The flux of muons at the SBL experimental site in the NGS beam line is the sum of two components:
This second contribution cannot be eliminated. It can, however, be reliably
estimated [11]
from the predicted neutrino flux and the density
of the molasse, taken to be 
, that acts as shielding
material. For a
proton momentum of 450 GeV/c, the estimated background flux is 10 muons per
protons over a
detector surface. About 
of these
muons come from the neutrino interactions in the last 40 m. of the shielding,
just in
front of the experimental site.
The first background is estimated to be much smaller than
the second component. In the NGS design, the stopping power of
the molasse is reinforced by the
presence of the hadron stopper, made of graphite and iron. A detailed
shielding calculation reported in Reference [5]
indicates, for a 450 GeV/c proton beam, a flux from punch-through
muons of 
pot on an SBL detector.
This is negligible with
respect to the contribution from neutrino interactions, thus keeping the total
muon flux per protons to about 
.
This flux is substantially smaller than the one seen by the present neutrino detectors in the WANF. It will thus present no problem to the detector but, on the contrary, will be useful for troubleshooting, tuning and alignment of the detector.