Development of High Granularity Timing Detector for ATLAS experiment

The proposed project aims at developing and qualifying detectors/sensors which will be used in the High Granularity Timing Detector (HGTD) of ATLAS experiment after the Large Hadron Collider upgrade around 2024.

The upgrade represents an enormous challenge in terms of coping with large particle fluxes emerging from the collisions. On average 200 p-p collisions occur every 25 ns. A resulting track and jet densities complicate the analysis of the underlying physics reactions that took place. Association of detector hits to tracks and jets from different collisions is a demanding process requiring huge CPU power on one side and creation of large combinatorial background (uncertainty) on the other. A way to solve that problem is separation of individual collisions also in terms of time of occurrence within each bunch crossing. With a full space-time point associated to each detector hit a far more efficient tracking can be achieved. Moreover, a much more precise analysis of the energy flow is possible for products of collisions that occur close in space with known timing. Precise timing detector with large enough granularity would also allow identification of the jet origin process (underlying physics) from the topology of the particle time and space distribution within the jets.

As a part of the ATLAS experiment upgrade a construction of the so called High Granularity Timing Detector (HGTD) is envisaged. HGTD will be placed between the Inner Tracker (ITk) and Liquid Ar calorimeter (LAr). It would extend from pseudo-rapidity=2.3 to 4.2 (12-65 cm) at z=350 cm from the interaction point. The required time resolution of the tracks in HGTD is ~30-40 ps and it is foreseen that it will serve also as luminosity meter and beam condition monitor. That requires reading of the detector hits with 40 MHz.

The most promising choice of detector technology for HGTD is Low Gain Avalanche Detectors (LGAD). These detectors differ from standard silicon n+-p detectors by an extra p+ layer between n++ implant and p bulk, which causes high enough electric field for impact ionization. The principle of operation is similar to Avalanche Photo Diodes with several differences. The LGADs detectors can be segmented into macro-pixels (~1 mm2) or strips therefore a special implant design around electrode edges is required to prevent device breakdown. In order to achieve the desired time resolution of HGTD both time walk and noise jitter have to be reduced. It was shown by simulations and also in test beam measurements that for 45 mm thick LGADs of pad size around 1 mm2 with gain of ~60, time resolution of 26 ps can be achieved for a single layer and 15 ps for three layers combined.

One of the largest obstacles for their applications is relatively fast decrease of gain with irradiation, which is attributed to the decrease of initial dopant concentration in multiplication layer. The proposed project aims to take part in design, production and testing of LGAD detectors that would eventually lead to construction of HGTD. The project is split into several work packages: design and production of several iterations of sensor prototypes (including a possible fallback solution of small cell 3D silicon detector in the innermost part of HGTD with largest radiation damage effects), setting up the system for precise time measurements for minimum ionizing particles, irradiation campaigns, characterization of irradiated sensors properties, simulations of sensor operation and plans for running scenarios of HGTD using LGADs. Finally the project will aim to find way to mitigate radiation damage through the use of different dopants, defect engineering and running conditions.