J-PARC Experiment E36 at the Nuclear and Particle Physics Facillity

Precise Measurement of Γ (K+→ e+ν)/Γ(K+→ μ+ν)
using Stopped Positive Kaons

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Active target

In order to localize the K+ stopping point (decay vertex) and also to help tracking of decay particles, the K+ 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.