Abstract
For the High-Luminosity phase of LHC, the ATLAS experiment is proposing the addition of a High Granularity Timing Detector (HGTD) in the forward region, to mitigate the effects of the increased pile-up. The chosen detection technology is Low Gain Avalanche Detector (LGAD) silicon sensors that can provide an excellent timing resolution below 50 ps. The front-end read-out ASIC must exploit the large signal derivative and small noise provided by the sensor, while keeping low power consumption. This paper presents the results on the first prototype of a front-end ASIC, named ALTIROC0, which contains the analog stages (preamplifier and discriminator) of the read-out chip. The ASIC was characterised both alone and as part of a module with a 2×2 LGAD array of 1.1×1.1 mm2 pads bump-bonded to it. The various contributions of the electronics to the time resolution were investigated in test-bench measurements with a calibration setup. Both when the ASIC is alone or with a bump-bonded sensor, the jitter of the ASIC is better than 20 ps for an injected charge of 10 fC . The time walk effect, which arises from the different preamplifier response for various injected charges, can be corrected up to 10 ps using a Time Over Threshold measurement. The combined performance of the ASIC and the LGAD sensor, which was measured during a beam test campaign in October 2018 with pions of 120 GeV energy at the CERN SPS, is around 40 ps for all measured modules. All tested modules show good efficiency and time resolution uniformity.
Highlights
The High Luminosity (HL) phase of the Large Hadron Collider (LHC), to begin in 2027, is expected to deliver instantaneous luminosities more than three times higher than the ones reached during the Run II period
The jitter contribution to the time resolution, either with just the ASIC or with a module consisting of the ASIC and an Low Gain Avalanche Detectors (LGADs) sensor, was found to be better than 20 ps for a charge larger than 5 fC
A 6% improvement of the ASIC jitter for Qinj = 10 fC was achieved during measurements at T = −30 ◦C, which will be the default operating temperature for the High Granularity Timing Detector (HGTD)
Summary
The High Luminosity (HL) phase of the Large Hadron Collider (LHC), to begin in 2027, is expected to deliver instantaneous luminosities more than three times higher than the ones reached during the Run II period. The schematics of a simplified voltage sensitive amplifier configuration are presented in figure 1 In such configuration, the jitter can be calculated assuming that the detector is a constant current source Iin with a duration time of tdur. For a sensor drift-time of 600 ps, if the preamplifier rise-time is reduced or increased by a factor of 2 compared to the optimal matching value, the jitter would deteriorate by just about 12%. Given these considerations, in order to minimize the jitter, the sensor should have a small capacitance, a short pulse duration, and be capable of providing a large charge. The ATLAS baseline choice is LGADs with a pixel size of 1.3 × 1.3 mm and a 50 μm active thickness, to be operated with a starting (minimum) collected charge of at least 10 (4) fC, i.e a gain of 20 (8). The electronics noise en is largely determined by the current (Id) that can be flown in the preamplifier input transistor, as explained
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