First test results of TRD prototypes for CBM with alternating wires

The CBM (Compressed Baryonic Matter) at FAIR will be dedicated to the exploration of the QCD phase diagram in the region of high net-baryon densities using heavy-ion collisions. The CBM Transition-Radiation Detector (TRD) has to deliver a good tracking and electron identification performance in an unprecedented high particle-density environment. A thin Multi-Wire Proportional Chamber (MWPC) without drift region delivers the required fast detector response for the expected high signal rates. One key challenge is to achieve stability of the gas gain. To reduce its sensitivity to cathode deformations, an alternating wire structure, as proposed for the ALICE VHMPID (Nucl. Instrum. Meth. A698 (2013) 11-18), is exploited. Field wires are introduced between the sense wires to improve the field line distribution and its stability. An asymmetric structure, i.e. one cathode is closer to the wire plane than the other, provides fast removal of positive ions. Prototypes with standard and alternating field wires, symmetric and asymmetric structure, and different pad planes have been build and tested with an ⁵⁵Fe source to measure position and pressure dependent gain variations.