  The readout electronics of the ATLAS Hadronic Endcap Calorimeter will have to
withstand the about ten times larger radiation environment of the future
high-luminosity LHC (HL-LHC) compared to their design values. The GaAs ASIC
which comprises the heart of the readout electronics has been exposed to
neutron and proton radiation with fluences up to ten times the total expected
fluences for 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 the fluence. The linearity of the
ASIC response has been measured directly in the neutron tests with a triangular
input pulse of varying amplitude. The performance measurements and expected
performance degradations under HL-LHC conditions are presented.
