ESCAPE PROBABILITY OF LOW-ENERGY ELECTRONS AND POSITRONS EMITTED IN RANDOM DIRECTIONS BENEATH A PLANE SOLID-SURFACE

Simulations of escape probability and angular distribution have been m ade with the low energy Monte Carlo code LEEPS for electrons and posit rons with initial energies E = 0.5-20 keV, scattered in Be, Al, Cu and Au. The simulated escape probability T(x) for an electron or positron starting in random direction from a depth x beneath the plane surface of a semi-infinite homogeneous solid (atomic number Z) is well descri bed by T(x) = A exp(-x/R) exp (-(x/1.9R)(2)). By least-square fit to s imulated data, the parameters A and R are expressed as functions of Z and E. The formula gives about one or a few percent lower values than the simulated T(x) for start depths x smaller than the initial elastic mean free path; this is related to the larger simulated probability o f glancing angle escape for such small start depths. The scaling prope rties of T(x) are discussed, and comparison is made with previous calc ulations and with measurements. The parameter R is for electrons relat ed to the Bethe range r by R/r approximate to 0.587(1 + alpha)(-0.47), where alpha = r/lambda(tr) and lambda(tr) is the transport mean free path at the initial electron energy. For low and medium Z, alpha appro ximate to 0.15Z at energies 2-20 keV. (C) 1998 Elsevier Science B.V. A ll rights reserved.