The emission of slow positrons from magnesium oxide powder

From Monte Carlo analyses of measured time-of-flight spectra for slow positrons traveling down a tube which is first evacuated and then filled with low-density helium gas, it is shown that the positrons leave the MgO-coated source grid at angles close to the normal direction. This result is important for low-energy positron collision studies. An interpretation of the result based on the surface charge held by the oxide is proposed. (orig.)