The drift chamber is the innermost sub-detector of the BESIII detector. It is one of the most important sub-detectors. Its main functions are:
1）Precise momentum measurement. To achieve this, special cares should be taken to minimize the effects of multiple Coulomb scattering in the design;
2）Adequate dE/dx resolution for particle identification;
3）Good reconstruction efficiency for short tracks from interaction point;
4）Realization of charged particle trigger at level one;
5）Maximum possible solid angle coverage( ～ 90%4 π Sr.) for charged track measurement.
To meet the above requirements, the BESIII drift chamber is designed to be a small-cell, low-mass drift chamber operated using a helium-based gas mixture and located in a 1T magnetic field, with a spatial resolution better than 130 mm averaged over the cell, a dE/dx resolution better than 6% for pi/K separation up to a momentum of 700 MeV/c, and a transverse momentum resolution of about 0.5%@1 GeV/c.
The BESIII drift chamber adopts the drift cell with almost square shape. All the cells are arranged in 43 circular layers. The radial extent is from 73 to 771mm occupied successively by 8 stereo layers, 12 axial layers, 16 stereo layers and 7 axial layers. Each layer contains a field wire layer (with all field wires) and a sense wire layer (with alternating sense and field wires). The cell size is about 12×12 mm2 for the 8 inner layers, in order to have high granularity and less aging effect, and about 16.2×16.2 mm2 for the following 35 layers. The sense wire is 25μm gold-plated tungsten and the field wire is 110μm gold-plated aluminum. The working gas of the chamber is chosen to be He/C3H8(60/40), together with the use of other low-mass materials, so as to minimize the effect of multiple scattering. Experimental study shows that it can provide good spatial resolution and dE/dx resolution.
The chamber is designed to consist of two parts, an inner chamber and an outer chamber. The inner chamber can be replaced if it malfunctions due to radiation damage. To maximize the polar angle coverage while accommodating the intrusion of the interaction quadruples, the end-plate of the inner chamber is designed to be a tilt shape, while that of the outer chamber has a multi-stepped and tilted shape to reduce the deformation caused by wire tension in large radius. The inner diameter of the drift chamber is 118 mm for easy assembly of the beam pipe. The physical outer diameter is designed to be 1600 mm to achieve good momentum resolution. The length is 2300 mm with the outmost sense wire layer covers a polar angle of |cosθ|=0.83.