2.3.1 Beam Counters and Target
For an additional suppression of beam contamination and to accomplish a
well-defined time signal a 1mm plastic scintillator plate is placed in
the focal point of the pion beam. The beam counter B0, which is located
outside the view of Figure 2–1, is located directly behind the lead collimator
after the beam enters the experimental area. An active degrader B1 follows
3.5 m downbeam directly after the evacuated beam pipe. B1 consists of 4
cm thick plastic scintillator which was chosen for an optimum stopping
rate within the active target. In coincidence with B0 it identifies a valid
pion signal, due to a discrimination against the muon and positron contamination.
At 19.6 m total length of the beam line the difference in Time-of-Flight
amounts 6 ns between pions and muons and 2.5 ns between pions and positrons.
This coincidence - timed with the target signal - is also used to determine
the pion stop timing and rate.
The pion stops close to the centre of the 4 cm long cylindrical active
target and its decay products are registered. The target consists of 9
elements arranged in three segmented concentrical rings and has a total
diameter of 4 cm. Each segment is coupled via light guides to 1 cm diameter
Hamamatsu 5600 photomultiplier tubes. The same PMT type is used for the
read out of the degrader. Energy and timing information therefore is available.
The chosen target diameter keeps the conversion probability of the ~68
MeV photons low while it stops the muons of the p+®m+nm
which energy is about 4.2 MeV. The 69 MeV positron from the p+®e+ne
decay suffers energy loss between 4 MeV and 6.4 MeV, since
it has to pass 2/sin(q) cm of plastic scintillator
material, where q is the angle between the z-axis
(defined by the direction of the beam) and the positron track. The positron
from the pion beta decay will stop within the target due to its low kinetic
energy.