Upgrade Of The Time-of-flight Detector At F1 Cave On RIBLL2

The second Radioactive Ion Beam Line in Lanzhou?RIBLL2?is a fragment separator based on the projectile fragmention method.After running in operation,it has been used to successfully carry out a series of experimental studies in light mass nucleus region?A<40?.Due to the small installation space,the current start time detector at F1consists of an EJ200 plastic scintillator coupled to photomultiplier tube single-end readout.In the beam experiments,the hit positions of the particles made the time resolution worse,which had serious effects on the heavy particle mass identification on the RIBLL2 beam line.To improve the mass identificated in the heavy nucleus region,the performance of the start time detector at F1 needs to be upgraded.To identify the nuclei with a mass up to 80,the simulated results show that the time resolution of the F1 detector must be better than 140.3 ps and the position resolution should be better than 2mm.Silicon photomultipliers?SiPMs?have obtained a growing attention as an alternative to the traditional photomultiplier tubes in application on the fast detection of scintillation light thanks to their compactness,low cost.It is particularly suitable for the upgrate of the F1 detector in a small space.In this paper the time performance of a plastic scintillator detector with an area of 10×10 cm² and read out by multiple silicon photomultipliers?SiPMs?at both ends was studied by change SiPM connection and scintillator type and dimension.The results tested with a ²³⁹Pu source shown that a resolution less than 131 ps was achieved with a 0.1 cm thickness EJ232 scintillator detector read out on each side by 12 SiPMs conncted in series.According to these studies,the upgrading scenarios of the F1 detector,which will be read out by multiple SiPMs connected in series at four ends,was determined.Meanwhile a TOF wall consists of 50 scintillator strips with a width of 2mm should be constructed for position measurement.According to the upgrading scenarios the F1 detector has been developed.The A time resolution of??89.1 ps was achieved for the primary beam of ⁷⁸Kr at 300MeV/u.Moreover,the fragments with a mass up to 75 produced by the fragmentation of ⁷⁸Kr at300MeV/u were identified clearly at the RIBLL2 under full acceptance conditions.This result will help us to expand the research field from light mass nucleus region?A<40?to medium mass nucleus region?A?80?.The desired goal has been achieved by this new F1 detector.