In order to localize the K+
stopping point (decay vertex) and also to help tracking of decay particles,
stopping target is an active bundle of scintillating fibers. The fine
segmentation is also necessary to suppress backgrounds in reconstructed
trajectories. The bundle is made of about 256 pieces of 3 mm rectangular
scintillating fibers with a length of 20 cm forming a 6 cm diameter rod.
In each fiber a thin wavelength shifter (WLS) fiber is embedded in a groove
and the light through the WLS fiber is read by a multi pixel proportional
counter (MPPC) i.e. a Geiger mode APD from Hamamatsu. According to the
test experiment using π/μ beam at TRIUMF, an input of a sufficient number
of photons into the MPPC is confirmed. The construction of the target system
was funded in Canada and will be finished by the end of 2013 at TRIUMF.