Belle II Silicon Vertex Detector: Cooling

Full text: At the KEK, the High Energy Accelerator Research Organization in Tsukuba, Japan, currently the B-Factory KEKB and its experiment, the Belle detector get upgraded, aiming a higher luminosity. The improved Super B-Factory will reach a luminosity of 8 x 1035 cm-1s-1 which is 40 times higher than the previous luminosity world record achieved there. For precise decay vertex measurements, a new silicon vertex detector (SVD) is required. The newly designed Belle II SVD comes along with a modern cooling system. The new Belle II SVD with its new 'Origami' readout scheme requires an efficient cooling system for thermal control of the front-end electronics inside the detector volume. One of the most important figures of merit for vertex detectors is the amount of material seen by particles traversing the detector, therefore the material budget has to be kept as low as possible. To minimize the material budget, an evaporative CO2 cooling system has been chosen. CO2 provides excellent thermo-dynamical properties regarding its high heat transfer capability of evaporating CO2. This leads to a much smaller tube diameter and therefore lower mass inside the detectors active volume, compared to using fluorocarbons like conventional cooling systems do. The 2PACL (Two-Phase Accumulator Controlled Loop) developed at Nikhef provides a stable and easily controllable detector cooling system, completely fulfilling the experiments requirements. Integrating the cooling system into the heart of the Belle II detector is difficult and investigations are ongoing to meet the required material budget and space constraints. (author)