H-LowB: Horizontal Low Background Insert

A simple muon veto works well for moderately small samples, but for very tiny samples the allowable rate of `good' muons would be unacceptably low because the total incoming muon rate is limited by `pile up'. The answer to these difficulties is to reject both the incidence and the decay of the errant muons so they generate neither a `start' nor a `stop' signal to the experiment, thus ignoring them totally.

[Diagram of the H-LowB insert]
Start = M and not V
Stop(L) = L1 and L2 and not V
Stop(R) = R1 and R2 and not V

The axial or `horizontal' low background apparatus is simple both in design and operation. It consists, essentially, of a cup-shaped veto counter placed behind the sample, and which fits in a horizontal gas flow cryostat. There are side positron counters mounted outside the cryostat. A similar version of this veto cup is mounted in the Dilution Refrigerator.

The small sample is mounted on a very thin Mylar sheet (or a narrow silver arm). Muons that miss the sample stop in the bottom of the veto cup, triggering both the M and Veto counters, which is not a valid `start'. Muons that stop in the sample, however, trigger only the M counter and thus start the clock.

The Veto counter also serves as a veto to decay events: Muons in the sample have an unobstructed view of the positron counters on either side, and their decays will be duly recorded; but when muons in the cup decay, their positrons must trigger the Veto if they are to trigger the L or R counters and such vetoed events are not accepted.