Time resolution of radiation hard resistive plate chambers for the CBM experiment at FAIR

The Compressed Baryonic Matter (CBM) experiment at FAIR/Darmstadt aims at the investigation of highly compressed nuclear matter at moderate temperatures in relativistic heavy-ion collisions. CBM uses rare probes for studying fundamental questions of QCD. Currently the technical design of CBM foresees a time-of-flight (TOF) wall equipped with resistive plate chambers (RPCs) 10m downstream from the target. The inner part of the TOF wall covers 50 – 100 mrad around the beam axis with estimated flux densities of charged particles of ≈2 · 10⁴ s⁻¹ cm⁻², 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 electrons mimicking GeV minimum ionizing particles (MIPs) 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 1mA beam current). Conventional timing RPCs with standard float glass electrodes are only efficient up to flux densities of ≪ 10³ s⁻¹ cm⁻²2. Decreasing the RC time constant of the electrodes requires material 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 (10¹³ Ωcm). The NIPNE (Bucharest, Romania) group developed a prototype RPC with Pestov glass [5] electrodes (10¹⁰ Ωcm) read out via stripe lines.