Problem 587

Earlier today Marcus Mendenhall at Vanderbilt reported a bug with Penelope EM. We
have observed a separate but perhaps related problem when 100 MeV protons are
incident on a 2 micron layer of silicon. Using low energy EM processes, as one would
expect, there is no difference between a target with one layer 2 microns thick and
another with two layers each 1 micron thick. Using Penelope EM in place of low energy
EM with no other changes there is a noticable discontinuity of track density at the
imaginary boundary between the two identical material layers.  The result is a bi-lobed
display of delta electron tracks in the penelope case and a continuous tube of tracks in
the low energy em case. If as we suspect there may be a simply geometrical issue, this
could be a second manifestation of it. It may, of course, also be a totally separate
problem.  I have rated this as "major" because if the simple openGL track display is
indicative, the density of deposited energy will be notably incorrect in the vicinity of the
virtual boundary. However, since we see this in semiconductor-device size samples, it
may not affect the majority of users.