Full simulation of a testbeam experiment including modeling of the Bonn Atlas Telescope and Atlas 3D pixel silicon sensors

Abstract

3D silicon pixel sensors are a strong candidate for the sensor component of a new B-layer in the ATLAS detector, and for the ATLAS sLHC tracker, as these sensors can be highly radiation hard, fast, and sensitive to the edge. In order to characterize the sensors before large-scale application, samples are mounted in small fixed-target {\it testbeam} experiments. Here the samples are exposed to high-energy charged hadrons, and the response to this radiation is measured. The hit position in the sensor is estimated using a beam telescope, which measures the position of the particle while in flight up- and downstream of the sample. The hit position is then estimated by assuming that particle flies in a straight line between the telescope measurements and the sample.

This thesis presents a full Geant4 simulation of the interaction between the beam particles and the material in the testbeam, including but not limited to sensors. The output from the simulation is then used for detailed modeling of the signal formation and electronics response for both the 3D pixel sensor samples and the beam telescope sensor. Predictions from these models are compared to experimental data, indicating that the telescope models developed have a very high accuracy.

This work has made it possible to estimate the telescope tracking resolution in the sensor samples to approximately $6 ~\unitbr{\mu m}$. An off-line method that may reduce the telescope hit position measurement uncertainty by subtracting common mode noise is also described.

Having this simulation system enables experimenting with sensor sample response models while monitoring how they behave from the perspective of data analysis. The results so far, still inconclusive, indicates that polysilicon-filled electrodes in full 3D sensors retain a non-zero efficiency.