IFAE Delivers First HGTD Modules to CERN

The ATLAS Pixel group has been working on the development of new silicon detector technologies for the High Luminosity LHC upgrade. The group is set to make important contributions to the HGTD project which is critical for the ATLAS detector performance in the forward region. In the last year the first prototype modules were fully fabricated in the IFAE clean rooms, including the critical bump bonding step which was carried out in-house. These modules were delivered to CERN for the HGTD demonstrator program.

The increase of the particle flux at the HL-LHC scenario, with instantaneous luminosities up to 7.5 × 10 34 /cm/s^2 , will have a severe impact on the ATLAS detector performance. A new full-silicon tracker, called ITk, will be deployed to maintain the physics reach in view of the larger radiation and pile-up levels. However, in the forward region the HL-LHC still presents a challenge, since the ITk position resolution degrades. Thus, ATLAS plans to install a High Granularity Timing Detector (HGTD) in front of the LAr end-cap calorimeters to help with the pile-up mitigation, as well as to provide a measurement of the luminosity.

Four of the six modules fully assembled at IFAE for the HGTD demonstrator program.

HGTD consists of two silicon-sensor double-sided layers that will provide precision timing information for minimum-ionising particles with a resolution as good as 30 ps per track, to help disentangle tracks from different vertices in the same bunch crossing. Readout cells have a size of 1.3 mm × 1.3 mm, leading to a highly granular detector with 3.6 million channels. Low Gain Avalanche Detectors (LGAD) technology has been chosen, as it provides enough gain to reach the large signal over noise ratio needed. HGTD modules consist of two LGAD sensors with a matrix of 15x15 channels, each connected to the readout chip by small solder bumps (80 um diameter) through a process known as bump-bonding. The two LGAD-chip hybrids are joined by a flexible PCB which through wire-bonds realizes the communication and extracts the timing and hit information for the DAQ system to process.

First detector unit with IFAE modules delivered to CERN.

IFAE has been heavily involved in the development of LGAD sensors, the readout chip (ALTIROC) as well as the hybridization and general module assembly activities. IFAE is one of six HGTD module assembly sites, and has been leading the activities since the start of the project. A major milestone was reached in June 2023, when IFAE delivered six of the twelve modules needed for the first demonstrator detector unit, which reflects the well developed assembly process at the institute.

The HGTD demonstrator program will exercise the full readout and verify the thermal performance of the detector.

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