High efficiency Transition Radiation Detectors for high counting rate environments

One of the options currently considered for lepton identification with the CBM experiment [1] for FAIR is a shell of multiple layers of Transition Radiation Detectors (TRD). Being a heavy-ion fixed target experiment in the energy range of 5-35 A·GeV the goal of CBM is to look for rare probes using the unique performance of FAIR in terms of high intensity heavy ion beams. This requires a high granularity TRD with good performance specifically engineered for a high counting rate environment. In this context, the optimization of the e/π rejection factor relative to the number of layers and electronic read-out channels is a challenging requirement. Various prototypes [2] based on Multiwire Proportional Chambers (MWPC) were designed, built and tested. Up to intensities of 100 kHz/cm, no major deterioration of their performance in terms of pulse height and position resolution has been observed. However, this performance was reached at the expense of a low conversion efficiency for transition radiation in such a single layer MWPC based TRD. In order to circumvent this aspect, we designed and built several variants of a new prototype of TRD based on a double sided pad read-out electrode[3] with gas volumes on either side. Neglecting about 15% absorption in the central electrode, the double sided configuration is equivalent to a detector of 12 mm gas thickness, preserving the time response of the one of half the thickness [4].