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 will have to withstand an about 10 times larger radiation environment at the future high-luminosity LHC (HL-LHC) compared to their design values. The GaAs ASIC, which comprises the heart of the readout electronics and consists of a Pre-Amplification and a Summing stage (PAS), has been exposed to neutron and proton radiation with fluences corresponding to ten years of running of the HL-LHC. Neutron tests were performed at the NPI in Rez, Czech Republic, where a 36 MeV proton beam is directed on a thick heavy water target to produce neutrons. The proton irradiation was done with 200 MeV protons at the PROSCAN area of the Proton Irradiation Facility at the PSI in Villigen, Switzerland. In-situ measurements of S-parameters in both tests allow the evaluation of frequency dependent performance parameters, like gain and input impedance, as a function of fluence. The non-linearity of the ASIC response has been measured directly in the neutron tests with a triangular input pulse of varying amplitude. The results obtained allow an estimation of the expected performance degradation of the HEC. For a possible replacement of the PAS chips, alternative technologies are investigated and exposed to similar neutron radiation levels. In particular, IHP 250 nm SiGe CMOS technology has turned out to show good performance and match the specifications required. The performance measurements of the current PAS devices, the expected performance degradations under HL-LHC conditions, and results from replacement technologies will be presented.