  We study the position sensitivity of radial zigzag strips intended to read
out large GEM detectors for tracking at future experiments. Zigzag strips can
cover a readout area with fewer strips than regular straight strips while
maintaining good spatial resolution. Consequently, they can reduce the number
of required electronic channels and related cost for large-area GEM detector
systems. A non-linear relation between incident particle position and hit
position measured from charge sharing among zigzag strips was observed in a
previous study. We significantly reduce this non-linearity by improving the
interleaving of adjacent physical zigzag strips. Zigzag readout structures are
implemented on PCBs and on a flexible foil and are tested using a 10 cm by 10
cm triple-GEM detector scanned with a strongly collimated X-ray gun on a 2D
motorized stage. Angular resolutions of60-84 urad are achieved with a 1.37 mrad
angular strip pitch at a radius of 784 mm. On a linear scale this corresponds
to resolutions below 100 um.
