Development of a negative ion micro TPC detector with SF₆ gas for the directional dark matter search

A negative ion micro time projection chamber (NIμTPC) was developed and its performance studied. An NIμTPC is a novel technology that enables the measurement of absolute $z$ coordinates for self-triggering TPCs. This technology provides full-fiducialization analysis, which is not possible with conventional gaseous TPCs, and is useful for directional dark matter searches in terms of background rejection and the improvement of the angular resolution. The developed NIμTPC prototype had a detection volume of 12.8 $\times$ 25.6 $\times$ 144 mm${}^3$. The absolute $z$ coordinate was determined thanks to the different drift velocities between majority and minority carriers with a location accuracy of 16 mm. Simultaneously, there was a successful reconstruction of the three-dimensional (3D) tracks with a spatial resolution of 130 μm. This is the first demonstration of 3D tracking with the detection of absolute $z$ coordinates, and it is an important step in improving the sensitivity of directional dark matter searches.