Strip Detectors and Ghosts Hits

If we want to measure the 2D space point of a charged particle we can accomplish it with various types of detector technology. The small-strip thin-gap (sTGC) detectors used by ATLAS[1] provide reasonably good position resolution of $\sigma\approx100\mu$m and are significatly cheaper than competing technologies (like all silicon detectors).

However, the sTGC design is really two 1D detectors sandwiched to provide 2D space points. This design keeps the detectors simple and low cost, but has the disadvantage of introducing ambiguity when determinging the 2D space-point from the individual 1D measurements.

You can see this ambiguity in the demo below. Here the "real" hits are shown in green while the fake hits, e.g. "ghost" hits, are shown in red. The ghost hits appear at the crossing of all x-measurements with all y-measurements. For this reason, the number of ghost hits $(g)$ grows according to the combinations of real hits $(n)$ like: $$ g = n * (n - 1) $$

#real hits: {{ n_real }} and #ghost hits: {{ n_ghost}}


As you can see, the problem grows quickly. There are a few ways to mitigate the problem. For instance, the detector can be segmented, for a multi-detector setup subsequent detectors can be rotated with respect to one another (ghosts move while real hits dont), or by adding additional measurments at an angle with respect to the XY-strips.

Adding UV strips

Additional strips can be added at an angle (often called U or V strips) to help reduce the ambiguity. Lets look at the same situation with an additional strip (U-strip) at 45° from the X and Y-strips. In this case, the U-strip can disambiquate some of the ghost hits. Now ghost hits only exist at the crossing of the XY-hit if they also fall on a U-hit also.

Resolution of U-strip: {{resolution}} $\mu$m;

#real hits: {{ n_real }} and #ghost hits: {{ n_ghost_uv}} / {{n_ghost}} (with / without U-strip)

What angle is optimal for the U-strips?

Instead of 45° U-strips, lets look at some other angles to see how they compare for rejecting the ghost hits.

Angle of U-strip: {{angle}}°

#real hits: {{ n_real }} and #ghost hits: {{ n_ghost_uv2}} / {{n_ghost}} (with / without U-strip)





[1] Abusleme, A., et al. “Performance of a Full-Size Small-Strip Thin Gap Chamber Prototype for the ATLAS New Small Wheel Muon Upgrade.” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol. 817, May 2016, pp. 85–92. ScienceDirect, doi:10.1016/j.nima.2016.01.087.