Energy loss of positrons below the excitation threshold in Ar gas

We performedpositron age-momentumcorrelation measurements to investigate the positron slowing down process in Ar gas at 7.5 MPa. Increase in the S parameter was observed up to 1 ns, and then stayed on the same level. By comparing with the calculated energy loss of positrons via elastic scattering, it was concluded that the increase in the S parameter correspondedto the positron slowing down from 11.6 eV to 2.5 eV. The positron slowing down process just below the first electronic excitation energy of Ar gas can be observed using the S parameter.