Speaker
Description
The Belle II experiment is currently recording data from $e^+e^-$ collision at the SupperKEKB accelerator, which holds the world's highest luminosity of $5\times10^{34}~\mathrm{cm^{-2}s^{-1}}$ and will be upgraded to achieve a higher luminosity of $6\times10^{35}~\mathrm{cm^{-2}s^{-1}}$. An R&D program has been established to develop a new vertex detector (VTX) the one currently operating in Belle II. This upgrade will enable the experiment to cope with the higher hit rate and radiation level at the target luminosity while maintaining excellent tracking performance.
The VTX is a pixelated detector with 6 straight layers, whose radii range from $14~\mathrm{mm}$ to $140~\mathrm{mm}$, matching the radial acceptance of the current vertex detector, with minimal material budget. The layers are equipped with identical CMOS Depleted Monolithic Active Pixel Sensors (DMAPS), OBELIX, which is developed based on the TJ-Monopix2 DMAPS prototype. The pixel matrix design of OBELIX is the same as that of TJ-Monopix2 while the digital logic has been modified to adapt to the Belle II experiment, enabling a trigger rate of $30~\mathrm{kHz}$ with $10~\mathrm{\mu s}$ latency. The VTX must be able to cope with a high average hit rate of up to $120~\mathrm{MHz \cdot cm^{-2}}$ and endure high irradiation, with accumulated TID of $100~\mathrm{Mrad}$ and NIEL flux of $5\times 10^{14}~\mathrm{n_{eq}cm^{-2}}$. While the design of the minimal material budget detection layers is on-going, it is critical to assess the maximum operation temperature at the expected maximal radiation.
This presentation will describe the main VTX concepts for the sensor and the detection layer design. Then, measurements of the detection performance on TJ-Monopix2 samples irradiated with electrons and protons will be reported, in order to confirm the good performance of VTX detector after being irradiated in its long-term operation. As a result, even though degradation of detection efficiency is seen, it is confirmed that the detection efficiency can be recovered back to more than $99\%$ with high bias voltage and configuring with low detection threshold. Besides, milder radiation damage is seen with electron-irradiation compared with proton-irradiation.
