Alice Gabrielli ; Lorenzo Vitale

2024
University of Trieste Trieste

Abstract: This thesis aims at deepening and improving the characterisation procedure of diamond detectors used as radiation monitor and beam abort for the Belle II experiment at the SuperKEKB electron-positron collider. Belle II is one of the leading particle physics experiments at the intensity frontier. In order to accumulate 50 times more particle collisions than its predecessors, the Belle and BaBar experiments, the SuperKEKB collider is designed to reach unprecedented instantaneous luminosities. As a side effects the detector must operate with high particle rates both from collisions and from beam loss backgrounds. The Belle II radiation monitor system consists of 28 single-crystal artificial-diamond detectors mounted in the interaction region of SuperKEKB. Owing to its excellent radiation hardness, diamond has been widely used for solid-state particle detectors and dosimeters in harsh radiation environments. I assembled and fully characterised ten new diamond detectors. I installed eight of them on a new beam pipe, to allow the new upgraded vertex detector installation. The response of diamond sensors might differ from sample to sample, due to crystal imperfections from the CVD-growing process or to the details of the electrode contact formation. In order to assess the main properties of each diamond sensor a suitable characterisation procedure using , and X radiation sources has been developed. After asses the crystal quality and check the response of each sensor, I determined the current-to-dose rate calibration factor in steady conditions. I devised a current-to-dose-rate calibration method that employs a silicon diode as a reference, to greatly reduce uncertainties associated with the source activity and with the setup simulation. The method has been validated by measuring the calibration factors with X and β radiation, spanning a dose-rate range from tens of nrad/s to few rad/s. However, intense radiation bursts may lead to non-linear effects in the collection of ionisation charges and deeper investigations on diamonds' transient response to intense radiation pulses are needed. For this purpose, I studied the transient response of our diamond detectors to collimated, sub-picosecond, ∼ 1 GeV electron beam bunches, with a bunch charge of tens of pC, provided by the FERMI electron linac in Trieste. In these experimental conditions the ionisation generates large charge carrier density in the diamond bulk. This high charge causes a transient modification of diamond electrical properties, which affects the output signal shape. The observed signal evolution in the time domain shows fair agreement with a two-step numerical simulation of the diamond time response to these intense high-energy pulses. The results that I obtained represent important steps forward in the characterisation techniques for diamond sensors to be used for the radiation-monitor upgrade.

Note: Presented on 19 03 2024
Note: PhD