Time-of-Flight resolution of the CTOF@CLAS12 prototype counter equipped with magnetic-resistant fine-mesh photomultiplier tubes

The novel CLAS12 Detector is being currently developed for the CEBAF upgrade to 12 GeV. One its component will be the Central Time-of-Flight system (CTOF). The CTOF will provide the detection and the identification of charged particles emitted at the central angles from 40° to 130°. The design goal is to achieve the time-of-flight resolution σTOF = 50 ps. This study examined the CTOF prototype counter. The counter was made of Bicron-408 scintillation bar viewed by magnetic-resistant fine-mesh HP R7761-70 and R5924-70 photomultiplier tubes through 600 mm and 700 mm long light guides respectively. The measurement was done using 35 MeV protons of MC50 Cyclotron at Korea Institute of Radiological and Medical Sciences. The light transfer efficiencies of the light guides were determined to be 54% and 48%. The proton light output was extrapolated to that of minimum-ionizing particles (MIPs) using the calibration with cosmic-ray muons. The dependence of the time-of-flight resolution on the energy deposited by minimum ionizing particles was derived as σTOF(L) = (120.7±1.64ps)/(L(MeV))1/2. The expected CTOF resolution for MIPs was estimated to be σTOF ≤ 50 ps.