Performance studies of Thin Fine-Pitch Silicon Vertex Detector for Belle II Vertex Detector upgrade
Category: Master Thesis, Visibility: Public
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| Authors | Zihan Wang, Yutaka Ushiroda |
|---|---|
| Date | Jan. 1, 2022 |
| Belle II Number | BELLE2-MTHESIS-2023-005 |
| Abstract | The Belle II experiment located at KEK, Tsukuba (Japan) precisely measures the Standard Model parameters analyzing various flavor physics processes to search for new physics beyond the Standard Model. The Belle II aims to accumulate a large data set of 50 ab−1 and to make this target achievable in reasonable time scale, the KEKB collider was upgraded to SuperKEKB to reach an instantaneous luminosity of 6 × 1035 cm−2 s−1. As a part of the Belle II Vertex Detector (VXD), the Silicon Vertex Detector (SVD), which is placed close to the beam pipe, mainly contributes to vertex determination and tracking of charged particles. The Double-sided Silicon Strip Detector (DSSD) is applied in the SVD. Accompanied by the increase of accelerator instantaneous luminosity, the large beam background degrades the performance of the SVD due to increase in the hit occupancy and radiation damage. The beam background condition of the SVD at the nominal luminosity is predicted using the Monte Carlo (MC) simulation scaled with Data/MC ratios determined by dedicated beam background studies. However, the Data/MC ratio for the luminosity background has not been determined because, using the method in previous studies, the evaluated amount of the luminosity background in the measured data showed strange dependence to the luminosity. In this thesis, a new method of the Data/MC evaluation is developed and used to give the reliable result of the luminosity background measurement on the SVD. Using the new Data/MC ratios, the projection of the SVD hit occupancy at the Belle II nominal luminosity is also updated. The projection turns out that the safety factor to the limit of the occupancy is marginal as it is only 1.9, below the desired safety margin 5. To achieve a good performance under the high beam background level at the Belle II nominal lu- minosity, a new silicon strip detector, Thin and Fine-Pitch SVD (TFP-SVD), is proposed as one of the upgrade candidates of the VXD. A new DSSD, TFP-DSSD, and a new front-end ASIC, SNAP128, were developed in the TFP-SVD project. In this work, we developed a test platform dedicated for the performance evaluation of the prototype sensor and ASIC, and performed the performance characteri- zation. In this evaluation, the basic functionality of SNAP128 is confirmed, and the power consumption and analog pulse width meets the design requirements. Also, we found two issues. The first one is that this chip gets saturated when processing the positive signal. The second one is that the noise level at the target detector capacitance is over the design limit and is about 60% larger than the simulation expectation. Based on these results, possible improvements of the design is proposed in the next ver- sion of the chip. To evaluate the performance of the sensor, we designed and assembled an integrated test board. Using this test board, the full depletion voltage, leakage current and noise of readout chip connected to the sensor is measured. The measured full depletion voltage and leakage current agree with the expectations. One remaining issue is that the noise of chip is much larger than expectation when connected to the sensor, based on the noise–detector capacitance relationship derived in the chip performance evaluation. With a better understanding of the achievements and remaining issues in the first prototype, the design of the SNAP128 will be updated and the next version is planned to be taped out in November, 2022. For the noise issue on the sensor, hypothesis of the noise source are proposed and following exper- iments will follow to eliminate the possible noise sources and remeasure the noise of chips connected to the sensor. As the next step of the sensor evaluation, detector efficiency and signal charge measurement using the cosmic rays and accelerator beam is also under discussion. |
| Conference | Tokyo |
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