Test under high irradiation of new ASD chips for Phase II upgrade of the ATLAS muon spectrometer for the HL-LHC

The muon spectrometer of the ATLAS detector selects muons originating from weak interactions ($p_T$ $>$ 20 GeV/c). The comparatively huge background of low-$p_T$ muons should be rejected by an efficient trigger system based on the muon trajectory. When the LHC begins high luminosity operation in 2026, it will be necessary to switch to a more selective first level muon trigger based on fast track reconstruction, requiring continuous readout of the MDT precision tracking chambers. The front-end electronics have been redesigned with new amplifier-shaper-discriminator (ASD) and TDC chips for compatibility with the new trigger scheme and the 10 times higher first-level trigger rates. The new ASD chip in 130 nm GF CMOS technology has been characterized in the laboratory and tested on a muon chamber in a muon beam under a high gamma radiation background in CERN's Gamma Irradiation Facility. The irradiation affects the drift tube occupancy and the pulse shape of the muon signals and therefore the efficiency and intrinsic spatial resolution. Tests were performed with a range of background fluxes, up to ten times the peak expected value at the HL-LHC.