Active Collimator
(HiTime: Belle High-Timing Insert)

A different type of veto is an `active collimator' which rejects muons that are going to miss the sample after scattering from the thin muon counter. Its effectiveness depends critically on correctly distinguishing between muons that miss and those that hit, so it would work best when placed just before the sample. A good active collimator also rejects the decays of these muons so the data acquisition rate is not limited by pile-up. An example of such an arrangement is the high-timing counter array dedicated to the Belle spectrometer:

[Diagram of the Hi-Timing insert]
Start clock = M and not ( C or V or U or D or L or R )
Start pileup gate = M and not C
Stop(U) = U and not C [and not L]
Stop(D) = D and not C [and not R]
Stop(R) = R and not C [and not U]
Stop(L) = L and not C [and not D]

In this specialized high-timing counter arrangement, the sample is mounted at the center of the cryostat indicated by the intersecting green lines. Muons must pass through the thin muon counter M (black; 2) and a hole in the thick scintillator of the active collimator C (red; 3). Then they traverse two windows and a heat shield (magenta; 4-6) in the cryostat before reaching the sample.

If a muon misses the sample, it would strike the muon-veto counter V (blue; 8) or a positron counter (blue; 7) and be rejected. (Rejection by the positron counters leaves a hole in the data at t=0.) These rejected muons cannot have their decays vetoed, so they must trigger the pile-up gate just like good muons.

The muons which strike the active collimator stop in its upstream edge, triggering it and vetoing the muon counter. The veto is complete: neither the clock nor the pile-up gate are started, and the experiment is ready for another muon immediately. When those muons decay, the positron may fly harmlessly away from the sample without being detected, or it could go `downstream' and hit one of the positron counters (blue; 7). For the latter, it must pass through the active collimator counter C which vetos all the stop signals.

In a way, the Belle insert is a bad example of active collimation because its active collimator is effective only at low magnetic field. At high fields, which is the specialty of the instrument, the field focusing makes the active collimator redundant, and even harmful.