Beam Pipe

The precise determination of decay vertices is an essential feature of the Belle experiment. Multiple coulomb scattering in the beam-pipe wall and the first layer of the silicon detector are the limiting factors on the $z$-vertex position resolution, making the minimization of the beam-pipe thickness a necessity. Moreover, since the vertex resolution improves inversely with the distance to the first detection layer, the vertex detector has to be placed as close to the interaction point as possible and, thus, to the beam pipe wall. This is complicated by the fact that the beam pipe at the interaction region is subjected to beam-induced heating at levels as high as a few hundred watts. This requires an active cooling system for the beam pipe and a mechanism for shielding the vertex detector from this heat.
Figure shows the cross section of the beryllium beam pipe at the interaction point. The central part (-4.6 cm $\leq z \leq$ 10.1 cm) of the beam pipe is a double-wall beryllium cylinder with an inner diameter of 40 mm. A 2.5 mm gap between the inner and outer walls of the cylinder provides a helium gas channel for cooling. The machine vacuum is supported by the 0.5 mm thick inner wall. The outer wall is also 0.5 mm thick. The beryllium central section is brazed to aluminum pipes that extend outside of the collision region as shown in Fig. . The conical shape of the aluminum beam pipe allows the synchrotron x-rays generated in the QCS and QC1 magnets to pass through the detector region without hitting the beam pipe wall. The helium-gas cooling is adopted instead of water in order to minimize the material in the beam pipe.

Figure: The cross section of the beryllium beam pipe at the interaction point.

Figure: The arrangement of the beam pipe and horizontal masks.

Assuming a uniformly distributed 100 W heat load on the inner wall, the maximum temperature increase for the inner (outer) beryllium wall is calculated to be 25 (5) $^o$C for a 2 g/s He flow velocity at a 1.5 atm pressure and a 0.0007 atm pressure drop. The calculation assumes that the aluminum tubes in contact with the ends of the beryllium section are maintained at a fixed temperature by an independent cooling system. The total material thickness of the central beryllium section is 0.3 % of a radiation length. Outside the outer beryllium cylinder, a 20 $\mu$m thick gold sheet is attached in order to reduce the low energy x-ray background ($<$ 5 keV) from the high energy ring. Its thickness corresponds to 0.6 % of a radiation length.