A beam test of a prototype of the BESIII drift chamber in magnetic field

A prototype of the BESIII drift chamber was tested with He/C3H8(60/40)He/C3H8(60/40) gas mixture in a 1T magnetic field at the ππ-2 beam line of KEK 12 GeV PS. The drift distance–time relationship was extracted for various conditions. The performance of the chamber, such as the spatial resolution, the dE/dxdE/dx resolution and the cell efficiency, was studied in detail. The dE/dxdE/dx was measured as a function of βγβγ to calculate the particle separation power. Based on the test results, the operating voltage of the BESIII drift chamber is optimized to be 2200 V, resulting in a spatial resolution better than 110μm, a cell efficiency over 98%, a dE/dxdE/dx resolution better than 5% and the 3σπ/K separation at a momentum exceeding 700MeV/c. These results confirm the validation of the physics design of the BESIII drift chamber.