The most straightforward method of rejecting muons that do not stop in the sample is to place a scintillation counter downstream from the sample and reject all muons that hit this veto counter.
Start clock = M and not V
Stop(U) = U1 and U2
Stop(D) = D1 and D2
Stop(B) = B1 and B2
Stop(F) = F1 and F2
Start pileup gate = M
In a cryostat, the sample is mounted on thin mylar at the center, and the wrappings on the M and V counters are also very thin, so that muons that scatter from the M counter and miss the sample will not stop before triggering the V counter. The M counter should be as close to the sample as possible to minimize the scattering. Two cryostat inserts are available with this arrangement. Furthermore, muon vetos are incorporated into several more complex vetoing systems.
Since normal decay positrons also tend to trigger the V counter, there will be
a `hole' in the Forward histogram at time=0 due to the V counter accidentally
vetoing the corresponding muons. A more refined definition of `Start' is
Start clock = (M and not V) or (M and F1 and F2)
or the equivalent
Start clock = M and not ( V and not (F1 and F2) )
This is not perfect, though, because then the `hole' is filled with un-vetoed data. The best approach is to adjust the coincidene times carefully so that the hole at early times is only a few nanoseconds wide.
A variation on the vetoing is to combine the functions of the V and F counters. This is usually done with high momentum (backward) muons which are able to penetrate cryostat walls and thick counter wrappings.
Unfortunately, the decay positron from a rejected muon is detected just as well as one from a good muon, but time-differential µSR relies on matching each decay with each muon, so a rejected muon must be allowed to decay before accepting another muon. This is accomplished by using the M counter without veto to start a pile-up gate.
Vetoing muons that pass through a thin sample is more problematic. It works in principle, and works to some extent in practice, but there is always some fraction of muons that traverse the sample with little energy to spare. When these stop in the veto scintillator, they produce little or no pulse and are not detected; they may even stop in the thinnest counter wrapping. Careful set up and modest expectations are required.