Performance of a small AT-TPC prototype

small prototype of an active-target time projection chamber detector based on the gas electron multiplier (GEM) readout technique was designed and constructed. Simulation calculations were performed to analyze the electric field distribution and optimize the design. In particular, a guard ring installed above the GEM foil may play an important role in shaping the electric field in the gas vessel and improving the overall performance of the detector. The best results were obtained at a guard-ring voltage of − 950 V. By using a collimated α -particle source, an energy resolution of 3.6–4.0 keV can be achieved for deposited energies of 18–21 keV, respectively. A position resolution of less 0.2 mm was observed along the electron drift direction, whereas the resolution in the readout plane was approximately 0.45 mm. A time resolution of less than 20 ns was obtained. By using helium as the primary working gas as well as the target material, α + α elastic scattering events can be clearly imaged.