Performance study of a MAPS detector prototype based on test beam

A detector prototype based on MAPS (monolithic active pixel sensor) is under development for the upgrade of the BESIII inner tracker. Pixel detector ladders with low material budget and high chip position precision are being developed. Each ladder consists of ten MIMOSA28 chips thinned to 50 mu m, a flex cable and a carbon fibre support. In order to verify the design and quantify the performance of the ladders in terms of the spatial resolution, the detection efficiency, the gap between the neighbouring chips on the ladder and the material budget of the ladder, a detector prototype system was set up and tested at the T24 beamline at DESY. The system consists of five layers of ladders, readout electronics and data acquisition. The test results show that the spatial resolution is about 5 mu m, and a tracking efficiency of 96% is achieved, the loss of about 4% of the efficiency most likely coming from the readout and the DAQ system. The average gap between the active areas of the neighbouring chips is about 380 mu m. The actual gap and the chip location accuracy is better than 10 mu m if the insensitive part of the chip is taken into account. The material budget of one ladder is 0.35 +/- 0.03 (sys.)% X-0, which is consistent with the predicted value of 0.37% X-0. These test results validate a good performance of the prototype.