Precision survey of the readout strips of small-strip Thin Gap Chambers using X-rays for the muon spectrometer upgrade of the ATLAS experiment

The instantaneous luminosity of the Large Hadron Collider (LHC) will be increased by a factor of 5 following a series of upgrades of the CERN accelerator complex. In anticipation to the increased collision rate at the interaction point, the muon end-cap inner station of the ATLAS detector will be replaced by the so-called New Small Wheels (NSWs) during the LHC shutdown of 2019/2020. The NSWs combine the Micromegas and small-strip Thin Gap Chambers (sTGC) technologies. The sTGC detector modules are arranged in trapezoid wedges of 4 detector planes with fiducial areas varying between 3 and 6 m$^{2}$. Up to 1000 azimuthal cathode strips are read out on each plane for precise muon trajectory measurements. The positioning of individual strips must be known to within 100 microns to satisfy the performance targets of this ATLAS upgrade. Non-conformities of the sTGC strip-pattern are therefore measured on finished wedges using an X-ray gun precisely positioned at reference points. The working principles and experimental procedure of this technique will be shown as well as validation studies based on measurements carried out on sTGC wedges of the early production.