Time-of-flight system for a CTOF prototype counter with fine-mesh and standard photomultipliers

The upgrade of Continuous Electron Beam Accelerator Facility (CEBAF) is undergo since 2010. The updated CEBAF Large Acceptance Spectrometer (CLAS12) detector is being developed for the JLab upgrade program up to 12 GeV. One of the important component of CLAS12 will be the central time-of-flight system (CTOF). The CTOF will provide detection and identification of charged particles emitted at central angles from 40° to 130°. The design goal is to achieve a time-of-flight resolution σ TOF = 50 ps. This study evaluated the timing properties of the CTOF prototype counter. The performance of the upgraded prototype 3 × 3.2 × 66 cm3 scintillation bar of BC-408, coupled to fine-mesh R5924-70 and R7761-70 Hamamatsu photomultipliers via acrylic light guides, has been measured by using two different methods: cosmic rays at Kyungpook National University (KNU), and proton beams at the MC-50 cyclotron of the Korea Institute of Radiological & Medical Sciences (KIRAMS). The gain and the timing resolution of fine-mesh PMs were compared with a conventional Hamamatsu R2083 PM. In the cosmic-ray test, the TOF resolutions were measured to be σ FM-PM = 79 ± 0.75 stat ± 4 syst ps and σ R2083 = 73 ± 1.3 stat ± 4 syst ps for fine-mesh and ordinary photomultipliers, respectively. With proton beams, these values were σ FM-PM = 39.1 ± 0.55 ps and σ R2083 = 35.0 ± 0.54 ps at count rate 0.15 MHz. These remained constant at higher count rates up to 1.5 MHz. The average ratio of the fine-mesh to the ordinary PM resolutions was 1.123 ± 0.02.