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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Toroidal Spectrometer

The Supercnducting Toroidal Magnet provides a high magnetic field up to 1.8 T at the magnet center. The magnet was assembled with a very high precision to ensure the 12-fold roratinal symmetry. The coils are cooled indirectly with a two-phase flow of He supplied by an online refrigerator.

The heart of the positron and muon spectrometer is a superconducting toroidal magnet with 12 iron sectors separated by 12 gaps. Each iron sector is magnetized by a superconducting coil, and a field up to 1.8 T can be excited across the 20-cm uniform gaps. In the proposed experiment a central field strength of 1.4 T is employed. The field geometry is nearly dipole with some toroidal component superposition. Charged particles from the target located in the center of the magnet are bent by ≈90 degrees and tracked and momentum-analyzed by the GEM and MWPCs at the entrance and exit of the gap. The field map necessary for tracking was calculated using a 3-dimensional code TOSCA. The validity of the map was checked using the measured monochromatic momentum spectra of muons from Kμ2 and pions from Kπ2.