Speaker
Description
The Belle II experiment at the asymmetric energy e- and e+ collider SuperKEKB provides rich flavor physics programs of beauty, charm, light quarks and tau-lepton at the luminosity frontier. Thanks to its nano-beam scheme, SuperKEKb holds the world's highest instantaneous luminosity record of $5 \times 10^{34}$ $cm^{-2}s^{-1}$, and aims to push up to $6 \times 10^{35}$ $cm^{-2} s^{–1}$ after the upgrade of the interaction region, including the final focusing magnet, foreseen in 2032.
To cope with the harsh beam background conditions expected after the upgrade, a fully pixelated vertex detector (VTX) with monolithic CMOS sensor (OBELIX) is planned as one of the major upgrades of Belle II.
The OBELIX sensor, derived from TJ-Monopix2 originally developed for the ATLAS experiment, features a matrix of $896 \times 464$ pixels with $33 \times 33$ $\mu m^{2}$ pitch. The chip is designed to cope with 30 kHz trigger rate at 120 MHz/$cm^2$ hit rate, and radiation tolerance up to 1 MGy and $5\times 10^{14}$ $n_{eq}/cm^2$.
The VTX consists of six cylindrical layers of OBELIX sensors surrounding the beam pipe at radii from 14 $mm$ to 140 $mm$. The detection layer design is now being optimized considering the expected background hit rate and related power consumption at the target luminosity. A particularly challenging system aspect is to maintain the sensor at room temperature in order to preserve detection efficiency after irradiation, while minimising the material budget.
The ladders of the inner two layers (iVTX) are composed of self-supported four-consecutive OBELIX sensors inter-connected with a post-process redistributed metal layer. The last sensor is then connected to a readout flexible circuit board. The iVTX concept is exploring passive cooling using a thin layer of Thermal Pyrolytic Graphite (TPG) running below the sensors and thermally connected to end-mount blocks actively cooled.
The ladders of the outer four layers (oVTX) consist of aligned OBELIX chips connected to a flexible circuit board (flex for power and signals) and supported by a carbon fiber structure equipped with a cooling pipe and two end-mount blocks at both ends.
In this talk, we will present an overview of the project and its latest developments, especially focusing on passive cooling studies of iVTX with TPG and the development of low material budget flex using aluminum conductor.