Irradiation tests and expected performance of readout electronics of the ATLAS hadronic endcap calorimeter for the HL-LHC

The readout electronics of the ATLAS Hadronic Endcap Calorimeter (HEC) will have to withstand a much more demanding radiation environment at the future high-luminosity LHC (HL-LHC) compared to LHC design values. The heart of the HEC read-out electronics is the pre-amplifier and summing (PAS) system which is realized in GaAs ASIC technology. The PAS devices are installed inside the LAr cryostat directly on the detector. They have been proven to operate reliably in LHC conditions up to luminosities of 1000 fb-1, within safety margins. However, at the HL-LHC a total luminosity of 3000 fb-1 is expected, which corresponds to radiation levels being increased by a factor 3-5. On top of that a safety factor of at least 2 needs to be accounted for to reflect our confidence in the simulations. The GaAs ASIC has therefore been exposed to neutron and proton radiation with integrated fluences in excess of 4∙10^15 n/cm2 and 2.6∙10^14 p/cm2, several factors above the levels corresponding to ten years of HL-LHC running. In-situ measurements of S-parameters allow the evaluation of frequency dependent performance parameters, like gain and input impedance. The non-linearity of the ASIC response has been measured both at warm and at cold, i.e. at LAr temperatures. This allows an improved estimation of expected degradation of the HEC performance. The measured gain and non-linearity of the ASIC response were finally applied to Monte-Carlo simulations in order to understand their effect on jet measurements in HL-LHC conditions.