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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Tracker (MWPC)

For the C2, C3, and C4 chambers, we will reuse the existing MWPC chambers which were used in the previous E246 experiment. They are placed at the entrance (C2) and exit (C3, C4) of each magnet gap. Since the C2, C3, and C4 chambers have to be operated under the strong field of the spectrometer, a planar MWPC with cathode readout was chosen. The anodes are 20-μm gold-plated tungsten wires strung with 2 mm pitch along the momentum-sensitive directions. The two cathode planes are located above and below the anode plane. The distance between the cathode and anode planes is 6mm. The cathode planes are made of a 25- μm Kapton foil with copper coating stripes of 18 μm in thickness. The width of the strip is 9 mm in width and the spacing between strips is 1 mm. A mixed gas of Ar (50%) and ethane (50%) is used as the operating gas. High voltage of +4.0kV is supplied to the anode, and the cathode signal is used for the hit position determination. Typical position resolutions are ~200 μm.