The role of the active target is to monitor the lateral stopping distributions.
The active target consists of 69 plastic
scintillator fibers surrounded by 8 guard-ring elements of the same material.
The entire active target is a cylinder of 
diameter and
long as shown in figure 
. However, each of the
69 fibers has a square cross
section of 
and is covered with an acrylic
cladding of 
thick for optical isolation. The fibers are
then glued together with epoxy to make the active target.
Figure: The active target used
during the test run
of 1994 to measure the horizontal and vertical beam profiles. It contains 69
plastic scintillator fibers with 8 guard-ring elements of the same material
around them. Each fiber is isolated by an acrylic cladding and they are held
together with epoxy.
Each fiber is connected to a one-inch photomultiplier tube via a fiber optic light guide.
The stopping distributions are needed for an accurate Monte Carlo simulation of
the experiment and acceptance corrections for pion beta and
decays.