Design and Construction

Figure shows the Belle detector with the segmented magnetic flux return iron in the barrel and end-cap regions. KLM consists of alternating layers of charged particle detectors and 4.7 cm-thick iron plates. There are 15 detector layers and 14 iron layers in the octagonal barrel region and 14 detector layers in each of the forward and backward end-caps. The iron plates provide a total of 3.9 interaction lengths of material for a particle traveling normal to the detector planes. In addition, the electromagnetic calorimeter, ECL, provides another 0.8 interaction length of material to convert $K_L$'s. $K_L$ that interacts in the iron or ECL produces a shower of ionizing particles. The location of this shower determines the direction of $K_L$, but fluctuations in the size of the shower do not allow a useful measurement of the $K_L$ energy. The multiple layers of charged particle detectors and iron allow the discrimination between muons and charged hadrons ($\pi^{\pm}$ or $K^{\pm}$) based upon their range and transverse scattering. Muons travel much farther with smaller deflections on average than strongly interacting hadrons.