Time resolution of radiation hard Resistive Plate Chambers for the CBM experiment at FAIR

The Compressed Baryon Matter (CBM) experiment at FAIR/Darmstadt studies highly compressed nuclear matter at moderate temperatures in relativistic heavy-ion collisions. CBM uses rare probes for studying fundamental issues of QCD. Currently the technical design of CBM foresees a time-of-flight wall (TOF) equipped with resistive plate chambers (RPCs) 10 m downstream from the target. The inner part of the TOF covers 50-100 mrad around the beam axis with estimated flux densities of charged particles of ~2·10^4 per s and cm², thus radiation hard detector components are indispensable in the forward direction.

RPC tests were conducted at the superconducting electron linac ELBE at FZ Dresden-Rossendorf using MeV electron beams mimicking GeV minimum ionizing particles for detector studies. The pulse width of ~5 ps of the ELBE beam provides the possibility to use its radio frequency directly as a time reference. ELBE also provides high rates (up to 1 mA beam current).

Conventional timing RPCs with standard float glass electrodes are only efficient up to flux densities of < 1·10^3 per s and cm². Accelerating the charge transport requires electrodes with lower volume resistivity while maintaining the time resolution (< 100 ps).

Timing and rate parameters of various RPCs were tested at ELBE, including conventional RPCs with float glass electrodes, as well as RPC electrodes made of special glasses (silicated) with 3-4 orders of magnitude lower volume resistivity than float glass (10^13 Ohm·cm).

Results are reported with emphasis on alternative electrode materials, i.e. polypropylene material and Pestov glass. The NIPNE group developed a prototype RPC with Pestov glass electrodes (10^10 Ohm·cm) read-out via stripe lines or pads.

Also ceramics electrodes are suggested for high rate timing RPCs. New ceramics material have recently become available for laboratory and prototype tests.

These tests will provide data for a decision on the type of high rate RPCs for CBM at FAIR.