Low energy muons - how to limit their step length

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Dear All,

I would like to ask you for an advice with the following simulation.

THE EXPERIMENT TO SIMULATE:
I simulate a muSR experiment, i.e. muons with initial momenta of approx. 28 MeV/c
( = kinetic energy of 3.65 MeV) hit a sample (e.g. a Copper plate), stop inside
it and decay into e+ or e- and neutrinos. Unlike in high-energy particle physics,
muons at this energy scale have much shorter range than the e+ or e- from the
muon decay (e- and e+ have the energy range 0 - 52MeV).

THE PROBLEM:
When using the default cuts, muons hitting the sample stop within one single step in
the Cu sample. The graph showing the stopping range of the muons in the Cu plate
is not smooth at all (it has some artificial bumps, see
http://www-hep2.fzu.cz/~ksedlak/penetrationDepthInCuPlate_rough.png
if you are interested), even though the mean value and the spread of the muon range
distribution are more or less OK. Numerically, the mean value of the muon range is
approx. 200 microns.

THE TEMPORARY FIX:
I can fix the problem (non-physical shape of the range distribution) by setting
the "SetMaxAllowedStep" to 10 microns for the sample volume, and for all volumes
in front of the sample that could be potentially hit by the muons. However,
this is not very convenient, and in addition (I think) it also slows down
the tracking of electrons (and consequently the whole simulation), for which
such a small steps are not necessary.

UNSUCCESSFUL TRIAL OF THE FIX:
I tried to use command SetCutValue(cutForMuon, "mu+"); in the myPhysicsList::SetCuts(),
but it seems to be ignored for muons (as reported also at other places in this forum).

MY QUESTIONS:
1) What would be the ideal way/method to limit the step length of muons without affecting
the step length of other particles?
2) How do I find the proper step length cut for the muons in different materials?

Any comments are welcome,
Kamil Sedlak