  The results of calculations of the charged particles energy, radial and time
distributions at the maximum of electromagnetic showers initiated by electrons
with energies from 5 to 1000 GeV in Fe, W and Pb are presented. It is shown
that the shapes of energy distributions weekly depend on the electron energy,
radial distributions for different materials become close to each other if
radius is expressed in g/cm2 and the time spread of the shower particles is in
the picosecond range. Analysis of the data obtained allows us to conclude that
a high Z material placed in a high energy electron beam can be used as a source
of short and intense bunches of ultarelativistic positrons and electrons with
subpicosecond time spread.
