplied.