The Time-Of-Propagation (TOP) counter is a novel particle identification device, covering the barrel region of the Belle II detector. The task is to distinguish what are called pions and kaons, which is critical to correctly know which kind of B decays happened.

The main part of this detector is a 2-cm thick quartz bar, whose surface of the quartz bars is polished with extreme flatness; its roughness is less than 5 angstrom, or size of 5 atoms. When a charged particle, such as a pion and a kaon, passes through the quartz bar, it emits Cherenkov light with a certain angle (0c)
with respect to the incident particle direction as shown in the figure. The light propagates with total internal reflection to the end of the bar and its arrival timing is precisely measured. Due to slight difference in the angle of the Cherenkov light, its path length to the photo sensor will differ between a pion and a kaon as show in the figure. This results in slight difference in light arrival timing. This time difference is measured by a series of photo sensors called Micro Channel Plate Photo Multiplier Tubes (MCP-PMTs), which are attached at the end of the quartz bar. They detect each “single light” or “photon” with an extreme precision of 50 picoseconds (one twenty billionth seconds), which is the world-best timing precision for a single photon. This performance enables us to identify the slight time difference and to know which of a pion or a kaon the track of the charged particle derives from.

This detector consists of 16 identical modules, where each module contains the quartz bar with size of length of 2.5 m and width of 45 cm. Because the Cherenkov light is very weak and easily to be lost with tiny dusts, the module production was performed in a clean room as shown in the photographs. These modules are arranged to form a barrel structure with a radius of 1.2 m and cover the entire inner surface of the electromagnetic calorimeter (ECL).