title: Low gain avalanche diodes for timing applications
Creators: name: Kedych, Vadym
Publisher: name: UNSPECIFIED
Year: 2024

description: This work aims to develop a non-destructive beam time structure monitoring system for the superconducting Darmstadt linear accelerator (S-DALINAC). The S-DALINAC is a 3 GHz superconducting electron accelerator capable of operations in the energy recovery (ERL) mode. The ERL operation mode allows the recovery of the beam energy and uses it for an acceleration of consecutive bunches. It is reached by recirculating the beam to the main LINAC with a 180° phase shift with respect to the phase of the electric field in the accelerator cavities. During the operation in the ERL mode, one beamline recirculates two beams simultaneously, leading to the 6 GHz repetition rate inside this beamline. To monitor both beams, a detector system capable of resolving this time structure of 6 GHz has to be built.

For this purpose, a test setup based on the Low Gain Avalanche Diodes (LGADs), silicon sensors designed for high-precision simultaneous position and time measurement, was developed. Several groups have demonstrated a time resolution below 50 ps for LGADs, which indicates that this technology is a suitable candidate
for the beam time structure monitoring at the S-DALINAC. The read-out system was based on leading-edge discriminators and Field-Programmable Gate Array (FPGA) based Time-to-Digital converters (TDCs) developed at GSI in Darmstadt, Germany. These allow for the estimation of arrival time and signal width via the time-over-threshold method.

To demonstrate the feasibility of an LGAD-based beam time structure monitoring system, a simplified setup based on a single 1 cm×0.5 cm LGAD sensor with a limited number of active channels was prepared and tested at the experimental hall at the S-DALINAC. Due to the placement of the setup, the delivered beam had a 3 GHz time structure. During this proof-of-principle demonstration, the S-DALINAC 3 GHz beam time structure was successfully resolved for the first time.

An upgraded setup, which employed two 1 cm×1 cm LGAD sensors, was prepared to investigate the feasibility and performance of an LGAD-based beam time structure monitoring system. In contrast to the first proof-of-principle experiment, the entire system was read out using an upgraded version of the read-out system. Also, with this upgraded setup, the 3 GHz time structure was successfully resolved, and an intrinsic time resolution of 79 ps was demonstrated.

Since the setup installation for the proof-of-concept study had to be done outside of the accelerator hall, only the beam with the 3 GHz time structure could be measured. A design concept study using Geant4 simulations was performed as a next step to investigate the feasibility of an LGAD-based beam time structure monitoring
system for the ERL mode. To evaluate the LGAD’s performance in such conditions, a simulation with the simplified geometry of the potential measurement station was prepared. This simulation demonstrated the feasibility of an LGAD-based system for monitoring the 6 GHz beam time structure. Additionally, the best detector position inside the accelerator hall could be identified. In the upcoming runs at the S-DALINAC, it will be possible to verify the results of simulations and the concept.

Lastly, the feasibility of the machine learning (ML) approach for the LGADs data analysis was demonstrated. It was shown that the ML approach reduces the amount of data required for the LGAD correction without distorting the time resolution. Additionally, this approach can be used to implement an online LGAD correction procedure.