Yield of positrons created by high-energy electrons in crystalline and amor
phous tungsten targets is calculated as a function of a target thickness. T
he phase space inherent in positron sources is studied. Corresponding elect
ron-photon showers initiated by electrons with energies from 10 to 300 GeV
are considered. Some advantages of crystal targets especially pronounced at
high energy are shown. (C) 1999 Elsevier Science B.V. All rights reserved.